Casting

1. Metal Casting - Overview

In this chapter, we will briefly review the history of metal casting, followed by major casting processes, important cast metals and their applications, worldwide production and types of foundries. We will finally touch upon key steps in developing a new casting and how computers can help reduce the time involved. 1.1 An Ancient Art Casting is a 6000-year young process. It has been mentioned in several Sanskrit works such as Shilpashastra derived from Sthapatyaveda containing the principles of realizing all kinds of man-made structures, in turn derived from Atharvaveda, one of the four principal Vedas. The original authors are said to be Viswakarma and Maya, the ‘chief engineers’ of gods and demons, respectively. The Rigveda mentions equipment used in casting, such as dhamatri (cupola), gharma aranmaya (crucible) and bhastri (blower). The major application was in creating the idols used for worship; and very strict rules were laid down to achieve perfection in terms of talmana (proportions), mudra (stance) and bhava (expression). In particular, dhyana slokas defined the spiritual quality of each deity and the lakshanas described the form. Other products included lamps, doors, frames, cooking and agricultural implements. Earliest castings include the 11 cm high bronze dancing girl found at Mohen-jo-daro (dated about 3000 BC). The remains of the Harappan civilization contain kilns for smelting copper ingots, casting tools, stone moulds, cast ornaments, figurines and other items of copper, gold, silver and lead. Iron has been mentioned in Vedas as ayas, and iron pillars, arrows, hooks, nails, bowls and daggers dated 2000 BC or earlier have been found in Delhi, Roopar, Nashik and other places. Large scale state-owned mints and jewelry units, and processes of metal extraction and alloying have been mentioned in Kautilya’s Arthashastra (about 500 BC). Later Sanskrit texts talk about assessing and achieving metal purity. The Ras Ratnakar written by Nagarjuna in 50 BC mentions the distillation of Zinc, proved through recent excavations in Zawar, Rajasthan. The Iron Pillar of Delhi, standing 23 feet, weighing 6 tonnes and containing 99.72% iron without any signs of rust, is a remarkable example of metallurgical science in 5th century AD. The first cast crucible steel was also produced around this period. The Nataraja and Vishnu statues of Chola dynasty (9th century) stand testimony to the fine practice of intricate castings in mediaeval India. Most of these were made in pancha dhatu (copper, zinc, tin, gold and silver) using the madhuchista vidhana (lost wax) process. Outside India, the oldest casting in existence is a copper frog dated 3200 BC discovered in Mesopotamia. One of the first cast iron objects, a 270 kg tripod, was cast by Chinese in 600 BC. A colossal statue of the Great Buddha in tin lead bronze was completed in 1252 AD at Kamakura in Japan. The casting technology was transferred from India and Middle

© 2004 Prof. B. Ravi, IIT Bombay

East to Europe through Portugese explorers in 14th century, where it blossomed as a fine art. Vannocio Biringuccio, head of Papal Foundry in Rome (around 1500 AD) is considered as the father of foundry industry in the West. He has been quoted as saying: “The art of casting… is closely related to sculpture, … it is highly esteemed… it is a profitable and skillful art and in large part delightful.” Indeed, the bronze sculptures represent the craftsman’s artistry as well as the capability of the casting process. The ancient art is preserved to this date in places such as Swamimalai in Tamil Nadu, where over 200 units are engaged in bronze art casting. The process starts with sculpturing the wax models by sthapathies (artisans), claimed to hail from the clan of Viswakarma. A mixture of bees wax, vriksa rasa (natural resin from trees) and a little cooking oil is heated and poured into sheets, to facilitate cutting and adding to the models. Each model is unique. In some cases, when multiple (ten or more) orders for the same model are placed, then a cement/plaster mould is made for making a rough shape of the wax models. The carving of each wax model takes 1-4 weeks depending on the size and intricacy. The rules laid down in shilpa shastras and agamas are strictly followed for making the wax models (Fig. 1.1).

Fig. 1.1 Ancient lost wax casting method: wax model sculpting, clay covering, wire clasped mould for dewaxing, as-cast Ganesha, and finishing After carving, the wax model is carefully pasted over and covered with natural clay obtained from river banks, after wetting with water. For hollow castings, cores are used,

