AIRCRAFT PRODUCTION TECHNOLOGY COURSE DEPARTMENT OF AERONAUTICAL ENGINEERING

ACADEMIC YEAR: 2019-20 FACULTY NAME: L.SUSHMA SUBJECT: APT SEMISTER: I

S.NO. Unit/ Topic Sub Topic Names No. NO Classes Required

Various process employed in aircraft industry, Types of patterns, Process involved in , 3 -casting, centrifugal casting, and . 3 I. Working Principles and equipment used with emerging trends in arc , gas welding, 2

resistance welding, Laser welding, 2

EBM, EDM, 2

Soldering and techniques. 2

Classification of processes, Types of chips, working principles&types of , 2 , machines 2

II. grinding (designation of ), m/c, 2

CNC machining (overview of G-Codes, M-Codes). 2 operations- shearing, , super plastic and . 2 Bending, Automation in bend forming and different operations in bending like stretch forming, spinning, etc. 2

Principles of working and applications of jet machining, ultrasonic machining, electron beam, 2 III. EDM, and plasma arc machining, 2

Water jet machining, Ion beam machining. 3 Heat treatment of Aluminum alloys, titanium alloys, steels, case IV. hardening, 1 Prepared by L. SUSHMAASSOPROF 1 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

initial stresses and the stress alleviation procedures. 3 Corrosion prevention, protective treatment for aluminum alloys, steels, anodizing of titanium alloys, organic coating, and thermal spray coatings. 2

Jigs & Fixtures: Jigs, fixtures, stages of assembly, types and equipment for riveted joints, bolted joints (only). 1

Aircraft Tooling Concepts.- types of tools used in A/C industry. 1 V. NDT and Other Inspection Techniques: comparison of NDT & DT, process involved in Dye Penetrate Test, 2

X-ray, and magnetic particle and ultrasonic testing. 1 TOTAL

Text/ Reference Books

Reference 1. Aircraft Production Technology by Keshu and Ganapathi. 2. Production Technology by P.N.Rao 3. Workshop Technology- Khanana publications

Faculty Faculty In-Charge Head of the Dept

Prepared by L. SUSHMAASSOPROF 2 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE MALLA REDDY COLLEGE OF ENGINEERING & TECHNOLOGY L T/P/D C II Year B. Tech, ANE-I Sem 3 -/-/- 3 AIRCRAFT PRODUCTIONTECHNOLOGY Objectives: Student can acquire knowledge on various production technologies involved in aircraft manufacturing.

UNIT - I Casting and Welding Techniques: Various molding process employed in aircraft industry, Types of patterns, Casting Process involved in Sand casting, die-casting, centrifugal casting, investment casting and shell molding. Working Principles and equipment used with emerging trends in arc welding, gas welding, resistance welding, Laser welding, EBM, EDM, and brazing techniques.

UNIT - II Machining and Forming: Classification of machining processes, Types of chips, working principles (with schematic diagram only) , types-lathe, shaper, milling machines, grinding (designation of grinding wheel), drilling m/c, CNC machining (overview of G-Codes, M-Codes). Sheet metal operations- shearing, punching, super plastic forming and diffusion bonding. Bending, Automation in bend forming and different operations in bending like stretch forming, spinning, drawing etc.

UNIT - III Unconventional Machining: Principles of working and applications of abrasive jet machining, ultrasonic machining, electron beam, EDM, and plasma arc machining, Water jet machining, Ion beam machining.

UNIT - IV Heat Treatment and : Heat treatment of Aluminum alloys, titanium alloys, steels, case hardening, initial stresses and the stress alleviation procedures. Corrosion prevention, protective treatment for aluminum alloys, steels, anodizing of titanium alloys, organic coating, and thermal spray coatings.

UNIT - V Jigs & Fixtures: Jigs, fixtures, stages of assembly, types and equipment for riveted joints, bolted joints (only). Aircraft Tooling Concepts.- types of tools used in A/C industry. NDT and Other Inspection Techniques: comparison of NDT & DT, process involved in Dye Penetrate Test, X-ray, and magnetic particle and ultrasonic testing.

Prepared by L. SUSHMAASSOPROF 3 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

AIRCRAFT PRODUCTION TECHNOLOGY Course file

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Unit – I Introduction

Prepared by L. SUSHMAASSOPROF 5 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Aircraft production technology Production of modern aircrafts involves the manufacture of several thousands of individual parts to precise dimensions and assembling them together with brought out components. The parts to be manufactured include those required for the airframe structure and bracketry, pipelines, electic cable runs etc for installing aircraft systems. The individual parts of the aircraft structure are assembled in several stages such as minor subassemblies, sub assemblies and major assemblies to form the aircraft structure. Unlike mass manufacturing practices prevalent in general engineeringindustries, aircraft are produced in relatively small quantities. Very high standards of quality control are required to meet the stringent design specifications and ensure safety of human lives. All raw material and components should comply with airworthiness standards and traceable throughout the process of manufacture and service usage to enable proper analysis of defects / flying accidents. Extensive specific to type production tooling is employed for achieving the required close dimensions and tolerances, surface finish and interchangeability of components. Many manufacturing processes and production machines have been specially developed for aircraft industry, such as, heat treatment and surface treatment processes, CNC machines etc. some of these are later adopted in general engineering industries. Sophisticated management system has been evolved in aircraft industry to cope with the complex technologies, high risk and investment, long gestation periods for design, development and production for series preparation and prolonged pay back periods.

Manufacturing:Manufacturing implies making of articles or goods and providing services to meet the human needs. It creates value by useful application of physical and mental labour in the process. This, however, is too inadequate a definition to give a clear picture of domain of manufacturing which is more complex and broad based than what it appears from this definition.

It can be defined as a chain of interrelated activities and operations as Order processing Design Drawing Selection of materials Process planning Production Production control Quality control Management Marketing. Etc

Most of the metals used in industry are obtained as ores. These ores are subjected to suitable reducing or refining processes which convert the metal into a molten form. This molten metal is poured into moulds to give commercial , called ingots. These ingots are further subjected to one or more processes to obtain usable metal products of different shapes and sizes. All these further processes used for changing the ingots into usable products can be classified as follows

Classification Manufacturing Process: They are mainly classified into six groups:

 Primary Shaping (or) forming process: Primary shaping is manufacturing of a solid body from molten or gaseous state or forms an amorphous material. Amorphous materials are liquid, gaseous, powders, fibers, chips, melts and like. A primary shaping or forming tool contains a hollow space, which, with the allowance for contraction usually corresponds to the form of the product. Here, cohesion is normally created among particles. Some of the important primary shaping processes are;

1. Casting 2. Powder 3. Plastic technology

 Deforming processes: Deforming processes make use of suitable stresses like compression, tension, and or combined stresses to cause deformation of the materials to produce required shapes without changing its mass or material composition. In forming, no material is removed; they deformed and displaced. Some of the deforming processes are;

1. 3. 2. 4. Sheet metal working Prepared by L. SUSHMAASSOPROF 6 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

5. Rotary 7. 6. Thread Rolling 8.

 Machining (or) removing processes: The principle used in all machining processes is to generate the surface required by providing suitable relative motion between the work piece and the tool. In these processes material is removed from the unwanted regions of the input material. In this work material is subjected to a lower stress as compared to . Some of the machining processes are;

1. 5. EDM 2. Drilling 6. ECM 3. Milling 7. Shaping & Planning 4. Grinding 8. Ultrasonic machining

 Joining processes: In this two or more metal parts are united together to make sub-assembly or final product. The joining processes are carried out by fusing, pressing, rubbing, riveting or any other means of assembling. Some of the joining processes are;

1. Pressure welding 4. Resistance Welding 2. Diffusion welding 5. Explosive welding 3. Brazing 6. Soldering

 Surface finish processes: These processes are utilized to provide intended surface finish on the metal surface of a job. By imparting surface finishing processes, dimension of the part is not changed functionally, either negligible amount of the metal is removed from or certain material is added to the surface of the job. Surface cleaning processes is also accepted as surface finishing processes. Some of the surface finishing processes are

1. Plastic coating 7. Honing 2. Metallic coating 8. Tumbling 3. Organic finishes 9. Electro- 4. Anodizing 10. 5. Buffing 11. Sanding 6. Inorganic finishes

 Material properties modification processes

1. Heat and surface treatment 2. Annealing 3. Stress relieving

The selection of manufacturing process depends on: i. Type & nature of the starting material. ii. Volume of production. iii. Expected quality & properties of the components. iv. Technical viability of the process. v. Economy. vi. Other factors influencing are – Geometric shape, tooling, jigs fixtures, gauges, equipment and delivery date.

Prepared by L. SUSHMAASSOPROF 7 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE MOULDING PROCESSES

1. Classification based on the method used Classification based on the material used

a. Bench Molding i. Expandable Molding

a. Sand Molding b. Floor Molding Green Sand Moulding c. Pit Molding Dry Sand Moulding Skin Dried Moulding d. Machine Molding Sand Moulding

Cemented bounded Sand Molding

Loam Sand Moulding Jolting CorbondioxideMoulding Squeezing b. Shell Molding

Jolt and squeeze c. Vacuum molding

Slinging d. Investment casting andMoulding e. Plaster and

ii. Non expandable molding

a. b. c. Centrifugal d. Continuous e. Slushcasting Permanentcas casting casting ting a.BENCH MOLDING: is for small work, done on a bench of a height convenient to the molder. b.FLOOR MOLDING: When castings increase in size, with resultant difficulty in handling, the work is done on the floor. This type of molding is used for practically all medium and large size castings. c.PIT MOLDING: Extremely large castings are frequently molded in a pit instead of a . The pit acts as the drag part of the flask and a separate cope is used above it. They sides of the pit are brick kind, and on the bottom there d.MACHINE MOLDING: Machines have been developed to do a number of operations that the molder ordinarily does by hand. Ramming the sand, rolling the mold, forming the gate and drawing the can be done by these machines. There are three main types of machines: Jolting Squeezing Sand slingerOr combination of above three

JOLTING: In the jolting molding machine, the pattern and flask are mounted on a mould plate and flask is filled with sand. The entire assembly is raised a small amount by means of a cylinder and is then dropped against a fixed stop. The compacting of sand is achieved by the decelerating forces acting on it. SQUEEZING: this machine has a fixed flask and movable flask which compresses the sand in the Prepared by L. SUSHMAASSOPROF 8 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE molding flask. SAND SLINGER: in this machine the sand is thrown out by centrifugal force from a rapidly rotating single laded impeller and directed over the pattern in the flask. Any of the two are combined iPERMANENT MOULD In this method the same mould is used for large numbers of castings. Each casting is released by opening the mould rather than by destroying it. Permanent moulds need to be made of a material which can withstand the temperature fluctuations and wear associated with repeated casting. A good example of a product made with methods such of this is the ubiquitous ‗die-cast‘ child‘s toy (‗die‘ is another word for ‗mould‘). iiEXPENDABLE MOULD AND PATTERN With this type of casting, a pattern is made from a low melting point material and the mould is built around it. The pattern is then melted or burnt out as the metal is poured in. The mouldhas to be destroyed to retrieve the casting.This method is used to make moulds for casting high melting-point alloys like those used for jet engine turbine blades. A model (the pattern) of the blade is made in wax. The pattern is then coated in a thick slurry containing ceramic particles. The slurry dries, and is then fired in an oven: this hardens the ceramic (like firing a pot) and melts out the wax, leaving a hollow ceramic mould. The metal is then poured in to the mould, which is broken away after the metal has solidified and cooled.

CASTING is a manufacturing process by which a liquid material is (usually) poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solid casting is then ejected or broken out to complete the process. Casting may be used to form hot liquid metals or various materials that cold set after mixing of components (such as epoxies, concrete, plaster and clay). Casting is a 6000 year old process. The casting process is subdivided into two distinct subgroups: expendable and non-expendable mold casting.

Advantages of Castings

 Intricate can be achieved, molten metal can be made to flow into any small section in the mould cavity and as such any intricate shapes internal or external can be made with the casting process  Castings of ferrous and nonferrous metals are practically possible  Necessary tools required for casting mould are simple and inexpensive. As a result, for trial production or of a small lot, it is an ideal method it is possible in casting process, to the amount of material where exactly require. As a result, weight reduction in design can be achieved  Castings are generally cooled uniformly from all sides and therefore they are expected to have no directional properties  Castings of and weight, even up to two hundred tones can be made.

Steps involved in Casting are

1) Pattern making 2) Mould and core preparation (or)making 3) Making the provisions like gating, , runner, 4) Melting and pouring the metal 5) Allow the mould to solidify 6) Fettling i.e. Cleaning 7) Testing and inspection of the casting

Prepared by L. SUSHMAASSOPROF 9 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

CASTING TERMS: PATTERN: Pattern is the replica of the final object to be made with some modifications. The mould cavity is made with the help of the pattern. PARTING LINE: This is a line dividing the molding flasks that makes up the sand mould. In split pattern it is also the dividing between the two halves of the pattern BOTTOM BOARD: This the board normally made of wood which is used at the start of the mould making. The pattern is first kept on the board, sand is sprinkled on it and then ramming is done in the drag. SAND:Small amount of carbonaceous material sprinkled on the inner surface of the mould cavity to give a better surface finish to the castings. MOULDING SAND: Mixture of silica, clay and moisture in appropriate proportions to get the desired results and it surrounds the pattern while making the mould cavity. BACKING SAND:It constitutes the refractory material found in the mould. This is made up of burnt sand. FLASK:A moulding flask is one which holds the sand mould intact. Depending upon the position of the flask in mould structure it is referred to by various names - drag – lower moulding flaskcope – upper moulding flaskcheek – intermediate moulding flask

CHILLS:To control the solidification and metallurgical structure of the metal, it is possible to place metal plates—chills— in the mold. The associated rapid local cooling will form a finer-grained structure and may form a somewhat harder metal at these locations. In ferrous castings the effect is similar to quenching metals in work.

CORES:Cores are separate shapes of sand that are generally required to form hollow interiors of the casting or a hole through the casting. To produce cavities within the casting—such as for liquid cooling in engine blocks and cylinder heads—negative forms are used to produce cores. Usually sand-molded, cores are inserted into the casting box after removal of the pattern. Whenever possible, designs are made that avoid the use of cores, due to the additional set-up time and thus greater cost.

Requirements:

1. Cores must be strong enough to retain its shape without deforming, to withstand handling and to resist erosion and deformation during filling of the mould 2. Cores must be permeable to allow the core gases to escape easily 3. Cores should be highly refractory in nature to withstand high temperature of the molten metal 4. Cores must be sufficiently low in residual gas-forming materials to prevent excess gas entering the metal 5. Cores must be stable with a minimum of contraction and expansion to make a true form of casting 6. Cores should be sufficiently collapsible, i.e., they should disintegrate and collapsible after the metal solidifies, to minimize strains on the castings and to facilitate removal of the core from the castings during shakeout.

Prepared by L. SUSHMAASSOPROF 10 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE PATTERNPattern is the replica of the component to be produced by casting process and is used to prepare mould cavity.The success of a casting process depends a lot on the quality and the design of pattern. PATTERN MATERIALSgenerally pattern is prepared using any of the following 1. Wood 2. Metal 3. Plastic 4. Plaster & Polyurethane foam 5. Wax or Mercury.

1. WOOD: Most popular material for pattern making is wood.

Advantages Limitations Applications

Inexpensive Susceptible to shrinkage Used when numbers and swelling of castings are more and size is large. Availability Poor wear resistance For making patterns Ease of Fabrication in various Abraded easily by sand action complex forms Lightness Cannot withstand rough handling Easy to obtain good surface finish Weak compared to metal. Can be Preserved for quite long Absorb moisture, time by application of shellac consequently get warped and coating change shape and size Wood pattern has limited life because of distortion and dimensional Change will occur as it is having less resistance. They have high strength to weight ratio, and are tough and strong. Ease of maintenance.

Types of wood used for pattern making

Pine wood Teak wood Mahogany softwood and Light straight Hard wood straight grained grained wood unlikely to wrap Easy to shape Most suitable for patterns close grain structure Tendency to warp More durable than above woods Easy to work Moderate cost uniform grain structure low cost Durability Easy to shape Harder Costlier than above woods. Indian woods: Deodar, walnut, kali, cherry, birch wood do not required any seasoning and are harder.

Prepared by L. SUSHMAASSOPROF 11 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE 2.Metal

Advantages Limitations Applications

Most suitable for mass production Expensive than wood used were more number of as they are stronger and accurate castings are to be casted with Not affected by moisture and No Higher weight same patterns war page Less wear and withstand rough Tendency to get rusted handling Suitable for precision and intricate shapes Good machinability characteristics better surface finish Excellent wear resistance and strength to weight ratio

Types metals used in the pattern preparation

Aluminum Alloys: Higher preferred Meta, Excellent resistance to corrosion and swelling, good machinability, strength and wear resistance, cheap, low melting point and good surface finish. Grey Fine grained inexpensive durable, easily machined and withstand abrasive Carbon 2 to 4 %,si action, goodsurface finish,larger weight, harder and brittle and may easily 3.5% break, thin sections are difficult to machine.

Steelsi, mg, s, p.h Fair machinability, excellent wear & swelling resistance, good strength, high weight and poor resistance to rust and corrosion.

Brass and Easily worked and machined, can be joined by soldering and brazing, Good (CU and alloys) surface finishand will not rust, High strength and toughness, Can be made in to very thin sections Alloys & white Low shrinkage, good cast ability, low temp (260) and light weightIntricate metal shapes can e obtained, mostly used I die casting

3.PLASTER:- Made out of Gypsum cement (plaster of Paris) -High compressive strength-300 kgcm2 -controlled expansion on solidification - No shrinkage allowance required Types:- 1. Ultrcal2. Hydrocal3. Hydrostone

4. PLASTICS:- Phenolic thermo setting plastic. Plastic pattern is made from plaster of paris.

5. WAXES:- used in Investment casting Types:- 1. Paraffin wax;bees wax 2.Carnauba wax;ceresin wax 3. Shellac wax.

Prepared by L. SUSHMAASSOPROF 12 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Types of patterns PATTERNS There are various types of patterns depending upon the complexity of the job, the no of castings require and the molding procedure adopted

 Single piece pattern  Skeleton pattern  Split pattern  Segmental pattern  Match plate pattern  Shell pattern  pattern  Built-up pattern  Gated pattern  Boxed-up pattern  Loose piece pattern  Lagged-up pattern  Sweep pattern  Left and right hand pattern  Follow board pattern

Single piece pattern:

Pattern that is made without joints, partings or any loose pieces in its construction is called a single piece or solid pattern. A single piece pattern is not attach to a frame or plate and is therefore, some times none as a loose pattern. When using such patterns the molder as to cut his own runners, feeding gates and risers. This operation takes more time and they are recommended for large pattern of simple shapes.

Split pattern:

Many patterns cannot be made in a single piece because of the difficulties encountered in molding them. To eliminate this difficulty and for castings of intricate design or unusual shape, split patterns are employed to found the mould. These patterns are usually made in two parts, one part will produce the lower half and the upper part will produce the upper half of the mould. The two parts, which may or may not behalf the same size and shape, are held in there proper relative position by means of dowel pins. Fastened in one piece and fittings holes bored in the other. It is some times necessary to construct pattern for a complicating casting that requires three more parts. This type of pattern is known as multi-piece pattern the three part pattern may use a flask having three parts known as cope, cheek and drag

Applications: Use for casting spindles, cylinders, steam valve bodies water stop cocks, and tapes bearings, small pulleys and wheels

Prepared by L. SUSHMAASSOPROF 13 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Match plate pattern:

When split patterns are mounted with one half (cope) on one side of a plate and the other (drag) half directly opposite on the side of the plate, the pattern along with the gating and risering is called a match plate pattern. The pattern made of metal, and the plate which makes the parting line may be either metal or wood. So on one side cope is prepared and the other side drag is prepared after molding the match plate is removed, a complete mold with gating is obtained.

Applications: 1. Used for small castings with higher dimensional accuracy and large production. 2. Used for machine molding. 3. They are expensive since they increase productivity, the additional cost is justified.

Cope & drag pattern: in the production of large castings the complete moulds are too heavy to be handled by a single operator. Therefore cope and drag patterns are used to ease this problem to efficient operation. The patterns are made in two halves, split on a convenient joint line, and separate cope and drag patterns are built and mounted on individual plates to which gating, risering systems are attached on boards. This permits operator or a group to prepare cope half and drag half separately and assembled after molding. This planned distribution increase production appreciably.

Applications: 1. These patterns are used for heavy castings which are inconvenient for handling. 2. used for continuous production.

Gatted pattern: Numbers of castings are produced in a single multicavity mould by joining a group of patterns, and the gates or runners for molten metal are formed by the connecting parts between the individual patterns. This would eliminate the hand cutting of the runners and gates and help in improving the productivity of a molder.

Loose piece pattern: This type of pattern is used when the contour of the part is such that withdrawing the pattern from the mould is not possible. Hence during molding the obstructing part of the contour is

Prepared by L. SUSHMAASSOPROF 14 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE held as a loose piece by a wire. After molding is over, first the main pattern is removed and then the loose pieces are removed through the gap generated by the main pattern. This is a highly skilled job and is generally expensive and therefore, should be avoided where possible.

Applications: 1. Used for large shapes which is axi-symmetrical or prismatic in nature 2. Used for bell shaped castings, cylinders etc.

Skeleton pattern: patterns for large castings require tremendous wood for full patterns. So a ribbed construction with large number of and rectangular openings between the ribs which form a outline of the pattern to be made. The frame work is rammed with clays, packing sand or loam. After packing the sand the desired form is obtained with the help of a stickle.

Segmental pattern: they are sections of patterns so arranged to form a complete mould being moved to form each section of the mould. When making a mould using this pattern, a vertical spindle is firmly fixed in the centre of drag flask. The bottom of the mould is rammed and swept level. Then the segmental pattern is fastened to the spindle. is rammed between the pattern and flask, and in the inside, but not at the ends of the pattern. After ramming one section, it goes to the other section for ramming. And this is repeated until the entire mould perimeter has been completed. Applications: used for circular work such as rings, wheel rims, gears, etc.

Shell pattern: the pattern is mounted on a plate and parted along the centre line, the two sections are accurately doweled together. The shell pattern is a hollow construction like a shell and outside shape is used as a pattern to make mould, while the inside space is used as a core-box for making cores. Applications: Largely used in preparing pipe work, drainage fittings.

Prepared by L. SUSHMAASSOPROF 15 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Follow Board Pattern: A board with a cavity or socket in it which conforms to the form of the pattern and defines the parting surface of the drag.

COLORS USED FOR PATTERNS:

If the surface is to be casted – Black Machined surface _ Red Core print and seats _ Yellow Loose piece _ Yellow / red diagonal strips Stop-offs _ Yellow / lack diagonal strips

Table 4 : A Typical Composition of Molding Sand

Molding Sand Constituent Weight Percent Silica sand 92 Clay (Sodium Bentonite) 8 Water 4

Prepared by L. SUSHMAASSOPROF 16 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE MOLDING MATERIAL AND PROPERTIES

A large variety of molding materials is used in for manufacturing molds and cores. They include molding sand, system sand or backing sand, facing sand, parting sand, and core sand. The choice of molding materials is based on their processing properties. The properties that are generally required in molding materials are: Table 4

. Refractoriness: It is the ability of the molding material to resist the temperature of the liquid metal to be poured so that it does not get fused with the metal. The refractoriness of the silica sand is highest. . Permeability: During pouring and subsequent solidification of a casting, a large amount of gases and steam is generated. These gases are those that have been absorbed by the metal during melting, air absorbed from the atmosphere and the steam generated by the molding and core sand. If these gases are not allowed to escape from the mold, they would be entrapped inside the casting and cause casting defects. To overcome this problem the molding material must be porous. Proper venting of the mold also helps in escaping the gases that are generated inside the mold cavity. . Green Strength: The molding sand that contains moisture is termed as green sand. The green sand particles must have the ability to cling to each other to impart sufficient strength to the mold. The green sand must have enough strength so that the constructed mold retains its shape. . Dry Strength: When the molten metal is poured in the mold, the sand around the mold cavity is quickly converted into dry sand as the moisture in the sand evaporates due to the heat of the molten metal. At this stage the molding sand must posses the sufficient strength to retain the exact shape of the mold cavity and at the same time it must be able to withstand the metallostatic pressure of the liquid material. . Hot Strength: As soon as the moisture is eliminated, the sand would reach at a high temperature when the metal in the mold is still in liquid state. The strength of the sand that is required to hold the shape of the cavity is called hot strength. . Collapsibility: The molding sand should also have collapsibility so that during the contraction of the solidified casting it does not provide any resistance, which may result in cracks in the castings. Besides these specific properties the molding material should be cheap, reusable and should have good thermal conductivity.

Molding Sand Composition: The main ingredients of any molding sand are:

1. Base Sand: Silica sand is most commonly used base sand. Other base sands that are also used for making mold are zircon sand, Chromite sand, and olivine sand. Silica sand is cheapest among all types of base sand and it is easily available.

2. Binder: Binders are of many types such as: 1. Clay binders,2. Organic binders 3. Inorganic binders Clay binders are most commonly used binding agents mixed with the molding sands to provide the strength. The most popular clay types are:Kaolinite or fire clay (Al2O3 2 SiO2 2 H2O) and Bentonite (Al2O3 4 SiO2 nH2O)of the two the Bentonite can absorb more water which increases its bonding power.

