Influence of the Zr content on the processability of a high strength Al-Zn-Mg-Cu-Zr alloy by laser powder bed fusion

The objective of this research is to study the effect of the Zr on the L-PBF processability, microstructure, and microhardness of an AlZnMgCu-Zr alloy. Two AlZnMgCu-0.5 and 1.5 wt% Zr pre-alloyed powders were produced by casting followed by gas atomization. In addition, an excess of Mg and Zn was added to the target compositions to compensate for vaporization during L-PBF. The as-atomized powders and the as-built and heat treated specimens were characterized via scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. Crack-free samples with a relative density of 99.0 ± 0.1% were obtained in the alloy containing 1.5 wt%Zr. The as-built microstructure of this alloy consisted of small equiaxed grains without preferred grain orientation near the melt pool boundary and slightly columnar grains between adjacent melt pools. After a solution and aging heat treatment (T6), the newly developed AlZnMgCu-1.5Zr alloy has an outstanding Vickers micro-hardness of 223 ± 3 HV, which is significantly higher than that of the wrought Al7075-T6 aluminum alloy. This excellent mechanical behavior is attributed to the presence of large Al 3 Zr nucleant particles of up to 500 nm in size that inhibit grain growth during solidification and the subsequent heat treatment, and of a high density of nanoscale second phases (MgZn 2 and Al 3 Zr) located within the aged α-Al grains.