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made of sand plus charcoal, sesame oil, cow dung and natural (tree) resin. The claycovered models are placed in the sun to dry for 3-4 weeks. After this, the clay moulds are tied with metal wire (to prevent expansion and breakage during wax removal). For this purpose, cow dung cakes are used as fuel, and the liquid wax comes out from a hole created for this purpose. For making decorative castings, an alloy of copper (84%), zinc (14%) and tin (2%) is used (Chola bronze). If the sculpture is to be used for worship, then small amounts of gold and silver are also added (making it a pancha dhatu). The metal is melted in a crucible furnace using wood charcoal and coal as fuel. Hand-operated bellows are used to blow air into the burning furnace. The mould is preheated to the metal temperature before pouring. After cooling, the mould is broken to reveal the casting. The gates and risers are removed, followed by the painstaking job of chiseling, filing, finishing and polishing. This takes 4-10 weeks depending on the idol size and details. The large labour component reflects in the final cost, which can be 4-8 times the material cost. Very large idols (weighing several tones) can also be made by this process, though melting and pouring can be a problem because of small crucible size. 1.2 Major Casting Processes Today, there are a large number of industrial casting processes (see Fig. 1.2). These can be classified based on the mould material, method of producing the mould and the pressure on molten metal during filling (gravity, centrifugal force, vacuum, low pressure, high pressure). Permanent or metal moulds are used in gravity and pressure die casting processes, suitable for producing a large number of components. In expendable mould processes (sand, shell and investment), a new mould is required for every casting or a bunch of castings with a common gating and feeding system produced in the same mould. Expendable moulds can be made using either permanent pattern or expendable pattern. Permanent pattern can be made from wood, metal or plastic. In expendable pattern processes (also called investment processes), each pattern produces only one casting. Such patterns are made of wax, expandable polystyrene (EPS) or other polymer materials. The four most popular processes are briefly described below, followed by a comparison of their capabilities (Table 1.3). The first two employ dispensable moulds, whereas the last two employ permanent moulds. Sand Casting: In this process, sand mixed with binders and water is compacted around wood or metal pattern halves to produce a mould. The mould is removed from the pattern, assembled with cores, if necessary, and metal is poured into the resultant cavities. After cooling, moulds are broken to remove the castings. This process is suitable for a wide range of metals (both ferrous and non-ferrous), sizes and shape complexity. Investment Casting: Wax is injected into a metal mould to make patterns, which are connected to a common sprue to form a tree. The tree is repeatedly dipped in ceramic slurry and dried, followed by heating to remove the wax. The ceramic shell is preheated, filled with molten metal and broken after cooling to get the castings. This is suitable for castings in any metal with small and intricate shape and thin walls.

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CASTING PROCESSES

EXPENDABLE MOLD

PERMANENT MOLD

LOW PRESSURE

PRESSURE DIE

HOT CHAMBER

GRAVITY DIE

COLD CHAMBER

CENTRIFUGAL

TRUE CENTRIFUGAL

SPECIAL PROCESSES

VACUUM

SEMI CENTRIFUGAL

CENTRIFUGING

PERMANENT CORE

SLUSH CASTING

EXPENDABLE CORE

SQUEEZE CASTING

CHILLED CASTING

CONTINUOUS

PERMANENT PATTERN

WATER & CLAY BOND

RESIN BOND

SHELL MOLDING

HOT BOX

PLASTER BOND

SKIN DRY SAND MOLDING

SILICATE BOND

COLD BOX

CO2 PROCESS

GREEN SAND MOLDING

EXPENDABLE PATTERN

DRY SAND MOLDING

NO BOND

INVESTMENT (WAX) CASTING

FULL MOLD (LOST FOAM) CASTING

VACUUM "V” PROCESS

CERAMIC MOLDING

SHAW PROCESS

CORE SAND MOLDING

FLOOR AND PIT MOLDING

LOAM MOLDING

HIGH PRESSURE MOLDING

Fig.1.2 Hierarchical classification of various casting processes. 4

Gravity Die Casting (also called permanent mould process): Molten metal is poured under gravity into a cast iron mould coated with a ceramic mould wash. Cores can be made of metal or sand. After solidification, the mould is parted and casting is removed. This process is mainly suitable for non-ferrous metal castings having medium size, complexity and thickness. Pressure Die Casting: Molten metal is injected under pressure into a hardened steel die, often water-cooled. Metal cores are used to produce cavities and undercuts. After solidification, one half of the die is moved and the casting is pushed out by ejector pins. This process is suitable for non-ferrous castings of small to medium size, varying complexity and thin walls. Table 1.3: Capabilities of major casting processes Attribute \ Process Maximum size Dimensional tolerance Surface finish Minimum thickness Economic quantity Sample lead time

Sand several tons > 0.6 mm > 200 RMS > 6 mm any number 2-10 weeks

Investment up to 20 kg > 0.1 mm > 60 RMS > 1.5 mm > 100 8-10 weeks

Gravity Die up to 50 kg > 0.4 mm > 150 RMS > 4.5 mm > 500 8-20 weeks

Pressure Die up to 8 kg > 0.05 mm > 30 RMS > 0.8 mm > 2500 12-24 weeks

Other important processes include: centrifugal casting, in which molten metal is poured into a rotating mould and centrifugal force pushes the metal against the mould; lost foam or EPS or full mould process, in which sand is packed around an expendable polystyrene pattern and the molten metal burns out the pattern as it fills the mould; vacuum casting, in which molten metal is forced into the mould under vacuum; and squeeze casting, in which semi-solid metal is forced under pressure into the mould, useful for composites. The major casting processes are described in detail next. 1.3 Sand Casting Sand casting is the most widely used process for both ferrous and non-ferrous metals. Depending on the moulding method, it may be classified as green sand, dry sand or shell mould process. A typical green sand foundry involves three groups of activities (Fig. 1.5). Pre-casting includes sand preparation, core making, moulding and mould assembly. The casting stage involves furnace charging, melting, holding, melt treatment (such as inoculation) and pouring into moulds, which are then left to cool. Post-casting involves shakeout, cleaning, fettling, shot-blasting and inspection. Further operations may include heat treatment and machining. The major steps are briefly described here.