3. Moisture: Clay acquires its bonding action only in the presence of the required amount of moisture. When water is added to clay, it penetrates the mixture and forms a microfilm, which coats the surface of each flake of the clay. The amount of water used should be properly controlled. This is because a part of the water, which coats the surface of the clay flakes, helps in bonding, while the remainder helps in improving the plasticity. A typical composition of molding sand is given in (Table 4).

Prepared by L. SUSHMAASSOPROF 17 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE ELEMENTS OF GATING SYSTEMS: Gating system provides the way for the molten metal to flow in to the mould cavity when poured from the ladle. It comprises of

1. Pouring basin 2. Sprue 3. Sprue base well 4. Runner 5. Runner extension 6. In gate 7. Riser 8. Core

Function of gating system:

 To prevent erosion of the mould walls.  The mould should be completely filled in the smallest time possible without having to raise metal temperature nor use metal heads.  Unwanted material such as , and other mould material should not be allowed to enter the cavity  The metal should flow smoothly in to the mould without any turbulence. A turbulent metal flow tends to form dross in the mould.  The metal entry in to the mould cavity should be properly controlled in such a way that aspiration of the atmospheric air is prevented.  A proper thermal gradient should be maintained so that the casting is cooled without any cavities or distortions.  Metal flow should be maintained in such a way that no gating, sprue walls takes erosion.  Gating system design should be economical and easy to implement and remove after casting solidification.  The casting yield should be maximized.

Prepared by L. SUSHMAASSOPROF 18 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE 1. POURING BASIN: This part is made on top or is the first part of the mould. The metal is not directly poured in to the mould cavity because it may cause erosion. Molten metal is poured in to a pouring basin which acts a reservoir from which it moves smoothly into the sprue. The pouring basin is also used to stop the slag and dirt which floats on the top and allow the clean metal underneath it in to the sprue. It should be deep enough to avoid vortexing and turbulence and the molten metal in it should be full enough during the operation, otherwise a funnel is likely to form through which atmosphere air and slag may enter. Sometimes 1 wall or both the walls are kept inclined at 45 to the horizontal so that momentum is observed and vortex formation is avoided.

2. SPRUE: Vertical passage that passes through the cope and connects the pouring basin with the runner or gate at the parting line is called sprue. The molten metal when moving from top of the cope to the parting plane gains velocity and as consequence requires smaller area. So generally a tapered sprue is used so that it will not aspirate the air from the surroundingmould sand.

SPRUE DESIGN

The design of the pouring basin and sprue can affect turbulence. For best results you want to design your pouring basin and sprue so that you can keep the sprue full of molten metal throughout your pour. A sprue tapered to a smaller size at its bottom will create a choke which will help keep the sprue full of molten metal. If you don't use a tapered sprue you can put a choke in when you are making the runners, you will want to have the choke as close to the bottom of the sprue as possible. The choke will also increase the speed of the molten metal, which is undesirable. To address this problem you can create an enlarged area at the bottom of the sprue, called a sprue base. This decreases the speed of the molten metal. There are two basic types of sprue bases, enlargement and well.

Taper angle = 2◦ to 4◦ d = 2.5 times the width of the runner. d = diameter of sprue D = depth of the runner. D = depth of sprue Dw = 2 times of that of the runners. Dw = depth of sprue well A = base is 5 times the cross sectional area of the sprue exit A = Cross sectional area of well

(a 1/2 sq. in. sprue exit would mean you need a base with an area of 2.5 sq. in. which would be a 1.5 inch diameter). The bottom of the sprue base should be flat, not rounded like a bowl. If it's it will cause turbulence in the metal.

3.SPRUE BASE WELL This is a reservoir for metal at the bottom of the sprue to reduce the momentum of the molten metal. The molten metal as it moves down the sprue gains in velocity. Some of which is lost in the sprue base well by which the mould erosion is reduced. This molten metal changes direction and flows into the runners in a more uniform way.

4.RUNNER It is generally in the horizontal plane (parting plane) which connects the sprue to its ingates, thus letting the metal enter the mould cavity. The runners are normally made trapezoidal in cross section. The main reason for this is to trap the slag and dross which are lighter and thus trapped in the upper portion of the runners. For effective trapping of the slag. Runners should flow full. When the amount of molten metal coming from the down sprue is more than the amount flowing through the ingates, the runner would always be full and thus slag trapping would take place. But when the metal flowing through the ingates is more than that flowing through the runners, then the runner would be filled only partially and the slag would then enter the mould cavity.

Prepared by L. SUSHMAASSOPROF 19 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE RUNNER DESIGN One of the most important things to remember in your runners and gates is to avoid sharp corners. Any changes in direction or cross sectional area should make use of rounded corners. Also make sure the runners and gates are well rammed and smooth. This will help avoid sand erosion and turbulence. To ensure that the metal is not flowing too fast in the runners the rule of thumb is that the cross sectional area of the runners should be greater than the area of the choke. The walls of the runners should be as smooth as possible to avoid causing turbulence. The runners should be filled with metal before the gates are, one way to ensure this happens is to put the runners in the drag and the gates in the cope. If you need to have a choke in the runner to restrict flow it should be at least 6" from the first gate.

Gate ratio:

Defined as the ratio of sprue base area : runner area : ingate area Choke: The choke is that part which has smallest crossectional area. This controls the rate of metal flow to help in lower the flow velocity in runner and collect the slag and minimize erosion in runner. It may be located at any of the two places:

1. If located in the sprue well it is unpressurised gating system Gating ratio : 1: 4: 4 This reduces turbulence, and minimizes erosion and oxidation of metal. but slag is not collected and enters the mould cavity

2. If located in the ingate it is pressurized gating system Gating ratio : 1: 0.75: 0.50 This provides back pressure and the runner is always full so slag is entrapped.

5. RUNNER EXTENSION

The runner is extended a little further after it encounters the ingate.This extension is provided to trap the slag in the molten metal. The metal initially comes along with the slag floating at the top of the ladle and this flows straight, going beyond the ingate and then trapped in the runner extension.

6. GATES

Also called ingates, these are the opening through which the molten metal enters the mould cavity. The shape and the cross section of the ingate should be such that it can readily be broken off after casting solidification and also allow the metal to enter quietly into the mould cavity. Depending on the application, various types of gates are used in the casting design. They are:

N TOP GATE BOTTOM GATE PARTING GATE Step gate O 1 This is the typing of gating When molten metal enters This is the most Such gates used through which the molten the mould cavity slowly it widely used in sand for heavy and metal enters the mould would not cause any mould castings. large casting. cavity from the top erosion. 2 .Since the first metal It takes somewhat higher As the name implies, The molten entering the gate reaches time for filling of the mould the metal enters the metal enters the bottom and hotter and also generates a very mould at the parting mould cavity metal is at the top, a unfavorable temperature plane when part of through a favorable temperature gradient. the casting is in the number of gradient towards the gate cope and part in the ingates which is achieved. Also, the drag. are arranged in mould is filled very quickly. vertical steps.

3 But as the metal falls The preparation of the gating For the mould cavity The size of Prepared by L. SUSHMAASSOPROF 20 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE directly into the mould also requires special sprue as in the drag, it is a ingates are cavity through a height, it shown or special cores for top gate and for the normally is likely to cause mould locating the sprue in the cavity in the cope it increased from erosion. Also, because it drag. is a bottom gate. top to bottom cause turbulence in the such that metal mould cavity, enters the mould 4 it is prone to form dross These gates may cause Thus, this type of cavity from the and as such top gate is not unfavorable temperature gating tries to derive bottom most advisable for those gradients compared to the the best of both the gate and then materials which are likely top gating. types of gates,viz. progressively to form excessive dross top and bottom moves to the gates. higher gates. 5 It is not suggested for non- Thus the system may have to Of all the gates, this This ensures a ferrous materials and is use additional padding of is also the easiest gradual filling of suggested only for ferrous sections towards risers and and most economical the mould alloys. Conductive to a large riser sizes to in preparation. without any favorable temperature compensate for the mould erosion gradient but erosion may be unfavourable temperature and produces a high distribution. sound casting. 6 It is suitable only for . However, if the drag In designing a simple casting shape which Bottom gate is generally used portion of the mould casting, it is are essentially shallow in for very deep moulds cavity is deep, it is essential to nature. To reduce the likely to cause mould choose a mould erosion pencil gates erosion and suitable gate, provided in the pouring aggravate dross considering the cup. formation and air casting material, entrapment in the casting shape case if nonferrous and size so as alloys. produce a sound casting. 7 This types of gate requires **Offers smooth flow with a This can be it flows minimum of additional minimum of erosion but solely the walls into runners to lead metal into unfavorable temperature the mould cavity. the cavity, and as such gradient provides higher casting yields.

7. RISER: a. Risers are often provided in molds to feed molten metal into the main cavity to compensate for the shrinkage. b. Risers are important to ensure a flow of molten metal to the part being cast as it's starting to solidify. c. Without a riser heavier parts of the casting will have shrinkage defects, either on the surface or internally. d. As molten metal solidifies it shrinks. If it does not have a source of more molten metal to feed it as it shrinks you will get defects in your casting. e. A riser purpose is to provide that extra molten metal. Basically a riser is a vertical portion of the gating system, similar to a straight sprue that stores the molten metal until it is needed by the casting. This means the metal in the riser must stay liquid as longer than the metal in the part being cast.

There are two types of riser

Open Riser: Top of the open riser is open to atmosphere at the top of the surface, it is cylinder shape

Prepared by L. SUSHMAASSOPROF 21 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Advantages: 1. An open riser is easy to mold, 2. Air can be removed from it and it will not draw metal as a result of vacuum and collects non metallic inclusions which float on the surface. Disadvantages: 1. It is not placed in the drag and reduces yield of the casting. 2. As open to atmosphere losses heat 3. Difficult to remove from casting during cleaning the Casting.

Close/blind riser: Blind risers are domelike risers or cylindrical, found in the cope half of the flask, which are not the complete height of the cope.

Advantages: 1. Can be placed at any position of the mold 2. Can be easily removed from the casting.

Disadvantages: 1. Difficult to mold 2. May draw liquid metal from solidifying casting.

Vent:Small opening in the mold to facilitate escape of air and gases.

8. TYPES OF CORES:

1.HORIZONTAL CORES: This is a most common type of core which is usually cylindrical in form and is laid horizontally at the parting line of the mould. The ends of the core rest in the seats provided by the core prints on the pattern. 2.VERTICAL CORE: This is placed in a vertical position both in cope and drag halves of the mold. Usually top and bottom of the core are provided with a taper, but the amount of taper on the top is greater than that at the bottom. 3.BALANCED CORE: when the casting is to have an opening only one side and only on core print is available on the pattern a balanced core is suitable. The core print in such cases should be large enough to give proper bearing to the core. In case the core is sufficiently long it may be supported at the free end by means of a chaplet (Rods with flat or curved plates riveted to give support). 4.HANGING AND COVER CORE: If the core hangs from the cope and does not have any support at the bottom of the drag, it is referred to as hanging core. In this case, it may be necessary to fasten the core with a wire or rod that may extended through the cope. On the other hand, if it has its support on the drag it is called cover core. In this case the core serves as a cover for the mould, and also as a support for hanging the main body of the core. 5.WING CORE: A wing core is used when a hole or recess is to be obtained in the castings either above or below the parting line. In this case the side of the core print is given sufficient amount of taper so that the core can be placed readily in the mould. This core is sometimes designated as drop core, tail core chair core and saddle core according to its shape and position in the mould. 6.RAM-UP CORE: It is sometimes necessary to set a core with the pattern before the mould is rammed up. Such a core is located in an inaccessible position in both interior and exterior portions of castings. 7.KISS CORE: When the pattern is not provided with a core print and consequently no seat is available for the core, the core is held in position between the cope and drag simply by pressure of the cope. They are suitable when a number of holes of less dimensional accuracy with regard to the relative position of the holes are required.

Prepared by L. SUSHMAASSOPROF 22 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

DIFFERENT PATTERN ALLOWANCES:

1. Shrinkage allowance:

Castings shrink when they cool. Like nearly all materials, metals are less dense as a liquid than a solid. During solidification (freezing), the metal density dramatically increases. This results in a volume decrease for the metal in a mold. Solidification shrinkage is the term used for this contraction. Cooling from the freezing temperature to room temperature also involves a contraction. The easiest way to explain this contraction is that is the reverse of thermal expansion. Compensation for this natural phenomenon must be considered in two ways.

Shrinkage after solidification can be dealt with by using an oversized pattern designed for the relevant . Pattern makers use special "contraction " (also called "shrink rules") to make the patterns used by the foundry to make castings to the design size required. These rulers are 1 - 6% oversize, depending on the material to be cast. These rulers are mainly referred to by their actual changes to the size. For example a 1/100 would add 1 mm to 100 mm if measured by a "standard ruler" (hence being called a 1/100 contraction ruler). The shrinkage caused by solidification can leave cavities in a casting, weakening it. Risers provide additional material to the casting as it solidifies. The riser (sometimes called a "feeder") is designed to solidify later than the part of the casting to which it is attached. Thus the liquid metal in the riser will flow into the solidifying casting and feed it until the casting is completely solid. In the riser itself there will be a cavity showing where the metal was fed. Risers add cost because some of their material must be removed, by cutting away from the casting which will be shipped to the customer. They are often necessary to produce parts which are free of internal shrinkage voids. One method that assists in keeping the metal molten in the riser longer is the utilisation of an exothermic sleeve. Sometimes, to promote directional solidification, chills must be used in the mold. A is any material which will conduct heat away from the casting more rapidly that the material used for molding. Thus if silica sand is used for molding, a chill may be made of , iron, aluminum, graphite, zircon sand, chromite or any other material with the ability to remove heat faster locally from the casting. All castings solidify with progressive solidification but in some designs a chill is used to control the rate and sequence of solidification of the casting.

 All the metals shrink when cooled except bismuth. This is because of the inter atomic vibrations which amplify by an increase in temperature.  Shrinkage of metal during casting takes place in 3 stages 1. Shrinkage of liquid metal from pouring temperature to freezing temperature 2.Shrinkage of liquid metal during solidification 3.Shrinkage of solid metal from freezing temperature to room temperature Stage 1 and stage 2 are called liquid shrinkage, this is taken care by providing risers. Stage 3 is taken care by providing shrinkage allowance.  Rate of contraction with temperature is dependent on the material. Ex Steel contracts to higher degree compared to .  The contraction also depends upon the metallurgical transformation taking place during solidification. Ex in grey cast iron and spherodised graphite iron, the amount of graphitisation controls the actual shrinkage. When graphite is more, shrinkage is less i.e, Carbon content increases shrinkage decreases

Material allowance Solid reaction Thermal contraction White cast iron 16-23.0 mm/m aluminium 13 mm/m 7% 5.6% Aluminium alloys 7 5 Magnesium 13 mm/m Prepared by L. SUSHMAASSOPROF 23 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE 15.5 mm/m 5.5 6 Copper Magnesium alloy 16 mm/m 4.5 7.5 Grey cast iron - 0 3 Grey cast iron with high C 1.8 3 Steel 21 mm/m

2. Machine allowance:Castings may oxidised in the mould during heat treatment so scales etc are formed, even rough surfaces are seen, therefore machining is to be done,therefore the dimensions must be slightly higher to compensate this decrease in dimensions.

 Machine finish allowance should be given on pattern in order to compensate the amount of metal that is lost in machining or finishing the casting to obtain the final casting of required dimensions and surface finish.  The amount of this allowance depends upon the size of casting, type of machining operation such as grinding, turning, milling, etc.  Type of moulding process such as sand casting, die casting etc, and the degree of surface finish

3. allowance:

At the time of withdrawing the pattern from the sand mould, the vertical faces of the pattern are in continual contact with the sand, which may damage the mould cavity, as shown in figure 1. To reduce the chances of this happening, the vertical faces of the pattern are always tapered from the parting line as shown in figure 2. This provision is called draft allowance.

 Draft allowance varies with the complexity of the job.  In general inner details of the pattern require higher draft than outer surfaces.  More draft is needed in case of hand moulding compare to machine moulding.

4. Distortion allowance:

 Weaker section such as long flat portions, U, V, Sections are distortion prone.  Foundry practice should be given extra material provision for reducing the distortion. Alternatively, the shape of pattern itself should be given a distortion of equal amounts in the opposite direction of the likely distortion direction.

5. Shake or Rapping allowance:

Before withdrawal from the sand mould, the pattern is rapped all around the vertical faces to enlarge the mould cavity slightly which facilitates its removal. Since it enlarges the final casting made, it is desirable that the original pattern dimensions should be reduced to account for this increase.

 No quantifying method is available.  Depends on the personnel and practices involved.

It is the negative allowance and is to be applied only to those dimensions which are parallel to the parting plane.

Prepared by L. SUSHMAASSOPROF 24 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Solidification of metals:-

After the metal is pound in to mold, a series of events takes place during the solidification of the casting and its cooling to ambient temperature. These events influence on the size, shape, uniformity and chemical composition of the grains formed through out the casting, which influence the over all properties.

Solidification is a comprehensive process of transformation of the melt of an alloy into a solid piece of the alloy, involving crystallization of the liquid phase, segregation of impurities and alloying elements, liberation of the gases dissolved in the melt, shrinkage cavities and porosity formation.

Factors affecting the solidification:-  Types of Metal  Thermal properties of both metal and mold  Geometrical relationship between volume and surface area of casting.  Shape of the mold

Pure metals:- They have definite meting and freezing point it solidifies at constant temperature.

For ex:- Al ______6600C Iron______15370C Tungsten ______34100C

After the temperature of molten metal drops to its freezing point, it temperature remains constant while the‖ latent heat of fusion‖ is give off. The solidification front i.e. solid –liquid interface moves through the molten metal, solidifying from the mould walls, inward the centre. Once solidification has taken more at any point, cooling resumes the solidified metal is called casting.  For a pure metal as the mold walls, which are at ambient temperature, the metal cools rapidly? Rapid cooling produces a solidified skin or shell of fine equiaxed grains.  Development of a preferred texture at a cool mold wall.  Note that only favorably oriented grains grow away from the surface of mold.  The grains grow in a direction opposite to that of the heat transfer out through mold.  Those grains that have favorable orientation will grow preferentially and are called columnar grains.  As the driving force of the heat transfer is reduced away from the mold walls, the grains become equiaxed and course.  Those grains that have substantially different orientation are blocked from further growth.  Such grain development is known as homogenous nucleation, meaning that grains (crystals) grow upon themsels, starting from mould wall. Nucleation:-  Thermodynamically, it is believed that in any system, a phase change will take place only if there is a reduction in the total free energy of the system.  Solidification of a metal can be considered interms of the movement of atoms in to position in which their free energy is lower than it was in their previous positions in the liquid phase.

Fine and homogeneous grain structure is the most desirable for the common castings and ingots. It is achieved when the crystallization proceeds under the following conditions:

 Formation of a large number of stable nuclei;  Fast extraction of latent crystallization heat and the superheat of the liquid.

Prepared by L. SUSHMAASSOPROF 25 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

These conditions are realized when a melt comes to a contact with a wall of a cold metallic mold. Small equiaxed grains (chill crystals) form at this stage. Latent crystallization heat, liberating from the crystallizing metal, decreases the under cooling of the melt and depresses the fast grains growth.

At this stage some of small grains, having favorable growth axis, start to grow in the direction opposite to the direction of heat flow. As a result columnar crystals (columnar grains) form. Length of the columnar grains zone is determined by the constitutional undercooling. When the temperature of the melt, adjacent to the solidification front, increases due to the liberation of the latent heat, constitutional undercooling will end and the columnar grains growth will stop.

Further cooling of the molten alloy in the central zone of the ingot will cause formation of large equiaxed grains. Formation of the grain zones of an ingot is presented in the figure. The crystals, growing as a result of solidification of ordinary alloys, are in dendrite form.

Segregation

Composition of solidified alloy is not uniform. Concentrations of impurities and alloying elements are different in different parts of the casting. This difference is a result of different solubility of impurities in liquid and solid phases at the equilibrium temperature. Segregation is a result of separation of impurities and alloying elements in different casting regions.

CrystallizationSome metallurgical processes involve phase transition.

The typical example of phase transition is crystallization.

Crystallization is transformation of liquid phase to solid crystalline phase.

There are two general stages of phase transformation (crystallization) process – nucleation and growth:

Prepared by L. SUSHMAASSOPROF 26 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE 1. Nucleation

Nucleation is a process of formation of stable crystallization centers of the new phase. Nucleation may occur by either homogeneous or heterogeneous mechanism, depending on the value of undercooling of the liquid phase (cooling below the equilibrium freezing point).

Presence of foreign particles or other foreign substance in the liquid alloy (walls of the casting mold) enables to initiate crystallization at minor value of undercooling (few degrees below the freezing point). This is heterogeneous nucleation. If there is no solid substance present, undercooling of a hundred degrees is required in order to form stable nuclei or ―seeds‖ crystals, providing following crystal growth (homogeneous nucleation)

Undercooling value determines quantity of nuclei, forming in the crystallizing alloy. When a liquid comes into a contact with cold and massive mold wall (chill zone), it cools fast below the freezing point, resulting in formation of a large quantity of stable nuclei crystals. In order to promote the nucleation process, surface-active additives are used. They decrease interfacial energy of the nuclei crystals, causing formation of many more new stable nuclei.

2. Crystal growth

Number of stable nuclei per unit volume of crystallizing alloy determines the grain size. When a large number of stable nuclei are present in chill zone of mold, fine equiaxed grains form. Latent crystallization heat, liberating from the crystallizing metal, decreases the undercooling of the melt and depresses the fast grains growth. At this stage some of small grains, having favorable growth axis, start to grow in the direction opposite to the direction of heat flow. As a result columnar crystals (columnar grains) form. Contrary to the pure metals, in alloys different type of undercooling takes place. It is called constitutional undercooling.

Constitutional undercooling Since solubility of an alloying element in solid is lower, than in liquid at the same temperature, this element (solute) is rejected by the solidifying metal to the liquid phase, enriching the region of liquid adjacent to the crystallization front. For the most of the alloys: the higher the concentration of alloying element in the alloy, the lower its liquidus temperature (temperature at which crystallization of the alloy starts).

Thus crystallization temperature of the liquid, adjacent to the crystallization front, rises with increasing the distance from the front surface. Therefore there is a layer of the liquid, where its temperature is lower, than its crystallization temperature. This is the region of constitutional undercooling (see the figure below).

Prepared by L. SUSHMAASSOPROF 27 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Dendrites

If a protruding finger forms on the solidifying surface, its tip may reach the region of constitutional undercooling . In this case the protuberance starts accelerated growth, forming the main dendrite arms. Under certain conditions the same process may occur on the surface of the main dendrite arms, causing branching off the secondary arms and then arms of higher orders.

Dendritic solidification of pure metals:  Horizontal position is evolution of latent heat at the freezing point. Liquidus: The locus of temperatures above which only liquid is stable. Liquidus line: The temperature composition curve for the liquid phase in equilibrium with solid. Nucleation: The start of the growth of a new phase from which crystallization begins. Nucleus: The protons and neutrons of an atom. Solidification: Freezing of a melt. Solidus: The locus of temperature below which only solids are stable.  This type of cooling curve is obtained only if solid foreign particles i.e heterogeneous nuclear present in the liquid metal and rate of cooling is relatively slow.(oxide on the surface of the liquid) (Slight super cooling is needed).  When there is no suitable solid matter present, a liquid experiences, difficulty in starting to crystallize and may cool 0.1 – 1000C below its real freezing point i.e nuclear or‖ seed-crystals‖ then form followed by their growth which second stage of freezing point. Solidification or crystallization commences by the formation of small nuclear scattered at random in the cooling – liquid.  From the main arms of the crystal, secondary growths occur to give a crystal skeleton known as dendrite and the mode of crystallization is termed dendrite (i.e treelike) solidification. Solidification of metal Around Nucleus: The liquid between the arms of the dendritics solidifies giving homogeneous grains with no evidence of dentric growth. The imparities betrays the dendritic growth:  Mass w≥450 kgs for Grey C.I T=K(1.236+T16.65) √w sec  For steel castings t = (2.4335-0.3953 log w) √w sec. Powering time:-

Prepared by L. SUSHMAASSOPROF 28 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE SAND CASTING:Sand casting is one of the most popular and simplest types of casting that has been used for centuries. From castings that fit in the palm of your hand to train beds (one casting can create the entire bed for one rail car) it can be done with sand casting. Sand casting also allows for most metals to be cast depending in the type of sand used for the molds.

Sand casting requires a lead time of days for production at high output rates (1-20 pieces/hr-mold), and is unsurpassed for large-part production. Green (moist) sand has almost no part weight limit, whereas dry sand has a practical part mass limit of 2300-2700 kg. Sand Casting is basically done in these steps:

1. Place a pattern in sand to create a mold 2. Incorporate a gating system 3. Remove the pattern 4. Fill the mold cavity with molten metal 5. Allow the metal to cool 6. Break away the sand mold and remove the casting.

Sand Mold Making Procedure

A multi-part molding box (known as a casting flask, the top and bottom halves of which are known respectively as the cope and drag) is prepared to receive the pattern. Molding boxes are made in segments that may be latched to each other and to end closures. For a simple object—flat on one side— the lower portion of the box, closed at the bottom, will be filled with prepared casting sand or green sand—a slightly moist mixture of sand and clay. The sand is packed in through a vibratory process called ramming and, in this case, periodically screeded level. The surface of the sand may then be stabilized with a sizing compound. The pattern is placed on the sand and another molding box segment is added. Additional sand is rammed over and around the pattern. Finally a cover is placed on the box and it is turned and unlatched, so that the halves of the mold may be parted and the pattern with its sprue and vent patterns removed. Additional sizing may be added and any defects introduced by the removal of the pattern are corrected. The box is closed again.

