Introduction To Casting Processes 1b5r26

ADVANCED CASTING PROCESSES INTRODUCTION: Metal casting is a process in which molten metal or more commonly an alloy is poured into a mould or die that contains a cavity which represents the shape of the component or casting to be produced. In the case of a mould the cavity is produced by pattern which is a replica of the component required, whereas in the case of a die cavity replicating the shape of the component is made by machining. Internal or external features that can not be readily moulded are obtained by the use of cores. Moulds are generally made of sand and are expendable i.e. they are destroyed when the casting is removed. Dies are permanent and may be used to produce a large number of castings before they are unusable. Cores may be either expendable (made in sand) or permanent (metal cores). Patterns may also be permanent or expendable. Over the centuries several casting processes have been developed to meet the changing requirements from the industries. In the past few decades some of the processes like Low Pressure Die Casting, Investment Casting, Centrifugal Casting, Semisolid Metal Casting etc. have been developed to meet critical requirement of aerospace and automotive industries. CLASSIFICATION OF CASTING PROCESS: Casting processes can be broadly classified into three categories according to the nature of the pattern and / or mould (i.e. whether expendable or permanent). These three categories are outlined below: 1. PERMANENT PATTERN / EXPENDABLE MOULD PROCESSES 1.1

SAND CASTING PROCESSES 1.1.1 Green sand moulding process 1.1.2 Dry sand moulding process 1.1.3 CO2 / Silicate process 1.1.4 Shell moulding process

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1.1.5 Cold set process 1.1.6 Vacuum sealed moulding process 1.2

REFRACTORY AGGREGATE PROCESSES 1.2.1 Ceramic mould process 1.2.2 Plaster mould process

2. EXPENDABLE PATTERN / EXPENDABLE MOULD PROCESSES 2.1

SAND CASTING PROCESS 2.1.1 Evaporative pattern casting process

2.2

REFRACTORY AGGREGATE PROCESS 2.2.1 Investment casting process

3. PERMANENT MOULD CASTING PROCESSES 3.1

DIE CASTING PROCESSES 3.1.1 Gravity die casting 3.1.2 Pressure die casting 3.1.3 Low pressure die casting

3.2

SQUEEZE CASTING

3.3

CENTRIFUGAL CASTING

Advantages of the Metal Casting Process: Metal casting competes against several alternative-manufacturing processes like forming, machining, powder metallurgy etc. in the production of shaped metal components. Each of these processes has its own merits and demerits but the flexibility of the metal casting process in casting shape, size, weight range, design versatility, alloy range, and order quantity is unmatched. The reasons for the wide acceptance of casting process are: Š

Intricate geometries (both external and internal) may be cast with relative ease which reduces a number of other operations.

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Š

Some of the alloys can only be cast due to their poor cold or hotworkability.

Š

Objects may be cast in a single piece which would otherwise require assembly of several pieces if made by other methods.

Š

Casting process is highly suitable for mass production and economic viability is excellent.

Š

Range of sizes, shapes and weights are possible in both small and large batches by proper selection of a casting process.

Š

Castings have generally better isotropic properties. At the same time it is possible to produce components with directional properties such as turbine blades by directional solidification in investment casting process.

Of the various casting processes only two processes which are of recent nature will be discussed in detail. These are (i) casting processes using expendable patterns (ii) casting processes utilizing feed material in the semisolid condition. Traditionally Investment Casting process uses expendable wax pattern. Other relatively newer processes under the first category are Full Mould (FM) Process, Evaporative Pattern Casting (EPC), Expanded Polystyrene (EPS) Process, Replicasting Process which utilize expanded polystyrene as the pattern material.

Processes under the second

category are: Rheocasting, Thixocasting, and Thixomoulding Processes etc.

INVESTMENT CASTING PROCESS: Investment casting process is also known as the Lost Wax process. The process involves the use of an expendable wax pattern. Injecting wax in metal dies produces wax patterns. The wax patterns are assembled with gating and feeding system. Over this wax assembly, ceramic coating is built up through successive stages of dipping and stuccoing. The initial dip coat, containing a fine refractory, is allowed to set before the assembly is dipped in a secondary dip tank and stuccoed with coarser, dry particles in a raining cabinet or fluidized bed. Hydrolyzed ethyl silicate is generally used as the binder in the slurry containing fine refractory powder. After stuccoing, the binder is gelled either by chemical means or by drying in air. Once the required shell thickness is achieved the wax pattern is removed by heating the mould. Before dewaxing shells are dried for long time to attain sufficient green strength in the shell. The dewaxed shells are fired to remove moisture, to burn off residual wax and other 3

