Outline ME3072 – MANUFACTURING ENGINEERING II BSc Eng (Hons) in Mechanical Engineering • Introduction to Semester - 4 • Fusion-Welding Processes • Solid-State Welding Processes Fundamentals of Joining • of Welding Processes • Weld Quality • &

Prepared By : R.K.P.S Ranaweera BSc (Hons) MSc Lecturer - Department of Mechanical Engineering University of Moratuwa 2 (for educational purpose only)

Joining Processes Classification of Joining Processes

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1 Introduction to Welding • Attention must be given to the cleanliness of the metal surfaces prior to welding and to possible • Is a process by which two materials, usually metals oxidation or contamination during welding process. are permanently joined together by coalescence, which is induced by a combination of temperature, • Production of high quality weld requires: pressure and metallurgical conditions.  Source of satisfactory heat and/or pressure  Means of protecting or cleaning the metal • Is extensively used in fabrication as an alternative  Caution to avoid harmful metallurgical effects method for or and as a replacement for bolted and riveted joints. Also used as a repair • Advantages of welding over other joints: medium to reunite metals.  Lighter in weight and has a great strength • Types of Welding:  High resistance  Fusion welding  Fluid tight for tanks and vessels  Solid-state () welding  Can be altered easily (flexibility) and economically 5 6

• has been defined as the capacity of • Steps in executing welding: metal to be welded under the fabrication conditions  Identification of welds, calculation of weld area by stress imposed into a specific, suitably designed structure analysis, preparation of drawings & to perform satisfactorily in the intended service.  Selection of appropriate welding process  Welding procedure – welding sequence, testing, etc • The following metals have good weldability in the  Execution of welding with supervision & inspection descending order: Iron, , Cast Steel,  removal, weld dressing , Low Alloy and Stainless Steels.  Stress relieving by proper treatment • Welding is extensively used in the following fields:  Testing, preferably by nondestructive methods automobile industry, aircraft machine frames, tanks, • Process of joining similar metals with the help of structural work, machine repair work, ship building, filler rod of the same metal is called autogeneous line fabrication ,thermal power plants and welding, and joining of metals using filler rod of is refineries, fabrication of metal structures. called heterogeneous welding. 7 8

2 • Types of welded joints: • Welding positions: Flat position, Horizontal position  Lap joint Vertical position and overhead position.  Butt joint  Corner joint  Edge joint  T-joint

• have to protect themselves against spark, hot metal, , infrared and visible light rays, welding fumes, and other hazards.

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Fusion-Welding Processes • Oxyfuel Gas Welding (OFW) Refers to a group of welding processes that use, • Introduction as their heat source, the flame produced by the Is defined as the melting together & coalescing combustion of fuel gas and . of materials by means of heat, with or without  Types of Gas used: the application of pressure and with or without  Oxyacetylene – high temperature the use of .  – low temperature Thermal energy required for these operations is  Methylacetylene propadiene – low temperature usually supplied by chemical (oxy-fuel gas, Heat is generated in accordance with a pair of thermit) or electrical ( arc, resistance, electron chemical reactions: beam, laser beam) means. Welds undergo important metallurgical & physical changes that will effect its performance.

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3 Three basic types of oxyacetylene flames used in Oxyfuel-gas (a) General view of and (b) cross-section of a torch used in welding and cutting operations: (a) neutral flame; (b) oxidizing oxyacetylene welding. The valve is opened first; the flame; (c) carburizing, or reducing, flame. The gas mixture in (a) gas is lit with a spark lighter or a pilot light; then the oxygen valve is basically equal volumes of oxygen and acetylene. is opened and the flame adjusted.

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Filler Metals  Used to supply additional material to the weld zone  Available as rod or wire made of metals compatible with those to be welded  Consumable filler rods may be bare, or they may be coated with .  Purposes of the flux: - Retard oxidation of the surfaces of the part being welded, by generating gaseous shield around the weld zone - Helps to dissolve and remove oxides and other substances from the and form a stronger joint - Slag developed protects the molten puddle of metal against oxidation as it cools Basic equipment used in Oxyfuel-gas welding. To ensure correct connections, all threads on acetylene fittings are left- - Provides means of adding various alloying elements into the handed, whereas those for oxygen are right-handed. Oxygen weld metal to enhance the properties of the joint regulators are usually painted green, acetylene regulators red. - Stabilizes the arc by providing certain chemicals 15 16

4 Pressure Gas Welding • : Consumable Heat is obtained from electrical energy. Arc is produced between the tip of the electrode and the workpiece to be welded, by the use of an AC or a DC power supply. Arc produce temperatures about 30,000 0C

Schematic illustration of the pressure-gas welding process.

