Study on defects, densification, and microstructural evolution of additively manufactured Al-Zn-Mg-Cu-Hf alloy

The present work proposes a new alloy composition design, with 1.5–3.0 wt% Hf added into a conventional high-performance Al-Zn-Mg-Cu alloy, aiming to address the notorious high crack sensitivity faced by metal additive manufacturing (AM). The effects of processing parameters or Hf content on the defects (pores, cracks, and distortions), the densification behavior, and the microstructural characteristics of grains have been investigated. It is demonstrated that by optimizing the laser processing parameters (power and scanning speed), a laser powder bed fused (LPBF) Al-Zn-Mg-Cu-Hf alloy with a high relative density (> 99.9 %) and without unwanted lack-of-fusion pores, keyhole-induced pores, cracks, and distortions is obtainable. In addition, the study reveals that Hf significantly affects the grain morphology, size, and orientation through the formation of primary L1 ₂-Al ₃Hf phase, because L1 ₂-Al ₃ Hf can act as ideal heterogeneous nucleation sites for α-Al. With increased Hf addition, more equiaxed grains replace initial columnar grains, accompanied by a reduction of mean grain size and a more random grain orientation. Particularly, at the maximum Hf addition of 3.0 wt%, a fully equiaxed grain structure is accomplished and the texturing is minimized.