Procedure:

1. The pattern is placed on a molding board. 2. The drag is placed on the board with pins down. 3. Molding sand is then riddled in to cover the pattern. 4. The sand is pressed around the pattern until the drag is completely filled. 5. The sand is firmly packed by the drag rammer. 6. After ramming the excess sand is leveled off with a straight bar called a strike rod. 7. Small vent holes are made through the sand to within a fraction of an inch of the pattern to insure the escape of gases. 8. The drag is then turned over so that the cope may be placed in position. 9. Before turning a little sand is sprinkled over the mold and a bottom board is placed on top. After rolling over the drag the molding board is removed exposing the pattern. 10. The surface of the sand is smoothed over with a trowel and covered with a fine coating of dry parting sand. 11. The cope is then placed on the drag, the pins on either side holding it in proper position. 12. To provide, a place for the iron to enter the mold, a tapered pin known as sprue pin is placed an inch to one side of the pattern. 13. The operations of filling, ramming, and venting the cope proceed in the same manner as in the drag. 14. The sprue pin is withdrawn, and funnel shaped opening is scooped out at the top so that there will be a fairly large opening in which to pour the metal. 15. The cope half of the flask is then carefully lifted off and set to one side.

Prepared by L. SUSHMAASSOPROF 29 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE 16. Before the pattern is withdrawn, the sand around the edge of the pattern is usually moistened with a swab so that the edges of the mold hold firmly together when the pattern is withdrawn. 17. To loosen the pattern, a draw spike, is driven into it and rapped lightly in all directions. 18. The pattern is withdrawn by lifting the draw spike. 19. Before closing, a small passage, known as a gate must be cut between the cavity and the sprue opening. 20. Sometimes a hollow, known as riser is provided in the cope to supply hot metal as the casting cools and shrinks. 21. The mold surfaces may be sprayed, swabbed, or dusted with coating materials such as silica flour and graphite.

Molding Sand

The principal raw material used in molding is the molding sand because it provides several major characteristics that may not be obtained from other materials. Molding sand is defined as granular particles resulting from the breakdown of rocks, due to the action of natural forces, suchas frost, wind, rain, heat and water currents. Rocks have a complex composition and sand contains most of the elements of rocks. Due to this reason, molding sand differs considerably in different parts of the world. In nature it is found on the bottom and banks of rivers and lakes. Molding sand is classified into different categories according to the nature of its origin. The principal constituents of molding sands are as follows: Silica (SiO2) – 86 to 90%, alumina (Al2O3) – 4 to 8%, iron oxide (Fe2O3) – 2 to 5% with smaller amounts of the oxides of Ti, Mn, and Ca, and some alkaline compounds.

Natural Sand. It is also called green sand and is collected from natural resources. It contains water as the only binder. It has the advantage of maintaining moisture content for long time, having a wide working range of moisture content, permitting easy patching and finishing of moulds.

Synthetic Sand. It is an artificial and obtained by mixing relatively clay free sand, binder (water and bentonite) and other materials as required. It is a better molding sand as its properties can be easily controlled by varying the mixture content.

Composition of green synthetic sand for steel castings is as under.

New silica and – 25%, old sand – 70%, Bentonite – 1.5%, Dextrine – 0.25% and moisture – 3 to 3.5%.

Composition of dry synthetic sand for steel castings is as under.

Net silica sand – 15%, old sand 84%, Bentonite – 0.5%, and moisture – 0.5%.

In addition to it, there are certain varieties of special sands such as Zirconite, Olivin etc. These special sands are more expensive than silica and are, therefore, used only where their use is justified.

Types of Molding Sand.Molding sands may be classified, according to their use as under:

(i) Green sand. When sand is in its natural more or less moist state, it is referred to as green sand. It is a mixture of silica sand with 18 to 30% clay and 6 to 8% water. The clay and water give bonding strength to green sand.

It is fine, soft, light and porous. Beign damp, it retains the shape given to it under pressure during squeezing.

As the mould becomes dense by ramming, the structure is made porous by venting. Sharp edges are avoided in green sand molding, because these being weak, break when hot metal is poured.

Prepared by L. SUSHMAASSOPROF 30 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Green sand is generally used for casting small or medium sized moulds. Larger output can be obtained from a given floor space as the cost and delay involved in drying the moulds is saved. Coal dust is mixed in green sand to prevent defects in castings.

Silica (sio2) + clay (acts as a binder) + water Green sand Moulds: Uses green sand.

(ii) Dry sand. Dry and molding is employed for large castings. The moulds prepared in green sand are dried or baked to remove almost all moisture of the moist and sand. The structure in the molding boxes after drying becomes stronger and compact. Venting is therefore necessary but not to chat extent as in the case of green sand mould. For larger heavy moulds, cowdumg, horse manure, etc. are mixed with the sand of coarser grains. green sand + 1 to 2% cereal flour + 2% pitch. This total mixture is baked in a oven at 110 to 260C for several hours Dry sand molds: Fairly coarse molding sand mixed with a binding material is used. Flasks are of metal, since molds must be oven baked before being used. It is free from gas troubles due to moisture. Skin-dried and dry-sand molds are widely used in steel foundries.

(iii) Loam sand. It is a mixture of clay and sand milled with water to a thin plastic paste from which moulds are built up on a backing of soft bricks.

Loam sand contains upto 50% clay and dries hard. It also contains fire clay. It must be sufficiently adhesive to hold to the vertical surfaces of the rough structure of the mould. Chopped stay and manure are commonly used to assist in binding. The moisture content is from 18 to 20%.

Loam is dried very slowly and completely before it is ready for casting. It is used for casting larger regular shaped castings like chemical pans, drums etc. Silica (fine) + finely ground refractories clay (50%) + graphite + fibrous reinforcements.

Loam sand Moulds: It is first built up with bricks or large iron parts; these parts are then plastered over with a thick Loam mortar, the shape of the cavity being obtained with sweeps or skeleton patterns. The mold is then allowed to dry thoroughly. It needs long time to make and is not used extensively.

(vi) Skin dried sand: Same as dry sand ut only skin (upper surface) upto 25 mm is dried in a oven.

Skin-dried molds: Two methods First-The sand around the pattern to a depth of about ½ inch is mixed with a binder so that when it is dried it will leave a hard surface on the mold. The remainder of the mold is made up of ordinary green sand. Second-The entire mold is made with green sand and then its surface is coat with a spray or wash, which hardens when it is applied. Spray used are: linseed oil, molasses water, gelatinized starch etc. In both of them mold is dried either by air or by a torch to harden the surface and drive cut excess moisture.

(iv) Facing sand. It is used directly next to the surface of the pattern and it comes into contact with the molten metal. Since it is subjected to the most severe conditions, it must possess high strength and refractoriness. It is made of silica sand and clay, without the addition of used sand.

Different forms of carbon known as facing materials, (e.g., plumbago powder, Ceylon lead or graphite) are used to prevent the metal from burning into the sand. Sometimes they are mixed with 6 to 15 times fine molding sand to make mould facings.

Prepared by L. SUSHMAASSOPROF 31 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Facing sand layer in a mould usually ranges from 20 to 30mm. Facing sand comprises 10 to 15% of the whole amount of mould sand.

(v) Backing sand. The old, repeatedly used molding sand, black in colour due to addition of coal dust and burning or coming in contact with molten metal is known as backing sand or floor sand or black sand. It is used to fill in the mould at the back of facing layer. It is weak in bonding strength because the sharp edges of sand grain become rounded due to high temperature of molten metal and burning of clay content.

(vi) System sand. This is used in machine molding to fill the whole flask. Its strength permeability and refractoriness must be higher than those of backing sand.

(vii) Parting sand. The molding boxes are separated from adhering to each other by spreading a fine sharp dry sand called ‗parting sand‘. Parting sand is also used to keep the green sand from sticking to the pattern. It is clean clay-free silica sand. Burnt core sand could also be used for this purpose.

(viii) Core sand. It is used for making cores. It is silica sand mixed with core oil (linseed oil, rosin, light mineral oil and other binders). For the sake of economy pitch or flour and water may be used as core sand for large cores.

(ix) Co2-sand. In CO2 sand, the silica grains, instead of being coated with natural clay, are coated with sodium silicate. This mixture is first packed around the pattern and then hardened by passing CO2 through the interstices for about a minute. The sand thus sets hard and produces a strong mould.

Clean sands are mixed with sodium silicate and the mixture is rammed about a pattern. When CO2 gas is pressure-fed into the mold, the sand mixture hardens. Very smooth and intricate castings are obtained. Used for core making.

Na2. x SIO2 + NH2O + CO2 -----> NA2CO3 + x SIO2. n(H20)

X = 1.6 TO 4 This reaction is fast completes in 1 min or less

(x) Shell sands. Shell sands are synthetic sands coated with phenol or urea-formal-dehyde resins and cured against a heated pattern to produce very strong, thin shell. No back up sand is required to provide support for the weight of the casting. Since alloys solidify at high temperatures, the resins are not dissociated.

(xi) Facing sands. Usually facing sand is applied on the pattern, so that only it comes in contact with the molten metal. This sand is refractory enough so as not to get fused and burnt on coming in contact with the metal.

(xii) Backing sands. These are applied as back up mechanical support to facing sand. These are permeable to allow gases to escape.

(xiii) Mould washes. These are slurries of fine ceramic grains. These are applied over the mould surfaces to minimize fusing of the facing and grains. These also produce smoother surface on casting due to filling up of the interstices.

Prepared by L. SUSHMAASSOPROF 32 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Shell molding:The process was developed and patented by Croning in Germany during World War II and is sometimes referred to as the Croning shell process. Definition: Shell molding is the process in which, the sand mixed with a thermosetting resin is allowed to come in to contact with a heated metallic pattern plate, so that a thin and strong shell of mould is formed around the pattern. Then the shell is removed from the pattern and the cope and drag are removed together and kept in a flask with the necessary back up material and the molten metal is poured in to the mould. Shell molding is also similar to sand molding except that a mixture of sand and 3-6% resin holds the grains together. Shell molding also uses sand with a much smaller grain than green-sand. Set-up and production of shell mold patterns takes weeks, after which an output of 5-50 pieces/hr-mold is attainable.

Sand Mixture used: Generally, dry and fine sand (90 to 140 GFN) which is completely free of clay is used for preparing the sand. The grain size to be chosen depends upon the surface finish desired on the casting. Too fine a grain size requires large amount of resin which makes the mould expensive. The synthetic resins used in shell moulding are mostly thermosetting resins, which get hardened irreversibly by heat. The resin most widely used, are the phenol formaldehyde resins. Combined with sand, they have very high strength and resistance to heat. The phenolic resins used are of two stage type that is the resin has excess phenol and acts like thermoplastic material. During coating with the sand resin is combined with a catalyst such as hexa-methylene-tetramine (hexa) in a proportion of about 14 to 16 % so as to develop the thermosetting characterstics. The curing temperature for these would be around 150°C and the time required would be 50 to 60 s.There are a dozen different stages (steps) in shell mold processing that include:

1. Initially preparing a metal-matched plate 2. Mixing resin and sand 3. Heating pattern, usually to between 505-550 K 4. Inverting the pattern (the sand is at one end of a box and the pattern at the other, and the box is inverted for a time determined by the desired thickness of the mill) 5. Curing shell and baking it 6. Removing investment 7. Inserting cores 8. Repeating for other half 9. Assembling mold 10. Pouring mold 11. Removing casting 12. Cleaning and trimming.

Procedure:

 Preparing the shell mould by mixing the sand in such a way that each sand grain is thoroughly coated with resin, which is done by mixing all dry sand, additives if any and hexa are mixed in a muller for 1 min and then liquid resin is added and mixed in muller for 3 min. To this cold or warm air is introduced and the mixing is continued till all the liquid is removed from the mixture and coating of the grains is achieved to the desired degree.  Then the mixture is cooled at 150°C temperature, only metal patterns with the associated gating are used. The metallic pattern is heated to a temperature of 200 to 300 ° so that temperature variation across the whole pattern is within 25 to 40°C depending on the size. A silicone release agent is sprayed on the pattern and the metal plate. The heated pattern is securely fixed to a dump box, of necessary thickness is already filled in.  Then the dump box is rotated at 45° so that the sand falls on the heated pattern. The heat from the pattern melts the resin adjacent to it thus causing the sand mixture to adhere to the pattern.

Prepared by L. SUSHMAASSOPROF 33 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE  Then the dump box is turned to the initial position so the excess sand falls back in to the box leaving the formed shell intact with the pattern.  The shell along with the pattern is kept in a gas fired oven for curing.  The shells thus prepared are joined together by adhesive bonding to get complete pattern.  Now the molten metal is sent in to the shell and allow to solidify and then the shell

Advantages

 Better surface finish, Better dimensional tolerances &Reduced Machining.  Less foundry space, Semi skilled operators can handle the process&can be mechanized.  Castings are dimensionally accurate than sand castings. It is possible to obtain a tolerance of ±0.25 mm foe steel castings and ±0.35 mm for grey cast iron castings under normal working conditions.  Permeability of the shell is high and therefore no gas inclusions occur.  Very small amount of sand needs to be used.  Mechanization is readily possible because of the simple processing involved in shell moulding.  Very thin sections (upto 0.25mm) of the type air cooled cylinder heads can be readily made because of the high strength of the sand used.  Shell moulding process is an efficient, economical method of producing steel castings.  The shell process is ideally suited for medium to high volume production of castings ranging in weight from a few ounces up to 80 pounds.  Complex parts can be cast with less labor.  The sand-resin mix can be recycled by burning off the resin at high temperatures.

Disadvantages

 The raw materials are relatively expensive; the process generates noxious fumes which must be removed &The size and weight range of castings is limited.  The patterns are expensive and therefore are economically used in large scale production.  Complicated shapes cannot be made.&Aland mg products to cast items in the 45-90 kg range.

Applications

-Crankshaft fabrication,gear housings, cylinder heads, and connecting rods -Steel casting parts, fittings, Molded tubing fabrication&Hydraulic control housing fabrication -Automotive castings (cylinder head and ribbed cylinder fabrication).

Prepared by L. SUSHMAASSOPROF 34 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Precision Investment Casting: Investment casting (known as lost-wax casting in art) is a process that has been practiced from 5000 years ago, when bees wax formed the pattern, to today‘s high technology waxes, refractory materials and specialist alloys, the castings ensure high quality components are produced with the key benefits of accuracy, repeatability, versatility and integrity.

Investment casting derives its name from the fact that the pattern is invested, or surrounded, with a refractory material. The wax patterns require extreme care for they are not strong enough to withstand forces encountered during the mold making.

This Process of making casting is often referred to as low wax process. Casting can be made to very close tolerances in this process and do not require subsequent machining. This is the process where the mould is prepared around an expendable pattern. The steps involved are  The first step in this process is the preparation of the pattern for every casting made. To do this, molten wax which is used as the pattern material is injected under pressure of about 2.3 MPa into a metallic die which has the cavity of the casting to be made. The wax when allowed to solidify would produce the pattern. To this wax pattern, gates, runners and any other arrangements can be appended by applying heat.  To make the mould, the prepared pattern is dipped into a slurry made by suspending fine ceramic materials in a liquid such as ethyl silicate or sodium silicate. The excess liquid is allowed to drain off from the pattern. Dry refractory grains such as fused silica or zircon are ‗stuccoed‘ on this liquid ceramic coating. Thus a small shell is formed around the wax pattern. The shell is cured and then the process of dipping and stuccoing is continued with ceramic with slurries of gradually increasing grain sizes. Finally when a shell thickness of 6 to 15 mm is reached, the mould is ready for further processing. The shell thickness required depends on the casting shape and mass, type of ceramic and the binder used. The next step in the process is to remove the pattern from the mould, which is done by heating the mould to melt the pattern. The melted wax is completely drained through sprue by inverting the mould. Any wax remnants in the mould are dissolved with the help of the hot vapour of a solvent, such a trichloro-ethylene.  The moulds are then pre-heated to a temperature of 100C, depending on the size, complexity and the metal of casting. This is done to reduce any last traces of wax left off and permit proper filling of all mould sections which are too thin to be filled in a cold mould.  The molten metal is poured into the mould under gravity. Under slight pressure, by evacuating the mould first the method chosen depends on the type of casting.

Other pattern materials used are plastics and mercury in place of wax. In the process called ‗Mercast‘,the mercury is kept under -57C where the mercury is frozen. The complete mould preparation is to be undertaken at a temperature below -38C.The main advantage of mercury as a pattern material is that it does not expand when changed from solid to liquid state as wax. But the main disadvantage is keeping the pattern at such low temperature, which is responsible for its diminishing use.

Advantages: 1. Complex shapes which are difficult to produce by any other method are possible since the pattern is withdrawn by melting it. 2. Very close tolerances and better surface finish can be produced. This is made possible because of the fine grain of sand used next to the mould cavity. 4. Casting produced by this process are ready for use with little or no machining required. This is particularly useful for those hard – to – machine materials such as nimonic alloys. 5. With proper care it is possible to control grain size, grain orientation and directional solidification in this process, so that controlled mechanical properties can be obtained. 6. Suitable for mass production of small-sized castings.&Wax can be reused .Intricate contours, and in most cases the components are cast near net shape, so requiring little or no rework once cast.

Limitations: Prepared by L. SUSHMAASSOPROF 35 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE 1. Precise control is required in all stages of casting. 2. Unsuitable for castings of more than about 5 kg weight. 3. Expensive in all respects.& con not cast location of holes.

Applications:

1. This process was used in the olden days for the preparation of

 Arte facts, , Surgical instruments

2. Presently the products made by this process are

 Vanes and blades for gas turbines, Shuttle eyes for weaving, Pawls and claws for movie cameras  Wave guides for radars&Bolts and triggers for fire arms  valve bodies&Impellers for turbo chargers  Parts of aerospace in aircraft engines, fuel systems and instruments, complete aircraft door frames, with steel castings of up to 300 kg and aluminum castings of up to 30 kg.

Prepared by L. SUSHMAASSOPROF 36 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Suctionmoulding: In this method a vacuum is created by withdrawing air from the mould space. Subsequently moulding sand is sucked in, and the cavity is filled up. The sand can thereafter be rammed in the pattern. The process is used for casting iron, steel and aluminum. The weight of castings ranges from 200gm to 120 kg.Mould can be used for once and is made from wet artificial crashed sand.

Advantages:

1. Optimum mould compaction around the pattern. 2. Decreasing hardness of compacted sand from inside to the outside. 3. High surface finish. 4.Dimensionally stable castings. 5. Reduced cleaning

Disadvantages:

1. High cost of manufacturing. 2. Change-over time is high

Injection moulding: is used mainly for thermoplastic polymer materials. When heated, thermoplastics do not become as fluid as metals so they cannot be shaped by gravity-fed casting methods. The injection moulding process has been developed specifically for thermoplastics.

The process is illustrated in Figure, which shows the main features of an injection-moulding machine. The raw polymer, in the form of solid granules, falls under gravity from a hopper into a cylinder where it is propelled along by a rotating screw into an electrically heated section. As the material is heated, it softens and flows. When the cylinder contains enough material to fill the mould, the screw action is stopped. In the final stage, the screw moves axially, acting as a ram, injecting the material through a small nozzle, and down channels (runners) into the shaped cavity within a cooled mould. When heated, most polymers start to degrade before they reach a sufficiently high temperature to fill a mould adequately under gravity alone. Injection moulding imposes high shear flow rates on the polymer as it is squirted at high pressure into the die. This tends to align the long polymeric molecules and increase the fluidity of the polymer substantially. This shear thinning of the molten polymer is essential to injection moulding and can only be achieved if high injection pressures are used.

Prepared by L. SUSHMAASSOPROF 37 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Plaster casting (of metals):Plaster casting is similar to sand molding except that plaster is substituted for sand. Plaster compound is actually composed of 70-80% gypsum and 20-30% strengthener and water. Generally, the form takes less than a week to prepare, after which a production rate of 1-10 units/hr-mold is achieved with items as massive as 45 kg and as small as 30 g with very high surface resolution and fine tolerances. Parts that are typically made by plaster casting are lock components, gears, valves, fittings, tooling, and ornaments. Plaster casting is an inexpensive alternative to other molding processes due to the low cost of the plaster and the mold production. It may be disadvantageous, however, because the mold quality is dependent on several factors, "including consistency of the plaster molding composition, mold pouring procedures, and plaster curing techniques." If these factors are not closely monitored, the mold can result in distorted dimensions, shrinking upon drying, and poor mold surfaces.

Once used and cracked away, normal plaster cannot easily be recast. Plaster casting is normally used for nonferrous metals such as aluminium-, zinc-, or copper-based alloys. It cannot be used to cast ferrous material because sulfur in gypsum slowly reacts with iron. The plaster itself cannot stand temperatures above 1200oC, which also limits the materials to be cast in plaster. Prior to mold preparation the pattern is sprayed with a thin film of parting compound to prevent the mold from sticking to the pattern. The unit is shaken so plaster fills the small cavities around the pattern. The plaster sets, usually in about 15 minutes, and the pattern is removed. The plaster is dried at temperatures between 120o and 260oC. The mold is preheated and the molten metal poured in.

Plaster casting represents a step up in sophistication and requires skill. The automatic functions easily are handed over to robots, yet the higher-precision pattern designs required demand even higher levels of direct human assistance.

NON-EXPENDABLE MOLD CASTING differs from expendable processes in that the mold need not be reformed after each production cycle. This technique includes at least four different methods:

1. Permanent mould casting 2. Die Casting. 3. Centrifugal casting. 4. . 5. slush casting

PERMANENT MOLD CASTING

While in sand castings the moulds are destroyed after solidification of castings, the moulds are reused repeatedly in the permanentmould castings. In this method a permanent mould is prepared called die. So at about 100 to 25,000moulds can be prepared using the same die depending upon the alloy used and complexity of the job.Mould material should have properties like-

. High melting point to withstand erosion by the liquid metal at pouring temperature. . High enough strength not to deform in repeated use. . High thermal fatigue . Resistance to resist premature crazing (formation of thermal fatigue cracks ) . Low adhesion

The materials used for making the moulds are-

 Cast iron  Brass and ferrous alloys  Aluminum alloys  Magnesium alloys  Copper alloys Prepared by L. SUSHMAASSOPROF 38 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE  Zinc alloys  Molybdenum alloys  Graphiteetc

In general castings to be produced by permanent method should have

 Relatively simple shape.  Uniform wall thickness.  Should not contain undercuts  Must not have complicated coring

Permanent mold casting (typically for non-ferrous metals) requires a set-up time on the order of weeks to prepare a steel tool, after which production rates of 5-50 pieces/hr-mold are achieved with an upper mass limit of 9 kg per iron alloy item (cf., up to 135 kg for many nonferrous metal parts) and a lower limit of about 0.1 kg. Steel cavities are coated with a refractory wash of acetylenesoot before processing to allow easy removal of the work piece and promote longer tool life. Permanent molds have a limited life before wearing out. Worn molds require either refinishing or replacement. Cast parts from a permanent mold generally show 20% increase in tensile strength and 30% increase in elongation as compared to the products of sand casting.

For making any hollow portions, cores are also used. If the core made out of sand are used that process is called as semi-permanent moulding. If metallic core are used they should not be complex with undercuts, and must be withdrawn immediately after solidification, otherwise extraction will be difficult because of shrinkage.

In designing the moulds, care must be taken to see that progressive solidification is achieved towards riser. If the castings have heavy sections which likely to interfere with progressive solidification, mould section around that area may be made heavier around that area to extract more heat. Chiils supported by heavier air blast may also be used to remove excess heat. Alternatively, cooling channels may be provided at the necessary points to get proper temperature distribution. The likely problem the with water circulation are the formation scales inside the cooling channels and subsequent blocking after sometime.

The gating and risering system can be provided. The moulds are coated with refractory material of 0.8mm thickness to increase the life. The coating is normally mixture of silicate, kaoline, clay, soapstone and talc. It is either insulating type or lubricating type. Coating is applied by spraying or brushing for about 0.8mm.

o By preventing the soldering of metal to the mould o By minimizing the thermal shock to the mould material o By controlling the rate and direction of casting solidification

The only necessary input is the coating applied regularly. Typically, permanent mold casting is used in forming iron, aluminum, magnesium, and copper based alloys. The process is highly automated. The sizes of castings obtained are about15 to 350 kg and is suited for mass production.

Prepared by L. SUSHMAASSOPROF 39 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Sl. Advantages Disadvantages Applications No 1 Castings will have closer dimensional Inability of the metallic mould to AuAutomobile pistons tolerances yield to the contraction forces of the solidifying metal 

2 Fine grained castings with superior Difficulty in removing the castingStstators mechanical properties can be produced from the mould since it cannot be broken

3 Better surface finish Cannot be used large castings GeGear blanks and for alloys of very high melting temperature 4 Greater mechanical strength CoConnecting rods

5 LoLow percentage of rejection Ai Aircraft fittings

6 More economical production in large Hi High cost of moulds Cylinder blocks etc quantities

7 CaCan be used for small and medium sizedLaLack of permeability non-ferrous castings.

Prepared by L. SUSHMAASSOPROF 40 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE DIE CASTING

Die casting is the process of forcing molten metal under high pressure into mold cavities (which are machined into dies). Most die castings are made from nonferrous metals, specifically zinc, copper, and aluminum based alloys, but ferrous metal die castings are possible. The die casting method is especially suited for applications where many small to medium sized parts are needed with good detail, a fine surface quality and dimensional consistency.

