Microstructural development process and strengthening mechanism of Al-Ni-Zr high-strength alloy processed via laser powder bed fusion

In this study, we fabricated an Al-Ni-Zr alloy using laser powder bed fusion (PBF-LB), and systematically investigated its processability, microstructural development process, and the relationship between the microstructures and mechanical properties. Cracks originated from delamination were generated in the PBF-LB specimen along melt pool boundaries under preheating at 35°C, meanwhile preheating at 200°C could avoid the cracks due to suppression of thermal stress/distortion. The as-built PBF-LB specimen had a high relative density of greater than 99.9% and exhibited extremely excellent room-temperature strength, which is comparable to that of a heat-treated ultra-super duralumin. The PBF-LB specimen consisted of bimodal crystal grains with fine equiaxed and columnar morphology, owing to heterogeneous nucleation by the primary crystallized Al3Zr phase, which has crystallographic coherency with an α -Al matrix phase. Nickel solute element was crystallized at cell boundaries to form α-Al/Al3Ni eutectic phase. Meanwhile, zirconium and nickel solute elements were partially dissolved in the α-Al solid solution due to rapid solidification by laser melting. Microstructural observation results suggested that the major strengthening factors for the Al-Ni-Zr PBF-LB specimens should be grain refinement strengthening by the fine bimodal crystal grains and dispersion strengthening by the Al3Zr phase.