Casting and foundry

Introduction
Casting has significant advantages compared with other methods of component manufacture. Castings are generally
cheaper than components made in other ways. The casting process in one or other of its forms provides the designer with an unrestricted choice of shape made in a single stage. A casting can usually be made much closer to the chosen design, which provides savings in both material and finishing processes compared with other methods of manufacture. In addition, the cast structure has the highest resistance to deformation at elevated temperatures so that castings have higher creep strengths than wrought and fabricated components. This advantage can be enhanced by modifying and aligning solidification to produce highly creep-resistant structures suchi as bundles of crystals with one crystallographic axis oriented lengthwise or single crystals (see Chapter 7, Section 7.4 non-ferrous metals). Cast metal may also have superior wear resistance than the equivalent forged metal. These advantages have combined to ensure that casting has become the most important process for the manufacture of components in metals (and in some other materials). Castings may be cast in one of a variety of ‘sand’ moulds formed airound a pattern and classified as ‘sand castings’; in one of a variety of moulds formed around a fusible wax pattern and classified as ‘precision’ or ‘lost wax’ castings; in a metal mould and classified as ‘die castings’; or they may be formed by centrifugal force and classified as ‘centrifugal castings’. ‘Splat’ casting produces material with the optimum mechanical properties in the form of small flakes. Even when casting has not been adopted to generate the shape of a component it may well have entered into the process of manufacture at an earlier state having been used to produce the stock for mechanical working, forging, rolling or extrusion by a process such as ‘ingpt’, ‘billet’, ‘continuous’ or ’semi-continuous’ casting.
The advantages of castings have, in the past, been offset by significant disadvantages compared with wrought products.

Castings are considered to be less ductile than the equivalent wrought product, and they have a less consistent performance in fatigue: and inferior integrity. The difference in ductility may be more apparent than real. A forged or rolled component may have a higher ductility than a casting in the direction of forging or rolling but a significantly lower transverse ductility. This is a distinct advantage if the longitudinal direction has to resist the principal stress but it is not necessarily a sign of inferiority of the casting. However, some of the problems concerned with brittleness,
lack of integrity and an inferior and less consistent performance under fatigue loading stem from fundamental difficulties in the: casting process, These problems will be highlighted and analysed and the way in which modern developments ameliorate and eliminate them will be indicated in the accounts of individual techniques.

Problems inherent in casting technology

The problems inherent in casting technology may be classified under the headings ‘Shrinkage’, ‘Gas’ (originating from metal and mould), ‘Coarse structure and segregation’, ‘Inclusions’ (originating from metal and mould) and such defects as ‘Cold shuts’ where metal streams meet but do not unite, or dimensional errors which arise from movement of cores.

Shrinkage and contraction

Metals (with the exception of some alloys of antimony, tin and bismuth) contract in volume when they solidify and continue to contract as they cool to room temperature. If, therefore, a substantial body of metal is located alongside thinner sections which solidify earlier so that it cannot be fed from them that body will shrink by forming external sinks, internal shrinkage cavities or possibly interdendritic voids which may communicate through the walls. The influence of shrinka e cavities on the UTS of A357 alloy is shown in Figure 16.10’2!4 If the body is constrained excessively by the mould or by the rest of the casting while its metal is in a hot short condition it will crack.
(An alloy is ‘hot short’ when it comprises solid islands or dendrites surrounded by thin bands of molten metal and
therefore has no strength or ductility.) If it has reached a temperature at which it is ductile, it will distort and perhaps
fail to clean up during subsequent machining. Design of components to be cast should therefore, wherever
possible, avoid the introduction of isolated relatively heavy sections. Where it has not proved possible to achieve this,
casting design should ensure either that such sections are heavily chilled and the flow of metal arranged so that they
solidify at the same time as the rest of the casting, or that  adequate arrangements are made to feed them by liquid metal channelled to them for this purpose. Interdendritic shrinkage cavities may be present even in cast material which nay be considered to have been fed satisfactorily.

Cores should, as far as possible: be made weak enough to give against the contraction stresses imposed by the cast metal and should be removed from the casting as S Q Q ~as possible after the metal has solidified.

Gas

The effect of gas varies with the metal cast. Hydrogen dissolved in molten aluminium may generate cavities of its
own but generally increases the size of shrinkage cavities. In  steel, hydrogen causes a number of very dangerous effects described in Chapter 7, Section 7.3 (Corrosion). The requirement for good-quality castings demands its complete removal from both metals. Fortunately, hydrogen is reasonably easy to remove from molten metal, in the case of aluminium by bubbling gas through the melt before casting. Figure 16.103

shows the arrangement of a spinning rotor argon lance system which, it is claimed, will approximately halve the quantity of
hydrogen in 250 kg of molten aluminium in 10 min. Hydrogen  is removed from steel by vacuum treatment of the metal in the
molten state (see Chapter 7, Section 7.3, Ferrous metals). Oxygen is removed from molten steel by ‘killing’ with
silicon andor aluminium. In the case of ingot casting of unkilled or ‘rimming’ steel advantage is taken of the effect of
the bubbles of oxygen evolved when this steel solidifies to neutralize the solidification contraction and eliminate the pipe
on the top surface. It therefore becomes possible to forge and roll the whole of the ingot, and not go to the expense of
discarding the top 30%, as would have to be done with killed steel.

Coarse structure and segregation

Slow cooling of solidifying metal generates a columnar structure of coarse dendrites growing perpendicular to the cooled
surface which have (in an alloy with a margin between liquidus and solidus) a lower solute content than the melt. The space
between and in advance of the dendrites is therefore enriched in solute (in the case of steel this includes carbon and other
alloying agents) and the result is a coarse segregrated structure.

The result of variation of cooling rate on tensile properties of A357 alloy is illustrated in Table 16.1484 and the effect
of structural variations on fracture toughness in Figure 16.104.@ The cooling rate should be the maximum consistent
with good feeding and is compared diagrammatically for a selection of casting processes in Table 16.15.

Inclusions

Inclusions can originate from oxide formed on the metal surface from slag, flux, from the melting crucible or from the
mould and can have just as great an effect on metal properties as pores (see Figure 16.10584). Clearly, all possible precautions
must be taken during melting to provide as clean a metal as possible. In addition, weirs that trap inclusions smooth coherent flow (see Figure 16.106). Filters restrict metal flows, a filter 50 x 50 X 22 mm gives a flow rate of 6 kg s-’ of grey cast iron at 300 mm head. Impingement against the mould wall and consequent entrapment of mould material are avoided. Upward pouring, as in the low-pressure die casting and Cosworth processes, is a very good solution to this problem because it allows metal to be transferred upwards from below its surface without turbulence, thus minimizing inclusions. Once the casting has been formed the molten metal head is maintained until all parts of the casting have been fed, thus maintaining a reservoir of hot metal as long as required. With top-pouring methods, on the other hand, the top of the feeders may solidify as soon as the casting.

Cold shuts

‘Cold shuts’, .i.e. locations where two metal streams meet without uniting, must be avoided by good casting design.

