Strength-ductility synergy of selective laser melted Al-Mg-Sc-Zr alloy with a heterogeneous grain structure

In this work, a Sc/Zr modified Al-Mg alloy was processed by both selective laser melting (SLM) and directed energy deposition (DED). Due to different precipitation behavior of primary Al-3(Sc,Zr)-L1(2) nucleation sites, a heterogeneous grain structure was formed in SLMed sample, which consisted of ultrafine equiaxed grains bands and columnar grains domains, while a fully equiaxed grain structure was obtained in DEDed sample. Tensile results showed that the as built SLMed sample had a good combination of strength and ductility. The yield strength of SLMed sample (335 +/- 4 MPa) was about 2.8 times that of DEDed sample (118 +/- 3 MPa), however, the ductility in uniform elongation (23.6 +/- 1.9%) was still comparable to that of DEDed sample (23.8 +/- 2.6%). Based on the relationship between the heterogeneous grain structure and strain hardening behavior, the strength-ductility synergy mechanism of the SLMed Al-Mg-Sc-Zr alloy was discussed. Stress partitioning tests showed that the contribution of back stress hardening to flow stress was higher in SLMed sample than DEDed sample, while effective stress hardening showed an opposite trend. Despite the overall strain hardening ability of SLMed sample was limited by the high dynamic recovery rate of ultrafine equiaxed grains, additional back stress hardening, which was caused by strain partitioning between equiaxed grains bands and columnar grains domains, improved its strain hardening ability and resulted in the good combination of strength and ductility.