Melting mode-driven processing diagram for nanoparticle-enhanced high-strength aluminum alloy processed by laser powder bed fusion

Abstract

AA7075 + ZrH₂ (7A76) is a nanoparticle-enhanced high-strength Al alloy, designed to substantially prevent solidification cracking during laser powder bed fusion (LPBF). Significant knowledge gaps persist in understanding the effects of melting modes, the functionality of nanoparticles, and compositional variations in this material system. This study systematically investigates the melting modes in LPBF of 7A76 to achieve defect-free samples. Processing diagrams were generated using dimensionless heat input (E*) and velocity (v*) terms, alongside a physics-based temperature prediction model used to predict melting mode thresholds. A wide operation window was discovered within the transition melting mode region, resulting in defect-free 7A76, reaching a relative density of 99.98 %, reported for the first time. Furthermore, the transition melting mode was effective in lowering the Mg and Zn evaporation. Microstructural characterizations revealed that although melting and solidification during the LPBF process resulted in the dissolution of Zr into the printed alloy, some Zr-rich particles remained unmelted. This work represents the first observation of grain nucleation on the partially melted Zr-rich particles in this modified alloy. Additionally, this work sheds light on the successful printing of nanoparticle-enhanced, crack-prone aluminum alloys using processing diagrams, while elucidating the role of nanoparticles in this process.