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 Al3Zr 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 (MgZn2 and Al3Zr) located within the aged alpha-Al grains.

Automated summary

The addition of Zr, along with large Al3Zr nucleant particles and nanoscale second phases, significantly enhances the microhardness of the AlZnMgCu alloy, making it exhibit superior mechanical properties compared to the Al7075-T6 aluminum alloy.