Abstract
For hypereutectic Al/Si alloys, one of the advantages of thixoforming in comparison with casting routes is the relatively short processing times at high temperatures and hence limited coarsening of the Si phase. Coarse silicon particles give poor mechanical properties. Here two hypereutectic Al/Si alloys (magneto-hydro-dynamically (MHD) stirred A390 from Pechiney and an extruded A390 alloy from Showa in Japan) have been thixoformed to form pistons. Opening up the die entrance to the full width of the crown made the flow into the die more uniform and helped to reduce the tendency for large pores to form due to swirling of the slurry. Die heating reduced cold shuts in the skirt (thin section) of the piston. Placing inserts into the die to make holes for the piston pins (i.e. having an obstacle in the thicker regions) evened up the flow between the thick and the thin regions in the die. Massive pores experienced in earlier shots were then eliminated. Placing ceramic material in the die entrance considerably reduced the shrinkage porosity in the crown. The use of the Showa alloy, where the globular semisolid microstructure is achieved by a solid state deformation route rather than MHD, gave reduced shrinkage porosity and eliminated macrosegregation of the eutectic and the silicon. Computer modeling has aided optimization of the die.
Highlights
Thixoforming is the shaping of metal alloys in the semi-solid state
Gravity die casting of hypereutectic alloys is difficult because: the melt temperature has to be significantly higher than for a eutectic alloy (~750 ̊C vs. ~650 ̊C) leading to die attrition and coarse silicon particles in the microstructure; there is a tendency for hypereutectic silicon to segregate especially in the thick sections of the castings; it is necessary to add a P refiner to the melt to ensure the Si assumes an equiaxed shape, rather than solidifying as plates – this effect can fade over time; extensive machining is required after casting e.g. ~1-2mm off the skirt thickness
The results suggested that improved die filling would be obtained by thixoforming the pinholes in situ, with the fully open gate
Summary
Thixoforming is the shaping of metal alloys in the semi-solid state. The process was developed from studies into the flow properties of metallic alloy slurries at MIT (as reviewed in [1]). Two different routes are used: electromagnetic (MHD) stirring (Pechiney A390 alloy) and casting followed by extrusion in the semi-solid state (Showa A390 alloy). This latter route involves recrystallisation and partial melting and is termed the RAP route [2]. Thixoforming has the potential to make possible the mass production of aluminium-high silicon pistons (initially ~17% Si by weight). This alloy is less dense and has a lower thermal expansion coefficient than gravity die cast pistons which are based on an alloy with the eutectic silicon content of ~13%. Some other reports on semisolid processing pistons are given in the literature [3,4,5,6,7]
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