Effect of heat treatment on the anisotropy in mechanical properties of selective laser melted AlSi10Mg

Anisotropy in mechanical properties is one of the critical issues in metal additive manufacturing (AM) impeding the wider application of parts fabricated using this manufacturing technique. In this work, the anisotropy in mechanical properties of an AlSi10Mg alloy fabricated by AM was investigated in terms of grain morphology, microstructure and residual stress distribution, with a focus on changes occurring during post-AM heat treatment by isochronal annealing. It was found that microstructural inhomogeneity is the major factor that weakens the melt pool boundaries, and leads also to anisotropic mechanical properties. Volumes in the vicinity of melt pool boundaries are characterized by coarse Al–Si eutectic networks and high residual stresses, where the former are the main reason for the plastic anisotropy, and the latter increase the local strength. Post-AM heat treatment can alleviate the presence of anisotropy due to homogenization of the microstructure and a redistribution of residual stresses. The overall effect of heat treatment on mechanical strength is a balance between improvement through precipitation strengthening and weakening of grain boundary strengthening, solid solution strengthening and dislocation strengthening. Through an appropriate heat treatment process (annealing at 280 °C for 2 h), the anisotropy in mechanical properties can be effectively reduced and a good strength-elongation synergy can be obtained (strength of 350 MPa and elongation of ∼9%). This study provides new ideas for understanding and mitigating anisotropy of additively manufactured AlSi10Mg, and also has implications for the study of anisotropy of other materials.