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PATTERN-MAKING

MELTING CORE MAKING

POURING

MOLDING

Fig.1.3 Key activities in sand casting (courtesy: Kirloskar Ferrous India Ltd.) Sand Preparation: Moulding sand should have good flowability (for better reproduction of pattern details), adequate green strength (to prevent its collapse during moulding), dry strength (to prevent its collapse during mould filling), sufficient refractoriness (to withstand molten metal temperature), enough permeability (to allow entrapped air and gases generated inside the mould to escape) and collapsibility (for ease of shakeout). These are achieved by a suitable composition of sand, binders, additives and moisture. Silica sand is the most widely available and economical. Special sands include zircon

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sand (lower thermal expansion, higher refractoriness and higher thermal conductivity, but more expensive), olivine sand (with properties in between silica and zircon sand) and chromite/magnesite sand (high thermal conductivity). The most widely used binder is bentonite clay (sodium or calcium bentonite), which imparts strength and plasticity to silica sand with the addition of water. Additives include coal dust (to improve surface finish by gas evolution at metal-mould interface), iron oxide (for high temperature resistance), dextrin (for improved toughness and collapsibility) and molasses (for high strength and collapsibility). Modern sand plants automatically carry out mulling, mixing, aeration and testing of the sand. They also reclaim used sand through magnetic separation (to remove metal particles), crushing of lumps and finally removal of excess fines and bond (usually by washing in hot water or by mechanical impact). Core Making: Cores are surrounded by molten metal, and have higher requirement compared to mould sand in terms of strength (to support their own weight and the buoyancy force of metal), permeability and collapsibility (especially for curved holes, otherwise they will be difficult to clean out). The most widely used binder for core sands is vegetable oil (linseed and corn oil, sometimes mixed with mineral oils), which is economical, but requires heating in an oven to about 240 C for 2-3 hours to develop sufficient strength. Another widely used process uses sodium silicate binder mixed in dry sand free of clay; the sand mixture hardens immediately when CO2 gas is passed through it. The process is highly productive. The core develops high compressive strength but has poor collapsibility. Other processes are based on organic binders; mainly thermosetting resins such as phenol, urea and furan. This includes hot box and cold box processes. The core sand mixed with binder is filled into a core box either manually or using a sand slinger. For higher productivity core blowing machines are used, in which core boxes are mounted in the machine and sand is forced and pressed into the core box under a stream of high velocity air. This is followed by appropriate heating of the core box to impart the desired properties to the core. Moulding: This involves packing the moulding sand uniformly around a pattern placed in a moulding box (or flask). Most foundries are equipped with jolt-squeeze machines operated by compressed air. The combination of jolting and squeezing action gives good compaction of sand near the pattern (by jolting the sand into crevices) as well as the top where the squeeze plate comes in contact with the mould. Many modern foundries have high pressure moulding equipment, which use air impulse or gas injection to impact the sand on the pattern. These machines produce relatively less noise and dust compared to jolt and squeeze machines and have much higher productivity (several moulds per minute). A special type of high pressure moulding machine is the flaskless moulding machine pioneered by Disamatic, in which the parting plane is vertical and the mould cavity is formed between consecutive blocks of mould. Melting: Most widely used melting equipment include cupula, oil/gas fired furnaces (including crucible and rotary furnaces), direct arc furnace and induction furnace. The cupola is the simplest and the most economical, and most suited for grey iron. Layers of pig iron, coke and flux (limestone) are charged into the cupola; air for combustion is blown through several openings (tuyeres). Use of hot air blast and double row tuyeres

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improves cupola efficiency. Oil or gas fired crucible furnaces are suitable for melting small quantities of metal, usually non-ferrous. The crucible is usually made of graphite and clay. Rotary furnaces are made of steel shells lined with refractory, turning at a rate of 1-2 rpm. The charge is placed through a door in the middle; one end of the furnace is heated (by firing oil or gas) and the melt is taken out through the other end. Electric furnaces include direct arc and induction furnaces, which are more widely preferred by newer foundries owing to ease of control over temperature and composition, and high melting rate. In arc furnace, the heat is generated between the electrodes and transferred to the metal. In induction furnace, the heat is generated in the metal itself by eddy currents. Induction furnaces can be classified depending on the location of the induction coil (cored and coreless), and frequency of current (high or medium). 1.4 Investment Casting Derived from the ancient lost wax process, and adapted by dental and jewelry manufacturers in the West, the modern investment casting process was rediscovered during World War II. It can produce near net shape parts (requiring only finish machining) in any metal in low to medium order quantities. It is especially suitable for small intricate parts of expensive or difficult-to-machine alloys. The expendable patterns are produced by injecting wax in a die made of aluminium or steel. Die design and manufacture determines the complexity and accuracy of the pattern and casting, and the die cost affects the economic order quantity. With the advent of Rapid Prototyping or Free Form Fabrication technologies, investment casting has gained further attention, since expendable RP patterns can be used to produce a single casting within days starting from the digital definition.

Fig. 1.4 Industrial investment casting process (courtesy: Uni Deritend Ltd.)

