CASTING – Moulding sand, cores and gates Mr. Ramanuja C M Asst.Professor ATMECE, Mysuru Syllabus

• Sand Moulding : – Types of base sand, requirement of base sand, Types of sand moulds. • Sand moulds: – Moulding sand mixture, ingredients (base sand, binder & additives) for different sand mixtures, Method used for sand moulding. • Cores: – Definition, Need, Types. – Method of making cores, Binders used. – Concept of Gating & Risering, Principle involved and types. – Fettling and cleaning of castings, Basic steps involved. – Casting defects, causes, features and remedies. • Moulding machines : – Jolt type, squeeze type, Jolt & Squeeze type and Sand Slinger. MOLD MATERIALS  A mold material is one, out of which the mold is made.

 A mold material should be such that the mold cavity retains its shape till the molten metal has solidified.

 Castings can be made in: ◦ Permanent molds—made of ferrous metals and alloys (steel, Grey C.I. etc.). ◦ Temporary refractory molds — made up of refractory sands and resins.

 Permanent molds are normally employed for casting low melting point materials. Permanent molds are too costly.

 For the above mentioned reasons, most of the foundry industry has its castings produced using refractory mold materials like Refractory Sands.

 As compared to permanent molds, the refractory sand molds can cast high melting point materials and bigger objects, whereas permanent molds TYPES OF BASE SAND

 The primary and basic material used for preparing moulds is sand, due to its high refractoriness.

 Sand usually referred to as 'base sand‘

 Nearly 90 - 95 % of the total moulding sand is occupied by sand and the remaining is binder and additives.

 Basic types of base sand are given below ◦ Silica Sand ◦ Chromite sand ◦ Zircon ◦ Olivine sand Silica Sand

 Silica sand is essentially silicon dioxide (Si02) found in nature on the bottoms and banks of rivers, lakes and seashore.

 Silica deposits tend to have varying degree of organic and contaminants like limestone, magnesia, soda and potash that must be removed prior to its use, otherwise which affects castings in numerous ways.   Silica sand is available in plenty, less expensive and possess favorable properties.

 Thermal expansion leads to certain casting defects; the reason for which not being used in steel foundries.

 However, silica sand when mixed with certain additives like wood flour, (corn flour), saw dust etc., defects can be eliminated.

 These additives burn by the heat of the molten metal thereby creating voids that can accommodate the sand expansion. II. Olivine sand  Olivine sand is typically used in non-ferrous foundries.  With its thermal expansion about half of that of silica sand, makes it suitable for production steel castings .  But the high cost restricts its wide use

III. Chromite sand  This is African sand with cost being much higher compared to other sands.  Due to its superior thermal characteristics, it is generally used in steel foundries for both mould and core making.

IV. Zircon or Zirconium silicate  This sand possesses most stable thermal properties of all the above sands.  The choice for this type of sand arises when very high temperatures are encountered and refractoriness becomes a consideration.  But the major disadvantage is that, zircon has trace elements of Uranium and Thorium which is hazardous in nature thereby restricting its use in foundries. PROPERTIES OF MOLDING

 SANDSThe very important characteristic of a molding sand is that it should produce sound castings.

 For doing so, the molding sand should possess certain desirable properties and they are: ◦ Flowability ◦ Green Strength ◦ Dry Strength ◦ Hot Strength ◦ Permeability or Porousness ◦ Refractoriness ◦ Adhesiveness ◦ Collapsibility ◦ Fineness ◦ Bench Life ◦ Coefficient of expansion ◦ Durability 1. Flowability • Flowability is the ability of the molding sand to get compacted to a uniform density. • Flowability assists molding sand to flow and pack all-around the pattern and take up the required shape. • Flowability increases as clay and water contents increase.

2. Green Strength • It is the strength of the sand in the green or moist state • A mold having adequate green strength will retain its shape, Will not distort, Will not collapse, even after the pattern has been removed from the molding box. • Green strength helps in making and handling the molds. 3. Dry Strength • It is the strength of the molding sand in the dry condition. •A mold may either intentionally be dried or a green sand mold may lose its moisture and get dried while waiting for getting poured or when it comes in contact with molten metal being poured. •The sand (of molding cavity) thus dried must have (dry) strength to –withstand erosive forces due to molten metal, –withstand pressure of molten metal, and –retain its shape. •There should be an optimum balance between dry strength, and collapsibility of the molding sand. 4. Hot Strength • It is the strength of the sand (of mold cavity) above 212°F. • In the absence of adequate hot strength, the mold may Enlarge, break, erode or, get cracked. 5. Permeability or Porousness • The moisture, binders (organic compounds) and additives present in mould sand core produce steam and other gases. • Though much of these gases escape through vents and open feeder heads, yet a good amount of the same tends to pass off through the pore spaces of the molding sand. • Thus to provide a path for free escape of the gases, the molding sand should be permeable or porous. • Sands which are coarse (Bigger in size) or have rounded grains exhibit more permeability. • Soft ramming and clay addition in lesser amounts also improves permeability. • In the absence of adequate permeability, defects like surface blows, gas holes, mold blast etc. may be experienced. 6. Refractoriness • It is the ability of molding sand to withstand high temperatures (experienced during pouring) without – fusion, – Cracking – Buckling – experiencing any major physical change. • As compared to castings of low melting point alloys, refractoriness is much more essential in the production of high melting point alloy castings (e.g. steel etc.). 7. Collapsibility  Collapsibility is that property of the molding sand which determines the readiness with which the molding sand or mold, ◦ automatically gets collapsed after the casting solidifies, and ◦ breaks down in knock out and cleaning operations.  If the mold or core does not collapse, it may restrict free contraction of the solidifying metal and cause the same to tear or crack. 8. Fineness  Finer sand molds resist metal penetration and produce smooth casting surfaces.  Fineness and permeability are in conflict with each other and hence they must be balanced for optimum results.  Fineness and permeability, both the properties of the molding sand can be maintained by using mold coating on highly permeable mold cavity walls. 9. Bench Life  It is the ability of the molding sand to retain its properties during storage or while standing (i.e., in case of any delay). 10. Coefficient of expansion  Molding sands should possess low coefficient of expansion. 11. Durability  The molding sand should possess the capacity to withstand repeated cycles of heating and cooling during casting operations.  Molding sand should be chemically immune to molten metals.  Molding sand should be reusable. TYPES OF SAND MOULDS

