The microstructures and tensile mechanical properties of ultrafine grained and coarse grained Al-7Si-0.3Mg alloy rods fabricated from machining chips

Ultrafine grained (UFG) and coarse grained (CG) Al-7Si-0.3Mg (wt%) alloy rods were fabricated by hot extrusion of compacts of a nanocrystalline Al-7Si-0.3Mg alloy powder and coarse grained Al-7Si-0.3Mg alloy granules respectively. Both the nanocrystalline powder and the granules were prepared by processing Al-7Si-0.3Mg alloy (widely known as A356 cast alloy) machining chips produced in machining cast components of this alloy such as car wheels. Samples from the rods were also T6 heat treated with a condition of 535 °C/1 h-water quenching-165 °C/8 h. The as-extruded UFG rod exhibited a yield strength (YS) of 298 MPa, ultimate tensile strength (UTS) of 345 MPa and elongation to fracture (δf) of 5.9%, which changed to 223 MPa, 342 MPa and 10.2% respectively after the T6 heat treatment. In contrast, the as-extruded CG rod exhibited a YS of 101 MPa, UTS of 187 MPa and δf of 16.6%, which changed to 203 MPa, 275 MPa and 10.8% after the heat treatment. The UFG Al-7Si-0.3Mg alloy sample exhibits a dramatically different precipitation behavior from the CG Al-7Si-0.3Mg alloy sample during the T6 heat treatment, with the formation of fine platelet-shaped pre-β″ precipitates on Al {001} planes in the former sample in contrast of the formation of needle-shaped β″ precipitates along Al <001> directions in the latter sample. The change of precipitation behavior leads to a weaker precipitation hardening effect. This work demonstrated that a significantly higher tensile strength can be achieved with heat treated UFG Al-7Si-0.3Mg alloy than CG Al-7Si-0.3Mg without sacrificing tensile ductility.