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The three major steps in the process: wax pattern injection and assembly, ceramic shell construction, and finally dewaxing and pouring are briefly described here. Wax pattern injection and assembly: The pattern material is usually petroleum-based paraffin wax with some blends (like polymers and resins), mainly for higher dimensional stability and strength. It is recyclable (can be used for feeders and gating system) and environment-friendly. It is injected into the die at around 50 C under a pressure of 5-25 kg/cm2 depending on part size and section thickness. Simple dies contain a single cavity and are manually operated, economical for small order quantities. For large order quantities and intricate parts, fully automated multi-cavity dies are developed. The dies require a parting surface (leading to flash), draft or taper for easy removal of the wax pattern (though minimal) and sometimes cores (for holes and undercuts). But it is possible to make a wax pattern in pieces and join them together, thus providing additional freedom to produce complex castings to the desired appearance. The die cavity is made slightly larger to compensate for volumetric shrinkage of the wax pattern during injection and also of the molten metal during casting solidification. The wax patterns are removed from the die, hand finished to remove flash and welded using a simple gas flame torch around a sprue to form a cluster or tree. The sprue is designed to lead molten metal into the individual cavities as well as provide liquid metal for compensating volumetric shrinkage during solidification. Ceramic shell construction: The wax pattern tree is dipped into an agitated slurry of fine refractory material (typically zircon sand) and binder in a rotating drum, immediately followed by stuccoing or showering with dry sand (see Figure 2). It is important to get the slurry coating on the entire surface, including the inside surface of holes. The shell is left to dry for a few hours in an air-conditioned room. The process is repeated 8-15 times to finally produce a ceramic shell 6-10 mm thick depending on the part size and wall thickness. Initial layers are built with fine sand to obtain good surface finish, whereas subsequent layers are built with coarse sand to obtain high permeability. The control of time, temperature and moisture during drying is critical for obtaining a shell of sufficient strength in the shortest possible time. Dewaxing and pouring: The wax is removed from the shell by heating it to about 120 C in a pressurized steam autoclave to prevent shell cracking, followed by heating to over 1000 C to vaporize any residual wax, impart strength to the shell and make it ready for receiving the metal. The metal is melted (usually in an induction furnace) and poured into the red-hot shell (usually heated to the same temperature as the molten metal). After the metal has cooled sufficiently, the ceramic shell is removed by mechanical vibration and chemical cleaning, leaving a metal cluster identical to the wax pattern assembly. The parts are then separated from the cluster, the gates are eliminated and castings are prepared for secondary operations such as heat-treating, machining or applied finishes as needed. The process requires a combination of manual skill, technical expertise and shop-floor discipline. Several technological improvements have been made in the last few years, such as wax additives to improve its fluidity and strength, the use of fibre materials to

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improve the strength of ceramic shells, new core materials (including water solvable), vacuum assisted counter gravity casting (CLA) to improve mould filling, and controlled solidification to achieve the desired structure such as long columnar grains. 1.5 Die Casting The three major die casting processes are named based on the mould filling pressure as: gravity die casting (GDC), low pressure die casting (LPDC) and high pressure die casting (HPDC). Dies for GDC are usually made of cast iron and are simpler in construction. The dies for HPDC are made of hardened tool steel to withstand the high pressures involved and have many more elements. The die casting processes are obviously not suitable for ferrous metals owing to their high melting temperature. 1.6 Casting Applications Castings can range in size: from a few grams (for example, watch case) to several tones (marine diesel engines), shape complexity: from simple (manhole cover) to intricate (6cylinder engine block) and order size: one-off (paper mill crusher) to mass production (automobile pistons). The desired dimensional accuracy and surface finish can be achieved by the choice of process and its control. Castings enable many pieces to be combined into a single part, eliminating assembly and inventory and reducing costs by 50% or more compared to machined parts. Unlike plastics, castings can be completely recycled. Today, castings are used in virtually all walks of life. Major areas of applications are given below (see Fig. 1.3). The transport sector and heavy equipment (for construction, farming and mining) take up over 50% of castings produced. Transport: automobile, aerospace, railways and shipping Heavy equipment: construction, farming and mining Machine tools: machining, casting, plastics moulding, forging, extrusion and forming Plant machinery: chemical, petroleum, paper, sugar, textile, steel and thermal plants Defense: vehicles, artillery, munitions, storage and supporting equipment Electrical machines: motors, generators, pumps and compressors Municipal castings: pipes, joints, valves and fittings Household: appliances, kitchen and gardening equipment, furniture and fittings Art objects: sculptures, idols, furniture, lamp stands and decorative items Virtually any metal or alloy that can be melted can be cast. The most common ferrous metals include grey iron, ductile iron, malleable iron and steel. Alloys of iron and steel are used for high performance applications, such as temperature, wear and corrosion resistance. The most common non-ferrous metals include aluminum, copper, zinc and magnesium based alloys. The production and application of ductile iron and aluminum castings are steadily increasing. Aluminum has overtaken steel in terms of production by weight. The consumption of magnesium alloys is rapidly increasing in automobile and other sectors, owing its high strength to weight ratio. Another important and emerging metal titanium is stronger than steel, but has found limited applications owing to the

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difficulty in casting and machining. Table 1.2 lists the major metals in use today (by weight) along with their main characteristics and typical applications. Table 1.2: Major cast metals METAL Grey Iron

USE 54%

Ductile Iron

20%

Aluminum

12%

Steel

9%

Copper base

2%

Zinc base

1%

CHARACTERISTICS Heat resistance, damping, low cost, high fluidity, low shrinkage. Strength, wear and shock resistance, dimensional stability, machinability. Strength to weight ratio, corrosion resistance.

APPLICATIONS Automobile cylinder block, clutch plate, brake drum, machine tool beds, housings Crank shafts, cam shafts, differential housing, valves, brackets, rollers. Automobile pistons, oil and fuel pumps, connecting rod, clutch housings. machinability, Machine parts, gears, valves

Strength, weldability High ductility, corrosion resistance.

Marine impellers, valves, hydraulic pump parts.