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• Shielded Metal Arc Welding (SMAW) Current used generally ranges between 50 A to 300 A & power requirements are generally 10kW About 50% of all industrial and maintenance welding is currently performed by this process. Type of current:  DC – straight & reverse polarity Also known as stick welding.  AC  is generated by touching the tip of a Schematic coated electrode against the workpiece and then illustration of withdrawing it quickly to a distance sufficient to the shielded metal-arc maintain the arc. welding operations Schematic illustration of the (also known as shielded metal-arc welding stick welding, process. because the electrode is in the shape of a stick).

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5 • (SAW) Electrical current typically range between 300 A to 3000 A & weld speed is high as 5 m/min. Weld arc is shielded by granular flux , consisting of lime, manganese oxide, calcium fluoride, silica, and other compounds. It prevents spatter & sparks and suppresses the intense ultraviolet radiation and fumes. Flux also act as a thermal insulator promoting deeper penetration of heat into the workpiece. Consumable electrode is a coil of bare round wire 1.5 mm – 10 mm in diameter. Applications include thick plate welding for shipbuilding and for pressure vessels. Schematic illustration of the submerged-arc welding process and equipment. The unfused flux is recovered and reused. 21 22

• (GMAW)  Schematic illustration of Formerly called metal (MIG) welding. the gas metal-arc Weld area is shielded by an effective inert welding process, formerly known as MIG atmosphere of , , , or (for metal inert gas) various other gas mixtures. welding. In addition, deoxidizers are usually present in the electrode metal itself, prevent oxidation of the molten weld puddle. Suitable for welding a variety of ferrous and nonferrous metals. Basic equipment used in Metal can be transferred by three methods: gas metal-arc welding spray, globular and short circuiting. operations.

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6 • Flux-Cored Arc Welding (FCAW) Similar to GMAW, with the exception that the electrode is tubular in shape & is filled with flux. Produce a more stable arc, improve weld contour , and produce better mechanical properties of the weld metal.  are usually 0.5 mm – 4 mm in diameter & the power required is about 20 kW. Used for welding of variety of joints, mainly on steels, stainless steels and based alloys. Self-shielded cored electrodes are also available Schematic illustration of the flux-cored arc-welding process. This operation is similar to gas metal-arc welding.

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• Electrodes Is classified according to the strength of the deposited weld metal, the current (AC or DC), & the type of coating. Identified by numbers or letters or by color code. Typical coated electrode numbers are 150 to 460 mm in length & 1.5 to 8 mm in diameter. (Wire diameter must not vary more than 0.05 mm & Coatings must be concentric with wire) Electrodes are coated with claylike material that include silicate binders & powder materials such as oxides, carbonates, fluorides, metal alloys, and cellulose. Designations for Mild Steel Coated Electrodes 27 28

7 • Arc Welding: Non-consumable Electrode • Gas Arc Welding (GTAW) Unlike arc-welding processes, non-consumable Also know as tungsten inert gas (TIG) welding. electrode processes typically use a tungsten Filler metal is supplied from a wire & are similar electrode . to the metals to be welded.  is supplied from external source. Shielding gas is usually argon or helium. Stable arc gap is maintained because the Is used for wide variety of metals & applications, electrode is not consumed. particularly , , & . Power supply is either DC at 200 A or AC at 500 A and power requirements range from 8 kW to 20 kW.

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• (AHW) The gas tungsten-arc welding process, formerly known as Uses an arc in a shielding atmosphere of H 2. TIG (for tungsten inert gas) welding. Arc is between 2 tungsten or carbon electrodes. Hydrogen also cools the workpiece. • Arc Welding (PAW) A concentrated plasma arc is produced and is aimed at the weld area. Arc is stable and reaches temperatures as high Equipment for gas as 33,000 0C. tungsten-arc welding operations. Plasma is ionized hot gas, composed of nearly equal numbers of electron and ions

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8 Plasma is initiated between tungsten electrode • Thermit Welding (TW) and the orifice by a low current pilot arc. Involves exothermic reactions between metal Shielding is supplied by means of an outer oxides & metallic reducing agents and the heat shielding rings and the uses of gases, such as produced in this reaction is used for welding. argon, helium or mixtures. Common mixtures of materials used in welding Two methods of : transferred steel & cast iron are iron oxide, , arc method (a) or nontransferred arc method (b). iron and aluminium. Mixtures may also contain other materials to impart special properties to the weld. Is suitable for welding & repairing large and .

