Microstructures and mechanical properties of laser additive manufactured Al-5Si-1Cu-Mg alloy with different layer thicknesses

The change of layer thickness can effectively influence the solidification condition and thermal history of the Al-5Si-1Cu-Mg aluminum alloy fabricated by laser additive manufacturing, and thus affect the microstructures and mechanical properties. Samples with ultrafine grain structures and excellent mechanical properties were successfully fabricated, and the microstructure and mechanical property differences related to the variations of layer thickness were systematically investigated. Results indicate that when higher layer thickness is employed, columnar grain structure is established with {100} Al texture due to high temperature gradient, while decreasing the layer thickness promotes the formation of equiaxed grains. Lower layer thickness sample with finer near-equiaxed grain structure has improved elongations. However, the strength is unexpectedly poorer than the higher layer thickness sample with near-columnar grain structure. The comparably better plasticity but poorer strength is attributed to the relatively coarser microstructure within grains, i.e. larger primary dendrite arm spacing with Si phases distributing more dispersed, introduced by more heat effect of subsequent deposition when applying lower layer thickness. For near-columnar sample, the cracked Si phases with different spacings perpendicular to loading direction would result in different microcrack coalescence behaviors and thus cause plasticity anisotropy. Whereas for near-equiaxed sample, due to the tailored distribution of Si phases, the direction-dependent elongation discrepancy is reduced.