In the following two sections, we will take a closer look at various ferrous and nonferrous metals, their characteristics and industrial applications. 1.7 Ferrous Castings Ferrous castings include those of grey cast irons, ductile (spheroidal graphite) irons and steels, briefly described here. Grey cast irons: These are alloys of iron, carbon and silicon, containing more than 2% carbon (as flake graphite), up to 3% silicon and less than total 1% of alloying elements (mainly chromium, copper, magnesium, molybdenum, nickel, phosphorous, silicon, sulphur, titanium and vanadium). Grey cast irons exhibit low to moderate strength, low ductility and toughness, low modulus of elasticity, low notch sensitivity, high resistance to wear and seizure, excellent vibration damping capacity, excellent machinability, high thermal conductivity, moderate resistance to thermal shock and most important, excellent fluidity. These properties mainly depend on the distribution, size and amount of graphite flakes and the matrix structure. These factors are in turn influenced by manufacturing conditions, chemical composition, solidification time and cooling rate. The grey irons are graded on the basis of their minimum tensile strength in MPa of a test piece machined from a 30 mm diameter test bar cast separately. Major grades include FG150, FG200, FG220, FG260, FG300, FG350 and FG400, with the corresponding tensile strength increasing from 150 Mpa to 400 MPa. The Brinell hardness also increases from 150 HB for FG150 to 250 HB for FG400. The corresponding density is in the range

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7050-7300 kg/m3 and specific heat is 490-605 J/(kg K) at 700 C. Thermal conductivity decreases from 49.5 W/(m K) for FG150 to 40 W/(m K) for FG400 at 500 C. Coefficient of thermal expansion is about 12.5 µm/(m K). The modulus of elasticity and wear resistance increase with tensile strength, while machinability, resistance to thermal shock, damping capacity and metal fluidity decrease with increasing tensile strength. Typical applications of various grades of grey cast iron are as follows. FG150: Exhaust manifolds, grates, housings, machine bases, manhole covers and traffic signals. FG200 and FG220: Air-cooled cylinders, clutch housings, clutch plates, compressor frames, cylinder heads, flywheels, gearboxes, impellers, oil pumps, pipes and fittings, light duty brake drums, pistons, rams and transmission casings. FG260: Anvils, diesel cylinder blocks, medium duty brake drums, face plates, heavy-duty flywheels, heavy machine beds, steams pressure castings, valves and wheels. FG300: Heavy-duty brake drums and clutches, differential carrier castings, heavy gearboxes, tractor transmission cases, truck and tractor cylinder blocks and compressors. FG350: Camshafts, compressors, cylinder liners, heavy machine beds, light crankshafts, pumps and rams. FG400: Connecting rods, camshafts, crusher frames, high-pressure well pumps, hydraulic cylinders, pressure castings in chemical industries and sluice gate valves. High alloy cast irons are used for special purpose applications requiring resistance to abrasion, corrosion and heat. Ductile or spheroidal graphite irons: These irons have higher mechanical properties than a comparable grey cast iron with the same composition, because the carbon is in the shape of spheroidal graphite. This is achieved by inoculating low-sulphur molten iron having low silicon content with magnesium or cerium or both, followed by addition of silicon. Subsequent cooling can produce a variety of matrix structures with ferrite and pearlite being the most common. Compared to grey cast iron, spheroidal graphite irons have higher ductility, tensile strength, modulus of elasticity and resistance to elevated temperature oxidation. Machinability and corrosion resistance are comparable to grey cast iron, though damping capacity is lower. Fluidity is lower than grey cast iron but better than steel. Spheroidal graphite irons are designated based on the specified minimum tensile strength in MPa and the minimum elongation (in percentage) after fracture of a test piece. This includes SG350/22, SG400/18, SG400/15, SG450/10, SG500/7, SG600/3, SG700/2, SG800/2 and SG900/2. The corresponding tensile strength varies from 350 MPa to 900 MPa, while the Brinell hardness varies from 150 to 320. Density and specific heat of the various grades remain nearly constant at about 7100 kg/m3 and 600 J/(KgK) (between 20 and 700 C) respectively. Coefficient of thermal expansion between 20 and 400 C also remains constant at 12.5µm/(m K). Thermal conductivity of the various grades falls from around 36 W/(m K) for SG350/22 to around 33 W/(m K) for SG900/2. The SG irons are widely used in automobile and farming industry: axle housings, brake calipers, brake cylinders, camshafts, connecting rods, crankshafts, gears, pistons and yokes. They are also used to make bulldozer parts, conveyor frames, couplers, crawler

