Influences of different Zr additions on the microstructure, room and high temperature mechanical properties of an Al-7Si-0.4Mg alloy modified with 0.25%Er

In this work, three innovative age hardenable Al alloys based on the Al-7Si-0.4Mg-0.25Er system (A356 alloy + 0.25 wt% Er), with nominal additions of Zr between 0 and 0.6 wt%, are designed, cast, heat treated and characterized from a microstructural and mechanical point of view. Zr additions leads to a progressive reduction of the Secondary Dendrites Arm Spacing (SDAS) of the alloys, due to inoculant pro-peritectic intermetallics and growth rate restriction of dendrites during solidification, and a reduction in eutectic Si size. 0.6 wt% Zr also causes a morphological modification of dendrites, which display a globular shape, thanks to the inoculation action of primary Zr-containing intermetallics. Tensile tests performed in the peak aged and overaged conditions reveal the beneficial effect of Zr on the mechanical properties of the alloys; in particular, the alloy with the highest Zr content shows the highest mechanical properties in all the tested conditions. This work shows the potentialities of Zr as an alloying element to increase the microstructural stability and mechanical properties of Al-Si-Mg-Er alloy.