organic materials present in the slurry, to sinter the ceramic, and to preheat the mould to the temperature suitable for pouring. Various stages of investment casting process are schematically shown in Fig. 1. The process is suitable for various ferrous, non-ferrous and superalloy castings. Mostly, castings produced by this method are small and weigh only few grammes to kilogrammes. Thin sections of 1.5 mm are possible to cast with very good reproduction of fine details. Castings with very good surface finish (1-3µm) can be cast with good dimensional accuracy. However, the process is expensive and extended lead times are required to produce casting. EVAPORATIVE PATTERN CASTING PROCESS: Evaporative pattern casting (EPC) is a sand casting process that uses an unbounded sand mold with an expendable polystyrene pattern placed inside of the mold. This process is somewhat similar to investment casting in that an expendable material can be used to form relatively intricate patterns in a surrounding mold material. Unlike investment casting, however, evaporative pattern casting (EPC) involves a polystyrene foam pattern that vaporizes during the pouring of molten metal into a surrounding mold of unbounded sand. The evaporative pattern casting (EPC) process is known to Foundrymen by various names such as expanded polystyrene (EPS) moulding process, the evaporative foam process, the lost- foam process, or by trade names like Full mould and Replicast. In these processes expanded polystyrene is predominantly used as the pattern material, although some other polymeric materials have also been successfully used. Patterns can be manufactured either by cutting the EPS block using heated nichrome wire and subsequently finishing the pattern with cutting, grinding etc., or by steam moulding of the pre-expanded polystyrene beads in aluminum alloy moulding dies. Pattern pieces, gates, runners and feeders are assembled with the aid of adhesives and this assembly is given a refractory wash. Coated pattern assembly (cluster) is positioned in mould boxes and loose sand is compacted around the pattern through vibration of the sand. Molten metal poured into the mould vaporizes the pattern and takes the shape of the pattern. Generally, vacuum is applied to the mould prior to, and during

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pouring to assist the extraction of gases generated during pattern degradation and improve mould rigidity. After the casting has solidified it can be easily removed from the loose sand and sand can be recycled directly since no binder is present in the sand. Fig. 2 shows steps involved in making a casting using EPS pattern. The main advantage of the process lies in the fact that there are no parting lines, no cores and no sand binders are used. Process can be readily automated and expensive sand conditioning plant is not required. There is greater flexibility in deg the casting, casting weight can be reduced and machining is minimized due to absence of taper in the pattern. Disadvantages of the process are: -

Patterns can get easily damaged

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Patterns undergo shrinkage

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Carbon-pick in low carbon steel castings

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Quality of the casting is determined by the quality of the pattern

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Sand may collapse during pouring.

CASTING PROCESSES UTILIZING SEMISOLID FEED MATERIAL: Conventionally all the casting processes employ molten metal for pouring into mould or die. It was not possible to feed semisolid material until the work of Flemings and his coworkers at MIT to produce thixotropic alloy slurries in semisolid condition. It was found that if the alloy is continuously stirred while cooling from completely molten to semisolid state, the resulting slurry shows thixotropic behaviour. Due to this behaviour material behaves like fluids while under shear but behaves like solid when shear force is removed. This made it possible to feed the thixotropic slurry to a pressure die casting machine and make the sound castings. The process has many advantages due to lower temperatures involved with casting. These are: Š Lower die erosion and increased die life Š Lower gas absorption related defects Š Lower inclusion defects due to less turbulence. 5

Š Made possible to use heat treatable Al-alloys for pressure die casting Š Increased productivity due to less heat to be dissipated during solidification Š Reduced shrinkage porosity The important aspect the process is the production of semisolid slurry with thixotropic characteristic.

This slurry has globular solid dispersed in the liquid

matrix. Fig. 3 compares conventionally cast Al-alloy dendritic microstructure with globular microstructure of the alloy slurry showing thixotropic behaviour. Over the years several methods have evolved to obtain globular microstructure in the semisolid alloy. Some of the methods are: Š Mechanical stirring of the solidifying alloy: In this method the solidifying alloy is stirred mechanically either in batch mode or continuous casting mode. In the former case impellers of various designs can be used to impart shearing. The batch furnace should have accurate control over temperature and cooling rate to achieve desired microstructure. Technique for continuous slurry making is shown in Fig. 4. The molten alloy from the reservoir es through a narrow zone between cylindrical rotor and an annular gap and undergoes shear action.

The shearing zone also has

temperature gradient with reservoir end above liquidus and exit port at a temperature within semisolid range. Š Magnetohydrodynamic (MHD) stirring of the alloy in a batch or continuous casting process:

These methods involve electromagnetic

stirring of the alloy in place of mechanical stirring. Due to the use of electromagnets stirring is indirect and has no direct with stirrer. This results in less contamination of the slurry and process is better suited for automation and having controlled atmosphere in the furnace. Of the various techniques, MHD stirring technique has reached the commercial stage. Š Grain refining by chemical or ultrasonic vibrations:

These methods

involve use of grain refiners / high power ultrasonic vibrations to achieve fine grain structure which helps in globularisation process. Generally,

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these methods are used in conjunction with MHD stirring or Liquidus casting processes (described below) and not all alone. Š Liquidus Casting:

Controlled cooling of the alloy near liquidus

temperature. Due to the low superheat the resulting microstructure is usually fine and non-dendritic. This material when reheated, results in globular microstructure. Some of the processes using this concept are New Rheocasting (NRC), Cooling slope, Rotating Copper Rod and Twin Screw Extruder Process. Š Strain Induced and Melt Activated (SIMA) technique: Whereas all the processes described above utilize liquid or semi-solid material as the starting material to produce the thixotropic material, SIMA process uses solid starting material.