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sprockets, elevator buckets, railway wheels and hoist drums. Other general engineering applications include boiler segments, coal crushers, hammers, die blocks, frames and jigs, nuclear fuel containers, tank covers, tunnels segments and turret heads. Cast steels: Steels have less than 2% carbon, and some alloying elements such as manganese, silicon, lead, copper and chromium. The properties are controlled by carbon content and heat treatment procedures. In general, the strength and hardness increase with the carbon content, but at the expense of ductility and toughness. Steels with more than 1.6% manganese, 0.6% silicon and 0.6% copper are usually designated as alloy steels. Depending on the total amount of alloying elements, alloy steels are classified as lowalloy (less than 5% alloying elements), medium-alloy (5-10%) and high alloy (over 10%) steels. These possess higher strength, toughness, abrasion resistance and corrosion resistance. There are also four types of special purpose cast steels. Abrasion resistant cast steels are usually austenitic manganese steels. Cast steel for low temperature service (ex. containers for liquefied gases) are generally ferrite hardenable steels and austentic non-hardenable steels. Corrosion resistant cast steels are iron chromium and iron-chromium-nickel alloys used for pumps, valves and piping for corrosive chemicals. Cast steel for high temperature service (ex. gas turbine components) includes high-alloy ferrite and austentic steels. Steels exhibit varying values of tensile strength depending on the alloying elements and heat treating techniques. It ranges from 500 MPa for general purpose steels to 1250 Mpa for high tensile strength steels. Typical physical properties are: density around 7200 kg/m3, specific heat 0.8 J/(kg K), thermal conductivity 23.2 W/(m K) and coefficient of thermal expansion around 11 µm/(m K). 1.8 Non-Ferrous Castings This includes alloys of aluminium, copper, magnesium, zinc and other metals. Most of them have lower mechanical properties (compared to ferrous metals) including strength, modulus of elasticity and stiffness, but exhibit superior properties in terms of light weight, resistance to corrosion, electrical and thermal conductivity. Major non-ferrous metals and their alloys are briefly described here. Aluminium: It is a soft silvery white metal with about one third of density of ferrous metals and low tensile strength of around 50 MPa in its pure form. It is an excellent conductor of heat and electricity and has corrosion resistance in most environments including seawater, oils and many chemicals. It is non-toxic, non-ferromagnetic and it also has non-sparking characteristics. To improve its strength, hardness and fluidity, silicon, copper, magnesium and zinc are added. Aluminium-copper alloys have medium strength and fair fluidity. Addition of nickel and magnesium further increases their maximum strength and hardness. Aluminium-silicon alloys show excellent fluidity and good pressure tightness, but are difficult to machine in comparison to aluminium-copper alloys. Aluminium-magnesium alloys have high strength, good corrosion resistance and

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good machinability, relatively poor castability. This can be improved by adding a small amount of silicon. Aluminium-zinc-magnesium alloys have high strength, good machinability, good corrosion resistance and good finishing characteristics, but lower castability and not suited for use at elevated temperatures. Addition of copper and small amounts of chromium and manganese to aluminium-zinc-magnesium alloys helps achieve the highest strength aluminium casting alloys. The tensile strength of aluminium-silicon alloys ranges between 140-270 MPa. Aluminium-copper and aluminium-magnesium alloys have a slightly higher range of tensile strength up to 300 Mpa. The density of all aluminium alloys ranges between 2550 to 2950 kg/m3. There is very little variation in the coefficient of thermal expansion, which is around 22µm/(m K). Thermal conductivity however varies: aluminium-silicon alloys have higher values typically 125W/(m K) than aluminium-magnesium alloys, which have an average value of 100W/(m K). Aluminium-silicon alloys are widely used for air-compressors, automobile transmission components, aircraft pump parts, automotive and compressor pistons, escalator steps, thin-walled and intricate instrument casing and aircraft supercharger covers. Aluminiumcopper alloys are used in the production of air-cooled cylinder heads, artificial limbs, aircraft pistons, castings for hydraulic equipment and valve tappet guides. Aluminiummagnesium alloys are used for producing castings for marine, food processing and decorative applications. They are also used for rail-road and passenger car frames and other parts requiring strength and shock resistance. Copper: Copper has been in use for more than 6000 years and continues to be an important metal. Its applications are driven by high electrical and thermal conductivity, moderate strength coupled with ductility and high corrosion resistance. Its tensile strength can be increased from 200 MPa to 450 MPa by cold working. It is however, heavier than iron and has a strength/weight ratio lower than aluminium alloys. It cannot be used at elevated temperatures. Major alloys of copper include brasses and bronzes. Brasses are copper-zinc alloys with or without small amounts of other elements like lead, aluminium, iron manganese, nickel and tin. Brasses have moderate to high strength, good casting characteristics, good corrosion resistance and attractive colour. Addition of lead (LCB alloys) improves machinability. Addition of tin improves corrosion resistance. Aluminium is added to achieve increased fluidity and smooth surface finish. High tensile brasses (HTB) have a higher percentage of aluminium, iron, tin and nickel. Silicon brasses are copper-zincsilicon alloys having good bearing characteristics, higher corrosion resistance and good casting characteristics. Leaded tin bronzes (LTB) are copper-tin-lead alloys used for bearing applications where both wear resistance and good anti friction characteristics are desired. Phosphor bronzes are copper-tin-phosphorus alloys characterized by high hardness, good wear resistance, good toughness, good bearing properties and good corrosion resistance. Aluminium bronzes (AB) are copper-aluminium alloys containing iron, often with nickel and manganese. They have good ductility, good resistance to cavitation erosion and wear, excellent resistance to corrosion and oxidation, good bearing