Core movement
Cores are normally of lower density than the surrounding metal and may float when submerged or be displaced by the
metal flow. Core prints must be accurate in dimensions and adequate in size. Where necessary, core prints may be provided
with steel bushes which fit in corresponding bushes in the mould.

General
The problems described above have been long appreciated and techniques are being developed which will completely
overcome them. These techniques are not necessarily applicable to all metals or to every size and shape of casting, but
where they can be applied they will produce castings with consistency in fatigue behaviour adequate for any requirement.
The following sections describe the techniques used to produce castings and stock for mechanical working with
particular emphasis on the most recent developments and most advanced processes which provide the highest quality
products in the most efficient way. Not every difficulty has been eliminated with every metal and alloy cast, but this
account will indicate how castings which will perform fully as well as forgings (and, in some cases, better) can be produced
in an increasing proportion of materials.

It is not possible in an account of this length to detail the very large number of variations in casting process which are
and have been employed. Where a selection has had to be made the process or variation described is, as far as possible,
the most efficient, the most modern, and produces a product

of the highest material quality, mechanical properties, dimensional accuracy and surface finish.

Ingot, billet and slab casting

Conventional casting of steel

For many years stock for mechanical working was made by top pouring individual ingots or billets typical of which are the
square steel ingots shown in Figure 16.107.85 During this period great advances have been made in metal treating and
handling techniques. Cleaner steelmaking technology, better deoxidation control and secondary steel making in the ladle
and the arc furnace have been introduced and furnaces have been designed (for steel and also for aluminium) which yield
metal free from both surface and bottom oxides and slag. Hydrogen can be removed from molten steel by vacuum
treatment, and from molten aluminium as described above.New types of refractory stoppers, both vertical and sliding,

have made it possible to start, stop and control the rate of flow of the casting stream.
However good the metal quality, however carefully the dimensions, shape and casting parameters of the ingot were
chosen, and however well the ingot was fed (and however effective the measures taken to keep the top of the ingot hot)
ingots formed from killed steel developed a pipe (which sometimes bridged) in which was located considerable segregation
(see Figures 16.107 and 16.108@). There was also usually segregation in a band round the ingot where the
columnar crystals which advanced from the surface gave way to the equiaxed structure. Segregation also developed if cracks
appeared between the columnar crystals at the surface and material of a higher solute concentration bled out of the
cracks. (This formation of blebs enriched in impurity elements also occurred in aluminium ingots.) To avoid vertically orientated
surface cracks the perimeter section of large ingot moulds consists of a series of arcs intersecting at cusps.
In a top-poured ingot the metal has to fall to the base of the mould from a height at least equal to that of the top of the
mould. The resultant splashing caused surface defects which were so serious that stainless steel ingots, where surface
quality is paramount, used to be bottom poured in spite of the increase in pipe and segregation which resulted. The combined
effects of pipe, segregation surface defects and cracking of individually cast ingots resulted in a substantial proportion
of metal being discarded. The introduction of continuous (and semi-continuous) casting proved so effective in overcoming
these defects that this process is ra idly displacing individual casting in steel (see Figure 16.109 2 ) and other metals.

 Continuous casting of steel

Billets, blooms, slabs, thin slabs and strip may be continuously cast. Continuous casting eliminates longitudinal segregation
and for billet and bloom casting horizontal segregation may be reduced or eliminated by electromagnetic stimng of liquid
steel in the mould (M-EMS), in the strand (S-EMS) and/or in the final stage of solidification (F-EMS). (Centreline segregation
may, however, be a problem in slab where electromagnetic stirring is difficult to apply.) Defects in quality are
therefore restricted to surface defects, including cracking, possible internal cracks and inclusions derived from nonmetallic
particles carried in the molten metal. The practices and techniques devised to improve the efficiency of a continuous slab castin machine may be described with reference to Figure 16.110,' which shows a continuous slab casting machine with hot connection facility. Reoxidation of the steel stream from ladle to tundish and from tundish to mould is prevented by using a ladle shroud, a
submerged entry nozzle, and by flooding with argon. Flow from the tundish to the mould is controlled b replaceable
stoppers. The tundish design (Figure 16.1118;) has been improved to give longer residence time, to incorporate weirs
and dams and to allow bubbling of inert gas through porous plugs located at the bottom to improve flotation and removal
of inclusions. Tundish level control has been made automatic and the tundish can be heated for start-up.
Flow control from ladle to tundish and from tundish to mould has been integrated and automated, and level in the
mould controlled by a duplex electromagnetidradioactive system. The metal in the mould is protected and the metal
mould interface lubricated by a 50 mm layer of 'black powder'. 88 This is the name given to a series of proprietary fluxes
probably containing calcium silicate and fluoride and more than one variety of carbon. The geometry of the particles and

the distribution of their constituents is more critical to performance than their composition. Hollow sphere granules with
carbon concentrated on the exterior maintain good insuiation until the granules melt and then the flux lubricates the
mouldktrand interface.

Figure 16.112 shows a flux feeder with automatic gravity feeding. The mould is oscillated vertically to prevent the strand shell sticking to the copper mould and causing breakout.

Oscillation is controlled hydraulical!y to give a downstroke synchronized with the strand's downward velocity and
an up-stroke giving low 'negative strip' time, thus saving powder consumption and improving lubrication. This gives
good surface quality and requires low maintenance. The width of the mould can be changed remotely during slab casting

Figure 16.112 ‘Dapsol’ automatic flux feeder for continuous caster showing distribution of flux

without impairing surface quality so that a whole variety of product widths may be cast in one stream. Further operational
flexibility is obtained by ‘link casting’, which allows small batches of differing steel grades to be cast without restranding.
When a ladle of one grade has been cast a fabricated metal link is partially immersed in the liquid steel and the exposed
part becomes enveloped by the steel of another grade from the following ladle.

Additional flexibility is provided by multi-strand casting. Either dividers are fixed in a single-slab mould or individual
slab moulds are mounted within one slab machine. The first stage of solidification takes place in the water-cooled copper
mould. Air-mist cooling, in which compressed air has been used to atomize spray water to fine droplets, is becoming
increasingly adopted, first for slab and more recently for bloom casting. This has the advantage of greater uniformity,
higher heat transfer coefficient, less risk of blockage because of the larger outlet orifice, much higher turndown ratio and
greater facility for automatic control than water only.