• Moulds prepared with sand are called 'sand moulds' or 'temporary moulds', as they are broken for removing the casting. • The different types of sand moulds are: – Green sand mould – Dry sand mould and – No-bake sand mould 1. Green sand mould • The word 'green‘ signifies that the moulding sand is in the moist state at the time of metal pouring. • The main ingredients of green sand are silica sand, clay and moisture (water). • Additives may be added in small amounts to obtain desired properties of mould/casting. • Nearly 60 % of the total castings are prepared from green sand moulds. Advantages of green sand moulds  Preferred for simple, small and medium size castings.  Suitable for mass production  Least expensive  Sand can be reused many times after reconditioning with clay and moisture Disadvantages  Moulds/cores prepared by this process lack in permeability, strength and stability.  They give rise to many defects like porosity, blow holes etc., because of low permeability and lot of steam formation due to their moisture content.  Moulds/cores cannot be stored for appreciable length of time.  Not suitable for very large size castings.  Surface finish and dimensional accuracy of castings produced are not satisfactory.  Difficult to cast thin and intricate shapes.  Mould erosion which is common in green sand moulds is another disadvantage. 2. Dry sand mould

 The word 'dry' signifies that the mould is dry or free from moisture at the time of metal pouring.  The absence of moisture makes dry sand moulds to overcome most of the disadvantages of green sand moulds.  A dry sand mould is prepared in the same manner as that of green sand mould, i.e., by mixing silica sand, clay and water.  The entire mould/core is dried (baked) in ovens to remove the moisture present in them.  Baking hardens the binder thereby increasing the strength of moulds/cores.  The temperature and duration of baking ranges from 200 - 450°F and from a few minutes to hours respectively depending on the type of metal being poured and size of the casting. 2. Dry sand mould Advantages • Strength and stability of moulds is high when compared to green sand moulds. • Baking removes moisture and hence, defects related to moisture are eliminated. • Better surface finish and dimensional tolerance of castings. Disadvantages • Consumes more time, labor and cost due to baking process. Hence, not suitable for mass production. • Not suitable for large and heavy size castings, as they are difficult to bake. • Capital cost of bake ovens. • Under baked or over baked moulds/cores is another disadvantage.

3. No-Bake sand moulds

 A no-bake or self-setting sand mould is one that does not require baking.  The main ingredients of no-bake sand are silica sand, binder (resin type), hardener and a catalyst or accelerator (if necessary).  The bonding strength developed in moulds/cores is by means of a self-setting chemical reaction between the binder and the hardener.  In some cases, a catalyst or an accelerator is added to speed up the chemical reaction. Advantages3. No-Bake sand moulds  Higher strength - about 50 to 100 times that of green sand moulds.  Patterns can be stripped within a few minutes after ramming which is not possible in both green and dry sand moulds.  Moulds/cores can be stored for longer periods.  Highly simplified moulding. Hence, reduced need for skilled labour.  Better dimensional accuracy and stability.  Improved casting quality with increased freedom from defects.  Surface finish is excellent. In many cases, castings can be used in as-cast condition without machining. Disadvantages  Use of resins and catalysts causes lot of environmental problems both within (i.e., during mixing and pouring) and outside (dumped sand) the foundries.  Resins and catalysts are expensive.  Unsafe to human operators.  Due to high strength and hardness of moulds/cores, sand reuse is a slightly difficult process. Skin-dried molds

 Sands used for making skin dried molds contain certain binders like linseed oil which harden when heated.  The mold is made with the molding sand in the green condition and then the skin of the mold cavity is dried with the help of gas torches or radiant heating lamps.  Unlike dry mold, a skin dried mold is dried only up to a depth varying from 6 mm to 25 mm.  A skin-dried mold possesses strength and other characteristics in between green and dry sand molds.  If a skin-dried mold is not poured immediately after drying, moisture from green backing sand may penetrate the dried skin and make the same ineffective. MOULDING SAND MIXTURE- INGREDIENS FOR DIFFERENT SAND MIXTURES

 A moulding sand is a mixture of base sand, binder and additives.  ingredients of ◦ green sand ◦ no-bake sand mixture ◦ dry sand mixture Ingredients for Green Sand

 MixtureGreen sand mixture is composed of base sand, binder, moisture and additives.  Base sand ◦ Silica sand is used as the base sand. ◦ It possesses favorable properties, inexpensive and can be reused many number of times. ◦ The amount of silica sand added may vary from 85 - 92 % depending on the requirements.  Binder ◦ Bentonite (clay binder) is the widely used binder for bonding sand particles. ◦ It is activated in the presence of water. ◦ A best bond between the sand particles can be obtained with Bentonite varying from 6 - 12 % and water 3 - 5 %.  Additives ◦ Additives are added in small quantities to develop certain new properties, or to enhance the existing properties of moulding sand. ◦ Sea coal, silica flour, wood flour and iron oxide are a few commonly used additives. Ingredients for No-bake sand  mixtureIngredients of 'alkyd binder system' which is one of the most widely used binder system in Indian foundries is discussed below.  Base sand ◦ Silica sand is used as the base sand.  Binder ◦ The alkyd binder system consists of three parts: Part A (binder), Part B (hardener) and Part C (catalyst).  Part A (Binder): ◦ The binder is an alkyd resin which is obtained by reacting linseed oil with a polybasic acid like isopthalic and solvents like turpentine, kerosene or mineral spirit to improve flowability. ◦ Its addition ranges from 2 - 5 % based on weight of sand.  Part B (Hardener): ◦ The hardener is a reacted product between cobalt/lead salts and napthanic acid. ◦ Its addition ranges from 5 - 10 % based on weight of binder.  Part C (Catalyst): ◦ Methylene-diphenyl-Di-isocyanate commonly known as MDI is used as catalyst to speed up the chemical reaction. ◦ Its addition ranges from 20- 25 % based on weight of binder. Ingredients for Dry Sand Mixture