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Procedure - align the part to be joined  built a • Electron Beam Welding (EBW) mold allow to flow superheated products Heat is generated by high velocity narrow beam electrons & the kinetic energy of the electrons is converted in to heat as they strike the workpiece Requires special equipment to focus the beam on the workpiece in a vacuum. • (LBW) Utilizes a high power laser beam as the source of heat to produce a fusion weld. The beam has high energy density, therefore deep penetrating capability.

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9 Comparison of Conventional and Electron- or • Cutting Laser-Beam Welding A piece of metal can be separated in to two or more pieces or into various contours by the use of heat source that melts and removes a narrow zone in the workpiece. Oxyfuel Gas Cutting (OFC)  Cutting occurs mainly by the oxidation of the steel  Basic reaction with the steel are,

The relative sizes of the weld beads obtained by conventional (tungsten arc) and by electron-beam or laser-beam welding

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Solid-State Welding Processes • Introduction Solid-phase welds are produced by bringing the clean faces of components into intimate contact to produce a metallic bond with or without (a)Flame cutting of steel plate with an oxyacetylene torch, and a application of heat, but application of pressure is cross-section of the torch nozzle. (b) Cross-section of a flame- essential to induce plastic flow. cut plate showing drag lines. Arc Cutting  Air carbon arc cutting  Plasma arc cutting  Lasers and electron beams 39 40

10 • Cold Welding (CW) • (USW) Pressure is applied to the , through Faying surfaces of the two components are either dies or rolls. subjected to a static normal force and oscillating Also known as roll bonding. shearing (tangential) stress. Prior to welding, the interface is degreased, wire Shearing stresses are applied by the tip of a brushed, and wiped to remove oxide smudge. transducer and frequency of oscillation is Can be used to join small workpieces made of generally in the range of 10 kHz to 75 kHz. soft, ductile metals. Temperatures generated usually in the range of one-third to one-half of the .

Schematic illustration Can be used with wide variety of metallic and of the roll bonding, or cladding, process nonmetallic materials, including dissimilar metals and plastics.

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(a) (b) • (FRW) Steps of operation  On of the components remains stationary while the other is placed in a or and rotated at a high constant speed.  Two members to be joined are then brought into contact under an axial force.  Rotating member is then brought to a quick stop, while the axial force is increased.  Pressure at the interface and the resulting friction (a)Components of an ultrasonic welding machine for lap welds. produce sufficient heat for a strong joint to form. The lateral vibrations of the tool tip cause plastic deformation Types of FRW processes: and bonding at the interface of the workpieces. (b)Ultrasonic seam welding using a roller. Inertia friction welding, Linear friction welding and 43 44

11 Friction Stir Welding (RSW) (a)  Use of a third body to rub against the faying surfaces  Probe at the tip heat and mix or stir the material

(b)

(a)Sequence of operations in the friction welding process (b)Shape of fusion zone in friction welding, as a function of the The principle of the friction stir welding process. Aluminum-alloy force applied and the rotational speed. plates up to 75 mm (3 in.) thick have been welded by this process

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• Resistance Welding (RW) Resistance (RSW)  Tips of two opposing solid cylindrical electrodes touch Heat required for welding is produced by means a lap joint of two sheet metals, and resistance heating of electrical resistance across two components produces a spot weld to be joined.  Currents range from 3000 A to 40,000 A Actual temperature rise at the joint depends on (a) Sequence in the specific heat and on the thermal conductivity resistance spot welding. (b) Cross- of the metals to be joined. section of a spot weld, showing the weld Magnitude of the current in resistance welding nugget and the indentation of the operations may be as high as 100,000 A, electrode on the sheet although the voltage is typically only 0.5V – 10V. surfaces. This is one of the most commonly Similar or dissimilar metals can be joined. used process in sheet- and in automotive-body assembly. 47 48

12 Resistance Seam Welding (RSEW)  Is a modification of spot welding wherein the electrodes are replaced by rotating wheels or rollers.  Using a continuous AC power supply, the electrically conducting rollers produce a spot weld when ever the current reaches a sufficient high level in the AC cycle.  Can produce a joint that is liquid tight or gas tight.  Roll spot welding is an extension of RSEW.

