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Conventional forming processes for metal products are forging or near net shape casting. In the first process, ingots of the metal are formed in the solid state and in the latter process the products are cast from molten metal. Discoveries in the early seventies at MIT showed that metals can achieve thixotropic properties in the semisolid state. This means that the viscosity of a metal may be considered to be both shear and time dependant. By heating an ingot of thixotropic material into the two-phase region, the metal is allowed to flow like a liquid containing up to 60% solid particles. The fluid like behaviour of thixotropic metals has resulted in several suggestions for new casting processes, or modifications of conventional methods such as die casting. The most important benefits of semi-solid materials are, first of all, a laminar mould filling process resulting in less entrapped air.

High pressure die casting is characterised by rapid die filling and subsequent rapid cooling and solidification of the metal in the die. These characteristics are favourable for the mechanical properties of magnesium die casting alloys. Since the filling pattern and the cooling rate of the metal is highly dependent on both process parameters and geometry of the part, there is a natural variation in mechanical properties. Variations in filling pattern can be caused by differences in the filling conditions set up by the gating system, pre-solidification in the shot sleeve and during filling as well as variations in the timing of the pressure intensification. In the present work the effects of solidification during filling are discussed with emphasis on the resulting microstructures and the correlation with mechanical properties.

High pressure die casting is characterized by rapid die filling and subsequent rapid cooling of the molten metal in the die. These characteristics are favourable for magnesium die casting alloys. The high cooling rate favours the formation of a fine dendrite and grain structure, which in turn leads to substantial hardening; this refinement also provides improved ductility. Since the cooling rate of the metal is highly dependent on both the process parameters and the geometry of the part, the three-dimensional flexibility associated with the latter factor means that the cooling rate cannot be uniform. This cooling rate difference in turn can lead to some variation in the mechanical properties between geometrically different portions of a die cast component. This variation is an inherent property of the material, in contrast to casting defects like microporosity, non-metallic inclusions, filling defects, and formation of hot cracks.