 Ingredients for dry sand mixture is similar to that of green sand. Loam sand ingredients

 Loam sand contains much more clay as compared to ordinary molding sand. The clay content is of the order of 50% or so.  The ingredients of loam sand may be fine sands, finely ground refractories, clays, graphite and fibrous reinforcement.  A typical loam sand mixture contains silica sand 20 volumes, clay 5 vols, and moisture 20%.  Molds for casting large bells etc., are made up of brick framework and lined with loam sand and dried.  Sweep or skeleton patterns may be used for loam molding. MOLDING METHODS  Various molding methods are: ◦ Bench molding ◦ Floor molding ◦ Pit molding ◦ Machine molding a) Bench molding  Molding is carried out on a bench of convenient height.  Small and light molds are prepared on benches.  The molder makes the mold while standing.  Both green and dry sand molds can be made by bench molding,  Molds, both for ferrous and (especially) non-ferrous castings are made on bench molds.  Both cope and drag are rammed on the bench. b) Floor molding  Molding work is carried out on foundry floor when mold size is large and molding cannot be carried out on a bench.  Medium and large-sized castings are made by floor molding.  The mold has its drag portion in the floor and cope portion may be rammed in a flask and inverted on the drag.  Both green and dry sand moulds can be made by floor molding c) Pit molding  Very big castings which cannot be made in flasks are molded in pits dug on the floor.  Very large jobs can be handled and cast easily through pit molding.  The mold has its drag part in the pit and a separate cope is rammed and used above the (pit) drag.  The depth of the drag in pit molding is much more than that in floor molding.  In pit molding, the molder may enter the drag and prepare it.  A pit is of square or rectangular shape.  The sides of the (pit) drag are lined with brick and the bottom is covered with molding sand .  The cope (a separate flask) is rammed over the pit (drag) with pattern in position.  Gates, runner, pouring basin, sprue etc. are made in the cope.  The mold is dried by means of a stove(heater) placed in the pit.  Cope and drag are then assembled. A crane may be used for lifting and positioning the cope over drag.  Cope can be clamped in position.  Mold is ready for being poured. d) Machine molding • In bench, floor and pit molding, the different molding operations are carried out manually by the hands of the molder, whereas in machine molding, various molding operations like sand ramming, rolling the mold over, withdrawing the pattern etc. are done by machines. • Machines perform these operations much faster, more efficiently and in a much better way. • Molding machines produce identical and consistent castings. • Molding machines produce castings of better quality and at lower costs. • Molding machines are preferred for mass production of the castings whereas hand molding (bench, pit and floor) is used for limited production. • Machine molding is not a fully automatic process; many operations can though be performed by machines, yet some others have to be carried out by hands. • A few different types of molding machines are listed below: – Jolt machine – Squeeze machine – Jolt-squeeze machine – Sand Slinger

CORE Introduction  Core is an obstruction-which when positioned in the mold, naturally does not permit the molten metal to fill up the space occupied by the core. In this way a core produces hollow castings.  Cores are required to create the recesses, undercuts and interior cavities that are often a part of castings.  A core may be defined as a sand shape or form which makes the contour of a casting for which no provision has been made in the pattern for molding.  core as a sand shape is generally produced separate from the sand mold and is then baked (hardened) to facilitate handling and setting into the mold.  Cores may be made up of sand, metal, plaster or ceramics.

Different Functions (Purposes) of Cores • For hollow castings, cores provide the means of forming the main internal cavities. • Cores may provide external undercut features • Cores may be employed to improve the mold surface • Cores may be inserted to achieve deep recesses in the castings. • Cores may be used to strengthen the molds • Cores may be used to form the gating system of large size molds.

Essential Characteristics of (dry sand) Cores • A Core must possess – Sufficient strength to support itself and to get handled without breaking. – High permeability to let the mold gases escape through the mold walls. – Smooth surface to ensure a smooth casting. – High refractoriness to withstand the action of hot molten metal (metal penetration etc.). – High collapsibility in order to assist the free contraction of the – solidifying metal. – Those ingredients which do not generate mold gases. TYPES OF CORES Cores may be classified according to A. The state or condition of core D. The shape and position of the core 1. Green sand core 1. Horizontal core 2. Vertical core 2. Dry sand core 3. Hanging or cover core 3. No bake sand core 4. Balanced core B. The nature of core materials employed 5. Drop core or stop off core 6. Ram up core 1. Oil bonded cores 7. Kiss core. 2. Resin bonded cores 3. Shell cores 4. Sodium silicate cores C. The type of core hardening process employed

1. C02 process 2. The hot box process 3. The cold set process 4. Fluid or castable sand process 5. Furan-No-Bake system 6. Oil-No-Bake process A. The state or condition of core 1. Green sand cores • Green sand cores are formed by the pattern itself. • A green sand core is a part of the mold. • A green sand core is made out of the same sand from which the rest of the mold has been made i.e., the molding sand. 2. Dry sand cores • Dry sand cores (unlike green sand cores )are not produced as a part of the mold. • Dry sand cores are made separately and independent of the mold. • A dry sand core is made up of core sand which differs very much from the sand out of which the mold is constructed. • A dry sand core is made in a core box and it is baked after ramming. • A dry sand core is positioned in the mold on core-seats formed by core-prints on the patterns. • A dry sand core is inserted in the mold before closing the same. 3. No-bake sand cores • The sand used for preparing no-bake core is similar to that used for making no-bake sand moulds. • Synthetic resins like phenol or urea formaldehyde are used as binder for bonding silica sand. • Certain chemicals are used as hardeners and catalysts to bring about a chemical reaction with the binder due to which bonding of sand grains takes place. B. The nature of core materials employed 1. Oil bonded cores • Conventional sand cores are produced by mixing silica sand with a small percentage of linseed oil. 2. Resin-bonded cores • Phenol resin bonded sand is rammed in a core box. • The core is removed from the core box and baked in a core oven at 375 to 450°F to harden the core. 3. Sodium Silicate cores • These cores use a core material consisting of clean, dry sand mixed with a solution of sodium silicate C. The type of core hardening process employed 1. hot box process  It uses heated core boxes for the production of cores.  The core box is made up of cast iron, steel or aluminium and possesses vents and ejectors for removing core gases and stripping core from the core box respectively.  Core box is heated from 350 to 500°F.  Heated core boxes are employed for making shell cores from dry resin bonded mixtures. 2. The cold set process  While mixing the core-sand, an accelerator is added to the binder.  Curing begins immediately with the addition of accelerator and continues until the core is strong to be removed from the core box.  Cold set process is employed for making large cores. 3. Castable sand process  A setting or hardening agent such as dicalcium silicate is added to sodium silicate at the time of core sand mixing.  The sand mixture possesses high flowability and after being poured in the core box, it chemically hardens after a short interval of time. D. The shape and position of the core 1. Horizontal core  Fig. shows horizontal core.  A horizontal core is positioned horizontally in the mold.  A horizontal core may have any shape, circular or of some other section depending upon the shape of the cavity required in the casting.