(a) (b)

Examples of Seam Welding (a) Seam-welding process in which rotating rolls act as electrodes. (a) and (b) Seam-welded (b) Overlapping spots in a seam weld. cookware and muffler. (c) Roll spot welds. (d) Resistance-welded gasoline tank. 49 50

High-Frequency (HFRW) Resistance Projection Welding (RPW)  Similar to seam welding  Embossing one or more projections on one of the  High frequency current (up to 450 kHz) is employed surfaces to be welded to increase the electrical  Types: resistance High frequency resistance welding (fig. a) High frequency (fig. b)

Schematic illustration of resistance projection welding

Two methods of high-frequency butt welding of tubes Projection welding of nuts or threaded bosses and studs 51 52

13  (FW) Stud Welding (SW)  Heat generated from the arc as the ends of the two  Similar to flash welding members begin to make contact and develop an  Part to be joined serves as one of the electrodes electrical resistance at the joint while being joined to another component  Form a flash at the joint  Disposable ceramic ring (ferrule) is placed around the  Used to repair broken band-saw blades joint to concentrate the heat generated, prevent oxidation and retain the metal in the weld zone

Flash-welding process for end-to-end The sequence of operations in stud welding, which is used for welding of solid rods or tubular parts welding bars, threaded rods, and various fasteners onto metal plates 53 54

• (EXW) (c) (d) Pressure applied by detonating a layer of explosives that has been placed over one of the components to be joined Cold pressure welding by plastic deformation (a) (b)

(c) and (d) Cross-sections of explosion-welded joints

Schematic illustration of the explosion welding process: (a) constant interface clearance gap and (b) angular interface clearance gap

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14 • /Welding (DFW) • Standard Identification for Welds Process in which the strength of the joint results primarily from diffusion and secondarily from plastic deformation of the faying surfaces Ability to fabricate sheet-metal structures by combining diffusion bonding with superplastic  Eliminate use of mechanical fasteners  High stiffness to weight ratio  Good dimensional accuracy  Low residual stresses

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Metallurgy of Welding Solidification of the Weld Metal  Formation of columnar grains (dendritic) • Weld Joint  Grain structure and size depends on the specific alloy, the specific welding process employed, and the Three distinct zones in a fusion-weld joint: specific filler metal  Base metal  Heat Affected Zone (HAZ)  Weld metal

Grain structure in (a) a deep weld (b) a shallow weld. Note that the grains in the solidified weld metal are perpendicular to the surface of the base metal. In a good weld, the solidification line at the center in the deep weld shown in (a) has grain migration, Characteristics of a typical fusion weld which develops uniform strength in the weld bead

zone in oxy-fuel gas and arc welding 59 60

15 Heat Affected Zone (HAZ)  Corrosion at HAZ  Properties and microstructure of HAZ depends on: Rate of heat input and cooling Temperature to which the zone was raised

Intergranular corrosion of a 310-stainless-steel welded tube after exposure to a caustic solution. The weld line is at the center of the photograph. Scanning electron micrograph at 20 X Schematic illustration of various regions in a fusion weld zone (and the corresponding phase diagram) for 0.30% 61 62

Weld Quality • Incomplete Fusion & Penetration Incomplete fusion or lack of fusion produces • Porosity poor weld beads. Caused by, Incomplete penetration occurs when the depth  Gases released during melting of the weld area of the welded joint is insufficient.  Chemical reactions during welding  Contaminants In the shape of spheres or of elongated pockets. • Slag Inclusions Are compounds such as oxides, fluxes, and electrode coating material trapped in the weld.

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16 Penetration can be improved by, • Weld Profile  Increasing the heat input  Reducing the travel speed during the welding  Changing the joint design  Ensuring that the surfaces to be joined fit properly

Underfilling – when the weld is not filled with proper amount of weld metal Undercutting – melting away of the base metal & the consequent generation of a groove. Overlap – surface discontinuity , usually caused by poor welding practice and by the selection of improper materials.

