Effect of heat treatment on the microstructure and anisotropy in mechanical properties of A357 alloy produced by selective laser melting

Highlights: • Limited fraction of refined Fe intermetallics is identified the first time by EDS in as selective laser melted aluminium alloys • The yield strength of A357 alloy is modelled for various heat treatment schemes by taking stress relief as critical step • Anisotropy in ductility between horizontally and vertically built samples diminish upon long time solution heat treatment A357 alloy was manufactured by selective laser melting (SLM) and subjected to different heat treatments, by considering the stress relief step as critical, to establish the microstructure-mechanical property relationship and model the yield strength. For the as-SLM processed condition, the imposed fast cooling rate refines and limits the amount of Fe-rich intermetallic and eutectic Mg₂Si phases, and enables a higher level of supersaturation of solutes. The latter leads to significant yield strength contributions from solid solution strengthening and natural ageing. The inter-cellular Si network within the columnar grains remains unchanged but small Si-rich particles appear within cells after direct ageing. The Si network breaks up during stress relieving, leading to high ductility. The lower ductility for the vertically-built samples under as-SLM processed, directly aged, and stress relieved conditions is due mainly to the finer Si networks brittle nature. Porosity resulting from trapped hydrogen gas enlarges significantly with solutionisation, deteriorating the ductility. Grain growth is not obviously observed with heat treatment. Si particles coarsen preferentially at grain boundaries initially, however, they distribute more evenly by coarsening within the grains as well during prolonged solutionisation. Anisotropy in both yield strength and ductility subsequently disappears when a more homogeneous microstructure is obtained.