Evolution of Heterogeneous Microstructure and its Effects on Tensile Properties of Selective Laser Melted AlSi10Mg Alloy

To figure out the evolution of the heterogeneous microstructure across the molten pool boundaries (MPBs) and its effects on tensile properties of selective laser melted (SLM) AlSi10Mg alloy, two of the most commonly used scanning strategies, i.e., bidirectional scanning within a layer and with a 90° and 67° rotation for the successive layers (zigzag-90 and zigzag-67) were adopted. Grain morphology, solidification microstructure, texture and room temperature tensile properties of the SLMed AlSi10Mg alloy under the two scanning strategies were studied. The coarse columnar and fine equiaxed grains were observed in a single solidified track under the two scanning strategies. The columnar grains dominated the bulk of the molten pool (MP) while the equiaxed grains mainly distributed along the MPBs. The equiaxed grains along MPBs break the continuous growth of the columnar grains and lead to a weak texture. A new mechanism based on heterogeneous nucleation and constitutional supercooling was proposed to explain the formation of heterogeneous microstructure across the MPBs. The tensile test was performed in the horizontal (perpendicular to the build direction) and the vertical (parallel to the build direction) directions fabricated under the two scanning strategies. The mechanical properties showed obvious fluctuation, and the yield strength of the horizontal direction is generally higher than that of the vertical direction. The distribution of pores and MPBs is the main factors to influence the tensile properties.