The principles of computer software for dynamic spray control of secondary cooling devised by one manufacturer are
shown diagrammatically in Figure 16.113.87 The same manufacturer has developed a system of quality assurance in real
time that assigns quality values to segments of the strand during casting and enables selection of those slabs suitable for
direct rolling from those which have to be cooled for inspection and rectification.
In a machine such as that illustrated in Figure 16.110 slabs are bent, straightened and soft reduced while metal in the

interior of the strand is still molten. Complex stresses introduced in these processes may result in problems associated
with centreline and spot segregation caused by solute enrichment in certain steels for ‘offshore-structural’ and ‘resistance
to hydrogen-induced cracking’ plate. These problems are overcome by ‘continuous straightening’, which applies a constant
bendiing moment over the straightening zone, and special roll design and optimized cooling during soft reduction. 89
There are two further developments. Direct rolling eliminates a reheating furnace, saves energy, reduces scaling and
therefore increases yield and, if it can be automated, may save labour. On the other hand, the rolling mill must be sited near
the caster, the slab must be transferred rapidly, edge heating is required and scheduling of production is highly complex.
Direct rolling is already practisedg0 in certain plants but the compromise procedure, hot linking, in which the slab is cut
hot into appropriate lengths, insulated and reheated for rolling before it has fully cooled, is more widespread and provides
moderate :savings in energy and cost.

Low-head casting will enable a continuous casting machine to be sited in an old ingot casting shop, the low ferrostatic
head will reduce bulging strains which lead to internal defects and the smaller machine size will reduce weight and therefore
cost.

Near-net shape (15-25 mm thick) and final thickness (1-3 mm tlhick) strip casting have been the subject of considerable
study and processes which may include moving-belt moulds and moving casting belts may be realized in the
medium oir long term.

{Continuous cmting of aluminium When continuous casting of aluminium alloys was first introduced
it provided a very significant improvement in soundness and fineness of structure compared with the ‘can cast’ method
of ingot production, which was the best previously available

.
A very Iaxge number off systems - some vertical, some horizontal, some utilizing fixed moulds while some depend on
casting wheels, bands, segmented moulds or rolls - are summarized in Table 16.16.91 Table 115.17’~ places the systems in approximate order of cooling rate and the fineness of structure of the material they

produce. The twin-roll and stationary mould strip casters will produce 3-7 m dendritic structures in 12.5 mm thick strip.

Continuous casting of other metals
Gold, silver with palladium, copper, bronzes, brasses, zirconium copper, chromium copper, platinum, platinum rhodium
nickel, solders, cadmium and cobalt are continuously cast in the following size ranges:
Solid rod 2-300 mm dia.
Strip 1.6 mm thick and up to 1000 m wide
Pipe, bore 30 mm (min), OD 350 mm (max), wall 12 mm min
Throughputs 1000-2800 kg h-’ (dependent on diameter) ean
be achieved.
The casting techniques used are those introduced by Propergi, Haslett, Southwire and Ontokampi. Typical examples of
plant are shown in Figure 16.114, which illustrates a Technics Guss or Wertli systems caster for strip 2 cm thick and 60 cm
wide, and Figure 16.115, a Rautomead RT650 caster for small-diameter copper and alloy rod. Both are horizontal

casters and both utilize a graphite tundish or crucible from a hole in the bottom or side of which the molten alloy flows into
a high-demsity polished graphite mould. A protective atmosphere, usually nitrogen, may be maintained around both.
Heat is extracted from the mould by a water-cooled copper cooler an(d the rod or strip is withdrawn intermittently. The
key to success is precise control of molten metal temperature, die cooling and withdrawal cycle. Although, unlike in continuous
casting of steel, the die is stationary instead of oscillatory, movement of the strand relative to the die is similar,
consisting of a rapid pull alternating with a relatively long hold. In the case of the strip caster a pull of 1 cm in 1-2 s
alternates with a hold of 2 4 s and overall strip production is 10-15 cm per minute.

Higher casting speeds are possible with the quick withdrawals which can be obtained with a pneumatic-activated
pulsed movement via a ‘sprag clutch’ to feed rolls (see Fi ure is no? possible in an account of this len th to discuss cooled
strand relationships but Figure 16.11892f’3g ives some idea of the variations in cooling rate. Secondary cooling after the
mould may be used to avoid solid-state reactions such as precipitation in alloys where this may occur.
16.11692)a nd a supercooler heat sink (see Figure 16.1179 4).

Semi-continuous casting

Semi-continuous casting processes (‘vacuum arc remelting’ and ‘electroslag remelting’) are employed to produce very
high quality stock for mechanical working. In both, one or more cast or forged billets are made into one electrode (where
three parallel electrodes are employed three-phase current may be used). The electrode(s) are fed downwards and as they
melt, the metal melted by the arc (or by the heat generated by the resistance of a bath of slag) is transferred downwards to
the other electrode which is located in a cylindrical watercooled copper vessel.

In vaciium arc remelting (see Figure 16.119) the arc is contained in a vacuum and the consumable electrode is fed
downwards through a seal which retains the vacuum. In electroslag remelting (see Figure 16.120) a bath of fused slag is
retained above the other electrode in the copper vessel. In both cases, after the start continuous casting and cooling and
feeding conditions exist in the bath of metal which is formed, and climbs continuously up inside the vessel. The resulting
billet which is fed and rapidly cooled during solidification is the best stock that can be produced for mechanical working.
Electroslag remelting refines the metal by dissolving such non-metallics as alumina in the flux. Vacuum arc remelting
breaks and disperses non-metallic inclusions but does not remove them completely. It does, however, completely remove
gaseous impurities such as hydrogen and oxygen from steel.

In general, electroslag remelting is used for steel and nickel and cobalt alloys, vacuum arc remelting for refractory metals,
titanium, zirconium, tantalum and molybdenum, but it is also used for steel and nickel alloys.

Sand casting
In sand casting, of which there are many variants, molten metal is poured into a sand ‘mould’ which defines the exterior
shape (including any re-entrant surfaces) of the component. ‘Cores’ also formed of sand define the interior.
The components of the mould of which there are usually two, the ‘cope’ and the ‘drag’, are moulded in sand around
‘patterns’ and the cores are made in ‘core boxes’. Very simple examples of patterns and core box which might be used for
hand moulding of the pipe connection (Figure 16.121(a)) are shown in Figures 16.121(b) and (c). Usually each section of
the pattern is mounted on a ‘moulding board’ as shown in Figure 16.122(a). In the moulding board are drilled two holes
which register with corresponding holes in the other pattern section assembly and with ‘moulding flasks’ also shown in
Figure 16.122(a). Alignment of the moulding board and pattern is maintained with the flask by means of ‘moulding
pins’ which fit the holes accurately. The flask is filled with moulding sand, which is compacted and levelled off with the
top of the flask, which is then turned over to form the ‘drag’ or bottom section of the mould. The pattern is iifted out vertically
and the core, which has been made of sand blown into the core box and hardened, is placed in the mould supported
in ‘core prints’ which are formed by the extensions to the pattern shown in Figure 16.121(b).
The ‘cope’, the other section of the mould which has been prepared in the same way, is placed on top of the drag and
aligned by means of moulding pins. Molten metal is introduced into the mould by means of channels (described later).
After the metal has solidified, the casting is removed from the mould, shaken and brushed to remove sand (which if it
should have stuck to the casting must be scraped or ground off) and runners, risers and feeders appended to it for the
purposes of casting .fettled’ off. The procedure described imposes certain geometrical constraints on the shape which
can be cast.

Geometrical considerations
The pattern must be capable of being withdrawn vertically from the mould. Each nominally vertical surface must therefore
be given a draft to facilitate this withdrawal and, if a two-part mould only is to be used, no pattern can contain an
overhang.

