Influence of post-heat treatment on the microstructure and mechanical properties of Al-Cu-Mg-Zr alloy manufactured by selective laser melting

The properties of modified conventional wrought aluminum alloys cannot be significantly enhanced by normal post-heat treatment in that the fine-grained strengthening, arising from high cooling rate in SLM, is underutilized. In this work, compared with the normal T6 heat treatment, a novel simple direct aging regime was proposed to maintain the grain-boundary strengthening and to utilize the precipitation strengthening of secondary Al₃Zr. It was found that a heterogeneous grain structure, which consisted of ultrafine equiaxed (∼0.82 μm) and columnar (∼1.80 μm) grains at the bottom and top of molten pool, respectively, was formed in the SLM processed sample. After direct aging (DA), the ultrafine grains were maintained and a mass of spherical coherent L1₂-Al₃Zr particles with a mean radius of approximately 1.15 nm was precipitated. In contrast, after solution treatment and aging (STA), a significant grain coarsening occurred in the equiaxed grain region. Meanwhile, the coarsening L1₂-Al₃Zr particles, nano-sized S′ phases and GPB zones were detected in the STA sample. This subsequently induced that the yield strength of the DA sample (∼435 MPa) was higher than that of the STA sample (∼402 MPa) owing to the grain boundary strengthening and precipitation strengthening. Both the STA and DA samples exhibited a higher strength than that of the other SLMed Al-Cu-Mg series alloys; this was comparable to that of the wrought AA2024-T6 alloy (∼393 MPa). Both the STA and DA samples exhibited a higher strength than that of the other SLMed Al-Cu-Mg series alloys; this was comparable to that of the wrought AA2024-T6 alloy (∼393 MPa).