A horizontal core is supported in core seats at both ends. • Uniform sectioned horizontal cores are generally placed at the parting line. •2. Vertical core • A horizontal core is very commonly used in foundries.  Fig. shows a vertical core.  On the cope side, a vertical core needs more taper so as not to tear the sand in the cope while assembling cope and drag.  A vertical core is named so because it is positioned in the mold cavity with its axis vertical.

• The two ends of a vertical core are supported in core seats in cope and drag respectively. • A big portion of the vertical core usually remains in the drag • A vertical core is very frequently used in foundries. 3. Hanging or cover core  Fig. shows a hanging (cover) core  It is known as hanging core because it hangs; it is also called cover core if it covers the mold and rests on a seat made in the drag.  A simple hanging core is one which is not supported on any seat rather it hangs from the cope with the help of wires, etc. • A hanging core is supported from above and it hangs vertically in the mold cavity. • A hanging core has no support from bottom. • A hanging core is provided with a hole through which molten metal reaches the mold cavity. • Hanging cores can be made up of either green or dry sand.

4. Balanced core  Fig. shows a balanced core.  A balanced core is one which is supported and balanced from its one end only.  A balanced core requires a long core seat so that the core does not sag or fall into the mold.  A balanced core is used when a casting does not want a through cavity. 5. Drop or stop off core  Fig. shows a Drop or stop off core.  A stop off core is employed to make a cavity (in the casting) which cannot be made with other types of cores.  A stop off core is used when a hole, recess or cavity, required in a casting is not in line with the parting surface, rather it is above or below the parting line of the casting.  Depending upon its shape and use, a stop off core may also be known as tail core, saddle core, chair core, etc.

6. Ram-up core  A ram-up core is shown in Fig.  A ram-up core is one which is placed in the sand along with pattern before ramming the mold.  A ram-up core cannot be placed in the mold after the mold has been rammed.  A ram-up core is used to make internal or external (surface) details of a casting. 7. Kiss core  Kiss core is shown in Fig.  A kiss core does not require core seats for getting supported.  A kiss core is held in position between drag and cope due to the pressure exerted by cope on the drag.  A number of kiss cores can be simultaneously positioned in order to obtain a number of holes in a casting. Method of making the cores Core Making (Preparation) Procedure Steps involved: 1. Core Sand Preparation 2. Making the Cores 3. Baking the Cores. 4. Finishing of Cores. 5. Setting the Cores. 1. Core Sand Preparation • The core sand of desired type (dry sand, no-bake etc.,) and composition along with additives is mixed manually or using Muller of suitable type. 2. Making The Cores • Cores are prepared manually or using machines depending on the needs. • Machines like jolt machine, sand slinger, core blower etc., are used for large scale continuous production, while small sized cores for limited production are manually made in hand filled core boxes. • A core box is similar to a pattern that gives a suitable shape to the core. • Figure shows a core box used to produce rectangular shaped cores with procedure.

Steps Involved in making the core • Core box is usually placed on work-bench; it is filled with already mixed and prepared core sand, is rammed by hand and the extra sand is removed from the core box. • Weak cores may be reinforced with steel wires to strengthen them. • Core box is inverted over the core plate and this transfers the core from the core box to core plate which (i.e., core) is then baked in the oven (over the core plate itself). • Larger cores can also be made manually but on the floor (and not on bench). It needs more than one man to work and the cranes may also be used, if necessary 3. Core Baking • Cores are baked in ovens in order to drive away the moisture in them and also to harden the binder thereby imparting strength to the core. • The temperature and duration for baking may vary from 200 - 450°F and from a few minutes to hours respectively depending on the size of the core and type of binder used. 4. Core finishing • The baked cores are finished by rubbing or filing with special tools to remove any fins, bumps, lose sand or other sand projections from its surface. • The cores are also checked for dimensions and cleanliness. • Finally, if cores are made in parts, they are assembled by using suitable pastes, pressed and dried in air before placing them in the mould cavity.

Core binders • A core binder, – holds sand grains together – gives strength to cores – makes cores to resist erosion and breaking, – imparts adequate collapsibility to cores. • core binders are of the following types A. Organic binders B. Inorganic binders C. Other binders. A. Organic Binders 1. Core oil. They may be • Vegetable (i.e., linseed oil) • Marine animal (i.e., whale oil), and • mineral oil (used for diluting vegetable and marine animal oils) 2. Cereal binders • They are – Gelatinized starch. It is made by wet milling and contains starch and gluten. – Gelatinized corn flour. • Cereal binders contribute to green strength. 3. Water soluble binders • They are – Dextrin, made from starch. – Molasses, etc. 4. Wood product binders • They are – Natural resins (i.e., rosin, thermoplastic). – Sulfite binders. They contain Lignin, are water soluble compounds of wood sugars produced in the paper pulp process i.e., as a by-product of paper making. B. Inorganic Binders  They are ◦ Fire clay ◦ Bentonite ◦ Silica flour ◦ Iron oxide, etc.  These binders develop green strength, baked strength, hot strength and impart smooth surface finish.  They are finely pulverized materials.