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properties and good casting and welding characteristics. They are also useful at elevated temperatures. Gunmetals are characterized by good casting characteristics, moderate strength, good corrosion resistance and low coefficient of friction. Mechanical properties of copper alloy castings vary widely depending on composition. High tensile brass (HTB2) has the highest tensile strength of 740 MPa followed by aluminium bronze (AB2) at 640 MPa. On the other hand, leaded tin bronzes have the lowest tensile strengths around 200 MPa, though they have the highest density (9000 kg/m3). On a descending scale, the density of bronzes is around 8800 kg/m3 followed by brasses (8500 kg/m3) and finally aluminium bronzes (7600 kg/m3). The coefficient of thermal expansion of pure copper is very high 372 W/(m K), but brasses and bronzes have a much lower value around 20 µm/(m K). The thermal conductivity also changes with the extent and type of alloying: leaded brass is 81 W/(m K), lead bronzes is 47-71 W/(m K), aluminium bronzes is around 50 W/(m K) and silicon brass is 28 W/(m K). Casting temperatures range from 980 C for high tensile strength brass (HTB) to 1250 C for aluminium bronze. High conductivity copper is used for making electrode clamps for arc furnaces, cooling rings for blast furnaces and lance nozzles. Leaded brass finds its use in making ornamental castings, plumbing fittings and fixtures and switchgear brush holders. High tensile brass is used for gun mountings, rolling mill castings, hydraulic equipment, locomotive axle bones, marine propeller and cones, pump casing and rudder and rudder posts. Silicon brass and tin bronze are used for bearings, bushings, gears, rocker arms, nuts, valves brackets and brush holders. Lead tin bronze is used to produce engine components, low pressure valves, plumbing hardware and applications requiring corrosion resistance. Phosphor bronze is used in connecting rod small-end bushings, locomotive slide valves, bushings for heavy duty loads, gears, pump parts used in marine work, and worm wheels that are required to be shock resistant. Aluminium bronze is used to manufacture acid resistant pumps, bearings, bushings, non sparking hardware, valve seats, propeller blades and hubs for fresh and saline water service, structural applications and marine fittings. Zinc: Pure zinc is a bluish white metal, brittle at room temperatures and soft and ductile at temperatures over 100 C. It has very good corrosion resistance properties and used in the galvanizing of iron and steel. Because of its low melting point, good fluidity, dimensional stability and no adverse effect on die steel, zinc is widely used in die casting. Zinc-aluminium-copper alloys (like AC41A or ZnAl4Cu1) have moderate strength, high resistance to surface corrosion, good impact resistance and damping characteristics. Addition of aluminium (ZA8, ZA12, ZA27 with 8, 12 and 27% aluminium respectively) improves strength, hardness, bearing and damping properties. Tensile strength of zinc alloys ranges between 285 MPa for ZA4 alloy to about 425 MPa for ZA27 alloy. The corresponding hardness varies from 83 HB to 120 HB. The specific heat is about 460 J/(kg K), thermal conductivity is 117 W/(m K) and coefficient of thermal expansion is 27µm/(m K). The density varies from 6700 kg/m3 to 5800 kg/m3 for zinc-aluminium alloys. Casting temperatures are around 400 C.

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Typical applications of zinc alloys include bathroom fittings, bodies for fuel pumps, body mouldings, car door handles, car radiator grills, hydraulic machinery parts, control panels, speedometer frames, toys and windshield wiper parts. Zinc-aluminium alloys are used for bearings and bushings for high load low speed applications, electrical conduit fittings, transformer parts, liquid and gas valve housings, marine and mine hardware and pressure resistant components. Forming die alloys are used in the construction of limited life dies and punches for forming sheet metal parts. 1.9 Production and Foundries Over 75 million metric tons of cast components worth more than $150 billion are produced annually by over 35,000 foundries worldwide. About 15 million tones of castings are recycled every year. Table 1.1: Top ten producers of castings COUNTRY CHINA USA JAPAN RUSSIA GERMANY INDIA FRANCE ITALY MEXICO BRAZIL

PRODUCTION (million tons) 2002 1994 16.26 12.36 11.81 11.71 6.75 6.68 6.20 4.59 3.48 3.27 1.58 3.02 2.03 2.44 2.27 2.03 1.97 1.49

NUMBER OF FOUNDRIES 2002 1994 12000 13934 2700 3100 1724 1428 1900 673 889 4700 6000 525 507 1159 594 1787 1000 934

According to the worldwide census of casting production, the top nations include China, USA, Japan, Russia, Germany, India, France, Italy, Mexico and Brazil (Table 1.1). Other countries with annual production over one million tons include Korea, Spain and Taiwan, closely followed by Turkey and Canada. A large number of foundries are also operating in Ukraine and Poland. Great Britain witnessed falling production of castings over the last decade. The top ten producers together account for over 80% of the total production of castings as well as the number of foundries worldwide. Over the last ten years, the number of foundries in most countries has reduced, while the production has increased. The average productivity of foundries worldwide is about 2000 tonnes per year. German foundries have the highest productivity, with an average 6800 tonnes per year. Most foundries are of jobbing type, handling orders from different customers. They are geared for quick development of new castings and fairly large variations in order quantities. On the other hand, the castings produced in captive foundries are mainly consumed by the parent organization. The defining line is thinning as many jobbing foundries are now specializing in fewer products (say only cam shafts) and captive 16

foundries are beginning to cater to organizations other than the parent, for better capacity utilization and to maintain a better competitive edge. Both types of foundries are moving towards increased levels of automation. Foundries are also classified depending on capacity, as large, medium and small. The capacity may refer to either melting capacity (which depends on the furnaces installed and working) or actual production of good castings (which depends on order booking, overall yield and rejections). Large ferrous foundries can produce over 10,000 tons of castings per year, and are usually equipped with automated sand plants, moulding, melting, pouring and fettling equipment. On the other hand, small foundries may have capacities of 1000 tons or less per year with most of the operations being carried out manually. Over the last decade, there has been a steady increase in requirements of casting buyers (original equipment manufacturers and assemblers) in terms of quality assurance, shorter lead-time, smaller lot size and competitive pricing. Assemblers are eliminating inspection of incoming goods and expect the suppliers to be responsible for casting quality. The increasing use of NC machines for finishing operations requires dimensionally stable castings with uniform surface hardness to prevent damage to cutting tools. Because of shrinking product development cycles, foundries are expected to deliver the first sample in weeks instead of months. The adoption of Just-In-Time philosophy by assemblers to reduce their inventory costs requires foundries to deliver small lots and more frequently, while adhering to strict delivery schedules. The casting buyers want the foundries to continuously reduce their costs every year by adopting better technologies and methodologies. Foundries also have to contend with increasing pressure from regulatory bodies in terms of energy conservation, environment protection and operational safety. Many leading customers, particularly in the automobile sector, are moving toward longterm strategic partnerships with a few capable foundries instead of short term cost-based purchasing agreements with a number of foundries as in the past. This means that in order to survive and grow, foundries have to offer dimensionally stable and sound castings (preferably with self-certification), ensure reliable on-time small lot delivery and provide continuous reduction in prices. This is forcing foundries to specialise, in terms of casting alloys, part geometry (size/weight and complexity) and end application. Casting buyers and suppliers are also realizing the importance of ensuring compatibility between product design and process capability, by integrated product and process development through close collaboration starting from the product design stage. 1.10 New Casting Development The three major stages in developing a new casting include product design, tooling development and foundry trials. Product design: This influences virtually all other decisions and activities in product life cycle, and eventually the technical and economical value of the product. In particular, tooling design and manufacturing process can only be optimised within the framework

