Casting, Semi-Solid Forming and Hot Metal Forming

G. Govender , ... O.F.R.A. Damm , in Comprehensive Materials Processing, 2014

5.06.3.1.1 New Rheocasting

NRC is the oldest commercial rheocasting process. It was patented in 1996 by Japanese machine builder UBE Industries and presented at the GIFA International Foundry Trade Fair 1999. The process involves the casting of slightly superheated molten metal into a holder, producing a large number of nuclei, which are made to grow into a globular microstructure through targeted slow cooling ( Figure 8). This was followed by a temperature adjustment using induction heating and subsequent forming. The NRC process was the first rheocasting process to be industrialized and was licensed to a number of component manufacturers, including Stampal s. p. A. in Italy (39–41). A disadvantage of the process is the fact that the slurry-making process is integrated with the high-pressure die casting (HPDC) machine – in other words, the process is not compatible with existing HPDC machines and infrastructure. This substantially drives up the capital cost of the process, and it is currently not widely available on a commercial basis.

Figure 8. Schematic illustration of the NRC rheocasting process (UBE)..

 Reproduced from Innovative Casting Process Clearing the Way for New Casting Possibilities "New Rheocasting"; Ube Industries Ltd, Marketing brochure: Japan, 1999

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Melting

John Campbell , in Complete Casting Handbook (Second Edition), 2015

Rheocasting

Rheocasting is technique for producing a partially solid alloy but flowable alloy by careful cooling of the liquid alloy into the liquid  +   solid range while mechanically stirring to ensure that dendrites are broken up, forming compact, near-spherical forms. Such an approach was discovered by Spenser Mehrabian and Flemings at MIT in 1972. The process continues to find niches in the industrial and commercial world in the second millennium.

Thixocasting

Thixocasting was a variant of the rheocasting approach in which the partially solid mixture was precast in the form of semi-continuously cast 'logs'. The logs required to be sawn to correct length, reheated to the casting temperature, and finally cast. This approach has always suffered from the cost of the separate melting and rheocasting process steps. Furthermore, the foundry returns (runners, feeders and scrap castings) present a problem for recycling. The just-in-time processes for the production of partially solid mixes avoid both of these issues.

Just-in-time slurry production

New concepts require agonisingly long development periods before they become accepted by users. Companies such as Alcan Canada (Doutre et al., 2000; Lashkari and Ghomashchi, 2007), Ube Japan (Lee and Kim, 2007), Idra Italy (Yurko et al., 2003) and Rheometal, Sweden (Granath et al., 2008), among others (Midson, 2008), have continued to develop novel just-in-time slurry production techniques to counter one of the challenges of the process, the production of controlled qualities of slurry at commercially attractive production rates, and commercially attractive costs.

Strain-induced melt activation

Strain-induced melt activation produces a thixotropic condition by first deforming the solid alloy followed by a heat treatment into the mushy zone. The approach avoids melting and the entrainment of air and oxides suffered by competitive techniques for slurry production. Chen et al. (2010) describe its use for magnesium alloy AZ91, using equal channel angular pressing to produce a high but uniform strained condition, and thereby achieve impressive results. Even so, it seems likely its potential for commercial exploitation will be limited by its challenging economics, in common with other semi-solid routes.

Mg alloy slurry production from granules

The work by Fan et al. (2007), Czerwinski (2008) and others has demonstrated the feasibility of producing rheocastings in Mg alloys by the processing of alloy granules through a twin screw injection moulding machine as part of a pressure die casting process. The processing of Mg granules via a route more normally associated with the processing of plastics is possible partly because of their low strength and hardness, and partly because Mg reacts with the steel screws to only a limited extent, compared for instance, to Al alloys which dissolve iron readily. Once again, the uniform dispersion and shredding of the oxide skins from the original granules is seen to deliver a product that has uniformly reliable, if not optimum, properties. In fact, the precipitation of the so-called β-Mg 17Al12 phase at grain boundaries in thixotropic Mg alloys containing over 7%Al has been cited (D'Errico, 2008) as limiting ductility. It seems almost certain that this phase is precipitating on bifilms generated by the mixing process, and pushed ahead of the growing grains, effectively pre-cracking the grain boundaries.