When, as is the case with the three-arm flanged connection shown in Figure 16.123, the design provides for an overhanging
flange, whichever way the component is moulded one of the flanges will prevent Withdrawal of the pattern. The problem
may be overcome by blocking off the overhanging part of the pattern with a ‘core print’ (see Figure 16.124(a)) and
making a core which forms the shape blocked off and itself fits into this core print.

Although simple and neat, this is not a perfect solution.
Some draft for which permission must be obtained from the designer must be allowed on the vertical arm of the connection
and, as the position stands, with a flat top to the core the top flange will form a sharp angle with the pipe. Such a sharp edge
will constitute a stress concentration in operation but, more seriously, may cause cracking or shrinkage cavities as the
casting solidifies.

There are other ways of overcoming overhangs including:
1. The use of a three-part pattern and mould.
2. The use of a cover core (Figure 16.125(b)). These methods
are to be avoided if possible because they are
difficult to mechanize.
3. The incorporation into the pattern of segmented loose
pieces (Figure 16.125(c)).

This method introduces a significant amount of hand work into what should be a simple mechanized process. Also, sand
compaction under the overhang may be inadequate and the act of removing segments may damage the mould. A much
more satisfactory procedure is to modify the design of the casting to eliminate overhangs. The pipe connection in Figure
16.123 might have been designed as a T to align all the flanges in one plane. Some other examples94 are &own in Figures
16.126 and 16.127.

The need to provide for pattern withdrawal is not the only constraint in casting design. Cores are subjected to flow and
buoyancy forces imposed by the molten metal, and the design of the casting must be such that it is possible to support them.
Figure 16.128 gives examples of designs which respectively make it difficult and easy to provide such support. Design (b)
also makes it easier to provide for the escape of gas evolved during casting.

Running and feeding
In conventional sand casting metal is poured into a ‘pouring basin’ and enters the casting via a ‘sprue’ or ‘downgate’, a
runner bar and a number of ‘ingates’ (see Figure 16.129). The objective is to introduce metal as smoothly and as free from
turbulence as possible, so that it fills up and then solidifies uniformly and simultaneously upwards from the bottom of the
casting over as wide an area as possible thus minimizing differential contractions. It solidifies last in the feeding heads
which therefore contain all the shrinkage cavities. The ‘pouring basin’ helps to equalize the flow into the downgate and, it
is hoped, traps slag or dross.

The ‘sprue’ or ‘downgate’ is a component of the runner system necessary to connect the pouring basin (which must be
at the top of the cope) to the runner bar (which must usually be at the level of the parting line). It may be tapered from the
top to the bottom to allow for the effect of gravity increasing

the rate of flow downwards. Pouring basin and sprue are milled into the mould after moulding is completed.
The ‘runner bar’ is usually moulded into the face of the cope and the drag, and should extend almost all the way round the
casting. It is both larger in section than the sprue (to promote a smooth flow) and the total section of the ingates (to ensure
as far as possible that metal flows through them all at the same time). Filters, weirs and spinners are provided for preference
between the sprue and the start of the runner bar to trap slag and dross. The correct size and position of the ingates is vital
to the success of the casting and may vary according to the metal cast A recommended design for magnesium is shown in
Figure 16.13094. This metal has a low heat capacity and solidifies quickly. For this reason, the rule that the crosssection
of the ingates is less than that of the runner does not hold for this metal. Filling must be rapid enough to ensure that
the casting fills while the metal is molten without resorting to unreasonably high temperature.
Ideally, for all materials (with the exception of a few alloys, which include tin bronze) the aim is to get the whole of the
casting bo fill with metal while it is liquid. It will then solidify progressively from the thin sections remote from the feeders
and the metal will remain liquid last in the feeder heads at the top. This can be achieved best by placing the ingates into or next
to the base of the feeder heads (as in Figure 16.129). If a flange has been placed in between a feeder and an intermediate
section its roots should be chamfered. Otherwise a well of liquid metal heated from the other side of the flange will
persist until the metal round it has solidified to form a shrinkage cavity. Feeder heads should be larger than the
sections they have to feed, and if they are internal to the mould they should be equipped with exothermic devices to
keep them hot.

The majority of castings are poured from small solidification range alloys which tend to form skins of solid metal and
shrinkage cavities which have to be ‘chased’ into the feeders. Those alloys which have large solidification ranges form
dispersed porosity. This must be overcome by modifying the temperature gradient by control of the casting temperature, by
gating and by the use of chills to produce directional solidification. However, with some alloys (including tin bronzes) the skin
of the casting solidifies with such a steep temperature gradient that it is free from shrinkage, and the microporosity goes to
the centre of the section and canno: be chased into a feeder. In this case the foundry worker avoids directional solidification
which might concentrate porosity in one part of the casting and encourages an even rate of solidification throughout the
casting. In all other cases the design engineer should do his or her best to assist the foundry worker to achieve directional
solidification. If possible, section thickness should increase smoothly to some point (probably a flange) which is conveniently
fed. Thick sections, isolated in the centre of thinner sections, will cause difficulties. Sharp corners and re-entrant
angles will cause shrinkage, puncturing and cracks, and this is accentuated by variation in section.
The way in which various types of junction give rise to increases in mass is illustrated in Figure 16.131,95 and the way
in which they can be lightened and made more flexible by staggering is shown in Figure 16.132.95 The development of the type of filter shown in Figure 16.106 has raised the possibility that the entire gating system may be dispensed with. It is claimed that if a ceramic foam system is placed in a convenient riser as close as possible to the mould cavity, gravity, die and sand castings in aluminium, magnesium and some copper alloys may be direct poured to produce higherquality casting and save substantial quantities of metal. Figure 16.133 compares an aluminium alloy manifold poured through a conventional running system and through a 'Dypur' filter unit.

Modern sand handling and moulding procedures The traditional mould material for sand casting has been silica
sand possibly with a natural binder - new for the facing sand, and recycled to form the backing sand. This is not a wellcontrolled
material and optimum moulding and core-making performance requires precise control of grain size, binder and,
where appropriate, water content. Sand reclamation The precision material required by a modern
foundry requires a reclamation plant that will return pure, dry, uncoated, dust-free particles for mixing and moulding.
There are two basic processes: one attrites the sand and burns off residual binders; the other relies on dynamic im-

pulses, grain against grain, that rub off binder coatings and remove them as dust (see Figure 16.134). In either case, 1% of
sand only is lost. The small usage of sand allows an alternative to silica sand to be used. Zircon sand with 99% reclamation gives lower cost than silica sand used conventionally. The heat capacity of the zircon sand produces a finer cast structure giving solidification rates in aluminium equivalent to those in permanent moulds. Zircon sand is more satisfactory for service with higher
melting point materials. The thermal expansion of zircon sand is (see Figure 16.135) smaller and more consistent than that of
silica, and the lower tolerances which result promote savings in weight of casting and finished component. Metred amounts
of various kinds of binder (see below) are added. Moulding The processes used for sand moulding include
squeeze, jolt, jolt squeeze and sand sling. A method which is claimed to give the most uniform and accurate compaction is
the air impact process (see Figure 16.136). This process takes a metered quantity of sand to which bentonite clay has been
added.