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• Cracks Are classified as,  Hot cracks – occur while the joint is still at elevated temperatures  Cold Cracks – develop after the weld metal has solidified. Typical types of cracks are,  Longitudinal  Transverse  Crater  Underbead  Toe cracks Types of cracks (in welded joints) caused by thermal stresses that develop during solidification and contraction of the weld bead and the surrounding structure. (a) Crater cracks. (b) Various types of cracks in butt and T joints. 67 68

17 • Surface Damage • Distortions Cause Localized heating & cooling during welding  Metal may spatter during welding & be deposited as causes residual stresses in the workpiece. small droplets on adjacent surfaces.  Arc strikes at places other than the weld zone. Affect  Poor surface appearance  High surface roughness

Residual stresses developed during welding of a butt joint

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Brazing & Soldering • Introduction Are processes that do not rely on fusion or pressure at the interfaces; instead utilize filler material that requires some temperature rise in the joint. Temperatures for soldering are lower than those for brazing, and the strength of a soldered joint is much lower. Distortion of parts after welding: (a) butt joints; (b) fillet welds. Distortion is caused by differential thermal expansion and contraction Can be used to join dissimilar metals of intricate of different parts of the welded assembly. shapes and various thicknesses.

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18 • Brazing Basic Steps in Brazing  Filler metal (low-melting-point nonferrous metal) is placed at or between the faying surfaces to be joined, and the temperature is raised enough to melt the filler metal but not the workpieces.  Molten metal is allowed to fill closely fitting space by (a) Brazing and (b) braze welding operations. capillary action.

 Upon cooling and solidification of the filler metal, a 0 strong joint is obtained. Filler metal used for brazing melt above 450 C. Main types of brazing processes Strength of the brazed joint depends on,  Ordinary Brazing  Joint design  Braze Welding – in which filler metal is deposited at  Adhesion at the interfaces between the workpiece the joint with a technique similar to OFW. and filler material 73 74

Surfaces to be brazed should be chemically or Brazing Methods mechanically cleaned to ensure full capillary  Torch Brazing (TB) - Heat source is oxyfuel gas with a action carburizing flame Brazing Flux  Furnace Brazing (FB) – Brazing metal is preloaded in appropriate configuration before placing it in a furnace  Prevent oxidation and to remove oxide film from workpiece surfaces  Use “ wetting agents”, to improve both the wetting characteristics of the molten filler metal and the capillary action  Made of borax, boric acid, borates, fluorides, & chlorides

An example of furnace brazing: (a) before, (b) after. Note that the filler metal is a shaped wire

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19  Induction Brazing (IB) – Source of heat is induction  Dip Brazing (DB) – Dipping the assemblies to be heating by high frequency AC current, where the parts brazed into either a molten filler metal bath or a with preloaded filler metal are placed near the molten salt bath, at a temperature just above the induction coils for rapid heating melting point of the filler metal  Infrared Brazing (IB) Schematic  Diffusion Brazing (DFB) illustration of a continuous  Etc. induction-brazing setup, for increased productivity

 Resistance Brazing (RB) – Source of heat is the electrical resistance of the components to be brazed

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Joint Design

Joint designs commonly used in brazing operations. The clearance between the two parts being brazed is an important factor in joint strength. If the clearance is too small, the molten braze metal will not fully penetrate the interface. If it is too large, there will be insufficient capillary action for the molten metal to fill the interface. Examples of good and poor design for brazing 79 80

20 • Soldering Types of Soldering Techniques In soldering, the filler metal, called solder, melts  Torch Soldering (TS) at a relatively low temperature and as in brazing  Furnace Soldering (FS) solder fills the joint by capillary action.  Iron Soldering (INS) Heat sources are usually soldering irons, ovens,  Induction Soldering (IS) or torches.  Resistance Soldering (RS)  Dip Soldering (DS) Filler metal used for soldering melt below 450 0C.  Infrared Soldering (IRS) Types of Solders and their Applications  Ultrasonic Soldering (US) Tin- General purpose Tin- Aluminum  Reflow Soldering (RS) Lead-silver Strength at higher than room temperature  Wave Soldering (WS) Cadmium-silver Strength at high temperatures Zinc-aluminum Aluminum; corrosion resistance Types of Fluxes Tin-silver Electronics Tin-bismuth Electronics  Inorganic acids or salts 81  Resin based fluxes 82

Joint Design Reference Texts Joint designs commonly used for soldering. Note that examples (e), • MANUFACTURING ENGINEERING & (g), (i), and (j) are TECHNOLOGY mechanically joined Serope Kalpakjian, Steven R Schmid prior to being soldered, for improved strength. Addison Wesley Longman (Singapore) Pte. Ltd. Fourth Edition.

• MATERIALS & PROCESSES IN MANUFACTURING E Paul Degarmo, JT Black, Ronald A Kohser. Prentice-Hall India Ninth Edition.

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