In this process, material is cold worked to

introduce a high dislocation density and subsequently heated to semisolid temperature to produce globular structure. Some of the above methods are schematically shown in Fig. 4. The semi-solid slurry can be used in different ways to form the component.

Some of the

technologies utilizing semisolid slurry to form the component are: o Rheocasting: In this process the slurry is directly fed to shot sleeve of a pressure die casting machine and injected into the die cavity. o Thixoforming: In these processes slurry is first cast into billets which can be stored and later used by reheating to the semisolid state. ¾ Thixocasting: In this process the slurry is first cast in the form of

billets. Later these billets are cut into solid slugs

and reheated to the semisolid temperature. Reheated slug is used in the shot sleeve of the pressure die casting machine. When slug is sheared by the plunger the material behaves like fluid and fills the die cavity. ¾ Thixoforging: The process is similar to Thixocasting till slug reheating. Reheated slug is used to form component using closed die in a forging press. 7

o Thixomoulding: The process is specifically designed for producing near net shape components from magnesium alloy chips. The chips are heated to the semisolid temperature while being continuously sheared in a screw feeder type machine and subsequently fed to the die cavity through a nozzle. o Rheomoulding: The process is similar to Thixomoulding, major difference being in the feed material. In this process liquid metal is used as the feed material. Liquid metal cools down gradually while being stirred by the screw (either single or twin), thus producing globular structure. The thixotropic slurry produced at the end of the screw is fed to the die cavity through a nozzle to form the component.

FUTURE DEVELOPMENTS: Casting processes based on the expendable pattern (such as EPS or wax) are currently exploited for automotive, defence and aerospace sectors.

Current and

future trends will be towards utilizing rapid prototyping (RP) concepts to produce these expendable patterns. Casting approach where designers will be able to make last minute changes due to RP route of expendable pattern will be in demand. The ultimate in the component manufacturing will be achieved when castings will be directly deposited in RP manner. This appears feasible when we make use of thixotropic nature of the alloy slurry to retain the shape during direct deposition on a moving platform.

A process has already been patented in USA which utilizes

thixotropic alloy slurry to make the component by depositing the slurry in sequential layer manner. The process combines the advantages of handling alloy slurry with the freeform fabrication technology. SUMMARY: A brief of three casting processes has been presented. Two processes rely on the expendable nature of the pattern to make casting using molten alloy as the feed material. The third process described is special in the sense that it utilizes

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semisolid alloy slurry as feed material in a die cavity. Future trends stressing the need for adapting RP concepts to make expendable patterns or directly make the cast components are suggested. REFERENCES: 1.

A.G. Clegg, Precision Casting Processes, Pergamon Press, Oxford, 1991.

2.

P. Beeley, Foundry Technology, Butterworth Heinemann, Oxford, 2001.

3.

Metals Handbook, 9th Edition, ASM, vol. 15, 1988.

4.

M.C. Flemings, Met Trans A, 22 (1991), 957 - 981.

5.

D.H. Kirkwood, Int. Mat. Rev., 39(5), 1994, 173 - 189.

6.

Z. Fan, ibid., 47(2), 2002, 49 - 85.

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a

b

c

Fig. 1: (a) EPS pattern encapsulated e in ceramic sh hell, (b) Ceram mic shell afterr removal of the t pattern an nd (c) Al-allo oy casting obtaained by pouring Al in thee ceramic shelll

Fig. 2 Different D steps involveed in investtment castin ng process.. (Ref: www.hhitchiner.com m)

a

b

200 0 µm Optical micrograp Fig. 3: phs from Al-alloy A356.0 sh howing; (aa) dendrittic microstrructure wheen alloy is slowly coo oled from liquid l to a semisolid temperaturre and subssequently chill c cast from that tem mperature, (b) alloy iss specially processed to t obtain globular miccrostructuree. SIMA

MHD M (Co ont.)

Mechanical Stirring S

Cheemical Grain Refining

MHD (Batch)

Deformation

Annealing

Liquid

Rottating Copper Rod R

Fig. 4: Schematicc diagram showing s so ome of meth hods used tto produce Al-alloys with w globu ular microsttructure. (R Refs: www.rwth-aacheen.de ; www w.wpi.edu))