Definition: Die casting is the art of rapidly producing accurately dimensioned parts by forcing molten metal under pressure into split metal dies which resemble a common type of permanent mould. Within a fraction of second, the fluid alloy fills the entire die, including all minute details. Because of the low temperature of the die (it is water cooled), the casting solidifies quickly, permitting the die halves to be separated and the casting ejected. If the parts are small, several parts may be cast at one time in what is known as multiple cavity die.

Die: Die consists of two parts. One called the stationary die or cover die and the other which is fixed to the casting machine called as ejection diewhich is move out for extraction of the castings. The casting cycle starts when the two parts of the die are apart. The lubricant is sprayed on the die cavity manually or by the auto lubrication system. The two die halves are closed and clamped. The required amount of metal is injected into the die and after solidification under pressure the die is opened and the casting is ejected.

Die casting are of two types:

 Hot chamber machine  Cold chamber machine

Hot chamber machine: A hot chamber consists of a

1. Furnace – chamber holding the molten metal at high temperatures (heating is continued). 2. Gooseneck – made of grey alloy or ductile iron or of cast steel to pump the molten metal from the furnace in to the die cavity by moving up and down. 3. Plunger – to develop the necessary pressure to force the metal in to cavity. 4. Die – 2 die are used. 5. Nozzle – to let in to the cavity.

Procedure:The operation starts with the closing of the die, when the plunger is in the highest position in the goose neck, thus facilitating the filling of the gooseneck by metal. The plunger then starts moving down to force the metal in the gooseneck to the die. The metal is then held at same pressure till it is solidified. Then the die is opened, and if any cores, if present are also retracted. The plunger then moves back returning the unused metal to the gooseneck. The casting is then ejected at the same time when plunger uncovers the filling hole.

Cold chamber machine: A cold chamber consists of a

1. Ladle – To supply the molten metal periodically. 2. Plunger – to develop the necessary pressure to force the metal in to die cavity. 3. Die – 2 dies are used. 4. Nozzle – to let in to the cavity.

Procedure: The operation starts with the pouring of metal in the machine by ladle. Then the plunger then starts moving left to force the metal in the plunger to the die. The metal is then held at same pressure till it is solidified. Then the die is opened, and if any cores, if present are also retracted. The

Prepared by L. SUSHMAASSOPROF 41 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE plunger then moves back. The casting is then ejected at the same time when plunger uncovers the filling hole.

Advantages:

1. Very high rate of production. Typically 200 pieces per hour. 2. Close dimensional tolerances of order ± 0.025mm is possible. 3. Because of metallic dies a very good surface finish of order 1 micron can be obtained. 4. Very small thickness of 0.50mm can be filled as metal is injected under pressure. 5. The die has longer life, which is of the order of 30,0000 pieces for zinc alloys and 150,000 for aluminum alloys. 6. Unit cost is less and is very economical for large scale production. 7. Die castings having better mechanical properties compared to sand casting, because of fine grained skin formed during solidification. 8. Complex shapes can be moulded as it uses movable cores.

Disadvantages:

1. The maximum size of the casting is limited. The normal size are 4kg to 15 kg because of the limitation of machine capacity. 2. Suitable for non-ferrous alloys and normally for al, zn, mg and copper alloys. 3. The air in the die cavity gets entrapped in the casting and is therefore a problem often with the die castings. 4. The dies and machines are expensive so economy for mass production.

Applications:

 Carburetors  crank case

Prepared by L. SUSHMAASSOPROF 42 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE CENTRIFUGAL CASTING

Centrifugal casting is both gravity- and pressure-independent since it creates its own force feed using a temporary sand mold held in a spinning chamber at up to 900 N (90 g). Lead time varies with the application. Semi- and true-centrifugal processing permit 30-50 pieces/hr-mold to be produced, with a practical limit for batch processing of approximately 9000 kg total mass with a typical per-item limit of 2.3-4.5 kg.

This is a process where the mould is rotated rapidly about its central axis as the metal is poured into it. Because of the centrifugal force, a continuous pressure will be acting on the metal as it solidifies. The slag, oxides and other inclusions being lighter, get separated from the metal and segregates toward the centre. There are three types of centrifugal casting process. They are 1. True centrifugal casting 2. Semi-centrifugal casting 3. Centrifuging.

True centrifugal casting This is normally used for the making of hollow pipes, tubes, hollow bushes, etc. When are axisymmetric with a concentric hole. Since the metal is always pushed outward because of the centrifugal force, no core needs to be used for making the concentric hole. The axis of rotation can be horizontal, vertical or any angle in between. Very long pipes are normally cast with horizontal axis, where as short pieces are more conveniently cast with a vertical axis. A normal centrifugal casting machine used for making cast iron pipes in sand moulds is shown in Figure. First, the molding flask is properly rammed with sand to confirm to the outer contour of the pipe to be made. Any end details, such as spigot ends, or flanged ends are obtained with the help of dry sand cores located in the ends. Then the flask is dynamically balanced so as to reduce the occurrence of undesirable vibrations during the casting process. The finished flask is mounted in between the rollers and the mould is rotated slowly. Now the molten metal in requisite quantity is poured into the mould through the movable pouring basin. The amount of metal poured determines the thickness of the pipe to be the cast. After the pouring is complete, the mould is rotated at its operational speed till it solidifies, to form the requisite tubing. Then the mould is replaced by a new mould machine and the process continued. Metal mould can also be used in the true centrifugal casting process for large quantity production. A water jacket is provided around the mould for cooling it. The casting machine is mounted on wheels with the pouring ladle which has long spout extending till the other end of the pipe to be made. To start, the mould is rotated with the metal being delivered at the extreme end of the pipe. The casting machine is slowly moved down the track allowing the metal to be deposited all along the length of the pipe. The machine is continuously rotated till the pipe is completely solidified. Afterwards, the pipe is extracted from the mould and the cycle repeated. Advantages:- 1. The mechanical properties of centrifugally cast jobs are better compared to other processes, because the inclusions such as slag and oxides get segregated towards the can be easily removed by machining. Also, the pressure acting on the metal throughout the solidification, causes the porosity to be eliminated giving rise to dense metal. 2. Up to a certain thickness of objects, proper directional solidification can be obtained starting from the mould surface to the centre. 3. No cores are required for making concentric holes in the case of true centrifugal casting. 4. There is no need for gates and runners, which increase the casting yields, reaching almost 100%. Limitations:- 1. Only certain shapes which are axisymmentric and having concentric holes are suitable for true centrifugal casting. 2. The equipment is expensive and thus is suitable only for large quantity production.

Prepared by L. SUSHMAASSOPROF 43 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Semi-centrifugal casting Semi-centrifugal casting is used for jobs which are more complicated than those possible in true centrifugal casting, but are axisymmentric in nature. It is not necessary that these should have a central hole, which is to be obtained with the help of a core. The moulds made of sand or metal are rotated about a vertical axis and the metal enters the mould through the central pouring basin as in figure. For larger production rates, the moulds can be stacked one other, all feeding from the same central pouring basin. The rotating speeds used in this process are not as high as in the case of true centrifugal casting.

Centrifuging In order to obtain higher metal pressures during solidification, when casting shapes are not axisymmentric, the centrifuging process is used. This is suitable only for small jobs of any shape. A number of such small jobs are joined together by means of radial runners with a central sprue on a revolving table as in figure. The jobs are uniformly placed on the table around the periphery so that their masses are properly balanced. The process is similar to semi- centrifugal casting.

CONTINUOUS CASTING

Continuous casting is a refinement of the casting process for the continuous, high-volume production of metal sections with a constant cross-section. Molten metal is poured into an open-ended, water-cooled copper mold, which allows a 'skin' of solid metal to form over the still-liquid centre. The strand, as it is now called, is withdrawn from the mold and passed into a chamber of rollers and water sprays; the rollers support the thin skin of the strand while the sprays remove heat from the strand, gradually solidifying the strand from the outside in. After solidification, predetermined lengths of the strand are cut off by either mechanical shears or travelling oxyacetylene torches and transferred to further forming processes, or to a stockpile. Cast sizes can range from strip (a few millimetres thick by about five metres wide) to billets (90 to 160 mm square) to slabs (1.25 m wide by 230 mm thick). Sometimes, the strand may undergo an initial hot rolling process before being cut. Continuous casting is used due to the lower costs associated with continuous production of a standard product, and also increases the quality of the final product. Metals such as steel, copper and aluminium are continuously cast, with steel being the metal with the greatest tonnages cast using this method. Prepared by L. SUSHMAASSOPROF 44 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Generally the starting point of any product is the ingot which is subsequently rolled through number of mills before a final products such as slab or bloom is obtained.However, the wide adoption of continuous casting has changed that scenario by directly casting slabs, billets and blooms without going through the rolling process. This process is fast and also economical. In this process, the liquid steel is poured into a double walled, bottomless water cooled mould where a solid skin is quickly formed and a semi-finished skin emerges from the open mould bottom. The skin formed in the mould is about 10 to 25 mm in thickness and is further solidified by intensive cooling with water sprays as casting moves downwards. A typical arrangement of continuous casting plant is shown schematically in figure. The melton steel is collected in a ladle and kept over a refractory lined intermediate pouring vessel named tundish. The steel is then poured into water cooled vertical copper moulds which are 45 to 750 mm long. Before starting the casting a dummy starter bar is kept in the mould bottom as shown figure. After starting the casting process as the metal level rises in the mould to a desirable height, the stator bar is withdrawn at a rate equal to the steel pouring rate. The initial metal freezes onto the starter bar as well as the periphery of the mould. This solidified shell supports the liquid steel as it moves downwards. This steel shell is mechanically supported (rollers) as it moves down through the secondary cooling zone where water is sprayed onto the steel surface to complete the solidification process. After the casting is completely solidified, it is cut to the desired lengths by a suitable cutoff apparatus.

FULL MOLD PROCESS / LOST FOAM PROCESS / EVAPORATIVE PATTERN CASTING PROCESS

The use of foam patterns for metal casting was patented by H.F. Shroyer on April 15, 1958. In Shroyer's patent, a pattern was machined from a block of expanded polystyrene (EPS) and supported by bonded sand during pouring. This process is known as the full mold process. With the full mold process, the pattern is usually machined from an EPS block and is used to make primarily large, one-of-a kind castings. The full

Prepared by L. SUSHMAASSOPROF 45 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE mold process was originally known as the lost foam process. However, current patents have required that the generic term for the process be full mold.

In 1964, M.C. Flemmings used unbounded sand with the process. This is known today as lost foam casting (LFC). With LFC, the foam pattern is molded from polystyrene beads. LFC is differentiated from full mold by the use of unbounded sand (LFC) as opposed to bonded sand (full mold process).

Foam casting techniques have been referred to by a variety of generic and proprietary names. Among these are lost foam, evaporative pattern casting, cavity less casting, evaporative foam casting, and full mold casting.

In this method, the pattern, complete with gates and risers, is prepared from expanded polystyrene. This pattern is embedded in a no bake type of sand. While the pattern is inside the mold, molten metal is poured through the sprue. The heat of the metal is sufficient to gasify the pattern and progressive displacement of pattern material by the molten metal takes place.

The EPC process is an economical method for producing complex, close-tolerance castings using an expandable polystyrene pattern and unbonded sand. Expandable polystyrene is a thermoplastic material that can be molded into a variety of complex, rigid shapes. The EPC process involves attaching expandable polystyrene patterns to an expandable polystyrene gating system and applying a refractory coating to the entire assembly. After the coating has dried, the foam pattern assembly is positioned on loose dry sand in a vented flask. Additional sand is then added while the flask is vibrated until the pattern assembly is completely embedded in sand. Molten metal is poured into the sprue, vaporizing the foam polystyrene, perfectly reproducing the pattern.

In this process, a pattern refers to the expandable polystyrene or foamed polystyrene part that is vaporized by the molten metal. A pattern is required for each casting.

Process Description

1. The EPC procedure starts with the pre-expansion of beads, usually polystyrene. After the pre-expanded beads are stabilized, they are blown into a mold to form pattern sections. When the beads are in the mold, a steam cycle causes them to fully expand and fuse together. 2. The pattern sections are assembled with glue, forming a cluster. The gating system is also attached in a similar manner. 3. The foam cluster is covered with a ceramic coating. The coating forms a barrier so that the molten metal does not penetrate or cause sand erosion during pouring. 4. After the coating dries, the cluster is placed into a flask and backed up with bonded sand. 5. Mold compaction is then achieved by using a vibration table to ensure uniform and proper compaction. Once this procedure is complete, the cluster is packed in the flask and the mold is ready to be poured.

Advantages

The most important advantage of EPC process is that no cores are required. No binders or other additives are required for the sand, which is reusable. Shakeout of the castings in unbounded sand is simplified. There are no parting lines or core fins.

Prepared by L. SUSHMAASSOPROF 46 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE MELTING PRACTICES

Melting is an equally important parameter for obtaining a quality castings. A number of furnaces can be used for melting the metal, to be used, to make a metal casting. The choice of furnace depends on the type of metal to be melted. Some of the furnaces used in metal casting are as following.

 Crucible furnaces  Cupola  Induction furnace  Reverberatory furnace

1. CRUCIBLE FURNACE.

Crucible furnaces are small capacity typically used for small melting applications. Crucible furnace is suitable for the batch type foundries where the metal requirement is intermittent. The metal is placed in a crucible which is made of clay and graphite. The energy is applied indirectly to the metal by heating the crucible by coke, oil or gas.The heating of crucible is done by coke, oil or gas. .

2. COKE-FIRED FURNACE.

 Primarily used for non-ferrous metals  Furnace is of a cylindrical shape  Also known as pit furnace  Preparation involves: first to make a deep bed of coke in the furnace  Burn the coke till it attains the state of maximum combustion  Insert the crucible in the coke bed  Remove the crucible when the melt reaches to desired temperature

3.OIL-FIRED FURNACE.

 Primarily used for non-ferrous metals  Furnace is of a cylindrical shape  Advantages include: no wastage of fuel  Less contamination of the metal  Absorption of water vapor is least as the metal melts inside the closed metallic furnace

4. CUPOLAFURNACE

Cupola furnaces are tall, cylindrical furnaces used to melt iron and ferrous alloys in foundry operations. Alternating layers of metal and ferrous alloys, coke, and limestone are fed into the furnace from the top. A schematic diagram of a cupola is shown in Figure. This diagram of a cupola illustrates the furnace's cylindrical shaft lined with refractory and the alternating layers of coke and metal scrap. The molten metal flows out of a spout at the bottom of the cupola. .

Description of Cupola: The cupola consists of a vertical cylindrical steel sheet and lined inside with acid refractory bricks. The lining is generally thicker in the lower portion of the cupola as the temperature are higher than in upper portion

 There is a charging door through which coke, pig iron, steel scrap and flux is charged

Prepared by L. SUSHMAASSOPROF 47 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE  The blast is blown through the tuyeres  These tuyeres are arranged in one or more row around the periphery of cupola  Hot gases which ascends from the bottom (combustion zone) preheats the iron in the preheating zone  Cupolas are provided with a drop bottom door through which debris, consisting of coke, slag etc. can be discharged at the end of the melt  A slag hole is provided to remove the slag from the melt  Through the tap hole molten metal is poured into the ladle  At the top conical cap called the spark arrest is provided to prevent the spark emerging to outside

Operation of Cupola

The cupola is charged with wood at the bottom. On the top of the wood a bed of coke is built. Alternating layers of metal and ferrous alloys, coke, and limestone are fed into the furnace from the top. The purpose of adding flux is to eliminate the impurities and to protect the metal from oxidation. Air blast is opened for the complete combustion of coke. When sufficient metal has been melted that slag hole is first opened to remove the slag. Tap hole is then opened to collect the metal in the ladle.

.Figure: Schematic of a Cupola

5. REVERBERATORY FURNACE

Prepared by L. SUSHMAASSOPROF 48 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE A furnace or kiln in which the material under treatment is heated indirectly by means of a flame deflected downward from the roof. Reverberatory furnaces are used in opper, tin, and nickel production, in the production of certain concretes and cements, and in aluminum. Reverberatory furnaces heat the metal to melting temperatures with direct fired wall-mounted burners. The primary mode of heat transfer is through radiation from the refractory brick walls to the metal, but convective heat transfer also provides additional heating from the burner to the metal. The advantages provided by Reverberatorymelters is the high volume processing rate, and low operating and maintenance costs. The disadvantages of the Reverberatorymelters are the high metal oxidation rates, low efficiencies, and large floor space requirements. A schematic of Reverberatory furnace is shown in Figure

Figure : Schematic of a Reverberatory Furnace

6. INDUCTION FURNACE

Induction heating is a heating method. The heating by the induction method occurs when an electrically conductive material is placed in a varying magnetic field. Induction heating is a rapid form of heating in which a current is induced directly into the part being heated. Induction heating is a non-contact form of heating.

The heating system in an induction furnace includes:

1. Induction heating power supply,

2. Induction heating coil,

3. Water-cooling source, which cools the coil and several internal components inside the power supply.

The induction heating power supply sends alternating current through the induction coil, which generates a magnetic field. Induction furnaces work on the principle of a transformer. An alternative electromagnetic field induces eddy currents in the metal which converts the electric energy to heat without any physical contact Prepared by L. SUSHMAASSOPROF 49 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE between the induction coil and the work piece. A schematic diagram of induction furnace is shown in Figure. The furnace contains a crucible surrounded by a water cooled copper coil. The coil is called primary coil to which a high frequency current is supplied. By induction secondary currents, called eddy currents are produced in the crucible. High temperature can be obtained by this method. Induction furnaces are of two types: cored furnace and coreless furnace. Cored furnaces are used almost exclusively as holding furnaces. In cored furnace the electromagnetic field heats the metal between two coils. Coreless furnaces heat the metal via an external primary coil.

Advantages of Induction Furnace

 Induction heating is a clean form of heating  High rate of melting or high melting efficiency  Alloyed steels can be melted without any loss of alloying elements  Controllable and localized heating

Disadvantages of Induction Furnace

 High capital cost of the equipment  High operating cost

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Prepared by L. SUSHMAASSOPROF 50 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

DEFFCTS IN CASTINGS:-Any irregularity in the molding process causes defects in castings which may sometimes be tolerated, sometimes eliminated with proper molding practice or repaid using method such as welding and metallization. The following are the defects which occur in sand castings. i. Gas defects iv. Pouring metal defects ii. Shrinkage cavities v. Metallurgical defects. iii. Molding material defects

Gas defects:-

 Blow holes and open blows: These are the spherical, flattened or elongated cavities present inside the casting or on the surface as shown on the surface they are called open blows and inside, they are called blow holes. Because of the heat in the molten metal, the moisture is converted into steam, part of whish when entrapped in the reaches the surface.  Air inclusions: The atmospheric and other gases absorbed by the molten metal in the furnace, in the ladle, and during the flow in the mould, when not allowed to escape, would be trapped inside the casting and weaken it.  Pin hole porosity: This is caused by hydrogen in the molten metal. This could have been picked up in the furnace or by the dissociation of water inside the mould cavity.  Shrinkage cavities: These are caused by the liquid shrinkage occurring during the solidification of the casting.

Causes: Excessive moisture&Organic content of sand, Moisture on chills, chaplets or metal inserts, Poor venting andLess permeability of sand if used (hard ramming), High gas content of molten metal and improper feeding&Insufficient drying of mould and cores

Molding material defects:

 Cuts and washes: These appear as rough spots and areas of excess metal, and are caused by the erosion of molding sand by the flowing molten metal. It appears as a low projection on the drag face of a casting that extends along the surface, decreasing the height as it extends from one side of the casting to the other end. It generally occurs in bottom gating in which the sand as in sufficient hot strength and when too much metal is allowed to flow.  Metal penetration: When the molten enters the gaps between the sand grains, the result would be a rough casting surface. The main reason for this is that, either the grain size of the sand is too coarse, or no mould wash has been applied to the mould cavity.  Fusion:- This is caused by the fusion of sand grains with the molten metal, giving a brittle, glassy appearance on the casting surface. . Run out:- A run out is caused when the molten metal leaks out of the mould. This may be caused either due to faulty mould making or because of thefaulty molding flask. . Buckles:-It appears as a long, fairly, shallow, broad, vee depression occurring in the surface of flat castings.It extends in a straight line across the entire flat surface. Rat tail is caused by the compression failure of the skin of the mould cavity because of the excessive heat in the molten metal.This as molding sand has got poor expansion properties and hot strength or the heat in the pouring metal is too high. . Rat tails: It appears as a long, fairly, shallow, broad, angular depression occurring in the surface of flat castings. It extends in a straight line across the entire flat surface. . Swell: this occurs as slight smooth bulge usually found on vertical faces of castings resulting from liquid metal pressure. Under the influence of the metallostatic forces, the mould wall may move back causing a swell in the dimensions of the casting. This is caused due to low strength of mouldbecause of too high water content or when sand not rammed properly. . Drop: The dropping of loose molding sand or lumpsnormally from the cope surface into the mould cavity is responsible for this defect.

Prepared by L. SUSHMAASSOPROF 51 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE . Scab: Caused when portion of the face of a mould lifts or break downs thus the recess is filled by the molten metal.

Pouring metal defects:-  Mis-runs: Mis-run is caused when the metal is unable to fill the mould cavity completely and thus leaves unfilled cavities.  Cold shuts: A cold shut is caused when two metal streams while meeting in the mould cavity, did not fuse together properly, thus causing a discontinuity or weak spot in the casting.

Causes: 1. Sometimes a condition leading to cold shuts can be observed when no sharp corners exist in a casting. 2. These defects are caused essentially, by the lower fluidity of the molten metal or when the section thickness of the casting is too small.

Metallurgical defects:-  Hot tears:-Hot tears are hot cracks which appear in the form of irregular crevices with dark oxidized fracture surface. Causes: Since metal has low strength at higher temperature, any unwanted cooling stress may cause the rupture of the casting, If the metal does not have sufficient strength to resist ensile forces produced during solidification &Non uniform cooling and incorrect gating design

 Hot spots:-These are caused by the chilling of the casting. For example, with grey cast iron having small amounts of silicon, very hard white cast iron may result at the chilled surface.

Prepared by L. SUSHMAASSOPROF 52 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

FETTING The complete process of the cleaning of casting, called ‗fettling‘, involves the removal of the cores, gates and risers, cleaning of the casting surface and chipping of the unnecessary projections on surfaces. The dry sand cores can be removed simply by knocking off with an iron bar, by means of core vibrator, or by means of hydro blasting. The method depends on the size, complexity and the core material used. The gates and risers can be removed by hammering, chipping, hack sawing, abrasive cutoff or by flame or arc cutting. For brittle materials such as grey cast iron, the gates can easily be broken by hitting with a . For steel and other similar materials, sawing with any metal cutting like hack saw and band saw would be more convenient. For large size gates and risers, it may be necessary to use flame or arc cutting to remove them. Similarly, abrasive cut off may also be used for removal of gates.

For cleaning the sand particles sticking to the casting surface, sand blasting normally used. The casting is kept in a closed box and a jet of compressed air with a blast of sand grains or steel grit is directed against the surface, which thoroughly cleans the casting surface. The typical shot speeds reached of the order of 80 m/s. Another useful method fir cleaning the casting surface is the tumbling. Here the castings are kept in a barrel which is completely closed and than slowly rotated on a horizontal axis 30 to 40 rpm. The barrel is reasonable packed, with enough room for castings to move so that they will be able to remove the sand and unwanted fins and projections. However one precaution to be taken for tumbling is that, the casting should all be rigid with non frail or overhung segments which may get knocked of during the tumbling operation.

Prepared by L. SUSHMAASSOPROF 53 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Important Questions

1. Write short notes on shell molding and co2molding? 2. Discuss about the various types of casting process 3. Explain about the various patterns and moulding sand properties required 4. Describe the need of investment in aircraft industry and explain the process 5. Explain the cupola operation and also brief about the crucible furnace 6. Write the advantages and disadvantages of various casting processes 7. Discuss different centrifugal casting processes in detail 8. What is manufacturing and Classify the various manufacturing processes 9. Chocolateis available in hollow shapes. What process is used to make these candies 10. What are the benefits and drawbacks to heating the mold in investment casting before pouring the molten metal? 11. What are cores? What are the different types of cores 12. Explain the centrifugal casting? Explain the types of centrifugal castings 13. Explain the gating system and its functions? And discuss about the design and rule to design riser?

Example problems: Calculate the optimum powering time for a casting whose mass is 100kg and a thickness of 25 MM. Fluidity of Iron is 32 inches. Calculate for both steel and cast Iron.

Powering time for (C.I) t= K(1.41+T/14.59) √100 =3240(1.41+2514.59) √100 =24.988sec.

For steel t = (2.4335-0.3953 log w) )√w = (2.4335-0.3953 log 100) √100 =16.249 sec. Gating system design is to fill the mould in the smallest time .The time for complete filling of a mould termed as powering time. Steels loose heat faster than grey cast iron. Hence powering time should be very less. For non-ferrous metals freezing time would be longer, since they loose heat slowly and tend to form dross of poured too quickly. For grey C.I mass≤ 450 kg Powering time t = K (1.41+T14.59) √wK=K/40 K= fluidity of iron in neclus/40 T=Average section thickness in mm W= mass of the casting in kg.