Metal matrix composites

Solid/liquid mixtures can be made by the addition of particles of solid into the melt. The exploration of such mixtures was pioneered by Pradeep Rohatgi, who over the years since his time in 1965 in the International Nickel Company Laboratories, New York, has explored mixtures of practically everything with everything (see his Rohatgi Symposium reported by Gupta, 2006). Most matrices studied so far are those of Al alloys, but other work includes Zn, Mg and, later, superalloys (Rohatgi, 1990). The particle additions include graphite, SiC, Al2O3 and low cost additives such as fly ash, a waste product from power plants. Thousands of tonnes of such metal matrix composites (MMCs) are now processed for use in the transport industries.

The process most used to make mixtures is the so-called vortex method in which the solid additions are fed into a central vortex made in a rapidly stirred liquid metal. Some work has also been carried out using particles in suspension in a carrier gas, introduced under the melt surface from a lance.

All processes that introduce particles from the outside into the interior of the melt suffer the problem of having to penetrate the oxide skin on the melt. This means that particles usually enter the matrix as clumps, each clump being enclosed by a wrapping of the oxide film necessarily entrained from the surface (Figure 2.3). The problem with such clumps is that the reservoir of air they contain continues to thicken and strengthen their surrounding oxide envelope. The vigorous stirring required to break these large agglomerates down into smaller agglomerates automatically introduces yet more oxides into the melt via the central vortex. Tian et al. (1999) study clusters of alumina particles in liquid Al, finding clusters from one particle of 10   μm in diameter to clusters of particles 100   μm in diameter. In each case, the clusters are enclosed by amorphous alumina films.

The MMC studies by Emamy et al. (2009) also illustrate particle clusters inside oxide 'paper bags'. There is evidence from this work that the loss of fluidity with higher additions of particles is not primarily the result of the higher percentage of particles but the result of increased oxide bifilm content from the vortex stirring process. The huge increase in bifilm population in such vortex-produced MMCs is also seen to degrade ductility, even though, paradoxically, there may be some slight increase in strength.

Badia (1971) explores the production of MMCs in Al and Zn matrices by additions of a variety of solids including SiC, Al2O3, SiO2 and graphite. He found that coating of the particles with Ni was essential to achieve some kind of wetting so that the additions would remain in suspension; otherwise, the additions appeared to surface once again, rejected from the melt as a dry powder. The action of the Ni is not completely clear and may depend not only on its metallic nature, but also on its powerful exothermic reaction with Al to form nickel aluminides. Any benefit to the MMC of the introduction of a lightweight phase is lost however, because the 5 μm Ni coat on particles 5   μm in diameter corresponds to an addition containing 50% Ni. Badia used the vortex addition technique and subsurface injection from a lance, but preferred the vortex approach because continued stirring with the rotor could maintain the addition in suspension.

Practically all of the previous studies have focussed on the use of particle additions to a melt. Occasional exploratory studies have included a variety of fibres (Rohatgi, 1990) and SiC whiskers (Das and Chatterjee, 1981) for which surprising gains in strength have been recorded for levels of only 0.5 volume percent addition.

In contrast to these laboratory studies, in which the introduction of particles was attempted with relatively inefficient protective atmospheres, Alcan instigated a major production initiative in which they produced an Al alloy – SiC MMC in tonnage quantities in a high vacuum environment (Hammond, 1989) – although at the time the production conditions were a commercial secret. The uniform dispersion of the particulate phase in Duralcan, as it came to be known, indicated its relative freedom from oxide films, although work by Emamy and the author did reveal some residual bonding problems, even though these were much less than those commonly seen elsewhere. Later work (Hoover, 1991) confirmed the excellent specific stiffness, good tensile properties including fatigue, and good retention of properties at elevated temperature.

In situ metal matrix composites

The formation of MMCs by the precipitation of the strong, hard particles in situ in the matrix by a metallurgical reaction during solidification is an attractive technique. In this case, the strengthening particles suffer no poor cohesion with the matrix because of an intervening oxide film and its associated thin layer of air; the particles are bonded to the matrix with atomic perfection, having grown atom by atom from the liquid.

The difference in the bonding of an in situ–generated MMC based on TiB2 particles in an Al alloy matrix, and Duralcan (a good quality extrinsically generated MMC based on SiC particles) was shown clearly in work by Emamy and Campbell (1995). A non-fed casting, designed to create a reduced internal pressure, possibly a slightly negative pressure (corresponding to a hydrostatic tensile stress) was seen to cause significant dispersed microporosity in Duralcan, but very little in the in situ MMC. The difference between the relatively poor bond and the perfect bond was clear.