Modern core-making processes Core-making differs from moulding because the core must be transferred into the
mould, and must therefore be strong enough to be free standing while this process is carried out. (Typical cores are
shown in Figure 16.137.)

Originally, cores were of silica sand bonded with molasses and linseed oil with bentonite to provide green strength for
handling. Cores were packed in sand and baked to develop dry strength and to remove gases which would be evolved in
heating.

The most modern development is to use a small precise addition of resin binder to recovered sand. Figure 16.138
illustrates a typical plant arrangement. A metered quantity of mixed sand is blown into a core box, the two halves of which
are clamped together hydraulically. After filling, the binder is cured either by heat from a hot core box or by introducing an

amine gas accelerator. Both processes are rapid. Up to 80 cores an hour can be made by the cold process on a machine of
the type illustrated and the tolerances of the finished castings are the most precise that can be achieved by sand casting (see
below). Cores may be located into the moulds either by a steel pin and bush system or by engineered sand to sand locations.
Dressings for mould and core The size of the grains of a moulding sand is a compromise. A large grain sue is desirable
so that gases evolved near the mould surface are able to diffuse away and not cause blowholes in the casting. A small
grain size is required to produce a good surface finish. Where these objectives are incompatible a dressing must be applied to
the surface of the mould or core. This can consist essentially of a finer-grain version of the moulding sand but, where a high
melting point metal is to be cast, the use of a highly refractory base such as zircon flour has the great advantage of preventing
'bum-on' of the mould onto the casting.

There are a number of proprietary dressings, mostly consisting of suspensions of refractory powders in liquid, which can

be sprayed or brushed onto the mould. It is essential to ensure that the mould is dried after the dressing has been applied.
A recently developed process which is claimed to give satisfactory results projects a dry powder based on zircon sand
towards the mould surface by means of a gun. This charges the particles electrically by contact with its barrel and thereby
ensures a uniform adherent coating by electrostatic charge.

Specific sand casting processes
Green sand casting Green sand casting uses a clay bonded sand as described above for the mould, with (where required)
a dry sand core. The process is very versatile in the range of materials cast, size and shape of castings, and quantities
required. It is also economical and production can start as soon as a pattern becomes available. The sand must be a
compromise between the best possible finish, which requires a fine grade, and the possibility of gas evolved from the mould
causing blowholes, which requires a relatively coarse grade (unless a mould dressing, which requires an additional, probably
manual process, is used) - see above.

Core assembly processes The core assembly process is similar to green sand but the mould consists of one or more cores
made as described above. The ‘mould’ is stronger and tolerates

tolerates long, thin projections. There is less risk of gas evolution which may cause blowholes than from a green sand
mould. However, a number of cores cost more to produce and assemble than a mould which consists only of a cope and drag
and more parting line flash may result.

Cold set sand mould With the introduction of cold catalyst hardening resins the distinction between mould and core has
vanished (except in the actual geometry of production). The tolerances which can be worked to are the smallest of any sand
casting process. For aluminium cast in zircon sand tolerances are as low as
+0.15 mm up to 100 mm
+0.20 mm up to 300 mm
+0.25 mm up to 800 mm
+0.10 mm across each core joint.
These close tolerances permit much greater precision in design and make possible substantial savings in weight of finished
component, in weight of metal cast and also in cost of finishing processes.

The Cosworth Process% The Cosworth Process, developed for aluminium but equally applicable to magnesium and zinc
alloys, takes advantage of the close tolerances and high cooling rate conferred by zircon sand and cold setting resin
mould and core systems. It uses them in conjunction with an upward pouring system (see Figure 16.139).
Molten metal is held in a large holding furnace which smooths out variations in temperature and analysis. Metal is
degassed by bubbling through inert gas at the ingot charging station, and the metal is held under inert gas. Oxide particles
and inclusions are separated by floating or sinking. The mould is filled from below by means of an electromagnetic pump.
Metal enters the casting smoothly and without turbulence, and the pressure head is maintained during feeding. When feeding
is complete, the pressure head is removed and the liquid metal, in what in a conventional casting would be the feeder
head, runs back into the bath. This results in a greatly improved metal yield. To summarize, the advantages claimed for the Cosworth Process compared with the conventional green sand casting process are:

A finer metal structure with tensile strength improved by 30% and elongation almost doubled. Improved integrity, pressure-tightness and freedom from porosity inclusions and casting defects. Increased dimensional accuracy which results in saving
metal, component weight and machining time.

Additional saving in metal melted because metal in runneirs and feeders is saved. Premium-quality castings It has been claimed that metal quality in aluminium castings may be further improved by varying thie composition of the material of the mould (so that
very high cooling rates are obtained locally) and by optimizing gating and feeding. By these means, castings have been
produced with properties equivalent to those of forgings, but they are very expensive.

 
Other sand casting processes Green sand and core assembly processes produce the vast bulk of sand castings but there are
a number of other processes which require mention because they have been, and in some cases still are, extensively used.
They are:

Dry sand moulding: This process starts with a green sand mould which is baked in a stove (or by blowtorches) until
it becomes hard. It was used in the past for moulds for very large castings in which the metal pressure might wash
away green sand mould material. Drying also makes sure that moisture does not cause blowholes. Dry sand moulding
has almost been superseded by the development of high-pressure moulding which confers the requisite
strength on green sand.
Loam moulding: Loam moulding uses very moist green sand which, instead of being formed on a pattern which is
relatively expensive and time consuming to manufacture. is forimed to a profile against a ‘strickle’ which is rotated
on a spindle. The mould is later dried to remove excess moisture.

Randupson process The Randupson process uses a mixture of sand with cement and water for larger sized castings. The
moulds take about 24 hours to harden and the material can be crushed and re-used. However, this causes a substantial dust
hazard.

Sodium silicate process This is a cold-hardening process which bonds the sand with sodium silicate that is hardened by
passing CO2 through the mould. The process is used for both moulds and coires and has been developed by adding hardening
agents such as glyceryl acetate, cement, ferra-silicon and calcium silicate to the sand immediately before moulding. The
process gives higher strength than green sand without the need for storin,g. Tolerances are better than for green sand, production
rate is high and materials are cheap. Steel castings may tend to burn-on, cores are difficult to knock out and there is a
problem ,with dust.

Fluid and castable sand process Sand with sodium silicate and a self-setting agent is suspended in a detergent foam
poured around the pattern and allowed to set. No ramming or drying is necessary because of the low water content of the
sand, but the process can only be used for simple shaped castings. The mould needs dressing to produce a satisfactory
finish and a licence is required to operate the process.