Prepared by L. SUSHMAASSOPROF 54 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Welding History of Welding:  Late 19th Century  Scientists/engineers apply advances in electricity to heat and/or join metals (Le Chatelier, Joule, etc.)  Early 20th Century  Prior to WWI welding was not trusted as a method to join two metals due to crack issues  1930’s and 40’s  Industrial welding gains acceptance and is used extensively in the war effort to build tanks, aircraft, ships, etc.  Modern Welding  The nuclear/space age helps bring welding from an art to a science

Welding: Welding is a joining process which produces coalescence of materials by heating them to suitable temperatures with or without the application of pressure or by the application of pressure alone, and with or without the use of filler material. Welding is used for making permanent joints. It is used in the manufacture of automobile bodies, aircraft frames, railway wagons, machine frames, structural works, tanks, furniture, boilers, general repair work and ship building.

Weldability: The term weldability has been defined as the capacity of being welded into inseperable joints having specified properties such as definite weld strength, proper structure etc. The real criterion in deciding the weldability of the metal is the weld quality and the ease with which it can be obtained.Capacity of a metal or combination of metals to be welded into a suitably designed structure, and for the resulting weld joint(s) to possess the required metallurgical properties to perform satisfactorily in intended service . Good weldability characterized by: . Ease with which welding process is accomplished . Absence of weld defects . Acceptable strength, , and toughness in welded joint The major factors that weldability depends: Prepared by L. SUSHMAASSOPROF 55 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE  Metallurgical Capacity  Parent metal will join with the weld metal without formation of deleterious constituents or alloys  Mechanical Soundness  Joint will be free from discontinuities, gas porosity, shrinkage, slag, or cracks  Serviceability  Weld is able to perform under varying conditions or service (e.g., extreme temperatures, corrosive environments, fatigue, high pressures, etc.)

TYPES OF WELDING:

 Plastic Welding or Pressure Welding The piece of metal to be joined are heated to a plastic state and forced together by external pressure (Ex) Resistance welding, Gas welding

 Fusion Welding or Non-Pressure Welding The material at the joint is heated to a molten state and allowed to solidify. (Ex) Arc welding

Classification of welding processes: (i) Arc welding  Carbon arc  Metal arc  Metal inert gas  Tungsten inert gas  Plasma arc  Submerged arc  Electro-slag (ii) Gas Welding  Oxy-acetylene  Air-acetylene  Oxy-hydrogen

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(iii) Resistance Welding  Butt  Spot  Seam  Projection  Percussion

(iv)Thermit Welding

(v) Solid State Welding  Friction  Ultrasonic  Diffusion  Explosive

(vi) Newer Welding  Electron-beam  Laser (vii) Related Process  Oxy-acetylene cutting  Arc cutting  Hard facing  Brazing  Soldering GAS WELDING: Principle:  In Gas Welding the heat necessary for melting base metal and filler rod is obtained by gas flame.  The composition of the filler rod is same as that of the base metal.  Oxygen and Acetylene gas are mostly commonly used in this process.  The temperature generated during the process is 32000c  The principle of gas welding is in the figure.

Prepared by L. SUSHMAASSOPROF 57 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Figure: Principal of Gas Welding  Weld is obtained by selecting proper size of flame, filler material and method of moving torch.  Fluxes are added to the welded metal to remove oxides  Common fluxes used are made of sodium, potassium. Lithium and borax.  Flux can be applied as paste, powder,liquid.solid coating or gas.  In this Welding process Oxygen and Acetylene mixture are used to weld the metal pieces at very high temperatures with an additive of filler material. Gas Flame:  When Oxygen and Acetylene are supplied equally to the torch, a Neutral flame is produced.  The flame consists of two portions. They are 1. Inner cone 2. Outer envelope  Inner cone is having a temperature of 32000C and the reaction is

C2H2 + O2 2CO + H2 + heat  The outer envelope will have 21000C near to inner cone and 12500C at the end point of the flame and the reaction is

4CO + 2H2 + 3O2 4CO2 + 2H2O + heat In this welding process there are 3 types of flames. They are 1. Neutral Flame 2. Carburizing Flame 3. Oxidizing Flame

Prepared by L. SUSHMAASSOPROF 58 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Neutral Flame: Oxygen and Acetylene are supplied equally to the torch. This Consists of 2 definite zones. (i) Inner cone at a temperature of 32000C which is short distance from the tip of the torch. (ii) Outer envelope is luminous and of a bluish colour. Ex: used for welding steel, C.I., Al, Cu etc. Carburizing Flame: In this Flame excess of Acetylene is present. This Consists of 3 definite zones. 1. Sharply inner cone 2. An intermediate cone of whitish in colour is known as Feather. 3. The bluish outer envelope Oxidizing Flame: In this Flame excess of Oxygen is present. This Consists of 2 definite zones. 1. Smaller inner cone which has puplish tinge and 2. The bluish outer envelope or cone. Ex: used for welding brass. GAS WELDING EQUIPMENT:

Prepared by L. SUSHMAASSOPROF 59 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

The following are the equipment used for Gas welding. 1. Gas Cylinders with total pressure of Oxygen – 154 kgf/cm2 at 210C Acetylene – 16 kgf/cm2 2. Regulators Working pressure of oxygen 1 kgf/cm2 Working pressure of acetylene 0.15 kgf/cm2 Working pressure varies depends upon the thickness of the work pieces welded. 3. Pressure Gauges 4. Hose and hose fittings 5. Welding torch 6. Valves 7. Goggles, Gloves and Spark – lighter. 8. Welding tip. The welding torch is as shown in the figure given below.

Prepared by L. SUSHMAASSOPROF 60 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Arc Welding: A fusion welding process in which coalescence of the metals is achieved by the heat from an electric arc between an electrode and the work.The principle of arc welding is based upon the formation of an electric arc between a consumable electrode and the base metal.  Arc temperature of 65000 C comes from the Electric energy, but the temperature is hot enough to melt any metal.  Most Arc Welding processes add filler metal to increase volume and strength of weld joint .

Figure: Arc Welding Process What is an Electric Arc? An electric arc is a discharge of electric current across a gap in a circuit  It is sustained by an ionized column of gas (plasma) through which the current flows

Prepared by L. SUSHMAASSOPROF 61 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE  To initiate the arc in Arc Welding, electrode is brought into contact with work and then quickly separated from it by a short distance Flux: A substance that prevents formation of oxides and other contaminants in welding, or dissolves them and facilitates removal . Provides protective atmosphere for welding . Stabilizes arc . Reduces spattering

Power Source in Arc Welding:  Direct current (DC) vs Alternating current (AC)  AC machines less expensive to purchase and operate, but generally restricted to ferrous metals.  DC equipment can be used on all metals and is generally noted for better arc control.

Arc Welding Methods: The different methods of Arc Welding are  Metal – Arc Welding  Gas Metal Arc Welding  Gas – tungsten Arc Welding  Plasma – Arc Welding  Submerged – Arc Welding  Flux – cored Arc Welding  Electro – slag Arc Welding

Metal – Arc Welding (MAW): In Metal – arc welding a metal rod is used as one electrode, while the work being welded is used as another electrode. During the welding operation, this metal electrode is melted by the heat of the arc, and is fused with the base metal. The principle of Metal – arc welding circuit is as shown in the figure.

Prepared by L. SUSHMAASSOPROF 62 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Gas Metal Arc Welding (GMAW): Gas Metal Arc Welding (GMAW) is also called as Metal Inert Gas Welding(MIG Welding). Uses a consumable bare metal wire as electrode and shielding accomplished by flooding arc with a gas. Metal is transferred through protected arc column to the work.  Wire is fed continuously and automatically from a spool through the welding gun.  Shielding gases include inert gases such as argon and helium for aluminum welding, and

active gases such as CO2 for steel welding.  Bare electrode wire plus shielding gases eliminate slag on weld bead - no need for manual grinding and cleaning of slag.

Figure: Gas Metal Arc Welding (MIG Welding) GMAW Advantages over MAW: . Better arc time because of continuous wire electrode  Sticks must be periodically changed in SMAW . Better use of electrode filler metal than SMAW Prepared by L. SUSHMAASSOPROF 63 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE  End of stick cannot be used in SMAW . Higher deposition rates . Eliminates problem of slag removal . Can be readily automated Gas Tungsten Arc Welding (GTAW): Uses a non-consumable tungsten electrode and an inert gas for arc shielding and it is also called as Tungsten Inert gas arc welding (TIG Welding).  Melting point of tungsten = 3410C (6170F)  Used with or without a filler metal  When filler metal used, it is added to weld pool from separate rod or wire.  Applications: aluminum and stainless steel most common

Figure: Gas Tungsten Arc Welding (TIG Welding) Advantages:  High quality welds for suitable applications  No spatter because no filler metal through arc  Little or no post-weld cleaning because no flux Disadvantages:  Generally slower and more costly than consumable electrode AW processes

Plasma Arc Welding (PAW): Special form of GTAW in which a constricted plasma arc is directed at weld area . Tungsten electrode is contained in a nozzle that focuses a high velocity stream of inert gas (argon) into arc region to form a high velocity, intensely hot plasma arc stream . Temperatures in PAW reach 28,000C (50,000F), due to constriction of arc, producing a plasma jet of small diameter and very high energy density

Prepared by L. SUSHMAASSOPROF 64 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Figure: Plasma Arc Welding

Advantages: . Good arc stability . Better penetration control than other AW . High travel speeds . Excellent weld quality . Can be used to weld almost any metals

Disadvantages: . High equipment cost . Larger torch size than other AW . Tends to restrict access in some joints

Submerged Arc Welding (SAW):

Uses a continuous, consumable bare wire electrode, with arc shielding provided by a cover of granular flux . Electrode wire is fed automatically from a coil . Flux introduced into joint slightly ahead of arc by gravity from a hopper . Completely submerges operation, preventing sparks, spatter, and radiation

Prepared by L. SUSHMAASSOPROF 65 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Figure: Submerged Arc Welding Applications: . Steel fabrication of structural shapes (e.g., I-beams) . Seams for large diameter pipes, tanks, and pressure vessels . Welded components for heavy machinery . Most steels (except hi C steel) . Not good for nonferrous metals

Arc welding Equipment:

 Equipment: 1. A welding generator (D.C.) or Transformer (A.C.) 2. Two cables- one for work and one for electrode 3. Electrode holder 4. Electrode 5. Protective shield 6. Gloves 7. Wire brush 8. Chipping hammer 9. Goggles

Prepared by L. SUSHMAASSOPROF 66 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Figure: Schematic Representation of Arc Welding Equipment

Comparison of A.C. and D.C. Arc welding: Alternating Current Direct Current (from Transformer) (from Generator)

 More efficiency  Less efficiency

 Power consumption less  Power consumption more

 Cost of equipment is less  Cost of equipment is more

 Higher voltage – hence not  Low voltage – safer operation

safe  suitable for both ferrous &non  Not suitable for welding non ferrous metals

ferrous metals  preferred for welding thin  Not preferred for welding thin sections

sections  Positive terminal connected to  Any terminal can be connected the work

to the work or electrode  Negative terminal connected to the electrode Advantages  Most efficient way to join metals  Lowest-cost joining method  Affords lighter weight through betterutilization of materials  Joins all commercial metals

Prepared by L. SUSHMAASSOPROF 67 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE  Provides design flexibility Limitations  Manually applied, therefore high labor cost.  Need high energy causing danger  Not convenient for disassembly.  Defects are hard to detect at joints

Thermit Welding: Thermit welding is a fusion – welding process in which the weld is effected by pouring superheated liquid thermit steel around the parts to be united. In this process neither arc is produced to the parts nor flame is used. In this welding, only the heat of the thermit reaction is utilized to bring the surface of metal to be welded in a plastic state and mechanical pressure is applied to complete the weld. A mixture of finely divided aluminium and iron oxide called as thermit mixture is kept in a crucible hanging over a mould and this mixture is ignited using a highly inflammable powder having Barium peroxide. The reaction takes about 30 seconds only and heat is liberated which is twice the temperature of melting point of steel and the reaction is as follows

8Al + 3Fe304 9Fe + 4Al203 + Heat

Prepared by L. SUSHMAASSOPROF 68 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE  In this process, a wax pattern is shaped around the parts to be welded.  A sheet iron box is placed around the wax pattern and the box is filled & rammed with sand.  Pouring gates, heating gates & risers are cut in the sand and a flame is directed into a heating opening.  The wax pattern melts & drains out but the heating is continued to raise the temperature of the parts to be welded.  The preheating is done before the liquid metal is poured into the mould in order to prevent chilling of steel  The burner (or) torch is removed and the preheating gate is plugged with sand.  The superheated metal produced by the thermit reaction in a crucible is poured into the mould surrounding the surfaces of the weld.  After the welding temperature is reached mechanical pressure is applied to complete the weld  It used to great extent in welding of pipes, cables, rails, connecting rods etc. Applications: . Joining of railroad rails . Repair of cracks in large steel castings and . Weld surface is often smooth enough that no finishing is required

Prepared by L. SUSHMAASSOPROF 69 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Resistance Welding (RW): A group of fusion welding processes that use a combination of heat and pressure to accomplish coalescence Resistance Spot welding is comparatively modern welding process. It came in to the field of welding in between years 1900 - 1905. It is a most widely used resistance welding method. The main purpose of resistance spot welding method is to join two to four light overlapping metal sheets (which can have thickness up to 3 mm).

Principle:

All resistance welding like spot welding, seam welding, projection welding etc. are worked on same principle of heat generation due to electric resistance. When a current passes through electric resistance, it produces heat. This is same principle which is used in electric coil. The amount of heat produced is depends on resistance of material, surface conditions, current supplied, time duration of current supplied etc. This heat generation takes place due to conversion of electric energy into thermal energy. The heat generation formula is

H = I2RT Where H = Heat generated in joule I = Electric current in ampere R = Electric resistance in Ohm T = Time of current flow in second

This heat is used to melt the interface metal to form a strong weld joint by fusion. This process produces weld without application of any filler material, flux and shielding gases.

At first the job is cleaned and all types of contaminants like grease, oil, dirt, scale and paint are removed. The surface pf the electrodes are also made very clean. For clamping the metal sheets together two copper electrodes are used at the same time. The current passes through electrodes and then into the metal sheets. Because of the resistance, heat is generated in the air gap within the contact points. Since copper is great conductor heat is dissipated to the metal so quickly. As the metal (workpiece) is a poor conductor of heat in comparison to the copper electrode the heat remains in the air gap. So the heat remains in the one place creating a strong effect and the metal

Prepared by L. SUSHMAASSOPROF 70 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE is melted at that desired spot. The period of heat dissipation is very small and at this time metal gets melted and then become solid and thus the joint is formed. . Heat generated by electrical resistance to current flow at junction to be welded . Principal RW process is resistance spot welding (RSW) Advantages: . No filler metal required . High production rates possible . Lends itself to mechanization and automation . Lower operator skill level than for arc welding . Good repeatability and reliability Disadvantages: . High initial equipment cost . Limited to lap joints for most RW processes

Weld quality and defects: 1. Porosity . Trapped gases, contaminants . Preheat or increase rate of heat input . Reduce speed allowing gas to escape, cleaning 2. Slag inclusions . Oxides, fluxes, electrode coating trapped in weld zone . Clean weld bead during multi-weld processes . Provide enough shielding gas 3. Incomplete fusion/penetration . Preheat and clean joint . Clean weld area, enough shielding gas . Change joint design or type of electrode 4. Cracks, residual stresses . Temperature gradients, embrittlement of grain boundaries . Inability of weld metal to contract during cooling

Prepared by L. SUSHMAASSOPROF 71 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Resistance Spot Welding (RSW) Working Principle

Advantages of Resistance Spot Welding

Comparatively Low cost Resistance Spot Welding (RSW) method doesn't need highly skilled worker. Distortion or warping of parts is eliminated though it leaves some depressions or indentation. The joint made is highly uniform. Automatic or semi-automatic operation both can be done. There is no need for edge preparation. Welding can be done in quick succession. It just needs a few seconds to make the joint. Disadvantages of RSW The equipment cost is high so it can has an effect on the initial cost. Skilled welders or technicians are needed for the maintenance and controlling. Some metals need special surface preparation for making the RSW a success. The thick jobs are not easy to weld. Applications of Resistance Spot Welding Spot welding of thick steel plates has been done and it has replaced the need for riveting. The welding of two or more sheet metals can be joined by mechanical means more economically by using the spot welding methods. We don't need gas tight joints.

Prepared by L. SUSHMAASSOPROF 72 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Spot welding can be used for attaching braces, pads or clips with cases, bases and covers which are mainly product of sheet metal forming. Automobile and aircraft industries relies greatly of spot welding these days.

Resistance Seam Welding (RSEW) Working Principles The Resistance Seam Welding (RSEW) is very much similar to the Spot Welding (RSW) but here circular rotating electrodes are used. And here we get continuous weld which is air-tight (If the process is perfect). The seam-welding form of the resistance process is a series of overlapping welds. Two or more sheets of base metal are usually passed between electrode rollers, as shown in following Figure, which transmit the current and also the mechanical pressure required for producing a welded seam which is normally gas-tight or liquid-tight.

Prepared by L. SUSHMAASSOPROF 73 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Resistance Seam Welding (RSEW) Advantages and Disadvantages Advantages Gas tight as well as liquid tight joints can be made. The Overlap is less than spot or projection welding. The production of single seam weld and parallel seams can be got simultaneously.

Disadvantages The welding process is restricted to a straight line or uniformly curved line. The metals sheets having thickness more than 3mm can cause problems while welding. The design of the electrodes may be needed to change to weld metal sheets having obstructions.

Applications of RSEW Girth weld is possible in rectangular or square or even in circular shapes. Most of the metals can be welded (Except copper and some high percentage copper alloys) Butt welding can be done.

Projection welding Projection welding is a resistance welding process for joining metal components or sheets with embossments by directly applying opposing forces with electrodes specially designed to fit the shapes of the workpieces. The current and the heat generation are localized by the shape of the workpieces either with their natural shape or with specially designed projection. Large deformation or collapse will occur in the projection part of the workpieces implying high process/machine dynamics. Projection welding is same as spot welding except a dimple is produced on work pieces at the location where weld is desired. Now the work pieces held between electrode and a large amount of current pass through it. A small amount of pressure is applied through electrode on welding plates. The current pass through dimple which melt down it and the pressure causes the dimple flatten and form a weld. Projection welding is widely used in electrical, electronics, automotive and construction industries, and manufacturing of sensors, valves and pumps etc.

Prepared by L. SUSHMAASSOPROF 74 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Flash butt Welding: It is another type of resistance welding which is used to weld tubes and rods in steel industries. In this process, two work pieces which are to be welded will be clamped in the electrode holders and a high pulsed current in the range of 100000 ampere is supplied to the work piece material. In this two electrode holders are used in which one is fixed and other is movable. Initially the current is supplied and movable clamp is forced against the fixed clamp due to contact of these two work pieces at high current, flash will be produced. When the interface surface comes into plastic form, the current is stopped and axial pressure is increased to make joint. In this process weld is formed due to plastic deformation.

Prepared by L. SUSHMAASSOPROF 75 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Application:

 Resistance welding is widely used in automotive industries.  Projection welding is widely used in production of nut and bolt.  Seam welding is used to produce leak prove joint required in small tanks, boilers etc.  Flash welding is used to welding pipes and tubes.

Advantages and Disadvantages:

Advantages:

 It can weld thin (0.1 mm) as well as thick (20mm) metals.  High welding speed.  Easily automated.  Both similar and dissimilar metals can be weld.  The process is simple and fully automated so does not required high skilled labor.  High production rate.  It is environment friendly process.  It does not require any filler metal, flux and shielding gases.

Disadvantages:

 High equipment cost.  The thickness of work piece is limited due to current requirement.  It is less efficient for high conductive materials.  High electric power required.  Weld joints have low tensile and fatigue strength.

Prepared by L. SUSHMAASSOPROF 76 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Laser beam welding is a technique in manufacturing whereby two or more pieces of material (usually metal) are joined by together through use of a laser beam. Laser stands for Light Amplification by Stimulated Emission of Radiation. It is a non-contact process that requires access to the weld zone from one side of the parts being welded.

The weld is formed as the intense laser light rapidly heats the material - typically calculated in Milli-seconds.

The laser beam is a coherent (single phase) light of a single wavelength (monochromatic). The laser beam has low beam divergence and high energy content and thus will create heat when it strikes a surface

The primary types of lasers used in welding and cutting are:

 Gas lasers: use a mixture of gases such as helium and nitrogen. There are also CO2 or carbon dioxide lasers. These lasers use a low-current, high-voltage power source to excite the gas mixture using a lasing medium. Operate in a pulsed or continuous mode.

Carbon dioxide lasers use a mixture of high purity carbon dioxide with helium and nitrogen as the lasing medium. CO2 lasers are also used in dual beam laser welding where the beam is split into two equal power beams.  Solid state lasers: (Nd:YAG type and ruby lasers) Operate at 1micrometer wavelengths. They can be pulsed or operate continuously. Pulsed operation produced joints similar to spot welds but with complete penetration. The pulse energy is 1 to 100 Joules. Pulse time is 1 to 10 milliseconds.  Diode lasers

Lasers are used for materials that are difficult to weld using other methods, for hard to access areas and for extremely small components. Intert gas shielding is needed for more reactive materials.

Laser Beam Welding Examples

Prepared by L. SUSHMAASSOPROF 77 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Overview

Laser beam welding (LBW) is a welding process which produces coalescence of materials with the heat obtained from the application of a concentrate coherent light beam impinging upon the surfaces to be joined.

The focused laser beam has the highest energy concentration of any known source of energy. The laser beam is a source of electromagnetic energy or light that can be pro jetted without diverging and can be concentrated to a precise spot. The beam is coherent and of a single frequency.

Gases can emit coherent radiation when contained in an optical resonant cavity. Gas lasers can be operated continuously but originally only at low levels of power. Later developments allowed the gases in the laser to be cooled so that it could be operated continuously at higher power outputs. The gas lasers are pumped by high radio frequency generators which raise the gas atoms to sufficiently high energy level to cause lasing. Currently, 2000-watt carbon dioxide laser systems are in use. Higher powered systems are also being used for experimental and developmental work. A 6-kw laser is being used for automotive welding applications and a 10-kw laser has been built for research purposes. There are other types of lasers; however, the continuous carbon dioxide laser now available with 100 watts to 10 kw of power seems the most promising for metalworking applications.

The coherent light emitted by the laser can be focused and reflected in the same way as a light beam. The focused spot size is controlled by a choice of lenses and the distance from it to the base metal. The spot can be made as small as 0.003 in. (0.076 mm) to large areas 10 times as big. A sharply focused spot is used for welding and for cutting. The large spot is used for .

The laser offers a source of concentrated energy for welding; however, there are only a few lasers in actual production use today. The high-powered laser is extremely expensive. Laser welding technology is still in its infancy so there will be improvements and the cost of equipment will be reduced. Recent use of fiber optic techniques to carry the laser beam to the point of welding may greatly expand the use of lasers in metal- working. Laser Welding vs. Arc Welding

Laser beam welding energy transfer is different than arc welding processes. In laser welding the absorption of energy by a material is affected by many factors such as the type of laser, the incident power density and the base metal's surface condition.

Laser output is not electrical in nature and does not require a flow of electrical current. This eliminates any effect of magnetism and does not limit the process to electrically conductive materials.

Lasers can interact with any material. It doesn't require a vacuum and it does not produce x-rays. How it Works

 Pump source provides energy to the medium, exciting the laser such that electrons held with in the atoms are elevated temporarily to higher energy states.  The electrons held in this excited state cannot remain there indefinitely and drop down to a lower energy level.

Prepared by L. SUSHMAASSOPROF 78 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE  The electron looses the excess energy gained from the pump energy by emitting a photon. This is called spontaneous emission and the photons produced by this method are the seed for laser generation.  Photons emitted by spontaneous emission eventually strike other electrons in the higher energy states. The incoming photon "knocks" the electron from the excited state to a lower energy level creating another photon. These photons are coherent meaning they are in phase, of the same wavelength, and traveling the same direction. A process called stimulated emission.  Photons are emitted in all directions, however some travel along the laser medium to strike the resonator mirrors to be reflected back through the medium. The resonator mirrors define the preferential amplification direction for stimulated emission. In order for the amplification to occur there must be a greater percentage of atoms in the excited state than the lower energy levels. This population inversion of more atoms in the excited state to the conditions required for laser generation.  The focus spot of the laser is targeted on the workpiece surface which will be welded. At the surface the concentration of light energy converts into thermal energy (heat). The heat causes the surface of the material to melt, which progresses through the surface by a process called surface conductivity. The beam energy level is maintained below the vaporization temperature of the workpiece material.

The ideal thickness of the materials to be welded is 20mm. The energy is a laser is concentrated, an advantage when working with materials that have high thermal conductivity. Advantages

 Works with high alloy metals without difficulty  Can be used in open air  Can be transmitted over long distances with a minimal loss of power  Narrow heat affected zone  Low total thermal input  Welds dissimilar metals  No filler metals necessary  No secondary finishing necessary  Extremely accurate  Produces deep and narrow welds  Low distortion in welds  High quality welds  Can weld small, thin components  No contact with materials Limitations

 Rapid cooling rate may cause cracking in some metals  High capital cost for equipment  Optical surfaces of the laser are easily damaged  High maintenance costs

Prepared by L. SUSHMAASSOPROF 79 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Soldering is a group of processes that join metals by heating them to a suitable temperature. A filler nonferrous metal that melts at a temperature below 840ºF (449ºC) and below that of the metals to be joined is used. The filler metal is distributed between the closely fitted surfaces of the joint by capillary attraction. Soldering uses fusible alloys to join metals.(brazing occurs at temperatures above 840 Fahrenheit).