Hadian et al. (2009) studies the system Al-Mg2Si and the refinement of its Mg2Si particles by more rapid freezing and by the addition of Li. Although the Mg2Si particles are usually regarded as brittle, this study is notable for its image of an Mg2Si particle cracked (one assumes by the presence of a central bifilm on which it formed) but plastically peeled open, revealing the particle to be impressively ductile (Figure 6.22). This is expected to be a universal feature of in situ MMCs; all the hard, strong particles will probably deposit on a bifilm, and thus all will be expected to possess an in situ crack, despite the intrinsic strength and crack resistance that they would be assumed to possess.

The common Al-Si system similarly behaves as an in situ MMC in which the silicon particles act as the hard, strengthening phase in the ductile Al matrix. Once again, the presence of central cracks in the Si is attributed to the formation of the Si particles on bifilms in suspension in the melt. It is known that clean melts cannot form Si particles in this way, with the result that the Si is forced to precipitate at a lower temperature and a different form, as a finely spaced eutectic which we call a 'modified' Al-Si eutectic. The addition of such elements as Sr and Na similarly act to inhibit precipitation on the oxide bifilms, thus encouraging the modified structure (Campbell, 2009). When the Si phase forms in this way, it can now no longer be regarded as separate particles, even though this is its appearance on a polished micrographic section. The Si has the form of a continuous branching growth known as a coral form. This form of the Al-Si MMC can be particularly strong and tough if bifilms are somehow removed by careful processing because the development of the MMC structure now no longer depends on their presence.

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Metal and Ceramic Based Composites

S.T. Mileiko , in Composite Materials Series, 1997

13.2.5 Compocasting

Compocasting, or rheocasting, suggested by Mehrabian et al. [ 402] in early 70s, is used to produce metal-matrix composites, mainly short-fibre/aluminium-matrix composites. The process is now known to have some variations but the main steps are as follows [190, 292, 364]:

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flow-casting of a semi-liquid alloy at a temperature just above the solidus;

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vigorous mixing of the alloy and adding fibres into the liquid/solid mixture;

•

rolling, die-casting, extrusion, or some other procedure to shape a composite.

Obviously, a matrix characterized by a large difference between solidus and liquidus temperatures makes easier to process the composites, especially at the final step. A mixture is contained in a crucible with an opening at the bottom used to pour out the mixture for further processing. Vigorous mixing at the second step is necessary to keep the solid/liquid mixture fluid to prevent the formation and growth of primary phase dendrites, that is a kind thixotropic behaviour of the slurry. The fibres are added to the matrix when it contains about 50% of the solid phase. Going on with stirring at the stage of adding fibres prevents floccation of the fibres and promotes wetting as a result of disruption of the contaminated layer on the fibre. If the fibre volume fraction is sufficiently low, the mixture can be cast directly, otherwise it should be heated again.

The microstructure of a composite obtained in such a way is characterized by a globular shape of the primary phase and a homogeneous distribution of fibres within the eutectic areas [190], so that fibres are surrounded by the matrix of a composition different from the average one. Possibly, this is a reason for enhanced wettability in the fibre/matrix system.

Figure 13.13 presents a schematic example of rolling as a final step of the fabrication route as produced by Kang et al. [292] who fabricated a Saffil fibre-reinforced aluminium matrix heavily alloyed so that the difference between solidus (477°C) and liquidus (635°C) temperatures occurred to be large. This allowed to pour a mixture of the fibre and matrix heated up to a temperature within an interval of the existence of a liquid phase in the matrix, in the gap between rollers and to keep a solidification front at a constant position in the processing zone. There exists a combination of the pressure on the slab, roller speed, and initial temperature of the mixture that provides a composite microstructure which looks like a usual one.

Fig. 13.13. Rolling of a mixture of short fibre and metal matrix containing initially solid and liquid phases. After Kang et al. [292].

Compocasting combined advantages and shortcomings of the fabrication routes using liquid and solid phases. Simplicity of the process and facilities, as well as easy shaping have come from liquid state technologies, a possibility to clean fibre surface are from the powder metallurgy. On the other hand, the process inherits the problems arising due to fibre breaking during processing.

