Influence of Rare Earth Additions on the Microstructure and Mechanical Properties of Al7Si0.3Mg Alloys Processed by Semi-Solid Die Casting and Gravity Die Casting

Abstract

Microstructures with fine globular grains and refined eutectic structures are important to enhance the mechanical properties of A356 alloys processed by semi-solid and gravity die casting. Rare earth (RE) additions have been shown to be capable of refining both the α-Al particles as well as modify the eutectic phase of alloys. In semi-solid die casting, Al7Si0.3Mg alloys with RE concentrations (0, 0.1 and 0.4 wt.%) were used to prepare semi-solid slurries using the SEED (Swirling Enthalpy Equilibrium Device) method, and subsequently semi-solid die cast. The same compositions of alloys were also applied to gravity die casting. The microstructure and mechanical properties of castings in two processes have been characterized. Compared to the grains produced in gravity die casting, globular grains with small size (260 μm) in the semi-solid die casting significantly enhance the UTS and elongation of alloys. Although the size of grains had no change with increasing RE concentrations in alloys. The Al-Si eutectics were changed to refined morphology with the 0.1 wt.% RE addition, which enhanced the ductility of alloys in two processes. When increasing the RE addition to 0.4 wt.%, the RE-rich phases precipitated at grain boundaries, which decreased the UTS and elongation of alloys.