The kind of solder used depends on the metals to be joined. Hard solders are called spelter and hard soldering is called silver solder brazing. This process gives greater strength and will stand more heat than soft solder. Types of Soldering

 Torch soldering: soldering process using air-fuel or oxy-fuel torches. Application can be automatic or manual.  Furnace: parts are soldered by passing them through a furnace.  Iron  Induction  Resistance  Dip (small scale process for electronic components)  Infrared  Ultrasonic  Reflow or Paste  Wave (used to attach circuits to circuit boards) Soldering Tips

There are three types of soldering tool tips.

Brazing vs. Soldering

 Soldering: filler metals have a melting point below 840 F (450C)

Prepared by L. SUSHMAASSOPROF 80 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE  Brazing: filler metals have a melting point above 840 F (450C)

Welding refers to fusion of two metals, while soldering and brazing use adhesion.

In soldering and brazing the filler metal melts and flows into the joint. The base material remains intact or un-melted. The parts are fitted with tight tolerances, which produces a capillary action (capillarity) to draw the filler metal into the joint.

The advantages of brazing and soldering include:

 ability to join metals that cannot be welded. reheating can separate parts, particularly if one needs to be replaced  easier to separate joined parts  parts can be produced in a batch furnace  portable process for joining smaller parts

The downside of both soldering and brazing include:

 tight joint tolerance required for capillary action  lower strength vs. welding  larger metal parts need to be soldered or brazed in a big furnace  flux is required Solder Types and Applications

Types of Solder Applications Tin-lead General Purpose Tin-zinc Aluminum Lead-silver Strength at higher than room temperature Cadmium-silver Strength at high temperatures Zinc-aluminum Aluminum, corrosion resistance Tin-silver Electronics Tin-bismuth Electronics

Hard Soldering (cupro-techtic)

Hard soldering or silver soldering refers to solder that has silver content used to lower the melting point so that the molten metal flows more easily. This type of soldering requires a hot heat sorce, requiring a special torch. Oxyacetylene equipment can also be used, but there is a risk that some metals can be melted such as copper.

Hard soldering is considered to be one of the best methods for joining two copper parts. Soft Soldering

Prepared by L. SUSHMAASSOPROF 81 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE This process is used for joining most common metals with an alloy that melts at a temperature below that of the base metal. In many respects, this operation is similar to brazing in that the base is not melted, but is merely tinned on the surface by the solder filler metal. For its strength the soldered joint depends on the penetration of the solder into the pores of the base metal surface, along with the consequent formation of a base metal-solder alloy, together with the mechanical bond between the parts. Soft solders are used for airtight or watertight joints which are not exposed to high temperatures. Joint Preparation

Flux

All soldering operations require a flux in order to obtain a complete bond and full strength at the joints. Fluxes clean the joint area, prevent oxidations, and increase the wetting power of the solder by decreasing its surface tension.

The following types of soft soldering fluxes are in common use:

 rosin  rosin and glycerine.

These are used on clean joints to prevent the formation of oxides during the soldering operations. Zinc chloride and ammonium chloride may be used on tarnished surfaces to permit good tinning. A solution of zinc cut in hydrochloric (muriatic) acid is commonly used by tin workers as a flux.

Flux comes in powder, paste and liquid form. I also comes in noncorrosive and corrosive forms. The corrosive form works best, but requires a flux remover after your soldering is complete.

Prepared by L. SUSHMAASSOPROF 82 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Application of Solder

Soft solder joints may be made by using gas flames, wiping, sweating the joints, or by dipping in solder baths. Dipping is particularly applicable to the repair of radiator cores. Electrical connections and sheet metal are soldered with a soldering iron or gun. Wiping is a method used for joining lead pipe and also the lead jacket of underground and other lead-covered cables. Sweated joints may be made by applying a mixture of solder powder and paste flux to the joints. Then heat the part until this solder mixture liquifies and flows into the joints, or tin mating surfaces of members to be joined, and apply heat to complete the joint. How to Solder

1. Clean base metals with a brush or other method (see above) 2. Clamp the two metals to be joined together ensuring that there is a gap for the solder. The goal is to minimize movement. 3. Plug in the electric soldering iron or light the gas torch 4. Heat the base material with the iron or torch 5. Apply solder by touching the end of the solder to the joint. You'll see it melt and start to flow. Continue until joint is full of solder. 6. Remove soldering tool. 7. Do not move soldered joint until everything has cooled. 8. Use flux remover if you used a corrosive flux. If you did, after removing flux, clean joint area with water and soap. 9. Wash hands. Soldering Tips

1. Clean the area to be soldered on the base metals thoroughly 2. Test the joints before soldering for a good fit 3. Clamp the pieces being joined to eliminate movement while soldering 4. Check the required temperature needed for the solder or rods being used 5. Check the table above for the right solder to use for your project. The most popular solder is 50% lead, 50% tin or 50/50. It will melt at 470F. 6. Wear eye protection and do the soldering in a room with excellent ventilation. 7. Use a flux, a substance that removes impurities from the air

Prepared by L. SUSHMAASSOPROF 83 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Brazing is a group of welding processes which produces coalescence of materials by heating to a suitable temperature and using a filler metal having a liquidus above 840ºF (449ºC) and below the solidus of the base metals. The filler metal is distributed between the closely fitted surfaces of the joint by capillary attraction. Brazing is distinguished from soldering in that soldering employs a filler metal having a liquidus below 840ºF (449ºC).

When brazing with silver alloy filler metals (silver soldering), the alloys have liquidus temperatures above 840ºF (449ºC).

Brazing is only used on ferrous metals because the solder melts at 960 degrees celcius, a point above the melting point of non-ferrous metals.

Brazing must meet each of three criteria:

1. The parts must be joined without melting the base metals. 2. The filler metal must have a liquidus temperature above 840ºF (449ºC). 3. The filler metal must wet the base metal surfaces and be drawn onto or held in the joint by capillary attraction.

Here Heat is Applied Below the Joint in Order to Generate a Capillary Action That Draws the Filler Alloy into the Joint.

Principles

Capillary flow is the most important physical principle which ensures good brazements providing both adjoining surfaces molten filler metal. The joint must also be properly spaced to permit efficient capillary action and resulting coalescence. More specifically, capillarity is a result of surface tension between base metal(s), filler metal, flux or atmosphere, and the contact angle between base and filler metals. In actual practice, brazing filler metal flow characteristics are also influenced by considerations involving fluidity, viscosity, vapor pressure, gravity, and by the effects of any metallurgical reactions between the filler and base metals.

The brazed joint, in general, is one of a relatively large area and very small thickness. In the simplest application of the process, the surfaces to be joined are cleaned to remove contaminants and oxide. Next, Prepared by L. SUSHMAASSOPROF 84 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE they are coated with flux or a material capable of dissolving solid metal oxides present and preventing new oxidation. The joint area is then heated until the flux melts and cleans the base metals, which are protected against further oxidation by the liquid flux layer.

Brazing filler metal is then melted at some point on the surface of the joint area. Capillary attraction is much higher between the base and filler metals than that between the base metal and flux. Therefore, the flux is removed by the filler metal. The joint, upon cooling to room temperature, will be filled with solid filler metal. The solid flux will be found on the joint surface.

High fluidity is a desirable characteristic of brazing filler metal because capillary attraction may be insufficient to cause a viscous filler metal to run into tight fitting joints.

Brazing is sometimes done with an active gas, such as hydrogen, or in an inert gas or vacuum. Atmosphere brazing eliminates the necessity for post cleaning and ensures absence of corrosive mineral flux residue. Carbon steels, stainless steels, and super alloy components are widely processed in atmospheres of reacted gases, dry hydrogen, dissociated ammonia, argon, and vacuum. Large vacuum furnaces are used to braze zirconium, titanium, stainless steels, and the refractory metals. With good processing procedures, aluminum alloys can also be vacuum furnace brazed with excellent results.

Brazing is a process preferred for making high strength metallurgical bonds and preserving needed base metal properties because it is economical. Processes

Generally, brazing processes are specified according to heating methods (sources) of industrial significance. Whatever the process used, the filler metal has a melting point above 840ºF (450ºC) but below the base metal and distributed in the joint by capillary attraction.

The brazing processes are:

 Torch brazing  Furnace brazing  Induction brazing  Resistance brazing  Dip brazing  Infrared brazing  Blanket  Exothermic

Torch Brazing

Torch brazing is commonly used for smaller production runs or one assembly. This type of brazing is performed by heating with a gas torch set to the proper required composition, and an appropriate flux. This depends on the temperature and heat amount required. The fuel gas (acetylene, propane, city gas, etc.) may be burned with air, compressed air, or oxygen.

Brazing filler metal may be pre-placed at the joint in the forms of rings, washers, strips, slugs, or powder, or it may be fed from hand-held filler metal in wire or rod form. In any case, proper cleaning and fluxing are essential. Prepared by L. SUSHMAASSOPROF 85 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

For manual torch brazing, the torch may be equipped with a single tip, either single or multiple flame. Manual torch brazing is particularly useful on assemblies involving sections of unequal mass. Welding machine operations can be set up where the production rate allows, using one or several torches equipped with single or multiple flame tips. The machine may be designed to move either the work or torches, or both. For premixed city gas-air flames, a refractory type burner is used.

Furnace Brazing

Furnace brazing is used extensively where the parts to be brazed can be assembled with the brazing filler metal in form of wire, foil, filings, slugs, powder, paste, or tape is pre-placed near or in the joint. This process is particularly applicable for high production brazing. Fluxing is employed except when an atmosphere is specifically introduced in the furnace to perform the same function. Most of the high production brazing is done in a reducing gas atmosphere, such as hydrogen and combusted gases that are either exothermic (formed with heat evolution) or endothermic (formed with heat absorption). Pure inert gases, such as argon or helium, are used to obtain special atmospheric properties.

A large volume of furnace brazing is performed in a vacuum, which prevents oxidation and often eliminates the need for flux. Vacuum brazing is widely used in the aerospace and nuclear fields, where reactive metals are joined or where entrapped fluxes would be intolerable. If the vacuum is maintained by continuous pumping, it will remove volatile constituents liberated during brazing. There are several base metals and filler metals that should not be brazed in a vacuum because low boiling point or high vapor pressure constituents may be lost. The types of furnaces generally used are either batch or contiguous. These furnaces are usually heated by electrical resistance elements, gas or oil, and should have automatic time and temperature controls. Cooling is sometimes accomplished by cooling chambers, which either are placed over the hot retort or are an integral part of the furnace design. Forced atmosphere injection is another method of cooling. Parts may be placed in the furnace singly, in batches, or on a continuous conveyor.

A vacuum is a relatively economical method of providing an accurately controlled brazing atmosphere. Vacuum provides the surface cleanliness needed for good wetting and flow of filler metals without the use of fluxes. Base metals containing chromium and silicon can be easily vacuum brazed where a very pure, low dew point atmosphere gas would otherwise be required.

Induction Brazing

In this process, the heat necessary to braze metals is obtained from a high frequency electric current consisting of a motor-generator, resonant spark gap, and vacuum tube oscillator. It is induced or produced without magnetic or electric contact in the parts (metals). The parts are placed in or near a water-cooled coil carrying alternating current. They do not form any part of the electrical circuit. The brazing filler metal normally is pre-placed.

Careful design of the joint and the coil setup are necessary to assure that the surfaces of all members of the joint reach the brazing temperature at the same time. Flux is employed except when an atmosphere is specifically introduced to perform the same function.

The equipment consists of or clamps with the electrodes attached at the end of each arm. The tongs should preferably be water-cooled to avoid overheating. The arms are current carrying conductors attached by leads to a transformer. Direct current may be used but is comparatively expensive. Resistance welding

Prepared by L. SUSHMAASSOPROF 86 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE machines are also used. The electrodes may be carbon, graphite, refractory metals, or copper alloys according to the required conductivity.

Resistance

The heat necessary for resistance brazing is obtained from the resistance to the flow of an electric current through the electrodes and the joint to be brazed. The parts comprising the joint form a part of the electric circuit. The brazing filler metal, in some convenient form, is pre-placed or face fed. Fluxing is done with due attention to the conductivity of the fluxes. (Most fluxes are insulators when dry.) Flux is employed except when an atmosphere is specifically introduced to perform the same function. The parts to be brazed are held between two electrodes, and proper pressure and current are applied. The pressure should be maintained until the joint has solidified. In some cases, both electrodes may be located on the same side of the joint with a suitable backing to maintain the required pressure.

Dip Brazing

There are two methods of dip brazing:

 chemical bath dip brazing  molten metal bath dip brazing.

Chemical Bath Dip Brazing

In chemical bath dip brazing, the brazing filler metal, in suitable form, is pre-placed and the assembly is immersed in a bath of molten salt. The salt bath furnishes the heat necessary for brazing and usually provides the necessary protection from oxidation; if not, a suitable flux should be used. The salt bath is contained in a metal or other suitable pot, also called the furnace, which is heated from the outside through the wall of the pot, by means of electrical resistance units placed in the bath, or by the I2R loss in the bath itself.

Molten Metal Bath Dip Brazing

In molten metal bath dip brazing, the parts are immersed in a bath of molten brazing filler metal contained in a suitable pot. The parts must be cleaned and fluxed if necessary. A cover of flux should be maintained over the molten bath to protect it from oxidation. This method is largely confined to brazing small parts, such as wires or narrow strips of metal. The ends of the wires or parts must be held firmly together when they are removed from the bath until the brazing filler metal has fully solidified.

Infrared Brazing

Infrared heat is radiant heat obtained below the red rays in the spectrum. While with every "black" source there is sane visible light, the principal heating is done by the invisible radiation. Heat sources (lamps) capable of delivering up to 5000 watts of radiant energy are commercially available. The lamps do not necessarily need to follow the contour of the part to be heated even though the heat input varies inversely as the square of the distance from the source. Reflectors are used to concentrate the heat.

Assemblies to be brazed are supported in a position that enables the energy to impinge on the part. In some applications, only the assembly itself is enclosed. There are, however, applications where the assembly and the lamps are placed in a bell jar or retort that can be evacuated, or in which an inert gas atmosphere can be

Prepared by L. SUSHMAASSOPROF 87 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE maintained. The assembly is then heated to a controlled temperature, as indicated by thermocouples. The part is moved to the cooling platens after brazing.

Blanket Brazing

Blanket brazing is another of the processes used for brazing. A blanket is resistance heated, and most of the heat is transferred to the parts by two methods, conduction and radiation, the latter being responsible for the majority of the heat transfer.

Exothermic Brazing

Exothermic brazing is another special process by which the heat required to melt and flow a commercial filler metal is generated by a solid state exothermic chemical reaction. An exothermic chemical reaction is defined as any reaction between two or more reactants in which heat is given off due to the free energy of the system. Nature has provided us with countless numbers of these reactions; however, only the solid state or nearly solid state metal-metal oxide reactions are suitable for use in exothermic brazing units. Exothermic brazing utilizes simplified tooling and equipment.

The process employs the reaction heat in bringing adjoining or nearby metal interfaces to a temperature where pre-placed brazing filler metal will melt and wet the metal interface surfaces. The brazing filler metal can be a commercially available one having suitable melting and flow temperatures. The only limitations may be the thickness of the metal that must be heated through and the effects of this heat, or any previous heat treatment, on the metal properties.

Prepared by L. SUSHMAASSOPROF 88 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

II UNIT

Prepared by L. SUSHMAASSOPROF 89 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Metal cutting is the process of producing a job by removing a layer of unwanted material from a given workpiece. Fig. shows the schematics of a typical metal cutting process in which a wedge shaped, sharp edged tool is set to a certain depth of cut and moves relative to the workpiece.

Under the action of force, pressure is exerted on the workpiece metal causing its compression near the tip of the tool. The metal undergoes shear type deformation and a piece or layer of metal gets repeated in the form of a chip.

If the tool is continued to move relative to workpiece, there is continuous shearing of the metal ahead of the tool. The shear occurs along a plane called the shear plane.

All machining processes involve the formation of chips; this occurs by deforming the work material on the surface of job with the help of a cutting tool. Depending upon the tool geometry, cutting conditions and work material, chips are produced in different shapes and sizes. The type of chip formed provides information about the deformation suffered by the work material and the surface quality produced during cutting.

Types of Chips:

Continuous chips: While machining ductile materials, large plastic deformation of the work material occurs ahead of the cutting edge of the tool. The metal of the workpiece is compressed and slides over the tool face in the form of a long continuous chip.

Prepared by L. SUSHMAASSOPROF 90 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Discontinuous (segmented) chips: A discontinuous chip is a segmented chip produced in the form of small pieces. The discontinuous chips are produced when cutting brittle materials like cast iron, bronze and brass. The working on ductile materials under poor cutting condition may also sometimes lead to the formation of discontinuous chips.

Continuous chips with built-up-edge: The term built-up-edge refers to the small metal particles that stick to the cutting tool and the machined surfaces as result of high temperature, high pressure and high frictional resistance during machining. The building up and breaking down of the built-up-edge is periodic; its size first increases, then decreases and again increases-the cycle gets repeated rapidly.

Prepared by L. SUSHMAASSOPROF 91 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE LATHE MACHINE

Working Principle: The lathe is a which holds the workpiece between two rigid and strong supports called centers or in a or face plate which revolves. The cutting tool is rigidly held and supported in a tool post which is fed against the revolving work. The normal cutting operations are performed with the cutting tool fed either parallel or at right angles to the axis of the work.

Construction: The main parts of the lathe are the bed, headstock, quick changing gear box, carriage and tailstock.

Bed: The bed is a heavy, rugged casting in which are mounted the working parts of the lathe. It carries the headstock and tail stock for supporting the workpiece and provides a base for the movement of carriage assembly which carries the tool.

2. Legs: The legs carry the entire load of machine and are firmly secured to floor by foundation bolts.

Headstock: The headstock is clamped on the left hand side of the bed and it serves as housing for the driving pulleys, back gears, headstock spindle, live centre and the feed reverse gear. The headstock spindle is a hollow cylindrical shaft that provides a drive from the motor to work holding devices.

4. Gear Box: The quick-change gear-box is placed below the headstock and contains a number of different sized gears.

5. Carriage: The carriage is located between the headstock and tailstock and serves the purpose of supporting, guiding and feeding the tool against the job during operation. The main parts of carriage are: a). The saddle is an H-shaped casting mounted on the top of lathe ways. It provides support to cross-slide, compound rest and tool post. b). The cross slide is mounted on the top of saddle, and it provides a mounted or automatic cross movement for the cutting tool. c). The compound rest is fitted on the top of cross slide and is used to support the tool post and the cutting tool. d). The tool post is mounted on the compound rest, and it rigidly clamps the cutting tool or tool holder at the proper height relative to the work centre line. e). The apron is fastened to the saddle and it houses the gears, clutches and levers required to move the carriage or cross slide. The engagement of split nut lever and the automatic feed lever at the same time is prevented she carriage along the lathe bed.

6. Tailstock: The tailstock is a movable casting located opposite the headstock on the ways of the bed. The tailstock can slide along the bed to accommodate different lengths of workpiece between the centers. A tailstock clamp is provided to lock the tailstock at any desired position. The tailstock spindle has an internal taper to hold the dead centre and the tapered shank tools such as and .

LATHE OPERATIONS

Prepared by L. SUSHMAASSOPROF 92 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE The engine lathe is an accurate and versatile machine on which many operations can be performed. These operations are:

1. Plain Turning and Step Turning

2. Facing

3. Parting

4. Drilling

5. Reaming

6. Boring

7. Knurling

8. Grooving

9. Threading

10. Forming

11. Chamfering

12. Filling and Polishing

13. Taper Turning

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MILLING

Introduction: Milling is the cutting operation that removes metal by feeding the work against a rotating, cutter having single or multiple cutting edges. Flat or curved surfaces of many shapes can be machined by milling with good finish and accuracy. A milling machine may also be used for drilling, slotting, making a circular profile and gear cutting by having suitable attachments.

Working Principle: The workpiece is holding on the worktable of the machine. The table movement controls the feed of workpiece against the rotating cutter. The cutter is mounted on a spindle or arbor and revolves at high speed. Except for rotation the cutter has no other motion. As the workpiece advances, the cutter teeth remove the metal from the surface of workpiece and the desired shape is produced.

Horizontal Milling Machine Construction: The main part of machine is base, Column, Knee, Saddle, Table, Overarm, Arbor Support and Elevating Screw.

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1. Base: It gives support and rigidity to the machine and also acts as a reservoir for the cutting fluids.

2. Column: The column is the main supporting frame mounted vertically on the base. The column is box shaped, heavily ribbed inside and houses all the driving mechanisms for the spindle and table feed.

3. Knee: The knee is a rigid casting mounted on the front face of the column. The knee moves vertically along the guide ways and this movement enables to adjust the distance between the cutter and the job mounted on the table. The adjustment is obtained manually or automatically by operating the elevating screw provided below the knee.

4. Saddle: The saddle rests on the knee and constitutes the intermediate part between the knee and the table. The saddle moves transversely, i.e., crosswise (in or out) on guide ways provided on the knee.

5. Table: The table rests on guide ways in the saddle and provides support to the work. The table is made of cast iron, its top surface is accurately machined and carriers T-slots which accommodate the clamping bolt for fixing the work. The worktable and hence the job fitted on it is given motions in three directions: a). Vertical (up and down) movement provided by raising or lowering the knee. b). Cross (in or out) or transverse motion provided by moving the saddle in relation to knee. c). Longitudinal (back and forth) motion provided by hand wheel fitted on the side of feed screw.

In addition to the above motions, the table of a universal milling machine can be swiveled 45° to either side of the centre line and thus fed at an angle to the spindle.

6. Overarm: The Overarm is mounted at the top of the column and is guided in perfect alignment by the machined surfaces. The Overarm is the support for the arbor.

7. Arbor support: The arbor support is fitted to the Overarm and can be clamped at any location on the Overarm. Its function is to align and support various arbors. The arbor is a machined shaft that holds and drives the cutters.

8. Elevating screw: The upward and downward movement to the knee and the table is given by the elevating screw that is operated by hand or an automatic feed. Prepared by L. SUSHMAASSOPROF 95 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE MILLING OPERATIONS

1. Plain or slab milling

2. Face Milling

3. Angular Milling

4. Straddle Milling

5. Form Milling

6. Gang Milling

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Introduction: The drilling machine or press is one of the most common and useful machine employed in industry for producing forming and finishing holes in a workpiece. The unit essentially consists of:

1. A spindle which turns the tool (called drill) which can be advanced in the workpiece either automatically or by hand.

2. A work table which holds the workpiece rigidly in position.

Working principle: The rotating edge of the drill exerts a large force on the workpiece and the hole is generated. The removal of metal in a drilling operation is by shearing and extrusion.

Working Principle of Drill machine Sensitive Drill Machine/Drill Press

Types of Drilling Machines: A wide variety of drilling machines are available ranging from the simple portable to highly complex automatic and numerically controlled machines are as follows:

1. Portable drilling machine: It is a small light weight, compact and self contained unit that can drill holes upto 12.5 rnrn diameter. The machine is driven by a small electric motor operating at high speed. The machine is capable of drilling holes in the workpieces in any position.

2. Sensitive drill machine/press: This is a light weight, high speed machine designed for drilling small holes in light jobs. Generally the machine has the capacity to rotate drills of 1.5 to 15.5 rnrn at high speed of 20,000 rev/min.

Construction: The machine has only a hand feed mechanism for feeding the tool into the workpiece. This enables the operator to feel how the drill is cutting and accordingly he can control the down feed pressure. Sensitive drill presses are manufactured in bench or floor models, i.e., the base of machine may be mounted on a bench or floor.

The main operating parts of a sensitive machine/drill press are Base, Column, Table, and Drill Head.

Prepared by L. SUSHMAASSOPROF 97 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE 1. Base: The base is a heavy casting that supports the machine structure; it provides rigid mounting for the column and stability for the machine. The base is usually provided with holes and slots which help to Bolt the base to a table or bench and allow the work-holding device or the workpiece to be fastened to the base.

2. Column: The column is a vertical post that Column holds the worktable and the head containing the driving mechanism. The column may be of round or box section.

3. Table: The table, either rectangular or round. Drill machine/press in shape supports the workpiece and is carried by the vertical column. The surface of the table is 90-degree to the column and it can be raised, lowered and swiveled around it. The table can be clamp/hold the required the workpiece. Slots are provided in most tables to allow the jigs, fixtures or large workpieces to be securely fixed directly to the table.

4. Drilling Head: The drilling head, mounted close to the top of the column, houses the driving arrangement and variable speed pulleys. These units transmit rotary motion at different speeds to the drill spindle. The hand feed lever is used to control the vertical movement of the spindle sleeve and the cutting tool.

The system is called the sensitive drilling machine/press as the operator is able to sense the progress of drill with hand-faced.

Prepared by L. SUSHMAASSOPROF 98 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE SHAPING MACHINE

Introduction: The shaper is a machine tool used primarily for:

1. Producing a flat or plane surface which may be in a horizontal, a vertical or an angular plane.

2. Making slots, grooves and keyways

3. Producing contour of concave/convex or a combination of these

Working Principle: The job is rigidly fixed on the machine table. The single point cutting tool held properly in the tool post is mounted on a reciprocating ram. The reciprocating motion of the ram is obtained by a quick return motion mechanism. As the ram reciprocates, the tool cuts the material during its forward stroke. During return, there is no cutting action and this stroke is called the idle stroke. The forward and return strokes constitute one operating cycle of the shaper.