 They greatly increase the amount of oil necessary in oil sand mixes. Note: Inorganic binders have been discussed under 1st chapter C. Other Binders  They are ◦ Portland cement. It hardens at room temperature. ◦ Cements (i.e., rubber cements). They harden at room temperature ◦ Sodium silicate. The core hardens as carbon-di-oxide gas is passed through it. PRINCIPLES OF GATING SYSTEM  The term gating system refers to all passageways through which the molten metal passes to enter the mold cavity.  The gating system is composed of ◦ Pouring cups and basins ◦ Sprue ◦ Runner ◦ Gates ◦ Risers.  Fig. shows the various components of the gating system. • Since the way in which liquid metal enters the mold has a decided influence upon the quality and soundness of a casting, the different passages for the molten metal are carefully designed and produced. • A gating system should avoid sudden or right angle changes in direction. • Sudden change in direction causes mold erosion, turbulence and gas pick-up. • If possible the gating system should form a part of the pattern

REQUIREMENTS, PURPOSES OR FUNCTIONS OF THE GATING SYSTEM • A Gating system should, – fill the mold cavity completely before freezing; – introduce the liquid metal into the mold cavity with low velocity and little turbulence, so that mold erosion, metal oxidation and gas pickup is prevented; – incorporate traps for the separation of non metallic inclusions which are either introduced with the molten metal or are dislodged in the gating system; – regulate the rate at which liquid metal enters into the mold; – be practicable and economical to make and; – consume least metal. In other words, the metal solidified in sprue, runner, gates and risers should be minimum because gates, risers etc., are removed from the final casting; the gating system should provide for the maximum yield. GATES Characteristics • A gate is a channel which connects runner with the mold cavity and through which molten metal flows to fill the mold cavity. • A gate should feed liquid metal to the casting at a rate consistent with the rate of solidification. • The size of the gate depends upon the rate of solidification. • A small gate is used for a casting which solidifies slowly and vice-versa. • More than one gates may be used to feed a fast freezing casting. • A gate should not have sharp edges as they (i.e., edges) may break during pouring and (sand pieces) thus be carried with the molten metal into the mold cavity.. • Moreover, sharp edges may cause localized delay in freezing, thus resulting in the formation of voids and inclusions in the cast objects. • A gate may be built as a part of the pattern or it may be cut in the mold with the help of a gate cutter. Types of Gates • The major types of gates are, 1. Top Gate 2. Bottom gate 3. Parting line side gate 1. Top gate • Fig. shows a few types of top gates. • A top gate is sometimes also called as Drop gate because the molten metal just drops on the sand in the bottom of the mold • In top gate, a stream of liquid metal impinges against the bottom of mold cavity until a pool is formed and this is kept in a state of agitation until the mold is filled.

Advantages of Top Gating  Simplicity for moulding.  Low consumption of additional metal.  Generation of favourable temperature gradients to enable directional solidification from the casting towards the gate which serves as riser too. Disadvantages of Top Gating  The dropping liquid metal stream erodes the mold surface.  Dropping metal does cutting action, lifts portions of the surface and causes scab (Skin).  Splashing of molten metal associated with the liquid metal stream increase chances of oxidation.  There is lot of turbulence and pick-up of air and other gases.

2. Bottom Gate  Fig. shows a few types of bottom gates.  A bottom gate is made in the drag portion of the mold.  In a bottom gate, liquid metal fills rapidly the bottom portion of the mold cavity and rises steadily and gently up the mold walls.  Types of bottom gate are ◦ Simple bottom gate ◦ A horn gate

Simple bottom gate A horn gate Advantages of Bottom Gating  There is no scouring(Rubbing) and splashing in the bottom gate.  As compared to top gate, a bottom gate involves little turbulence and metal erosion.  Bottom gate produces good casting surfaces. Disadvantages of Bottom Gating  In bottom gates, liquid metal enters the mold cavity at the bottom. If freezing takes place at the bottom, it could choke off the metal flow before the mold is full.  A bottom gate creates an unfavorable temperature gradient and makes it difficult to achieve directional solidification especially when the bottom gate has a riser at the top of the casting.  A bottom gate involves greater complexity of molding.  The liquid metal cools as it rises the mold walls and results in cold metal and cold mold near the (top) riser and hot metal and hot mold near the gate. Solidification of Metals

Mr. Ramanuja C M Asst.Professor ATMECE, Mysuru Contents

• Solidification of Metals • Nucleation and Grain Growth • Cooling Curves • Homogenous Nucleation • Heterogeneous Nucleation • Dendrites • Shrinkage Solidification of a pure metal. Solidification of Metals

1. During solidification, the liquid changes in to solid during cooling. 2. The energy of liquid is less than that of the solid above the melting point. Hence liquid is stable above the melting point. 3. Below the melting point, the energy of liquid becomes more than that of the solid. 4. Hence below the melting point, the solid becomes more stable than the liquid. 5. Therefore at the melting point, liquid gets converted in to solid during cooling. 6. This transformation of liquid into solid below melting point is known as solidification. Solidification of Metals

1. Thermodynamically, both liquid and solid have equal energy at melting point and therefore both are equally stable at melting point. 2. Therefore, no solidification or melting will take place at the melting point. Liquid will remain liquid and solid will remain solid. 3. Some under-cooling will be essential for solidification. 4. This transformation occurs by nucleation and growth. Cooling curve for a pure metal showing possible undercooling. • The transformation temperature, as shown on the equilibrium diagram, represents the point at which the free energy of the solid phase is equal to that of the liquid phase. • Thus, we may consider the transition, as given in a phase diagram, to occur when the free energy change, ΔGV , is infinitesimally small and negative, i.e. when a small but positive driving force exists due to undercooling. Nucleation and Growth of Crystals

• At the solidification temperature, atoms from the liquid, such as molten metal, begin to bond together and start to form crystals. • The moment a crystal begins to grow is know as nucleus and the point where it occurs is the nucleation point. • When a metal begins to solidify, multiple crystals begin to grow in the liquid. • The final sizes of the individual crystals depend on the number of nucleation points. • The crystals increase in size by the progressive addition of atoms and a)Nucleation of crystals, b) crystal growth, c) grow until they impinge upon irregular grains form as crystals grow together, adjacent growing crystal. d) grain boundaries as seen in a microscope.

