Er addition to Al-Si-Mg-based casting alloy: Effects on microstructure, room and high temperature mechanical properties

The paper deals with the production, heat treatment, microstructural and mechanical characterization of innovative Al-Si-Mg alloys based on Al-7Si-0.4 Mg (A356) with additions of rare earth Er, with the aim of increasing the microstructural stability and mechanical performances at high temperature. Three alloys with different Er concentration (between 0, and 0.41 wt%) were investigated in the as-cast, peak aged and overaged conditions. Microstructural analyses demonstrated that Er additions affect eutectic morphology by fragmenting and spheroidising Si particles. Tensile tests at RT, 423 and 473 K proved that Er-containing alloys have improved mechanical strength with respect to the reference alloy; in particular, among the investigated alloy, the one with 0.22 wt% Er displayed the best combination of mechanical properties in all the tested conditions. Additions of 0.41 wt% Er, on the other hand, led to reduced castability of the alloy, with increased fraction of casting defects, and to the formation of primary Er-rich precipitates with high aspect ratio, causing a drop in ductility and reduced mechanical properties. Tests results demonstrated the importance of suitable chemical additions in improving the mechanical behaviour of cast Al-Si-Mg alloys. It can be concluded that the alloy containing 0.22 wt% Er could be proposed for structural applications at room and elevated temperatures, effectively extending the operating temperatures of Al-Si-Mg-based alloys.