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Semi-solid metal (SSM) casting of aluminum components is currently establishing itself as a viable process for critical applications in the automotive industry. SSM casting processes compete favorably on both cost and performance with other casting techniques including gravity permanent mold (GPM), conventional high pressure die casting (HPDC) and squeeze casting. In this paper the various SSM casting routes in use today are reviewed. The two categories of SSM processes are thixocasting (involves the use of electro-magnetically stirred or grain-refined billets) and rheocasting (slurry produced directly from the liquid phase). The former requires a billet that needs to be reheated and processed, whereas the latter is cast directly from the liquid state. Also described here are new approaches to slurry making. These include the Slurry on Demand (SoD) process from AEMP, the Sub Liquidus Casting (SLCR) process from THT, and Diffusion Solidification.

Semi-solid processing (SSM) has many advantages in that the alloy is cast at lower temperatures (i.e., in the two-phase region) giving rise to reduced die wear, as well as giving rise to novel microstructures. The resultant SSM processed castings are dendrite-free and do not contain hot tears; rather, the SSM structure is globular, and the liquid phase surrounding the globules acts as a “lubricant” during processing. Moreover, the flow of the slurry into the die cavity is more laminar than turbulent, since the starting metal is in the mushy region. This concept of SSM processing was realized by the development of a continuous process titled: CRP - Continuous Rheo-conversion Process. In this process, one allows the incipient solidification of alloy melt(s) under the combined effects of forced convection and rapid cooling rates. In the CRP, two liquids held at particular level of superheat, are passively mixed within a reactor.

Squeeze casting is considered a “high integrity” casting process because it imparts qualities (higher tensile properties, in particular ductility due to reduced or absence of porosity in the matrix, and the ability to heat treat) to a metal that are difficult to achieve with conventional casting techniques including gravity permanent mold (GPM) and high pressure, high velocity (HPDC) die casting. In recent years, the squeeze casting process has been widely used with various aluminum alloys to manufacture near-net shape automotive components requiring high strength, ductility or pressure tightness. However, with the emphasis on weight reduction, lower cost and improved performance of structural components, alternative lightweight materials including magnesium are now being seriously considered. Unfortunately, the use of magnesium as a structural material has been hindered by the lack of data on mechanical properties and the lack of new improved casting methods.

With the ongoing demand for vehicle weight reduction, the need for lighter weight and high integrity automotive components continues to increase. Aluminum offers a low weight alternative to steel and cast iron. However, the conversion to aluminum has been focused mainly on engine components, wheels, steering systems, and suspension. Structural body components, which are often manufactured from steel assemblies, account for a large portion of the overall vehicle weight. Converting steel body components to aluminum can significantly reduce vehicle weight, thus improving performance and fuel economy. Variants of the conventional high pressure die casting processes (HPDC) are being used to manufacture thin-walled (2-4mm) aluminum castings for structural applications.

The basic data on the mechanical properties of a casting are frequently obtained from a tensile test, in which a suitable specimen machined from the casting is subjected to increasing axial load until it fractures. The engineering tension test is widely used by casting manufacturers as an acceptance test for customer specifications. However, tensile bars machined from castings often provide undesirable information, thereby leading one to question the part integrity. This paper, therefore, discusses the various factors that affect tensile properties obtained from specimens machined from actual castings.