Cooling Curve

• A cooling curve is a graphical plot of the changes in temperature with time for a material over the entire temperature range through which it cools. Cooling Curve for Pure Metals

• Under equilibrium conditions, all metals exhibit a definite melting or freezing point. • If a cooling curve is plotted for a pure metal, It will show a horizontal line at the melting or freezing temperature. Cooling Curve of Alloys

• In this method, alloys with different compositions are melted and then the temperature of the mixture is measured at certain time intervals while cooling back to room temperature. • A cooling curve for each mixture is constructed and the initial and final phase change temperatures are determined.

Cooling Curve

• Then these temperatures are used for the construction of the phase diagrams Cooling curve for the solidification of a pure metal. Cooling curve for a solid solution.

• In case of alloys, the solidification does not take place at a constant temperature. • In such cases, the solidification occure in a range of temperature. Series of cooling curves for different alloys in a completely soluble system. The dotted lines indicate the form of the phase diagram Phase Diagram of Solid Solution Cooling Curves for Solid Solution Mechanism of Solidification of Metals

• The solidification of metals occur by nucleation and growth transformation. • In nucleation and growth transformation, the nuclei of the solid phase are formed and then they grow. Nucleation and Growth Transformation

• Nucleation – The physical process by which a new phase is produced in a material. In the case of solidification, this refers to the formation of tiny stable solid particles in the liquid. • Growth - The physical process by which a new phase increases in size. In the case of solidification, this refers to the formation of a stable solid particle as the liquid freezes. Nucleation and Growth Transformation • The Nucleation and Growth Transformation may be of two types • 1. Homogeneous Nucleation • 2. Heterogeneous Nucleation Homogeneous Nucleation

• Homogeneous Nucleation – Formation of a critically sized solid from the liquid by clustering together of a large number of atoms at a high undercooling (without an external interface). Solidification of Metals

• The transformation temperature, as shown on the equilibrium diagram, represents the point at which the free energy of the solid phase is equal to that of the liquid phase. • Thus, we may consider the transition, as given in a phase diagram, to occur when the free energy change, ΔGV , is infinitesimally small and negative, i.e. when a small but positive driving force exists. Cooling curve for a pure metal showing possible undercooling. Energy barrier separating structural states. Free Energy Changes

• The second phase has lower free energy than the first phase • Activation energy may be required for the transformation to occur as shown above. Nucleation and Growth Transformation

The olue free eergy ΔGV – free energy difference between the liquid and solid 3 Δ GV = 4/3πr ΔGv

The surfae eergy ΔGs – the energy needed to create a surface for the spherical particles 2 ΔGs = 4πr γ γ → speifi surfae eergy of the partile

Total free energy Change

ΔGT = ΔGV + ΔGs Nucleation and Growth Transformation • Embryo - An embryo is a tiny particle of solid that forms from the liquid as atoms cluster together. The embryo is unstable and may either grow in to a stable nucleus or re-dissolve. • Nucleus – It is a tiny particle of solid that forms from the liquid as atoms cluster together. Because these particles are large enough to be stable, nucleation has occurred and growth of the solid can begin. Critical Size of Nucleus

• The minimum size that must be formed by atoms clustering together in the liquid before the solid particle is stable and begins to grow. r* : critical radius

here ΔGT reaches the maximum) • liquid metal is cooled below freezing point, slow moving atoms bond together to create homogeneous nuclei • Nucleus – larger than critical size, can grow into a crystal • Embryo – smaller than critical size, continuously being formed and redissolved in the molten metal • For Critical size of nucleus d(ΔGT)/dr = 0 when r = r* Or r* = - γ/ΔGv

critical nucleus size

• Critial uleus size aily deteried y ΔGV • Amount of undercooling increases, the critical nucleus size decreases the relationship is

R* = γTm / ΔHf ΔT

Where γ: surface free energy Tm: freezing temperature ΔHf : latent heat of fusion ΔT: amount of undercooling critical radius versus undercooling critical nucleus size - Example

• calculate the critical radius of homogeneous nucleus forms from pure liquid Cu. • Assume 2 ΔT = 0.2ΔTm , γ = 1.77 × 10-7 J/cm o 3 Tm = 1083 C, Δ Hf = 1826 J/cm • calculate the number of atoms in criticalsized nucleus at this undercooling critical nucleus size - Solution

• ΔT = 0.2ΔTm = 1356 K × 0.2 = 271 K 2 γTm 2(1.77 × 10-7 J/cm )(1356 K ) • r* = ─── = ───────────── = . × 10-8 cm 3 ΔHf ΔT (1826 J/cm )(271 K)

• volume of nucleus = / π . × 10-8 cm) 3 = 3.82 × 10-21 cm3 • Cu: FCC structure, unit length a = 3.61 × 10-8 cm • 4 atoms per unit cell • volume of unit cell = (3.61 × 10-8 cm) 3 = 4.70 × 10-23 cm 3 3.82 × 10-21 cm 3 • number of atoms = ─────── × 4 = 325 atoms 4.70 × 10-23 cm 3 critical nucleus size

• d(ΔGT)/dr = 0 when r = r*

• r* = - γ/ΔGv

(a) Effect of nucleus size on the free energy of nucleus formation. (b) Effect of undercooling on the rate of precipitation. Homogeneous Nucleation

• Quantitatively, since ∆ Gv depends on the volume of the nucleus and ∆ GS is proportional to its surface area, we can write for a spherical nucleus of radius r 3 2 ∆ G = π r / ∆ Gv + 4 π r γ • where ∆ Gv is the bulk free energy change involved in the formation of the nucleus of unit volume and γ is the surface energy of unit area. Critical Size of Nucleus

• When the nuclei are small the positive surface energy term predominates, while when they are large the negative volume term predominates, so that the change in free energy as a function of nucleus size. This indicates that a critical nucleus size exists below which the free energy increases as the nucleus grows, and above which further growth can proceed with a lowering of free energy; ∆ Gmax may be considered as the energy or work of nucleation W. Both rc and W may be calculated since

