Laser powder directed energy deposition (LP-DED) NASA HR-1 alloy: Laser power and heat treatment effects on microstructure and mechanical properties

This study investigated the microstructure and tensile properties of the NASA HR-1 (Fe-Ni-Cr) alloy using the laser powder directed energy deposition (LP-DED) process. Laser power and spot size were varied for deposition, and it was noted that low laser power could result in higher cooling rates and finer microstructure. The melt pool depth and width and the defect content increased with the increase in laser power. Out of several laser power (i.e., 350, 750, 1070, 2000, and 2620 W), the samples deposited using 750 W demonstrated the highest material density. In addition, the effects of heat treatment on the microstructure and tensile properties were investigated. It was found that a heat treatment schedule consisting of stress relief, homogenization, solution treatment, and aging could thoroughly homogenize the microstructure (i.e., remove the as-solidified dendritic microstructure), result in relatively uniform, equiaxed grains, and increase the material hardness. Importantly, this heat treatment schedule efficiently reduced titanium segregation, preventing the formation of deleterious needle-shaped η (Ni3Ti) precipitates in NASA HR-1, known to be detrimental to mechanical properties and resulted in almost comparable tensile properties with varying laser power.