Expendable pattern processes

A number of casting processes derive the shape of the finished casting not from a re-usable pattern but from a material which is removed by the action of heat before or during casting. This eliminates the limitations which arise from the need to withdraw
a pattern. The mould (or shell, see below) can be made in one piece so that drafts can be eliminated (unless they are
needed to make the pattern which may, however, be flexible). Parting-line mismatch and flash are eliminated, and undercuts
are easier to deal with.

On the other hand, the operation of making the pattern occurs once for every casting instead of once per component
design. This may be satisfactory for prototypes, or one-off castings, for which the expendable pattern may be cheap and
easy to produce, and for large runs where the expendable pattern is mass produced. The additional operation may prove
costly for intermediale quantities.

The full-mould expendable-pattern process This uses lightweight, foamed plastic patterns moulded or machined and, if
required, assembled by cementing sections together. The pattern, incorporating runners and risers, is surrounded by
any form of cold setting sand, or by dry unbonded sand, and is removed as a vapour by the heat of the incoming metal. This
forms an interface with the evaporating plastic and prevents collapse of the mould even though the sand is unbonded.

Shell-moulding techniques
These utilize, instead of a solid mould, a shell a few millimetres thick. This may be either one-piece formed around an
expendable pattern, or two-piece formed around a solid pattern. In the ‘Croning’ process sand coated with a thermosetting
resin is projected onto a metal pattern heated to 250°C. After removal from the pattern, the halves of the shell are fixed
together, possibly backed with sand or steel shot, and the metal poured.

In the ‘Shaw‘ process the pattern is dip-coated with a mixture of a fine refractory such as sillimanite and ethyl
silicate. The bond produced by this mixture is rubbery when green, which makes stripping from the pattern easy. The
mould is flash fired to remove alcohol, fired at 8OC-1000°C and poured.

Both these processes give good reproduction of details and exceptional accuracy of dimensions, so finishing operations
are minimized. Patterns are, however, expensive; size of part is limited; and large runs in the order of 500 castings are
needed to make them economic.

 Casting processes incorporating both shell

moulding and expendable patterns ‘Investment’, ‘precision’ or ‘lost wax’ c as t i n g ~ ~L~os~t ’w ax
casting is employed to produce components to any conceivable shape in nickel, copper, titanium and aluminium alloys, steel
and precious metals. It is not limited by the need to withdraw a pattern. There are no mould parting lines and very fine detail
may be reproduced. The surface smoothness of the casting is 1-3 pm rms and dimensioned tolerances as low as IS0 10 IT.
For a casting process lost wax is expensive but the cost of a finished component (if indeed it can be made in any other
way) is competitive.

The manufacture of weapons, tools and ornaments by lost wax casting continued, with only minor changes, for almost
5000 years up to the introduction, in 1932 by Austenal Laboratories Inc., of hydrolysed ethyl silicate binder for
quartz dental moulds. Development was intensified early in the Second World War by the requirements for blades first for
aircraft turbosuperchargers and later for aircraft turbines. The high creep strength alloys are difficult to forge and machine and cast blades with a structure of equiaxed crystals, columnar crystals with a preferred orientation, or one single crystal9'
have much superior creep resistance to forged blades. The investment casting process employs a model or pattern
of the object to be cast in wax (or an alternative material such as frozen mercury, a low melting point metal or a polymer)
which can easily be removed from the mould which is formed round it. The wax pattern(s) may be made in robot moulding
machines with up to 100 tonnes clamping pressure and assembled singly or in clusters with casting gates andor
runners and vents. They are next dipped repeatedly in coating materials consisting of refractory slurry and liquid binder.
There has been several variants of coating materials which use either alkaline silica sol or acidic alcoholic hydrolysed
ethyl silicate air dried and ammonia set binders. In some processes these two methods are used for alternate coats. At
the time of writing, however, the rapid ammonia set process has been most widely adapted. The filler is fine silica for the
primary coat. Subsequent coatings have a higher ratio of liquid binder to solid refractory and the gradings become progressively
coarser.
Moulds, typically 8 mm thick, are dried at room temperature and then plunged into furnace chambers at 1050°C. This
melts the surface wax before the main body has time to expand  and, after dewaxing, cures the ceramic mould.
Some aircraft turbine blades require to be cast hollow to permit the passage of cooling air. These are made by forming
the wax round a core of strong ceramic the same shape as the cavity required in the cast component. The completed shell
mould may be inserted free-standing in the casting furnace or may be supported by dry granular refractory.
The actual casting may be carried out in normal atmosphere, in vacuo, or in a protective atmosphere. The pouring
and cooling can be varied according to the material structure required9*. 99 (see Chapter 7, Section 7.4). Centrifugal
assistance (see Section 16.5.6.3), or mould heating, may be incorporated to give improved casting detail. The ceramic core
of a hollow casting is removed by chemical leaching. In the Middle Ages the lost wax process was used to cast
equestrian statues greater than life size, but the great bulk of modem castings are measured in inches. However, integral
turbine wheels and vane rings are now cast in one piece and large structural aerospace components can be made in titanium
or nickel base alloys.

The 'Replicast 20, 21' ceramic shell process'O0 The Replicast process resembles the full mould expendable pattern process
in using an expanded polystyrene replica of the required article; and the precision casting process in using a ceramic
shell. It has the flexibility of the lost wax process (including absence of parting lines, draft or tapers) but is claimed to be
more suitable for larger castings because the patterns are light in weight, cheaper and less liable to distortion and fracture as
size increases. In addition, the ceramic shells may be produced thinner (4-6 mm) and therefore cheaper because they are
subjected to a much lower stress during removal of the polystyrene than is generated by the removal of wax. The
shells are positioned on a box supported by loose sand, vibrated to fill all cavities, and then held under vacuum during
casting. It is claimed that a casting can be made within 5 hours of the replica being removed from the die.
Dimensional tolerances and surface finish claimed for the Replicast process compared with other casting processes are
illustrated in Figures 16.140 and 16.141. It is claimed that the reduction in casting wall section and the casting-in of the bolt
holes of the casting shown in Figure 16.142 gave a weight saving in the castings delivered of 23%. Other benefits which
derive from the Replicast process as compared with casting in a green sand mould are:

1. Improved positioning of feeder heads is possible.
2. The chilling effect of the ceramic enhances feeding and is
claimed to eliminate hot tears.
3. The strength and integrity of the ceramic are claimed to
eliminate inclusion defects and gas evolution.

The major advantage claimed is, however, the reduction in fettling ,and machining compared with green sand casting. The
limitations and drawbacks are problems in making parts in excess of 250 kg, high labour costs at front-end, higher die
costs and a longer lead time for manufacture of new parts. As a consequence, Replicast is best suited to components of fairly
complex geometry with stringent acceptance criteria and with a high machining content destined for high-duty applications.
The Steel Castings Research and Trade Association (SCRATA) operate a demonstration foundry.

