Process optimization and mechanical property evolution of AlSiMg0.75 by selective laser melting

A central composite design of experiment has been carried out to investigate the effect of selective laser melting (SLM) process parameters on the relative density of AlSiMg0.75 alloy and obtain samples with relative density of 99.0624%. Square samples and flat tensile samples were manufactured with optimized process parameters to study the evolution of microstructure, microhardness and tensile property. The microstructure of as-built samples shows fine grains and coarse grains with Al-rich cellular structure and Si-segregated network structure in top view. However, a large number of dendrite grains with very small space are observed in side view. After heat treatment, the cellular and dendrite structure broke into particles of different sizes with microhardness reduction and Si phase appearance by XRD and DSC. The heat treated samples show lower tensile strength and yield strength with the reduction of residual stress and Si solubility, and the destruction of small structures. But the elongation is higher than the as-built ones. The fracture morphology of as-built samples shows cleavages with small and shallow dimples, which indicates a mixture of brittle and ductile fracture. The fracture morphology of heat treated samples shows big and deep dimples, which indicates a ductile fracture.