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established during this stage. It essentially involves specifying three types of requirements. The first is functional requirements driven by product geometry, including the overall shape and individual features, specified in terms of their dimensions, relative location and orientation. The second is property requirements, which include thermophysical (density, thermal conductivity, electrical resistance, etc.), mechanical (tensile strength, impact resistance, wear resistance, etc.) and chemical (corrosion resistance) properties. The properties are mainly driven by the material composition and structure (which can be modified by various treatments). The third is production and quality requirements including order quantity, lead-time, surface finish, dimensional tolerance and internal soundness. These are driven by parameters related to tooling design and manufacturing process planning. The above requirements are evolved and refined through three steps in product design: conceptual design, detailed design and prototyping. Conceptual design essentially focuses on basic geometric elements to achieve the functional requirements. Detailed design involves selecting the material(s) and defining the geometry (features and their dimensions), including the extent of their geometric variation (manufacturing tolerances). This is followed by prototype fabrication to test the form, fit and function of the product. The production and quality requirements are then finalised. Several iterations of conceptual design, detailed design and prototyping are carried out to achieve the optimal combination of functional requirements, quality and cost. Tooling development: It is a critical activity linking product design and manufacturing. The tooling comprises patterns and core boxes (for sand casting) or dies (for die casting and investment casting). Tooling design can be further classified as design of main cavity (or the pattern for producing the cavity), other cavities and accessories. The main cavity, produced by bringing together two or more segments of moulds, involves selection of the best orientation of part in the mould and determining the parting line. The mould may have a single or multiple cavities depending on part size, requirement and other considerations. Internal cavities in the part, such as holes and undercuts (portions which hinder removal of pattern from mould or part from die), are produced by cores. This requires identification of cored features, design of cores (including their supports, called prints in sand casting) and core boxes for producing the cores. Proper allowances have to be incorporated in the mould cavity and cores considering part shrinkage, distortion and subsequent machining. Draft or taper has to be given to facilitate easy removal of the pattern from sand mould or casting from permanent mould. Other cavities include feeders or risers (number, location, shape, dimensions) to compensate for volumetric shrinkage and gating channels (sprue, runner and ingates) to lead molten metal into the mould. Accessories include cooling, guiding and ejection systems, especially for dies. The method for manufacturing the tooling depends on its material, complexity, quality and time/cost considerations. Conventional machining combined with manual finishing is still widely used, but gradually being replaced by numerically controlled machining owing to better consistency and higher productivity. In recent years, rapid prototyping or free form fabrication technology is being used to produce tooling for one-off castings.

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The rapid prototyped model can be duplicated using rapid or ‘soft’ tooling methods suitable for small order sizes. Foundry trials: After tooling development, trial castings are produced in the foundry. This involves preparing the moulds or dies, melting and pouring the metal, and removing the castings after solidification. The castings are inspected using destructive and nondestructive methods for external and internal defects, if any. The most common destructive method is cutting the casting in different places and observing the crosssection. Non-destructive methods include radiography, ultrasound and eddy current testing. Based on the results, the tooling (usually gating and feeding) may be modified and process parameters (usually pouring temperature, time and pressure variation in case of die casting) may be tuned to improve casting quality to the desired level. Typically, 3trials are required for most new castings, each trial taking up a working week. The sample castings are sent to the customer for approval and then the regular production commences. However, even after several trials and approval of sample castings, there can be a high incidence of casting defects during regular production. Internal defects (such as shrinkage, gas porosity and blow holes) are usually discovered at the machining stage in the assembler company, often leading to production bottlenecks. If such defects cannot be eliminated by modifications to process parameters or tooling design, then it becomes necessary to modify the product design, which is prohibitively expensive at this late stage. The average lead-time for the first good sample casting is several weeks, of which tooling development and proving accounts for nearly 70%. The lead-time can be reduced by more than half, especially for intricate castings, using computer-aided systems for product design, tooling development and process optimisation. Using a solid modelling program, a 3D model of the cast product can be created on a computer, visualised from various angles and its mass properties can be computed. The model can be subjected to various loads to predict internal stresses and deformations, and the part geometry can be optimised for its functional requirements. The tooling models can be generated by modifying the part model by splitting across a parting line and applying draft and various allowances. The models of feeders and gating system can be added to create the complete casting model. Mould filling and casting solidification can then be simulated to predict internal defects. The process parameters, tooling design and part model can be modified and verified by simulation to achieve the desired quality without pouring trial castings.

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