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Casting, Semi-Solid Forming and Hot Metal Forming

S.P. Midson , in Comprehensive Materials Processing, 2014

5.13.7.7 Generate Semisolid Structure

As shown in Figure 1 , rheocasting obviously includes the additional step of cooling the liquid alloy into the semisolid temperature range, while concurrently generating the globular microstructure. This obviously creates additional cost for rheocasting, so the more cheaply this step can be performed the better. Certainly some rheocasting processes generate the globular structure directly in the ladle while transferring the alloy from the holding furnace to the die casting machine, so the incremental cost will be very small. However, other rheocasting processes involve more complex steps, and so the additional cost will be higher. Therefore, it is difficult to generalize about the magnitude of the additional cost associated with the generation of the semisolid slurry, and each rheocasting process needs to be examined individually.

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Casting, Semi-Solid Forming and Hot Metal Forming

A.H. Ahmad , ... D. Brabazon , in Comprehensive Materials Processing, 2014

5.07.2.6.1 New Rheocasting

The steps in the new rheocasting (NRC) process are described below (69–71). Figure 7 presents a schematic diagram depicting the steps in the NRC process. The process starts with melting the raw material and pouring with low superheat into a chill environment so that more nuclei occur throughout the melt volume. Superheating the molten alloy to less than 50 °C above the liquidus temperature is done in this process (69). The molten metal then is cooled slowly to avoid dendritic nucleation. After this, the molten metal is held for some time within the semisolid temperature range. After it has reached the appropriate solid fraction, it is then transferred from the cooling station and charged into an inclined sleeve. Finally, this SSM is cast into the required part mold by die casting. The dimension of the charge holding wall was seen as crucial where an increment of wall thickness produced the unwanted dendritic microstructures (72). This process has been used for aluminum and magnesium castings.

Figure 7. Schematic of the new rheocasting (NRC) process for the production of globularly microstructured feedstock.

 Reproduced from Adachi, M.; Sato, S.; Sasaki, H. Method of Shaping Semisolid Metals. European Patent, EP0841406, 2001.

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Fracture in the liquid/solid state

John Campbell , in The Mechanisms of Metallurgical Failure, 2020

2.7.2 Thixocasting

Thixocasting was a variant of the Rheocasting approach in which the partially solid mixture was precast in the form of semicontinuously cast "logs." The logs required to be sawn to correct length, reheated to the casting temperature at which they would retain their shape, but be easily sheared to convert to a flowable slurry, and finally injected or poured into a die. This approach has always suffered from the cost of the separate melting and Rheocasting process steps. Furthermore, the foundry returns (runners, feeders, and scrap castings) present a problem for recycling. The just-in-time processes for the production of partially solid mixes, described below, avoid both of these issues.

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Casting, Semi-Solid Forming and Hot Metal Forming

E.J. Zoqui , in Comprehensive Materials Processing, 2014

5.09.3.1.5 A390 Aluminum Alloy

A390 is the only hypereutectic aluminum–silicon alloy used in the thixoforming process. In the rheocasting process, its high silicon content causes the initial formation of the solid phase to be silicon particles with an FCC structure (diamondlike) rather than α particles. Figure 15 shows the expected phase diagram for the composition range presented in Table 1. The yellow area indicates the aforementioned formation of silicon particles immersed in liquid, with the eutectic reaction beginning only at a lower temperature. The solid α phase is expected to begin to form below 571 °C, and due to the very low solubility of silicon in the aluminum chlorofluorocarbon (CFC) structure, formation of AlFCC is expected. The separation of silicon from the α phase will also form the AlFeSi phase, which will coexist with the α phase, AlFCC, and silicon from approximately 558–519 °C. The complex eutectic reaction continues with the precipitation of Al5Cu2Mg down to 519 °C, at which point the AlFeSi dissolves into the α matrix phase. Below this temperature, the precipitation of Al7Cu2Fe is expected to consume all the remaining liquid. Thus, the complete eutectic reaction will occur in a temperature range of 568–510 °C. The precipitation of the classical Al2Cu particle, which is the main element responsible for hardening via T6 temper, is expected to occur below 510 °C.

Figure 15. Aluminum–silicon phase diagram of A390 alloy produced by Thermo-Calc® software, showing the expected iron (Fe), copper (Cu), manganese (Mn), and magnesium (Mg) content usually found in these alloys.

The as-cast microstructure expected at room temperature is composed of a mixture of silicon particles plus the eutectic phase consisting of AlFCC/α lamellar structure with silicon containing small amounts of Al2Cu, Al5Cu2Mg, and Al7Cu2Fe. This structure requires solubilization to improve its mechanical properties, and T6 temper usually involves reheating the alloy up to 500 °C for 5–8 h, followed by quenching and 5–6 h of aging at 170 °C.