Construction: The main parts of the Shaper machine is Base, Body (Pillar, Frame, Column), Cross rail, Ram and tool head (Tool Post, Tool Slide, Clamper Box Block).

Base: The base is a heavy cast iron casting which is fixed to the shop floor. It supports the body frame and the entire load of the machine. The base absorbs and withstands vibrations and other forces which are likely to be induced during the shaping operations.

Body (Pillar, Frame, Column): It is mounted on the base and houses the drive mechanism compressing the main drives, the gear box and the quick return mechanism for the ram movement. The top of the body provides guide ways for the ram and its front provides the guide ways for the cross rail.

Cross rail: The cross rail is mounted on the front of the body frame and can be moved up and down. The vertical movement of the cross rail permits jobs of different heights to be accommodated below the tool. Sliding along the cross rail is a saddle which carries the work table.

Prepared by L. SUSHMAASSOPROF 99 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Ram and tool head: The ram is driven back and forth in its slides by the slotted link mechanism. The back and forth movement of ram is called stroke and it can be adjusted according to the length of the workpiece to be-machined.

Prepared by L. SUSHMAASSOPROF 100 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

To grind means to abrade, to wear away by friction, or to sharpen.

In manufacturing it refers to the removal of metal by a rotating abrasive wheel. Wheel action is similar to a . The cutting wheel is composed of many small grains bonded together, each one acting as a miniature cutting point (1)

Types of grinders

Cylindrical grinders: This machine is used primarily for grinding cylindrical surfaces, although tapered and simple format surfaces may also be ground. They may be further classified according to the method of supporting the work .Diagrams illustrating the essential difference in supporting the work between centers and are shown in figure 23.2.In the center less type the work is supported by the work rest the regulating wheel, and the grinding wheel itself. Both types use plain grinding wheels with the grinding face as the outside diameter. The depth of cut is controlled by feeding the wheel into the work. Roughing cuts around 0.002 in(0.05 mm) per pass may be made but for finishing this should be reduce to about 0.0002 in (0.005 mm) per pass or less. In selecting the amount of in feed, consideration is given to the size and rigidity of the work, surface finish and the decision of whether or not to use a coolant.

Where the face of the wheel is wider than the part to be ground it is not necessary to traverse the work.

This is known as plunge cut grinding. The grinding speed of the wheel is terms of surface feet per minute that is, Vc= π Dc x N Where Vc=Cutting or grinding speed(m/min) Dc=Diameter of grinding wheel (m)

Centerless Grinders: Center-less grinders are designed so that they support and feed the work by using two wheels and a work rest as illustrated in figures 23.2 and 23.4.The large wheel is the grinding wheel and the smaller one the pressure or regulating wheel. The regulating wheel is a rubber-bonded abrasive having the frictional characteristics to rotate the work at is own rotational speed. The speed of this wheel, which may be controlled, varies from 50 to 200 ft/min (0.25-1.02 m/s). Both wheels are rotating the same direction. The rest assists in supporting the work while it is being ground, being extended on both sides to direct the work travel to and from the wheels. The axial movement of the work past the grinding wheel is obtained by tilting the wheel at a slight angle from horizontal. An angular adjustment of 0o to 10o is provided in the machine for this purpose. The actual feed can be calculated by this formula(1). F=π d Nsinα

F=Feed (mm/min) N=rpms d=Diameter of regulating wheel(mm) α=Angle inclination of regulating wheel

The work done on an internal grinder is diagrammatically shown in 23.6 tepered holes or those having more than one diameter may be accurately finished in this manner. There are several types of internal grinders Fig. Centerless internal grinding

Surface grinding: Grinding flat or plane surfaces is known as surfaces grinding. Two general types of machines have been developed for this purpose; those of the type with a reciprocating table and those having a rotating worktable. Each machine has the possible variation of a horizontal or vertical positioned grinding wheel spindle. The four possibilities of construction are illustrated below figure. Fig. Types of machines Tool and cutter grinder In grinding tools by hand a bench or pedastal type of grinder is used. The tool is hand held and moved across the face of the wheel continually to avoid excessive grinding in one spot. For sharpening miscellaneous cutters a universal type grinder is used.

Prepared by L. SUSHMAASSOPROF 101 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE SHEET METAL FORMING

Introduction Sheet metal is simply metal formed into thin and flat pieces. It is one of the fundamental forms used in metalworking, and can be cut and bent into a variety of different shapes. Countless everyday objects are constructed of the material. Thicknesses can vary significantly, although extremely thin thicknesses are considered foil or leaf, and pieces thicker than 6 mm (0.25 in) are considered plate.

Sheet metal processing The raw material for sheet metal manufacturing processes is the output of the rolling process. Typically, sheets of metal are sold as flat, rectangular sheets of standard size. If the sheets are thin and very long, they may be in the form of rolls. Therefore the first step in any sheet metal process is to cut the correct shape and sized ‘blank’ from larger sheet.

Introduction Sheet metal processes involve plane stress loadings and lower forces than bulk forming. Almost all sheet metal forming is considered to be secondary processing. The main categories of sheet metal forming are  Shearing  Bending  Drawing Sheet metal forming processes are very common in industry.Some products that are manufactured using sheet metal forming include:  House appliances  Car bodies  Aircraft fuselages.

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Shearing: Shearing is a sheet metal cutting operation along a straight line between two cut-ting edges by means of a power shear and is as shown in figure 1. It involves cutting sheet metal, as well as plates, bars and tubing of various cross sections, into individual pieces by subjecting it to shear shear stress in the thickness direction by using and Die.This process using Punch and Die indicating the various process variables is as shown in the figure 2.Shearing is the process of separating adjacent parts of sheet metal through controlled fracture.

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Figure 2: Shearing process with a Punch and Die indicating process variables The separation of metal by the movement of two blades operated based on shearing forces. A narrow strip of metal is severely plastically deformed to the point where it fractures at the surfaces in contact with the blades.The fracture then propagates inwardto provide complete separation. 1. Proper gives clean fracture surface. 2. Insufficient gives ragged fracture surface. 3. Excessive gives greater distortion, greater energy required to separate metal.

Characteristic features of Shearing: The overall features of a typical sheared edge for the two sheared surfaces are

Prepared by L. SUSHMAASSOPROF 104 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Blanking and punching Blanking and punching is similar sheet metal cutting operations that involve cutting the sheet metal along a closed outline. If the part that is cut out is the desired product, the operation is called blanking and the product is called blank. If the remaining stock is the desired part, the operation is called punching. Punching is a metal fabricating process that removes a scrap slug from the metal work piece each time a punch enters the punching die. This process leaves a hole in the metal work piece.

Figure 6: Punching & Blanking operations Ex: producing a washer illustrates both Punching and Blanking operations is as shown in the figure 7 below

Bending: Bending is one of the most common forming operations. Parts are often further shaped by the relatively simple process of bending, either in one or several places. Stretching on the outer surface is typical of this processandcompression on the inner surface. The terminology used in bending is as shown in figure 8. Bend allowance, Lb is the length of the neutral axis in the bend and is used to determine the blank length for a part.

Lb = α(R + kT) Where α is the bend angle in radians, T is sheet thickness, R is bend radius and k is a constant

Prepared by L. SUSHMAASSOPROF 105 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Figure 8: Bending Terminology The engineering strain on a sheet during bending is given by the relation

As R/T decreases, the tensile strain at the outer fibre increases and the material eventually cracks and is as shown in the figure 9 below.

Figure 9: Effect of inclusions on cracking in the direction of bending operation Springback: Because all materials have finite modulus of elasticity, plastic deformation is followed. When the load is removed, by some elastic recovery. This recovery is called as springback in bending and is as shown in figure 10. Around the neutral plane,the stresses must be elastic because complete tensile andcompressive stress-strain curves of the material aretraversed on both bend sides. When the forming tool isremoved from the metal, the elastic components of stresscause springback which changes both the angleand radiusofthe bent part.

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Figure 10: Springback in bending

Springback can be calculated approximately in terms of radii Ri and Rfas

Where Ri is the radius of curvature before release of load,

Rf is the radius of curvature after release of load, Y is Yield Stress, E is Modulus of Elasticity & T is Sheet Thickness.

Stretch Forming: In Stretch forming, the sheet metal is clamped along its edges and then stretched over a die or form block, which moves upward, downward or sideways, depending on the particular machine. It used to make aircraft wings, skin panels, automobile door panels and window frames.Aluminium skins for the Boeing 767 and 757 fuselages are made by stretch forming using a blank under a tensile force as high as 9MN. The Stretch forming Equipment is as shown in the figure11 given below. Forming by using tensile forcesto stretch the material over a tool or form block. It is used most extensively in theaircraft industry to produce partsof large radius of curvature.(normally for uniform crosssection). It requires materials withappreciable ductility.Springback is largely eliminatedbecause the stress gradient isrelatively uniform.

Prepared by L. SUSHMAASSOPROF 107 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Figure 11: Stretch Forming Equipment

The step by step procedure of stretch forming process is illustrated below in the figure 12. The steps of stretch forming process are  Loading  Pre – stretching  Wrapping  Release

Prepared by L. SUSHMAASSOPROF 108 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Figure 12: step by step procedure of stretch forming process

Deep Drawing: Drawing is a sheet-metal operation to make hollow-shaped parts from a sheet blank. The metalworking process used forshaping flat sheets into cup-shapedarticles.Pressing the metal blank of appropriate size into a shaped die with a punch.A circular sheet blank with a diameter Do and thickness is placed over a die opening with the radius.In deep drawing, the blank is allowed(even encouraged) to draw into thedie and the thickness of the sheetmetal is nominally unchanged.As compared to deep drawing, theblank is clamped and depth isattained at the expense of the sheetthickness in stretching. The blank is held in place with a blank holder or hold down ring under a certain force.

The Deep drawing Process for producing cylindrical cups with initial blank diameter (Do) and various parameters are as shown in figure 13.

Prepared by L. SUSHMAASSOPROF 109 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Figure 13: Deep drawing process producing cylindrical cups

Figure 14: Deep drawing process on a circular sheet metal blank The variables in deep drawing are 1. Properties of the sheet metal

Prepared by L. SUSHMAASSOPROF 110 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE 2. Sheet thickness 3. Blank holder force 4. Speed of the punch 5. Friction at the punch, die and work piece interfaces. 6. Clearance between punch and die. 7. Ratio of the blank diameter to punch diameter. 8. Corner radii of punch and die

Spinning: Spinning, a type of stretch forming, is relatedto bending. Spinning is an old process which involves the forming of axisymmetric parts over a , by the use of various tools and rollers. In spinning, a circular blank isheld against a male die/form, which in turnis rotated by a mechanism similar to a lathespindle.The metal blank is clamped against a form block, which is rotated at high speed. The blank is progressively formed against the block, by a manual tool or by means of small- diameter work rolls. There are 3 basic types of spinning process. They are  Conventional spinning  Shear Spinning  Tube Spinning

Conventional spinning: In Conventional spinning, a circular blank of flat or preformed sheet metal is held against a rotating mandrel, while a rigid tool deforms and shapes the material over the mandrel. The tool may be either manually or by computer – controlled hydraulic mechanism. The conventional spinning process and its parts are also in the figure 15 given below. Part diameters may range upto 6m.

Prepared by L. SUSHMAASSOPROF 111 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Shear Spinning: Shear spinning produces an axisymmetric conical or curvilinear shape, while maintaining the parts maximum diameter and reducing its thickness. It also known as Power spinning or flow turning or Hydrospinning. In this process an axisymmetric conical or curvilinear shape is generated in a manner whereby the diameter of the remains constant.Parts typically upto 3m in diameter can be spun to close dimensional tolerances. Typical parts made are rocket – motor casings and missile nose cones. Shear spinning process for making conical parts is as shown in figure 16.

Tube Spinning: In Tube spinning, the thickness of cylindrical parts is reduced by spinning them on a cylindrical mandrel using rollers and is as shown in figure 17.

Prepared by L. SUSHMAASSOPROF 112 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Super plastic forming: Super plastic forming is a process that employes common metal working is polymer processing techniques which produces complex shapes. The process improves productivity by eliminating mechanical fastners, and produces parts with good dimension accuracy and low residual stress. The technology is now well advanced for titanium structures for aerospace application. Commonly used die materials in super plastic forming are low alloys steels, cast tool steels, ceramics, graphite, plaster of paris. Ex: The majority of application for super plastic forming produces titanium parts for military aircraft, such as Toronado and Mirage 2000 Aircrafts.

Advantages:  Lower strength is required of the tooling, because of low strength of the material at forming temperatures, hence tooling costs are lower.  Little or no residual stress occurs in formed parts.  Complex shape formed out of one piece with close tolerances and elimination of secondary operations

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Limitations:  The material must not be super plastic at service temperature.  Because of the extreme strain rate sensitivity of the super plastic material is at sufficient low rates.

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UNIT III

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Unconventional machining

 In conventional machining process the ability of cutting tool is utilized to stress the materials beyond the yield point to start the material removal process.  In conventional machining cutting tool must be harder than the work piece material.  The advent of hard materials for aerospace applications have made the machining process by conventional methods very difficult and time consuming. This is due to Material Removal Rate (MRR) decreases with increased hardness of the work material.

Hence the machining process which utilizes other machining methods are termed as unconventional machining or Non-traditional machining or advanced machining processes or modern machining processes.

The main reasons for choosing non-traditional machining processes are:

1. To machine high steel alloys. 2. To generate desired complex surfaces and 3. To achieve high accuracy and surface finish.

Classification of unconventional machining processes was mainly on the basis of the nature of energy employed in machining process. They are 1. Chemical Processes

1. Chemical Machining (CM) 2. (PCM)

2. Electrochemical Processes

1. Electro-Chemical Machining (ECM) 2. Electro Chemical Grinding (ECG)

3. Electro-Thermal Processes

1. Electrical Discharge Machining (EDM) 2. Electron Beam machining (EBM) 3. Plasma Arc Machining (PAM) 4. Laser Beam Machining (LBM)

4. Mechanical Processes

1. Ultrasonic Machining (USM) 2. Abrasive Jet Machining (AJM) 3. Water Jet Machining (WJM) 4. Abrasive Water jet Machining (AWJM) aser Beam Machining (LBM) is a form of machining process in which laser beam is used for the machining of metallic and non-metallic materials. In this process, a laser beam of high energy is made to strike on the

Prepared by L. SUSHMAASSOPROF 116 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE workpiece, the thermal energy of the laser gets transferred to the surface of the w/p (workpiece). The heat so produced at the surface heats, melts and vaporizes the materials from the w/p.Light amplification by stimulated emission of radiation is called LASER. Working Principle

It works on the principle that when a high energy laser beam strikes the surface of the workpiece. The heat energy contained by the laser beam gets transferred to the surface of the w/p. This heat energy absorbed by the surface heat melts and vaporizes the material from the w/p. In this way the machining of material takes place by the use of laser beam.

Laser: light amplification by stimulated emission of radiation is called laser.

When the electrons in the excited state do not jumps back to the ground state by its own. This situation is called meta-stable state. When a photon is fired to the meta- stable state of atoms, this stimulates an electron at excited state and it jumps back to its ground state giving of two photons (one photon that we fired and other produced by the electron). These two photons stimulate other atoms electrons and produces more photons- a chain reactions starts and number of photon increases. This process is called stimulated emission as we are stimulating other electrons to get photons. Here we are getting two light photons from a single photon i.e. amplifying the light (increasing the light).

Hence the light beams produced by this method is called laser (light amplification by stimulated emission of radiation). Types of Laser

On the basis of the media used for the production of the laser it is classified as

1. Gas Lasers: In these types of laser, gases are used as the medium to produce lasers. The commonly used gases are He-Ne, argon and Co2. 2. Solid State Lasers: The media of the solid state lasers are produced by doping a rare element into a host material.

Ruby laser is an example of solid state laser in which ruby crystal is used as medium for the generation of laser beam.

The other media used in the solid state lasers are

(i) YAG: For yttrium aluminum garnet which a type of crystal. (ii) Nd:YAG – Refers to neodymium-doped yttrium aluminum garnet crystals Main Parts

The various main parts used in the laser beam machining are

1. A pumping Medium: A medium is needed that contains a large number of atoms. The atoms of the media are used to produce lasers. 2. Flash Tube/Flash Lamp: The flash tube or flash lamp is used to provide the necessary energy to the Prepared by L. SUSHMAASSOPROF 117 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE atoms to excite their electrons. 3. Power Supply: A high voltage power source is used to produce light in flashlight tubes. 4. Capacitor: Capacitor is used to operate the laser beam machine at pulse mode. 5. Reflecting Mirror: Two types of mirror are used, first one is 100 % reflecting and other is partially reflecting. 100 % reflecting mirror is kept at one end and partially reflecting mirror is at the other end. The laser beams comes out from that side where partially reflecting mirror is kept. How Laser is Produced

 A high voltage power supply is applied across the flash tube. A capacitor is used to operate the flash tube at pulse mode.  As the flash is produced by the flash tube, it emits light photons that contain energy.  This light photons emitted by the flash tube is absorbed by the ruby crystal. The photons absorbed by the atoms of the ruby crystals excite the electrons to the high energy level and population inversion (situation when the number of exited electrons is greater than the ground state electrons) is attained.  After short duration, this excited electrons jumps back to its ground state and emits a light photon. This emission of photon is called spontaneous emission,  The emitted photon stimulates the excited electrons and they starts to return to the ground state by emitting two photons. In this way two light photons are produced by utilizing a single photon. Here the amplification (increase) of light takes place by stimulated emission of radiation.  Concentration of the light photon increases and it forms a laser beam.  100 % reflecting mirror bounces back the photons into the crystal. Partially reflecting mirror reflects some of the photons back to the crystal and some of it escapes out and forms a highly concentrated laser beam. A lens is used to focus the laser beam to a desired location. Working of Laser Beam Machining

A very high energy laser beam is produced by the laser machines. This laser beam produced is focused on the workpiece to be machined.

When the laser beam strikes the surface of the w/p, the thermal energy of the laser beam is transferred to the surface of the w/p. this heats, melts, vaporizes and finally removes the material form the workpiece. In this way laser beam machining works. Advantages

 It can be focused to a very small diameter.  It produces a very high amount of energy, about 100 MW per square mm of area.  It is capable of producing very accurately placed holes.  Laser beam machining has the ability to cut or engrave almost all types of materials, when traditional machining process fails to cut or engrave any material.  Since there is no physical contact between the tool and workpiece. The wear and tear in this machining process is very low and hence it requires low maintenance cost  This machining process produces object of very high precision. And most of the object does not require additional finishing  It can be paired with gases that help to make cutting process more efficient. It helps to minimize the oxidation of w/p surface and keep it free from melted of vaporized materials. Produces a very high energy of about 100 MW per square mm of area. Prepared by L. SUSHMAASSOPROF 118 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE  It has the ability to engrave or cut almost all types of materials. But it is best suited for the brittle materials with low conductivity. Disadvantages

 High initial cost. This is because it requires many accessories which are important for the machining process by laser.  Highly trained worker is required to operate laser beam machining machine.  Low production rate since it is not designed for the mass production.  It requires a lot of energy for machining process.  It is not easy to produce deep cuts with the w/p that has high melting points and usually cause a taper.  High maintenance cost. Application

1. The laser beam machining is mostly used in automobile, aerospace, shipbuilding, electronics, steel and medical industries for machining complex parts with precision. 2. In heavy manufacturing industries, it is used or drilling and cladding, seam and spot welding among others. 3. In light manufacturing industries, it is used for and drilling other metals. 4. In the electronic industry, it is used for skiving (to join two ends) of circuits and wire stripping. 5. In medical industry, it is used for hair removal and cosmetic surgery.

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Abrasive Jet Machining (AJM):

In AJM, the material removal takes place due to the impingement of the fine abrasive particles. These particles move with a high speed air (or gas) stream. Figure 6.1 shows the process along with some typical parameters of the process. The abrasive particles are typically of 0.025 mm diameter and the air discharges at a pressure of several atmospheres.

Mechanics of AJM:

When an abrasive particle impinges on the work surface at a high velocity, the impact causes a tiny brittle fracture and the following air (or gas) carries away the dislodged small work piece particle (wear particle). This is shown in Figs. 6.2a and 6.2b. Thus, it is obvious that the process is more suitable when the work material is brittle and fragile. A model for estimating the material removal rate (mrr) is available. The mrr due to the chipping of the work surface by the impacting abrasive particles is expressed as –

where Z is the number of abrasive particles impacting per unit time, d is the mean diameter of the abrasive grains, v is the velocity of the abrasive grains, ρ is the density of the abrasive material, Hw is the hardness of the work material (the flow stress), and X is a constant.

Prepared by L. SUSHMAASSOPROF 120 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Process Parameters of AJM:

The process characteristics can be evaluated by judging – (i) the mrr, (ii) the geometry of the cut, (iii) the roughness of the surface produced, and (iv) the rate of nozzle wear.

The major parameters which control these quantities are:

(i) The abrasive (composition, strength, size, and mass flow rate),

(ii) The gas (composition, pressure, and velocity),

(iii) The nozzle (geometry, material, distance from and inclination to the work surface).

We shall now discuss each of these parameters as also their effects: i. The Abrasive:

Mainly two types of are used, viz., – (i) aluminium oxide and (ii) silicon carbide. However, generally aluminium oxide abrasives are preferred in most applications. The shape of these grains is not very important, but, for a satisfactory wear action on the work surface, these should have sharp edges. Al2O3 and SiC powders with a nominal grain diameter of 10-50 μm are available. The best cutting is achieved when the nominal diameter is between 15 μm and 20 μm.

A reuse of the abrasive powder is not recommended as the – (i) cutting capacity decreases after the first application, and (ii) contamination clogs the small orifices in the nozzle. The mass flow rate of the abrasive particles depends on the pressure and the flow rate of the gas. When the mass fraction of the abrasives in the jet (mixing ratio) increases, the mrr initially increases, but with a further increase in the mixing ratio, it reaches a maximum and then drops (Fig. 6.3a). When the mass flow rate of the abrasive increases, the mrr also increases (Fig. 6.3b).

ii. The Gas:

The AJM units normally operate at a pressure of 0.2 N/mm2 to 1 N/mm2. The composition of gas affects the mrr in an indirect manner as the velocity-pressure relation depends on this composition. A high velocity obviously causes a high mrr even if the mass flow rate of the abrasive is kept constant.

Prepared by L. SUSHMAASSOPROF 121 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE iii. The Nozzle:

The nozzle is one of the most vital elements controlling the process characteristics. Since it is continuously in contact with the abrasive grains flowing at a high speed, the material must be very hard to avoid any significant wear. Normally, WC or sapphire is used. For a normal operation, the cross-sectional area of the orifice is between 0.05 mm2 and 0.2 mm2.

The shape of the orifice can be either circular or rectangular. The average life of a nozzle is very difficult to ascertain. A WC nozzle lasts between 12hr and 30hr, whereas a sapphire nozzle lasts for 300hr approximately.

One of the most important factors in AJM is the distance between the work surface and the tip of the nozzle, normally called the Nozzle Tip Distance (NTD). The NTD affects not only the mrr from the work surface but also the shape and size of the cavity produced. Figure 6.5 shows the effect of NTD. When the NTD increases, the velocity of the abrasive particles impinging on the work surface increases due to their acceleration after they leave the nozzle.

This, in turn, increases the mrr. With a further increase in the NTD, the velocity reduces due to the drag of the atmosphere which initially checks the increase in the mrr and finally decreases it. Figure 6.6 shows how the NTD affects the mrr.

Prepared by L. SUSHMAASSOPROF 122 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

The abrasive jet machines are manufactured and marketed by a single manufacturer (namely, S.S. White Co., New York) under the name ―Airbrasive‖.

Electron Beam Machining (EBM):

Basically, electron beam machining is also a thermal process. Here, a stream of high speed electrons impinges on the work surface whereby the kinetic energy, transferred to the work material, produces intense heating. Depending on the intensity of the heat thus generated, the material can melt or vaporize. The process of heating by an electron beam can, depending on the intensity, be used for annealing, welding, or metal removal.

Very high velocities can be obtained by using enough voltage; for example, an accelerating voltage of 150,000 V can produce an electron velocity of 228,478 km/sec. Since an electron beam can be focused to a point with 10-200μm diameter, the power density can go up to 6500 billion W / mm2. Such a power density can vaporize any substance immediately. Thus, EBM is nothing but a very precisely controlled vaporization process. EBM is a suitable process for drilling fine holes and cutting narrow slots.

Holes with 25-125μm diameter can be drilled almost instantaneously in sheets with thicknesses up to 1.25 mm. The narrowest slot which can be cut by EBM has a width of 25μm. Moreover, an electron beam can be maneuvered by the magnetic deflection coils, making the machining of complex contours easy. However, to avoid a collision of the accelerating electrons with the air molecules, the process has to be conducted in vacuum (about 10-5 mm Hg); this makes the process unsuitable for very large work pieces.

To indicate the wide range of applications of the electron beam, a plot of the power density versus the hot spot diameter is given in Fig. 6.69. It is obvious that the range of the electron beam is the largest. This is why the electron beam is used not only for machining but also for the other thermal processes.

Prepared by L. SUSHMAASSOPROF 123 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

The electrons are emitted from the cathode (a hot tungsten filament), the beam is shaped by the grid cup, and the electrons are accelerated due to a large potential difference between the cathode and the anode. The beam is focused with the help of the electromagnetic lenses. The deflecting coils are used to control the beam movement in any required manner.