Permanent mould or ‘die’ casting

Die casting comprises the transfer of molten metal to a die which is a metal block, or assembly of blocks, containing a
machined cavity which is the shape of the required casting together with a runner and feeder(s). There may be, inside the
cavity, one or more metal or sand cores. Die casting imposes a quicker rate of cooling on the metal as it solidifies than sand
casting and produces a finer structure with improved tensile and ductile properties. A dressing is usually applied to the die
runner ;and riser. This may be based on a refractory such as alumina, silica or zirconia where a decreased chill is required,
or graplhite where a high chill is beneficial. In either case a binder which may be based on sodium silicate is required. The
dimensional accuracy of die castings is generally superior to that of sand castings. This leads to savings in metal and
machining costs. Most metal alloys can be die cast, but the bulk of die castings are made in metals of medium melting
point, mainly alloys of aluminium, zinc and magnesium.

Gravity die casting
In gravity die casting molten metal is poured into a die, usually from a hand ladle. Moulds are usually two part split vertically
and locked together during casting. Figure 16.B431°1 shows one half of a typical die for casting magnesium alloy. The
pouring gate is located in the parting line of the die. The junctiori between the down-sprue and the gate may house
either at metal filter or a weir to trap inclusions. The die is filled from the runner and the metal flows in horizontally to
the metal in the die cavity to achieve directional solidification. Note that most metals are more fluid than magnesium alloys
and will normally require smaller section runners. The feasibility of direct-pouring gravity-lie castings down the feeder
using a filter unit such as is shown in Figure 16.106 is worth consideration.

Good mechanical properties significantly superior to those of sand castings can be achieved in well-designed and carefully
produced die castings in aluminium, magnesium and zinc. For examplte, the properties of ‘Y alloy’ are 275MPa UTS 1%
elongation as compared with 220 MPa UTS 0.5% elongation sand cast, and 400MPa 15% elongation wrought.
Gravity die castings may also be made in low melting point alloys, copper alloys and cast iron. The process is not normally
applied to steel.

Die castings can be made in weights from 150 g to 200 kg but the optimum is from 1 to 50 kg, section thickness from 3
to 50 mm, IS0 dimensional tolerances IT12-14, machining allowancz roughly 4 mm, and surface smoothness 2.5-25
pm r.m.s.

Low-pressure die casting bears the same relationship to gravity die castings as the Cosworth process does to conventional
sand casting. It also has the same advantages. It was developed initially to improve the quality of gravity die casting in
aluminium and zinc alloys, and it could be applied to many other metals. Figure 16.144 illustrates the principle of the
process.

The mould or die is sited above a bath of molten metal in a gas-tight chamber. A riser tube extends from well below the
surface of the metal to the base of the mould cavity. Metal is constrained to rise smoothly and without turbulence until it
completely fills the mould by using the pressure of the atmosphere above the melt, usually less than 69 kNm-’.
Solidification contraction is fed from the reservoir of molten metal under pressures in the rider tube. The pressure is
released when solidification is complete in the mould and the molten metal in the tube drains back into the furnace.

Feeding is greatly improved compared with gravity die casting because of the improved thermal pattern, and this
improves strength and pressure-tightness, The absence of turbulence and the metal source from below the surface
reduce the chance of inclusions. Yield is very high (9&95%) because only a small butt of metal requires to be trimmed from
the gate. This compares with 4G60% for gravity die casting. The process is very suitable for automation using, if required,
multi-cavity moulds. The disadvantage compared with gravity die casting is the higher capital cost of the equipment.

High-pressure die casting

Alloys of the majority of metals can be high-pressure die cast. In its best-known form this process is known simply as ’die
casting’ and is used extensively for alloys of zinc, aluminium and magnesium. It is equally applicable to the low melting
point metals tin, lead and cadmium. The process has recently been extended to enable castings
up to 3 kg in weight to be produced in carbon (up to 0.5%C) and stainless steels, copper and copper alloys, cobalt and
air-melting grades of nickel alloys. There are three variants of the process: high-pressure cold chamber die casting; highpressure
hot chamber die casting; and the GKN ‘Ferro Die’ process for casting steel and other relatively high melting point
metals.

There are two variants of cold chamber diecasting machine:

the horizontal and the vertical, the working principles of which are illustrated in Figures 16.145 and 16.146.”’ In both
variants, a metered quantity of metal is poured into the shot sleeve and injected into the die at 5-500 M Nm-2 pressure by a
piston. When the metal has solidified, the die is opened and the casting ejected. It is desirable in the case of the horizontal
machine that the piston should move in two stages; slowly at first to expel air, and quickly when the metal reaches the gate.

In hot chamber die casting (Figure 16.147) the shot tube is located below the surface of a reservoir of liquid metal and fills
automatically when the plunger is withdrawn. This greatly increases the rate of production, and, presumably, reduces
those defects caused by chill in the shot tube. The hot chamber machine is simple, easy to operate and
permits even higher production rates than the cold chamber machines. It operates at lower pressures and is particularly
suitable for thin-walled castings in magnesium alloy and the lower aluminium content zinc alloys. Aluminium and the
higher aluminium content zinc alloys attack the parts of the machine with which they come in contact so that they are not
really suitable for hot chamber high-pressure die casting. High-pressure die castings can be made in the weight range
0.1 g to 10 kg. They have an extremely good surface finish and a high dimensional accuracy (IS0 tolerance IT 11-14).
Smaller wall thicknesses may be cast (magnesium > 1.2 mm, aluminium > 0.7 mm, zinc > 0.2 mm) than by any other casting
process. Pressure die castings require very little machining and their machining allowance may be as low as 0.25 mm.
Labour costs are low, but die costs and die preparation times high. Dies last up to 100 000 shots before failing
(together with the shot tube and piston) by crazing. The process is only economic for large-scale production.
The most serious disadvantage of high-pressure die castings is the large amount of internal porosity. Castings have a
so-called ‘skin’ 0.1-0.2 mm thick, formed by the rapid freezing of the first metal injected into the die cavity. Beneath this skin
can be found spherical gas porosity, interdendritic porosity and ’flow line’ porosity (see Figure 16.148102) which occurs
between the skin and the central region of the casting. The origin of these defects lies in the shot chamber where
chilling of the metal can cause solidification to occur and large agglomerates of solid can cause blockages in the gate region.
A chart which may be used to determine the possible causes of

specific defects in magEesium high-pressure die casting is shown in Figure 16.149. lo2
Better properties are achievable by other casting routes and this, and the porosity, limit the useful application of die
castings to non-structural components. High-pressure aluminium ailoy die castings cannot be heat treated or anodized
because the porosity would cause blistering.

In general, high-pressure die castings are used in non-loadbearing situations such as zip fasteners, car door handles and
carburettor bodies. Computer printer and other rapidly moving parts are die cast in magnesium alloys.