2 d ∆G/dr = π r ∆Gv + π rγ = 0 when r = rc and thus

rc = -2γ / ∆G v

• Substituting for rc gives

3 2 W = π γ /3 ∆Gv

• The probability of an atom having sufficient energy to jump the barrier is given, from the Maxwell–Boltzmann distribution law, as proportional to exp [Q/kT] where k is Boltza’s costat, T is the teperature ad Q is usually expressed as the energy per atom in electron volts.1 • The rate of reaction is given by Rate = A exp [- Q/kT] where A is a constant • The surface energy factor is not strongly dependent on temperature, but the greater the degree of undercooling or supersaturation, the greater is the release of chemical free energy and the smaller the critical nucleus size and energy of nucleation. • This can be shown analytically since ∆Gv = ∆H - T∆S, • and at T = Te, ∆Gv = 0, so that ∆H = Te ∆S. It therefore follows that

∆Gv =(Te -T) ∆S = ∆T∆S • and because ∆Gv is proportional to ∆T, then W is proportional to ∆S3 / ∆T2 • Heterogeneous Nucleation – Formation of a critically sized solid from the liquid on an impurity surface. • heterogeneous nucleation occurs in a liquid on the surface of its container, insoluble impurities and other structural materials that lower the critical free energy required to form a stable nucleus Heterogeneous Transformation

• In practice, homogeneous nucleation rarely takes place and heterogeneous nucleation occurs either on the mould walls or on insoluble impurity particles. • A reduction in the interfacial energy would facilitate nucleation at small values of ∆T. • This occurs at a mould wall or pre-existing solid particle

Chill-cast ingot structure crystal growth and grain formation

• ulei → rystals → grais • polycrystalline – solidified metal containing many crystals • grains – crystals in solidified metal • grain boundaries – the surfaces between the grains • two major types of grain structures: (1) equiaxed grains – crystals grow about equally in all directions, commonly found adjacent to a cold mold wall (2) columnar grains – long, thin, coarse grains, created when metal solidifies rather slow in the presence of a steep temperature gradient. columnar grains grow perpendicular to the mold surface Ingot Structure

Al ingot Nucleation and Growth Transformation in solid solution Nucleation and Growth Transformation • The factors which determine the rate of phase change are: • (1) the rate of nucleation, N (i.e. the number of nuclei formed in unit volume in unit time) and • (2) the rate of growth, G (i.e. the rate of increase in radius with time) Dendrites

• In metals, the crystals that form in the liquid during freezing generally follow a pattern consisting of a main branch with many appendages. A crystal with this morphology slightly resembles a pine tree and is called a dendrite, which means branching. • The formation of dendrites occurs because crystals grow in defined planes due to the crystal lattice they create. • The figure to the right shows how a cubic crystal can grow in a melt in three dimensions, which correspond to the six faces of the cube. • For clarity of illustration, the adding of unit cells with continued solidification from the six faces is shown simply as lines. • Secondary dendrite arms branch off the primary arm, and tertiary arms off the secondary arms and etcetera.

Dendrites Dendrites

• During freezing of a polycrystalline material, many dendritic crystals form and grow until they eventually become large enough to impinge upon each other. • Eventually, the interdendriticspaces between the dendrite arms crystallize to yield a more regular crystal. • The original dendritic pattern may not be apparent when examining the microstructure of a material. • However, dendrites can often be seen in solidification voids that sometimes occur in castings or welds, as shown in the next slide..

Dendrites Shrinkage

• Most materials contract or shrink during solidification and cooling. Shrinkage is the result of: – Contraction of the liquid as it cools prior to its solidification – Contraction during phase change from a liquid to solid – Contraction of the solid as it continues to cool to ambient temperature. • Shrinkage can sometimes cause cracking to occur in component as it solidifies. • Since the coolest area of a volume of liquid is where it contacts a mold or die, solidification usually begins first at this surface. • As the crystals grow inward, the material continues to shrink. • If the solid surface is too rigid and will not deform to accommodate the internal shrinkage, the stresses can become high enough to exceed the tensile strength of the material and cause a crack to form. • Shrinkage cavitation sometimes occurs because as a material solidifies inward, shrinkage occurred to such an extent that there is not enough atoms present to fill the available space and a void is left.

THANK YOU INVESTMENT CASTING AND MELTING FURNACES

 Mr. Ramanuja C M  ASST.PROFESSOR  ATMECE , MYSURU INVESTMENT CASTING

Advantages: Excellent surface finish; high dimensional accuracy; nearly unlimited intricacy; almost any metal; no flash or parting line

Limitations: Expensive patterns and molds; high labor costs; limited size

Common metals: Mainly aluminum, copper and steel; also used with stainless steel, nickel, magnesium and precious metals INVESTMENT CASTING

Size limits: As small as 1/10 oz; usually less than 10 lb

Thickness limits: As thin as .025 in, less than 3 in

Tolerances: .005 in on the first inch; .002 in per additional inch

Draft allowance: none required

Surface finish: 50-125 µin PRESSURE CASTING

Pressure casting forces the metal up into the mold chamber by applying a small amount of pressure VACUUM CASTING PERMANENT MOLD CASTING (PRESSURE/VACUUM)

Advantages: Good surface finish and dimensional accuracy; metal mold causes rapid cooling and fine grain structure; molds can be used up to 25 000 times

Limitations: High initial mold cost; shape, size and complexity are limited; mold life is very limited with metals with high melting points

Common metals: Alloys of aluminum, magnesium and copper most common; iron and steel can be used in graphite molds; alloys of lead, tin and zinc also used PERMANENT MOLD CASTING (PRESSURE/VACUUM)

Size limits: Several ounces to about 150 lb

Thickness limits: Minimum depends on material but generally thicker than 1/8 in; maximum about 2 in

Tolerances: .015 in for the first inch and .002 in for each additional inch; .01 in added across the parting line

Draft allowance: 2 - 3°

Surface finish: 100 - 250 µin DIE CASTING

Another form of permanent mold casting; molten metal is forced into the mold cavity at pressures ranging from .7 MPa - 700 MPa DIE CASTING DIE CASTING EXAMPLE OF A DIE CASTING MOLD CENTRIFUGAL CASTING

