The effect of hot isostatic pressing on the microstructure and tensile properties of an unmodified A356-T6 cast aluminum alloy

In this paper, the effect of HIPping process on the microstructure and tensile properties of an unmodified sand cast A356-T6 aluminum alloy was studied. The microstructure and tensile fracture surfaces of the alloy were examined by transmission electron microscope (TEM), scanning electron microscope (SEM) and optical microscope. The results show that sub-grain boundaries are formed by HIPping process, and some silicon precipitates are formed at the sub-grain boundaries during aging hardening. The needle-shape precipitates are Mg2Si particles according to SED pattern analysis. The lattice misfit between Mg2Si and aluminum matrix is about 0.256% for [1 1 1]Al//[4 1 0]Mg2Si[1 1 1]Al//[4 1 0]Mg2Si HIPping process significantly reduces porosity volume fraction and pore sizes and thus improves ductility. However, the tensile strength is improved very marginally due to the brittle nature of the unmodified coarse microstructure. The sub-grain boundary formed in the HIPping process has not shown significant influence on the tensile properties. For the studied alloy with large secondary dendrite arm spacing (SDAS) (above 80 μm), the tensile fracture exhibits a transgranular mode (along the cell boundaries) with quasi-cleavage feature.