In case of drilling holes the hole diameter depends on the beam diameter and the energy density. When the diameter of the required hole is larger than the beam diameter, the beam is deflected in a circular path with proper radius. Most holes drilled with EBM are characterized by a small crater on the beam incident side of the work. The drilled holes also possess a little taper (2°—4°) when the sheet thickness is more than 0.1 mm. Some idea about the performance characteristics of drilling holes with EBM can be obtained from Table 6.5.

While cutting a slot, the machining speed normally depends on the rate of material removal, i.e., the cross- section of the slot to be cut. The sides of a slot in a sheet with thickness up to 0.1 mm are almost parallel. A taper of 1° to 2° is observed in a slot cut in a thicker plate. A small amount of material splatter occurs on the beam incident side. Table 6.6 gives some idea about the slot cutting capabilities of the electron beam.

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The power requirement is found to be approximately proportional to the rate of metal removal. So, P ≈ CQ, C being the constant of proportionality. Table 6.7 gives the approximate values of C for different work materials.

A very rough estimation of the machining speed for the given conditions is possible, using Table 6.7.

Mechanics of EBM:

Electrons are the smallest stable elementary particles with a mass of 9.109 x 10-31 kg and a negative charge of 1.602 x 10-19 coulomb. When an electron is accelerated through a potential difference of V volts, the 2 2 change in the kinetic energy can be expressed as 1/2me (u –u0 ) eV, where me is the electron mass, u is the final velocity, and u0 is die initial velocity. If we assume the initial velocity of the emitting electrons to be negligible, the final expression for the electron velocity u in km/sec is – u ≈ 600√V (6.67)

When a fast moving electron impinges on a material surface, it penetrates through a layer undisturbed. Then, it starts colliding with the molecules, and, ultimately, is brought to rest (Fig. 6.71). The layer through which the electron penetrates undisturbed is called the transparent layer.

Prepared by L. SUSHMAASSOPROF 125 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Only when the electron begins colliding with the lattice atoms does it start giving up its kinetic energy, and heat is generated. So, it is clear that the generation of heat takes place inside the material, i.e., below the transparent skin. The total range to which the electron can penetrate (δ) depends on the kinetic energy, i.e., on the accelerating voltage V. It has been found that –

Where δ is the range in mm, V is the accelerating voltage in volts, and p is the density of the material in kg / mm3.

Effects of EBM on Materials:

Since machining by an electron beam is achieved without raising the temperature of the surrounding material (except an extremely thin layer), there is no effect on the work material. Because of the extremely high energy density, the work material 25-50μm away from the machining spot remains at the room temperature. Apart from this, the chance of contamination of the work is also less as the process is accomplished in vacuum.

Plasma Arc Machining (PAM):

A plasma is a high temperature ionized gas. The plasma arc machining is done with a high speed jet of a high temperature plasma. The plasma jet heats up the work piece (where the jet impinges on it), causing a quick melting. PAM can be used on all materials which conduct electricity, including those which are resistant to oxy-fuel gas cutting. This process is extensively used for profile cutting of stainless steel, monel, and super alloy plates.

A plasma is generated by subjecting a flowing gas to the electron bombardment of an arc. For this, the arc is set up between the electrode and the anodic nozzle; the gas is forced to flow through this arc.

The high velocity electrons of the arc collide with the gas molecules, causing a dissociation of the diatomic molecules or atoms into ions and electrons resulting in a substantial increase in the conductivity of the gas which is now in plasma state. The free electrons, subsequently, accelerate and cause more ionization and heating. Afterwards, a further increase in temperature takes place when the ions and free electrons recombine into atoms or when the atoms recombine into molecules as these are exothermic processes.

So, a high temperature plasma is generated which is forced through the nozzle in the form of a jet. The mechanics of material removal is based on – (i) heating and melting, and (ii) removal of the molten metal by the blasting action of the plasma jet. Prepared by L. SUSHMAASSOPROF 126 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE For more details, see the standard handbooks and reference books. Here, we shall list the basic characteristics to familiarize the reader with the process. Ultrasonic Machining:

Principle:

It works on the same principle of ultrasonic welding. This machining uses ultrasonic waves to produce high frequency force of low amplitude, which act as driving force of abrasive. Ultrasonic machine generates high frequency vibrating wave of frequency about 20000 to 30000 Hz and amplitude about 25-50 micron. This high frequency vibration transfer to abrasive particle contains in abrasive slurry. This leads indentation of abrasive particle to brittle work piece and removes metal from the contact surface.

Power Source: As we know, this machining process requires high frequency ultrasonic wave. So a high frequency high voltage power supply require for this process. This unit converts low frequency electric voltage (60 Hz) into high frequency electric voltage (20k Hz).

Magnetostrictive transducer: As we know, transducer is a device which converts electric single into mechanical vibration. In ultrasonic machining magnetostrictive type transducer is used to generate mechanical vibration. This transducer is made by nickel or nickel alloy.

Booster: The mechanical vibration generated by transducer is passes through booster which amplify it and supply to the horn.

Tool: The tool used in ultrasonic machining should be such that indentation by abrasive particle, does not leads to brittle fracture of it. Thus the tool is made by tough, strong and ductile materials like steel, stainless steel etc.

Tool holder or Horn: As the name implies this unit connects the tool to the transducer. It transfers amplified vibration from booster to the tool. It should have high endurance limit.

Abrasive Slurry: A water based slurry of abrasive particle used as abrasive slurry in ultrasonic machining. Silicon carbide, aluminum oxide, boron carbide are used as abrasive particle in this slurry. A slurry delivery and return mechanism is also used in USM.

Working process:

Now we know about basic part and idea of ultrasonic machining. In this machining material is removed by indentation of abrasive particle on work-piece. It works as follow.

Prepared by L. SUSHMAASSOPROF 127 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Diagram of Ultrasonic Machinie

 First the low frequency electric current passes through electric supply. This low frequency current converts into high frequency current through some electrical equipment.  This high frequency current passes through transducer. The transducer converts this high frequency electric single into high frequency mechanical vibration.  This mechanical vibration passes through booster. The booster amplify this high frequency vibration and send to horn.  Horn which is also known as tool holder, transfer this amplified vibration to tool which makes tool vibrate at ultrasonic frequency.  As the tool vibrates, it makes abrasive particle to vibrate at this high frequency. This abrasive particle strikes to the work piece and remove metal form it.

This is the whole working process of ultrasonic machining.

Application:

 This machining is used to machine hard and brittle material like carbide, ceramic, glass etc.  This is used in machining of die and tool of drill, wire drawing machine etc.  Used in fabrication of silicon nitrite turbine blade.  It is used to cut diamond in desire shape.  It is used machining of machining non-conductive hard material which cannot be machined by ECM or EDM due to poor conductivity.

Advantages:

 Hard material can be easily machined by this method.

Prepared by L. SUSHMAASSOPROF 128 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE  No heat generated in work so there is no problem of work hardening or change in structure of work piece.  Non-conductive metals or non-metals, which cannot be machined by ECM of EDM can be machined by it.  It does not form chips of significant size.

Disadvantages:

 It is quite slower than other mechanical process.  Tool wear is high because abrasive particle affect both work-piece and tool.  It can machine only hard material. Ductile metal cannot be machine by this method.  It cannot used to drill deep hole.

Ion beam machining

Ion beam machining is generally a surface finishing process in which the material removal takes place by sputtering of ions. It is also called etching process. This is different process from electric discharge, electron beam, laser beam and plasma arc machining.

Working Principle :

This process is very simple. It consists in bombarding the work with accelerated ions which collide with the surface atoms of the work. Each bombarding ions, as a result of collisions, dislodges surface layer. It consists of an electron gun discharging free electrons into a chamber filled with argons gas. The gas is ionized by electrons. The top of the chamber is known as ion-beam generating apparatus. At the other end, the work piece is fixed to a table which can be oscillated and rotated so that different points on the work surface can be subjected to ion beam.

Accuracy :

 Etching rates vary up to 2000 Å per min.

Prepared by L. SUSHMAASSOPROF 129 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE  Accuracy of the etching process is considerably high mainly due to the small amount of material removal.  Tolerances in the vicinity of + 50 Å to - 50 Å are possible.

Applications of IBM :

 It is applied mostly in micro-machining of electronic components.  Typical materials that can be etched included glass, alumina, quartz, crystal, silica, agates, porcelains, numerous metals, cermets and oxides.  It is also be used to deposit materials such as platinum, tungsten and silicon oxide insulators on other material substrate.

Advantages of IBM :

 IBM is almost universal.  No chemical reagents or etching are required.  Etching rates are easily controlled.  There is no undercutting as with other chemical etching process.

Disadvantages of IBM :

 IBM is relatively expensive.  Etching rates are slow.  No heat is generated so there is little possibility of some thermal or radiation damage.

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UNIT 4

Prepared by L. SUSHMAASSOPROF 131 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE The Heat Treatment of Steel

Heat treatment of steel refers to time- and temperature-controlled processes that relieve residual stresses and/or modifies material properties such as hardness (strength), ductility, and toughness. Other mechanical or chemical operations are sometimes grouped under the heading of heat treatment. The common heat-treating operations are annealing, quenching, tempering, and case hardening.

Annealing

When a material is cold- or hot-worked, residual stresses are built in, and, in addition, the material usually has a higher hardness as a result of these working operations. These operations change the structure of the material so that it is no longer represented by the equilibrium diagram. Full annealing and normalizing is a heating operation that permits the material to transform according to the equilibrium diagram. The material to be annealed is heated to a temperature that is approximately 100°F above the critical temperature. It is held at this temperature for a time that is sufficient for the carbon to become dissolved and diffused through the material. The object being treated is then allowed to cool slowly, usually in the furnace in which it was treated. If the transformation is complete, then it is said to have a full anneal. Annealing is used to soften a material and make it more ductile, to relieve residual stresses, and to refine the grain structure.

The term annealing includes the process called normalizing. Parts to be normalized may be heated to a slightly higher temperature than in full annealing. This produces a coarser grain structure, which is more easily machined if the material is a low-carbon steel. In the normalizing process the part is cooled in still air at room temperature. Since this cooling is more rapid than the slow cooling used in full annealing, less time is available for equilibrium, and the material is harder than fully annealed steel. Normalizing is often used as the final treating operation for steel. The cooling in still air amounts to a slow quench.

Quenching

The absence of full annealing indicates a more rapid rate of cooling. The rate of cooling is the factor that determines the hardness. A controlled cooling rate is called quenching. A mild quench is obtained by cooling in still air, which, as we have seen, is obtained by the normalizing process. The two most widely used media for quenching are water and oil. The oil quench is quite slow but prevents quenching cracks caused by rapid expansion of the object being treated. Quenching in water is used for carbon steels and for medium-carbon, low-alloy steels.

The effectiveness of quenching depends upon the fact that when austenite is cooled it does not transform into pearlite instantaneously but requires time to initiate and complete the process. Since the transformation ceases at about 800°F, it can be prevented by rapidly cooling the material to a lower temperature. When the material is cooled rapidly to 400°F or less, the austenite is transformed into a structure called martensite. Martensite is a supersaturated solid solution of carbon in ferrite and is the hardest and strongest form of steel.

If steel is rapidly cooled to a temperature between 400 and 800°F and held there for a sufficient length of time, the austenite is transformed into a material that is generally called bainite. Bainite is a structure intermediate between pearlite and martensite. Although there are several structures that can be identified between the temperatures given, depending upon the temperature used, they

Prepared by L. SUSHMAASSOPROF 132 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE are collectively known as bainite. By the choice of this transformation temperature, almost any variation of structure may be obtained. These range all the way from coarse pearlite to fine martensite.

Tempering

When a steel specimen has been fully hardened, it is very hard and brittle and has high residual stresses. The steel is unstable and tends to contract on aging. This tendency is increased when the specimen is subjected to externally applied loads, because the resultant stresses contribute still more to the instability. These internal stresses can be relieved by a modest heating process called stress relieving, or a combination of stress relieving and softening called tempering or drawing. After the specimen has been fully hardened by being quenched from above the critical temperature, it is reheated to some temperature below the critical temperature for a certain period of time and then allowed to cool in still air. The temperature to which it is reheated depends upon the composition and the degree of hardness or toughness desired.8 This reheating operation releases the carbon held in the martensite, forming carbide crystals. The structure obtained is called tempered martensite. It is now essentially a superfine dispersion of iron carbide(s) in fine-grained ferrite.

The effect of heat-treating operations upon the various mechanical properties of a low alloy steel is shown graphically in Figure.

The effect of thermal mechanical history on the mechanical properties of AISI4340 steel (Prepared by the International Nickel Company)

Case Hardening

Prepared by L. SUSHMAASSOPROF 133 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE The purpose of case hardening is to produce a hard outer surface on a specimen of low carbon steel while at the same time retaining the ductility and toughness in the core. This is done by increasing the carbon content at the surface. Either solid, liquid, or gaseous carburizing materials may be used. The process consists of introducing the part to be carburized into the carburizing material for a stated time and at a stated temperature, depending upon the depth of case desired and the composition of the part. The part may then be quenched directly from the carburization temperature and tempered, or in some cases it must undergo a double heat treatment in order to ensure that both the core and the case are in proper condition. Some of the more useful case- hardening processes are pack carburizing, gas carburizing, nitriding, cyaniding, induction hardening, and flame hardening. In the last two cases carbon is not added to the steel in question, generally a medium carbon steel, for example SAE/AISI 1144.

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UNIT 5

Prepared by L. SUSHMAASSOPROF 135 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE

Aircraft Assembly: The aerospace manufacturing sector is traditionally based on manual operations rather than automation because of the level of accuracy required in assembling aerospace structures. Canadian manufacturers are now realizing that to remain competitive they need to reduce costs by incorporating automation and intelligence into their processes. The NRC Institute for Aerospace Research (NRC Aerospace) is helping Canadian aerospace companies develop and adopt cost- effective, flexible, reconfigurable approaches for aerospace structure assembly using robotics and automation. This will increase the scope of work that aerospace subcontractors can carry out for original equipment manufacturers. Projects are currently underway to develop low cost reconfigurable robotized cells for aircraft component assembly and large-scale machining operations. Virtual manufacturing is also being investigated.

Aircraft Manufacturing Assembly:

Benefits:  Validates Design/Assembly Integrity Prior to Commitment  Validates Operation Sequences & Tooling Concepts  Identifies Assembly Anomalies  Drives & Validates Design Release Schedule

Prepared by L. SUSHMAASSOPROF 136 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE  Enables Optimization of Assembly Processes  Reduces Downstream Production Planning (Assembly)  Creates Consistent Virtual/Simulation Based Work Instructions  Captures Best Assembly Practices

Fixture: • A is a special tool used for – Locating the work, – Clamping the work, – Supporting the work, – Holding all the elements together in a rigid unit during a manufacturing operation. • The most important considerations are: Accuracy and rigidity, followed by ease of use, and economy in construction. : A Jig is a type of Fixture with means for positively guiding and supporting tools. For both Jigs & Fixtures: – Origin: traced back to Swiss watch and clock industry! – Objective: to provide interchangeability, reduction of cost, and accuracy of the manufactured Individual detail Parts, Sub-assemblies, Sections, or main components, and finally the complete structure. Advantages: • Ensure the interchangeability and accuracy of parts manufactured, • Minimize the possibility of human error, • Permit the use of medium-skilled labor, • Reduce the manufacturing time, • Allow the production of repeat orders without retooling. Types: • Assembly Fixtures, • Machining Jigs & Fixtures, • Drilling Jigs, boring Jigs, etc. • Welding Fixtures, • Trim Jigs, • Control or Master Jig,

Prepared by L. SUSHMAASSOPROF 137 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE • Apply Jig; attaching to a larger jig or an assembly of parts, etc. ... Step-by-Step Procedure for Jigs & Fixtures: For every part of structure, • A mockup is first designed and is made from material such as wood, plaster, etc.,… • The mockup is used to design and to build the fixtures ensuring the contours and the external form of the structure, • The Master Jig is then designed in order to complete the fixture providing the important reference points, Jig and pinpoints, and to provide a Reference for the regular checks of the fixture, • Thermal expansion of fixtures is important.

Applications

Various projects are currently underway related to development of low-cost reconfigurable robotized cells for aircraft component assembly. NRC Aerospace experts are working with Bombardier Aerospace on the design of a fully integrated vision system for drilling sequence and panel inspection, and design of robotized cell auxiliary hardware components. & Its Types: Aircraft raw materials come in different but limited sizes due to manufacturing limitations as well as economical distribution. The designer has to choose materials which are available, can be transported to the manufacturing facility (even the homebuilder's basement or garage), can be cut to required sizes with the minimum tools, and can be handled without causing too many rejects due to mishandling ... and still end up with an aircraft of appreciable size, adequate strength and good looks. Aircraft can't just be made out of one big sheet of material and "wrapped together." Rather, various parts have to be formed out of different types of material and joined together. Each of those parts carries a load and the fastener that brings these parts together has to carry the load from one part to the other. If we have, for example, 1,000 lbs. to be carried over from one skin to another, we can choose various ways of achieving this (see figure 1).

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The designer of an aircraft chooses the solutions best adapted to the materials used - a continuous joint with wood and composites, a single bolt or heavy (thick) fittings with steel; or riveted joints on relatively light gauge materials and/or when the joints are long (to avoid the weight penalty of many steel bolts). For over 50 years, riveted aluminum structures have been very successful, and are found to varying degrees on virtually all aircraft (whether the complete airframe or just an instrument panel). They do not fail under static or repeated loads and they do not corrode if the are well chosen and properly set. How to set the rivets correctly can be learned quite easily and should be explained by the designer when he sells drawings or kits to build an aluminum aircraft. The choice of rivets is very simple: only 2017 alloy rivets are commercially readily available (these are the "AD" rivets mentioned in earlier columns). They have good corrosion resistance and are compatible with 2024 and 6061 materials. Now, let's look at why they are also a good structural fastener. (See figure 2). First the hole is drilled slightly oversized (via the use of number drills) so that the rivet can easily be introduced after deburring (see Figure 2, item E).

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But it also has some drawbacks: 1. You need special equipment (you'll need to buy an air compressor, rivet gun(s), rivet snaps and bucking bars); 2. You need some expertise and prior practice (you’ll need a good teacher for this - errors can be costly in more ways than one); 3. It is noisy (your family and neighbors may object to your setting rivets in your basement or garage after 10 p.m. or on Sunday morning . . . and that is just when you have the time for it); 4. You need access to both sides of the parts to be assembled (and this is obviously not always easy or possible: How will you get the bucking bar inside an aileron of a small aircraft?). You’ll often need a helper to "buck" the rivet on the other side, or have long skinny arms and/or a full assortment of bucking bars.

Types

There are a number of types of rivets, designed to meet different cost, accessibility, and strength requirements:

Solid rivets

Figure3: Universal head solid rivet

Prepared by L. SUSHMAASSOPROF 140 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Solid rivets are one of the oldest and most reliable types of fasteners, having been found in archaeological findings dating back to the Bronze Age. Solid rivets consist simply of a shaft and head which are deformed with a hammer or rivet gun. The use of a rivet compression or crimping tool can also be used to deform this type of rivet; this tool is mainly used on rivets close to the edge of the fastened material, since the tool is limited by the depth of its frame. A rivet compression tool does not require two people and is generally the most foolproof way to install solid rivets. Solid rivets are used in applications where reliability and safety count. A typical application for solid rivets can be found within the structural parts of aircraft. Hundreds of thousands of solid rivets are used to assemble the frame of a modern aircraft. Such rivets come with rounded (universal) or 100° countersunkheads. Typical materials for aircraft rivets are aluminium alloys (2017, 2024, 2117, 7050, 5056, 55000, V-65), titanium, and nickel-based alloys (e.g. Monel). Some aluminum alloy rivets are too hard to buck and must be softened by annealing prior to being bucked. "Ice box" aluminum alloy rivets harden with age, and must likewise be annealed and then kept at sub-freezing temperatures (hence the name "ice box") to slow the age-hardening process. Steel rivets can be found in static structures such as bridges, cranes, and building frames. The setting of these fasteners requires access to both sides of a structure. Solid rivets are driven using a hydraulically, pneumatically, or electromagnetically driven squeezing tool or even a handheld hammer. Applications in which only one side is accessible require the use of blind rivets.

Semi-tubular rivets

Figure4: Oval head semi-tubular rivet Semi-tubular rivets (also known as tubular rivets) are similar to solid rivets, except they have a partial hole (opposite the head) at the tip. The purpose of this hole is to reduce the amount of

Prepared by L. SUSHMAASSOPROF 141 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE force needed for application by rolling the tubular portion outward. The force needed to apply a semitubular rivet is about 1/4 of the amount needed to apply a solid rivet. Tubular rivets can also be used as pivot points (a joint where movement is preferred) since the swelling of the rivet is only at the tail. Solid rivets expand radially and generally fill the hole limiting movement. The type of equipment used to apply semi-tubular rivets range from prototyping tools (less than $50) to fully automated systems. Typical installation tools (from lowest to highest price) are hand set, manual squeezer, pneumatic squeezer, kick press, impact riveter, and finally PLC-controlled robotics. The most common machine is the impact riveter and the most common use of semitubular rivets is in lighting, brakes, ladders, binders, HVAC duct work, mechanical products, and electronics. They are offered from 1/16-inch (1.6 mm) to 3/8-inch (9.5 mm) in diameter (other sizes are considered highly special) and can be up to 8 inches (203 mm) long. A wide variety of materials and platings are available, most common base metals are steel, brass, copper, stainless, aluminum and most common platings are zinc, nickel, brass, tin. All tubular rivets are waxed to facilitate proper assembly. The finished look of a tubular rivet will have a head on one side, with a rolled over and exposed shallow blind hole on the other. Semi-tubular rivets are the fastest way to rivet in mass production but require a capital investment.

Blind rivets

Figure5: Three aluminium blind rivets: 1/8", 3/32", and 1/16" Blind rivets are tubular and are supplied with a mandrel through the center. The rivet assembly is inserted into a hole drilled through the parts to be joined and a specially designed tool is used to draw the mandrel into the rivet. This expands the blind end of the rivet and then the Prepared by L. SUSHMAASSOPROF 142 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE mandrel snaps off. (A POP rivet is a brand name for blind rivets sold by Emhart Teknologies.) These types of blind rivets have non-locking and are avoided for critical structural joints because the mandrels may fall out, due to vibration or other reasons, leaving a hollow rivet that will have a significantly lower load carrying capability than solid rivets. Furthermore, because of the mandrel they are more prone to failure from corrosion and vibration. Unlike solid rivets, blind rivets can be inserted and fully installed in a joint from only one side of a part or structure, "blind" to the opposite side. Prior to the adoption of blind rivets, installation of a solid rivet typically required two assemblers: one person with a rivet hammer on one side and a second person with a bucking bar on the other side. Seeking an alternative, inventors such as Carl Cherry and Lou Huck experimented with other techniques for expanding solid rivets. The blind rivet was developed by the United Shoe Machinery Corporation.[1] Due to this feature, blind rivets are mainly used when access to the joint is only available from one side. The rivet is placed in a pre-drilled hole and is set by pulling the mandrel head into the rivet body, expanding the rivet body and causing it to flare against the reverse side. As the head of the mandrel reaches the face of the blind side material, the pulling force is resisted, and at a predetermined force, the mandrel will snap at its break point, also called "Blind Setting". A tight joint formed by the rivet body remains, the head of the mandrel remains encapsulated at the blind side, although variations of this are available, and the mandrel stem is ejected. Most blind rivets have limited use on aircraft and are never used for structural repairs. However, they are useful for temporarily lining up holes. In addition, some "home built" aircraft use blind rivets. They are available in flat head, countersunk head, and modified flush head with standard diameters of 1/8, 5/32 and 3/16 inch. Blind rivets are made from soft aluminum alloy, steel, copper, and Monel.

Drive rivet

A drive rivet is a form of blind rivet that has a short mandrel protruding from the head that is driven in with a hammer to flare out the end inserted in the hole. This is commonly used to rivet wood panels into place since the hole does not need to be drilled all the way through the panel, producing an aesthetically pleasing appearance. They can also be used with plastic, metal, and other materials and require no special setting tool other than a hammer and possibly a backing block (steel or some other dense material) placed behind the location of the rivet while hammering it into place. Drive rivets have less clamping force than most other rivets. Prepared by L. SUSHMAASSOPROF 143 AIRCRAFT PRODUCTION TECHNOLOGY COURSE FILE Flush rivet

A flush rivet is used primarily on external metal surfaces where good appearance and the elimination of unnecessary aerodynamic drag are important. A flush rivet takes advantage of a hole, they are also commonly referred to as countersunk rivets. Countersunk or flush rivets are used extensively on the exterior of aircraft for aerodynamic reasons. Additional post- installation machining may be performed to perfect the airflow.

Friction-lock rivet

One early form of blind rivet that was the first to be widely used for aircraft construction and repair was the Cherry friction-lock rivet. Originally, Cherry friction-locks were available in two styles, hollow shank pull-through and self-plugging types. The pull-through type is no longer common, however, the self -plugging Cherry friction-lock rivet is still used for repairing light aircraft. Cherry friction-lock rivets are available in two head styles, universal and 100 degree countersunk. Furthermore, they are usually supplied in three standard diameters, 1/8, 5/32 and 3/16 inch. A friction-lock rivet cannot replace a solid shank rivet, size for size. When a friction-lock is used to replace a solid shank rivet, it must be at least one size larger in diameter.the reason behind this is that friction-lock rivet loses considerable strength if its center stem falls out due to vibrations or damage.

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