The GKN ‘Ferro Die’ process

The GKN ‘Ferro Die’ process is carried out on what is essentially a horizontal cold chamber machine modified by
making the parts that come into contact with molten metai (including the die) of molybdenum. The ladle is replaced by a
vertical tube furnace heated by an induction coil. A weighed siag of solid metal is placed in the furnace. As soon as the slug
melts and fails into the shot tube it is injected into the die. The process is economic for the production of a minimum of 500
castings.,

Squeeze casting

Squeeze casting gives (pending the development of rheocasting and thixocasting) the most favourable properties obtainable obtainable by any method of casting. Squeeze cast products bear comparison in properties with forgings. It has so far been
applied to alloys of aluminium and magnesium to manufacture castings and also metal matrix composites.
In squeeze casting the metal being cast is solidified under pressure between metal dies. In ‘direct’ squeeze casting a
metered quantity of molten metal (which must be clean, free from gas and preferably filtered) is poured into the female die
casting or mould and the male die is driven into the metal at a controlled speed so that it forms the required shape in a
non-turbulent manner. Pressures between 50 and 150 MPa are maintained on the metal while it solidifies. The dies are coated
with a graphite coat 2nd heated. The process is illustrated graphically in Figure 16.150.’03 Direct squeeze casting has the following advantages over all casting processes described so far:

The resulting casting is completely free from gas and shrinkage porosity. The very high cooling rate which the applied pressure
facilitates favours control of microstructure by means of temperature control of mould and liquid metal. Very
fine-grain structures may be produced without the use of grain-refining additions. No feeders or risers e are required.
In the indirect squeeze casting process, metal is injected (as in pressure die casting) into the die cavity by a small-diameter
piston. One process arrangement is illustrated in Figure 16.151103, but other methods have been proposed. Unlike
pressure die casting, the pressure remains on throughout solidification, which takes place progressively from the part of
the casting away from the punch. Although it is more difficult to obtain such good properties as with the direct process, high
quality automobile parts have been produced. In both direct and indirect versions of squeeze casting there
are no internal or external defects which require NDT. Mechanical properties are as good (or sometimes better) than
those of forgings. The production cycle times for squeeze castings are less by as much as 66% of those for comparable
die castings because of the high heat transfer coefficients.

Many aluminium alloys can be produced wi?h very substantially improved properties by squeeze casting. Table 16.18’03
shows how the properties of 7010 alloy vary with squeezing pressure. The effect of grain size on the fracture toughness of
this alloy is shown in Figure 16.152.’03 Figure 16.153’03 shows how the consistency of the SN curve
of 70/mm grain size 7010 compares with that of plate material and also the way in which ?he fatigue strength varies with the
grain size.

Magnesium alloys can be fabricated by squeeze casting and Figure 16.154 gives a comparison between squeeze and other
casting processes in A91 alloy. The squeeze cast properties are better, but in this and in other cases squeeze casting does not
appear to provide an advantage comparable with that obtainable with aluminium alloys. More research is required before
the full potential of squeeze casting is realized.

Rheo-casting and thixo-casting

In both rheo-casting and 'thixo-casting' a slurry (prepared by stirring an alloy while it is in the semi-solid condition between
the solidus and liquidus) is cast. Dendrites which may have formed as the alloy is solidifying are broken up and the slurry
flows as a thixotropic liquid whose viscosity decreases as it flows but recovers when it is allowed to stand so that the
material stiffens.

The first slurries produced, containing up to 60% of particles, were formed by mechanical stirring when the allo had
partially solidified., More recently it has been shown”’ that induction electromagnetic stirring will produce the necessary
agitation. Slurries may also be prepared by a powder route in which elemental powders are mixed, cold pressed and then
heated to a temperature at which an alloy (created by diffusion between the powders) melts. Intermittent stirring may then
produce a structure in which spheroidal particles are contained within a liquid matrix.lo7 Slurries may also be obtained by
deformation, recrystallization and incipient melting and by gas atomizing a fine stream of metal in nitrogen or argon (as in the
Osprey process). Whichever process is found to be suitable for the specific alloy will, if successful, produce a fine equiaxed
structure which should lead to optimum properties in the resultant casting.

The casting processes are illustrated diagrammatically in Figure 16.155. In rheo-casting a metered quantity of the slurry
may be poured directly into the shot chamber of a die-casting machine or into a squeeze casting die. In thixo-casting the slug
is allowed to solidify as a billet which is cut up into unit charges. These are reheated to the inter-solidus/liquidus temperature
and either die or squeeze cast. (The billet can, of course, he forged instead of cast, but thixo-forging is outside
the scope of this section.)

These casting processes will normally lead to castings with properties significantly better than traditional casting techniques
(particularly if thixo-casting is combined with squeeze casting), but there may be additional advantages.
If the powder route is used, the temperature to which the alloy must be heated is lower than if it is completely melted.
This greatly eases the duties imposed on furnaces and refractories.
This proved to be a great advantage in the manufacture of dental castings in titanium alloys where the temperatures
required were about 1250°C as compared with 1650°C for complete melting. The properties of a series of alloys thixoand
conventional-cast are listed in Table 16.19.1°8 16.5.5.7 Casting metal matrix composites
Casting processes are among many which have been used for manufacturing metal matrix composites. Conventional casting
is used to manufacture particulate alumina and silicon and tungsten carbides in aluminium, and short-fibre and whisker
alumina, silicon carbide and carbon in aluminium. The volume fraction is limited to approximately 20% by the increase in
viscosity of the molten metal.

Squeeze casting is used for the Toyota car pistonlog as described in Chapter 7, Section 7.6 (Composites). The main
problem is the high pressure involved which limits the size and shape produced.

The liquid pressure forming (LPF) process may be used to produce composites with aluminium, magnesium and lead
alloys and the reinforcing materials listed in Table 16.20.’09 The process is based on low-pressure die casting and uses gas
pressure to force molten metal into a preform housed in a split steel die which has previously been evacuated (see Figure
16.156’09).

 Centrifugal casting processes
Three types of casting process rely on centrifugal force rather than gravity or pressure to impel molten metal into a mould or
die. This has the advantage that a force which can be greater than gravity, and which may be increased or decreased at will,
is available to run and feed the casting.

 Centrifugal casting
Centrifugal casting produces hollow cylinders (generally of cast iron or steel) by pouring molten metal into a launder
projecting into the end of a horizontally or vertically rotating cylinder. This is usually made of steel but sometimes of
graphite or ceramic. The metal is held against the mould wall by centrifugal force. Feeding is good and dross tends to
migrate to the bore where it can be machined off. Dimensional accuracy is good; production rate is rapid and dimensions
range from very large pipes and cylinders to small ‘pots’ which can be machined to form cylinder liners and piston rings. The

casting shape is, however. limited to cylinders and the equipment is expensive.

Semi-centrqugal casting
Semi-centrifugal casting produces rotationally symmetric castings, the moulds for which can be stacked vertically and
poured in the common axis while rotating. The centrifugal force available makes this very suitable for pulleys and gear
blanks wlhich require the best quality metal on the periphery.This process is very suitable for the production of gears and its
use shoulld increase.

Centrifugally assisted casting
In this process a number of moulds, often of the investment type, are located symmetrically round a crucible. When the
metal has been melted the assembly is rotated at a controlled speed so that the metal runs smoothly into the moulds. When
the moulds are filled the speed is increased so that detail is well reproduced and the castings are satisfactorily fed. The
process is excellent for casting small articles of jewellery.