Figure 16 shows the expected solid to liquid transition as a function of temperature. In this case, the eutectic knee occurs at 564/567 °C, in which range 6–8% of silicon particles are formed. As this temperature gradually decreases, large amounts of eutectic phase will be formed until solidification is complete, as stated before. Therefore, controlling the processing of this alloy requires very strict control of the temperature to achieve good results in thixoforming and/or rheocasting operations. The expected final mechanical properties are described in Table 6.

Figure 16. Semisolid transformation of A390 alloy obtained by Thermo-Calc® simulation software.

Table 6. Basic mechanical properties of the A390 aluminum alloys commonly used in semisolid casting

Property A390 – F A390 – T6
Yield strength (MPa) 248–270 345–365
Ultimate tensile strength (MPa) 317 345–365
Elongation (%) <1.0 <0.2
Hardness (BHN) 120–145 150

Data obtained from NADCA. Product Specification Standards for Die Castings Produced by Semi-Solid and Squeeze Casting Process, 4th ed.; NADCA (North American Die Casting Association) Standards: Wheeling, IL, USA, Publication No. 403, 2006; and http://www.matweb.com/ (accessed Feb, 2012).

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Casting, Semi-Solid Forming and Hot Metal Forming

P. Kapranos , ... T. Haga , in Comprehensive Materials Processing, 2014

5.02.4.5.2 Semisolid Fabrication Methods

Incorporation of the reinforcement particles within a semisolid alloy is claimed to be advantageous because the solid mechanically entraps the reinforcement, thus avoiding agglomeration, settling, or flotation (100,101) . This involves adapting the rheocasting technique (102–104), a casting process of metallic alloys that produces a unique cast microstructure with nondendritic, near globular primary solid phase. In this process, vigorous shearing is applied to a molten alloy as it cools into the solidification range. The shearing breaks the dendrites into individual round particles that become more or less spherical by coarsening (104). In conventional casting, the casting becomes stiff when the solid fraction of the primary phase is about 15%. However, the rheocast slurry maintains very low viscosity at much higher solid fractions, depending on the shear rate and cooling rate. This enables the slurry to be cast at lower temperatures and provides many advantages over conventional liquid casting such as reduced hot cracking and reduced shrinkage. When a rheocast ingot is reheated to a temperature in the semisolid range, it still maintains its cast shape but becomes soft enough for further processing. A rheocast ingot reheated to the semisolid state can be die cast or forged into near net-shape products with good dimensional accuracy, finer microstructures, and better mechanical properties than those obtained by conventional processing. The umbrella term for the semisolid processes is thixoforming, and it is discussed in a separate chapter.

The rheocasting technique was extended to produce metal matrix composites (102–104) since reinforcements such as ceramic particulates, short fibers, or whiskers have poor wettability to molten metals and are very difficult to fabricate by mixing reinforcements into liquid metal. The reinforcement may be easier incorporated into semisolid alloy slurries of a matrix formed by rheocasting. Once the reinforcements are introduced into the semisolid slurry, they are entrapped mechanically by primary solid particles. The chemical interaction between the reinforcement and liquid matrix can proceed with time, and finally, the reinforcements are fully trapped into the composite slurry. This method of MMC production is called compocasting. The compocasting process is very effective in making cast composites with higher particle content (80,82,105,106). The reinforcement particles are gradually added while stirring continues at a constant rate. Aluminum A356-carbon nanotubes composites (CNTs) were successfully produced through a special reinforcement addition technique and the compocasting route (107). Vogel et al. (108) gave the term 'stir-casting' to the production of metals with near spheroidal microstructures through the shearing action induced by stirring. Ceschini et al. studied the tensile properties and the low-cycle fatigue behavior of 7005 aluminum alloy reinforced with 10 vol.% of Al2O3 particles (W7A10A composite) and 6061 aluminum alloy reinforced with 20 vol.% of Al2O3 particles (W6A20A composite) (109). These composites were produced by DURALCAN (USA) using a proprietary molten metal process based on the compocasting method.

Optical microscopy characterization of the microstructures showed different particle size in the two composites, as shown in the micrographs of Figure 8(a) and 8(b).

Figure 8. Optical micrographs of the W6A20A (a) and W7A10A (b) composites.

