LAUSR

Abstract In this work the tensile behaviour of selective laser melted (SLMed) aluminium alloy A357 in the as-fabricated and heat-treated states is explained using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and transmission electron backscatter diffraction (t-EBSD). The as-built sample has an ultrafine microstructure, with high residual stresses and non-equilibrium solid solute concentration of Si in the supersaturated Al matrix. Consequently, the tensile properties of the SLMed Al alloy A357 are comparable or better than traditional cast counterparts. The Al grains in the SLMed alloy consist of sub-micron sized Al cells, and both high angle and low angle boundaries are initially occupied by eutectic nano-sized Si particles, which are beneficial for strength but detrimental for ductility. With subsequent solution heat treatment, the Si particles on the low angle cell boundaries (LACBs) dissolve while those at the high angle grain boundaries (HAGBs) coarsen. Simultaneously internal stresses decrease, as does solute content in the matrix. The evolution of these microstructural features explains the improved tensile ductility (at its maximum >23%) and reduced tensile strength for the heat treated SLMed aluminium alloy A357 samples.