Semisolid metal processing (SSM) or thixoforming is a new technology that offers several advantages over liquid processing and solid processing. This process utilizes semisolid behavior as well as reduces macrosegregation, porosity, and forming forces during shaping process. A lot of research work has been carried out by various researchers in order to exploit the potential of this process to produce different products especially for automotive industry. This paper will summarise the rheological behavior of aluminium alloys in semisolid slurries, thixoformability of modified aluminium alloys, the effect of feedstock production method on mechanical properties, and the importance of developing low-cost raw materials for semisolid processing. 1. Introduction Thixoforming is a viable technology for forming alloys in semisolid state to near net-shaped products. Thixoforming basically consists of three phases, as shown in Figure 1; (a) producing a material with a globular microstructure, (b) heating the material to the forming temperature, and (c) forming the material in a die-casting press. The process relies on the thixotropic behavior of alloys with a spheroidal rather than a dendritic microstructure in semisolid state [1]. Flemings and his students were the first to discover accidentally this thixotropic behavior in the early 1970s while performing continuous hot tearing test of solidifying Sn-15% Pb [2]. Figure 1: Semisolid metal processing routes. In thixotropic condition, an alloy decreases in viscosity if it is sheared but it will thicken again if it is allowed to stand [3–6]. This process requires uniform heating and partial remelting of the alloy slug to obtain a homogeneous consistency throughout [7]. This technology promises some advantages such as prolonged die life due to less thermal shock and also provides more laminar cavity fill which could lead to reduced gas entrapment [8]. This process also can improve the usage of feedstock materials and it can contribute to the reduction of processing cost of thixoformed parts [9]. Viscous flow is improved in semisolid metal processing because this process can provide additional laminar cavity fill, which can reduce gas entrapment. Aluminium alloys are among the most prominent and well-known materials used in the mechanical construction and automotive industries, as shown in Figure 2. According to Kenney et al. [10], the energy needed to heat aluminium alloys for casting is 35% greater than that required to heat the same aluminium alloy for the conditions needed for semisolid forming. Millions of
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