The maximum axis dimension of the particles follows a Weibull distribution, with shape parameter b = 2.80 and scale parameter g = 24.70, with a mean value of 11 μm, for the W7A10A and b = 2.63 and g = 12.40, with a mean value of 22 μm, for the W6A20A. In both composites, the Al2O3 particles were of nonuniform size, irregularly shaped, and randomly dispersed in the alloy matrix.

Agglomeration or clustering of the particles was also observed, resulting in particle-rich and particle-depleted regions. This material inhomogeneity is generally higher in MMCs manufactured by molten metal processes with respect to those produced by powder metallurgy.

A novel quick quenching stir-caster has been designed and built for processing Al–SiC composites in the liquid and semisolid state by Naher et al., Figure 9 (110). Temperature controlled compocasting experiments were performed on Al–10%SiC, and it was found that stirring the MMC slurry in a semisolid state during the solidification process helps to incorporate ceramic particles into the alloy matrix. This quick quenching compocasting method was successful in fabricating Al–SiC metal matrix composites (110).

Figure 9. Schematic drawing of a stir-caster: (1) stirring motor, (2) stirrer rod, (3) furnace, (4) stirrer impeller, (5) crucible, and (6) actuator.

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Semisolid Processing

D.H. Kirkwood , in Encyclopedia of Materials: Science and Technology, 2001

3 Thixoforming Processes

An early proposal was to use the slurry as formed on cooling by directly charging it from the slurry generator into the shot chamber of a die-casting machine and then injecting it into the die. This was termed rheocasting, but because of the difficulty of matching the production rates of slurry manufacture and die casting, it is no longer considered an attractive route.

The alternative approach is to allow a rheocast billet to solidify fully, to cut it up into slugs of appropriate size, which are heated back to the semisolid condition when required at a later date. They regenerate the spheroidal particle structure during this operation and are ready to be formed into complex shapes. This may take place (see Fig. 5) between closed dies (thixoforging), or by loading into the shot chamber of a die-casting machine and injecting into a die (thixocasting). In addition, a process which at present is only used for magnesium alloys (thixomolding), involves loading alloy chips into a heated Archimedes' screw. The screw drives the chips down the barrel under heating and shearing conditions, which generate good semisolid material, and then forces the slurry into a die in an operation similar to plastic injection molding.

Figure 5. SSM forming processes.

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Casting of aluminium alloys

S. Otarawanna , A.K. Dahle , in Fundamentals of Aluminium Metallurgy, 2011

Semi-solid metal processing (SSP)

SSP is a type of die casting process wherein a partially solid metal, typically solid fraction (fs ) of ~0.5 18 , is intentionally prepared and injected into a die cavity. The semi-solid material is prepared to have globular primary phase morphology (Fig. 6.3a) which exhibits thixotropic behaviour and therefore is more 'castable' than a dendritic material (Fig. 6.3b–c) 20 . This can be obtained by either stirring (typically mechanical stirring or electromagnetic stirring) or grain refinement and thermal history control 18 .

6.3. Comparison of different A356 microstructures obtained from: (a) above-liquidus casting, (b) thixocasting and (c) rheocasting [18]. Note that the thixocast microstructure contains eutectic entrapped within the primary (Al) globules whilst the rheocast does not.

 (courtesy of North America Die Casting Association)

Despite the fact that there are numerous techniques for SSP, they can be categorised by the processing route into two groups: (1) thixocasting, and (2)rheocasting. In the thixocasting route, a billet containing globular microstructure is prepared by a billet manufacturer. Upon reheating, the billet is ready to be placed into the shot sleeve of an HPDC-like machine and injected into the die cavity. For rheocasting, the thixotropic slurry is prepared on-site before subsequently being transferred into a pressure casting machine. Thixocasting billets are sold at a premium price, up to twice the ingot-alloy price, and cannot be reused by the die-caster. A material loss of 8–10% during reheating has been reported 21 . This billet-costissue leads to a global trend to develop and use the rheocasting processes.

With more viscous injected alloy and lower injection velocity compared to conventional HPDC, SSP provides a more stable flow front and therefore less entrapped air during die filling. As the injected alloy already contains a substantial fs , subsequent solidification shrinkage is clearly less than in conventional HPDC. Die life is normally longer due to lower alloy temperature. Due to the fact that rheological properties of semi solid alloy are a function of fs , only some alloys (with clearly defined eutectic point and fs , is not a strong function of temperature in the working fs range) are suitable for SSP and good control of temperature is also required.

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