Uses a rotating mold to form hollow cylindrical parts such as pipes, gun barrels and lamp posts VERTICAL CENTRIFUGAL CASTING CENTRIFUGAL CASTING

Advantages: Can produce a wide range of cylindrical parts; good dimensional accuracy and cleanliness

Limitations: Limited shape; spinning equipment may be expensive

Common metals: Iron, steel, stainless steel, alloys of aluminum, copper and nickel

CENTRIFUGAL CASTING

Size limits: Up to 10 ft in diameter and 50 ft in length

Thickness limits: Wall thickness .1 – 5 in

Tolerances: Outer diameter within .1 in; inner diameter within about .15 in

Draft allowance: 1/8 in / ft

Surface finish: 40 - 100 µin SEMICENTRIFUGAL CASTING

Uses a rotating mold to form parts with radial symmetry, such as wheels with spokes SQUEEZE CASTING

A combination of casting and forging; a die applies pressure as the metal solidifies CASTING SINGLE CRYSTALS

Uses a slow crystal-growth solidification procedure to produce parts made of a single crystal with no grain boundaries

A helical constriction only allows one crystal of favorable orientation to grow into and fill the mold chamber CASTING SINGLE CRYSTALS RAPID SOLIDIFICATION

Cools metal rapidly at rates as high as 106 K/s so that it cannot crystallize and instead forms an amorphous glasslike structure MELTING FURNACES

Cupola Crucible Furnace Induction Furnace MELTING FURNACES

Cupola A vertical cylindrical furnace used for melting cast iron MELTING FURNACES

Crucible furnace Melts metal without direct contact with a burning fuel mixture MELTING FURNACES

Induction furnace Uses an alternating magnetic field to heat the metal DESIGN CONSIDERATIONS DESIGN CONSIDERATIONS DESIGN CONSIDERATIONS DESIGN CONSIDERATIONS CASTING ALLOYS WELDING

Mr. Ramanuja C M Asst.Professor ATMECE, Mysuru OUTLINE

Introduction Welding Process Fusion Welding Arc Welding Resistance Welding Oxyfuel Welding Laser Welding Solid-State Welding Diffusion Welding Friction Welding Ultrasonic Welding Welding Metallurgy Welding Defects WELDING WELDING APPLICATIONS WELDING PROCESS

A concentrated heat source melts the material in the weld area; the molten area then solidifies to join the pieces together

Sometimes a filler material is added to the molten pool to strengthen the weld TYPES OF WELDING

Fusion Welding Use heat to melt the base metals and may add a filler metal

Solid-State Welding Uses heat and pressure, or pressure alone, to join the metals; the temperature does not reach the melting point TYPES OF WELDING PHYSICS OF WELDING

In fusion welding, a source of high- density heat energy raises the temperature of the surfaces enough to cause localized melting; if the heat density (power ÷ surface area) is too low, the heat is conducted away as fast as it is added and melting does not occur ARC WELDING

Uses an electric arc to heat and melt the work metals ARC WELDING ARC WELDING ARC WELDING RESISTANCE WELDING

Uses heat and pressure to join metals; the heat is generated by resistance to an electrical current at the welding point RESISTANCE WELDING

Example of a resistance welding machine OXYFUEL WELDING

Uses a high-temperature flame from the combustion of acetylene and oxygen LASER WELDING

Uses a laser beam to melt the metals; can be used for deep, narrow welds LASER WELDING

Laser welding of a pipe DIFFUSION WELDING

Uses heat and pressure to join the metals by solid-state diffusion; the temperature is less than half the melting temperature

force heat

atomic movement

force FRICTION WELDING

Uses pressure and frictional heat caused by mechanical rubbing, usually by rotation FRICTION WELDING ULTRASONIC WELDING

Uses rapid vibrations to break up surface films and heat the surfaces, allowing them to bond WELDED JOINT

Fusion zone The area of base metal and filler metal that has been completely melted

Weld interface A thin area of base metal that was melted or partially melted but did not mix with the filler metal

Heat affected zone The surrounding area of base metal that did not melt, but was heated enough to affect its grain structure WELDING METALLURGY

The base metal(s) and filler metal mix together during melting, forming an alloy when they solidify

The solidification of the metals can be considered as casting a small amount of metal in a metal mold WELDING METALLURGY STRESSES AND DISTORTION WELDING DEFECTS

Cracks Fractures in the weld itself or in the metal adjacent to it

Cavities Porosity and shrinkage voids; similar to casting defects

Solid inclusions Nonmetallic solid material embedded in the weld metal WELDING DEFECTS

Incomplete fusion A weld bead that does not fill the entire joint cross-section

Imperfect shape / unacceptable contour A weld that does not have the proper shape for maximum strength

Miscellaneous defects Arc strikes (damage from direct contact with an electrode), excessive spatter (drops of molten metal that solidify on the base parts), and others INSPECTION AND TESTING

Visual inspection Visually examining the weld for surface defects

Nondestructive evaluation Uses various methods that do not damage the specimen

Destructive testing Methods in which the weld is destroyed during the test or to prepare the specimen VISUAL INSPECTION

Visual inspection checks for: - conformance to dimensional specifications of the part design - warpage - cracks, cavities, incomplete fusion and other defects visible from the surface NONDESTRUCTIVE EVALUATION

Dye-penetrant and fluorescent-penetrant tests use a dye or fluorescent substance to make small defects more visible

Magnetic particle testing (limited to ferromagnetic materials) use small magnetic particles to find distortions in the magnetic field caused by defects

Ultrasonic testing uses the transmission of sound through the specimen; discontinuities scatter or absorb the sound

Radiographic testing uses X rays or gamma rays to detect flaws DESTRUCTIVE TESTING

Mechanical tests use a weld joint in a conventional testing method, such as a tensile test or shear test

Metallurgical tests involve creating metallurgical specimens, such as micrographs, to examine the features of the weld SUMMARY

Fusion welding melts the material then allows it to solidify and join it together

Solid-state welding uses pressure, and sometimes heat, to allow the metal to bond together without melting

Welding allows the production of parts that would be difficult or impossible to form as one piece