Additive Manufacturing: Additive Manufacturing of Copper-Bearing High-Strength Low-Alloy Steels using Laser Powder Bed Fusion with Post-Heat Treatments

Copper-bearing high-strength low-alloy (HSLA) steels possess high strength, excellent low-temperature toughness, and good weldability. The high strength is attained due to Cu clusters and M₂C precipitate formation during aging, while the superior weldability is mainly due to the low carbon content. The superior properties achieved due to the synergistic effects from strengthening precipitates and alloy composition facilitate these steels to be suitable for naval applications. In this work, HSLA steels were fabricated using the laser powder bed fusion (LPBF) process. Pre-alloyed powders are manufactured based on uncertainty quantification of alloy composition using an ICME framework. The critical printing parameters such as laser power and scan speed were optimized, and the optimum processing window for achieving the least porosity was identified. The porosity was found to be ~0.5% for the optimized builds. The post-heat treatment was guided by thermodynamic calculations after verification by key experiments. In order to achieve the best performance, the optimized post-heat treatment is different from the one applied on traditionally manufactured HSLA steels. Atom probe tomography (APT) results showed that the fraction of Cu and M₂C (M: Mo, Cr) was the highest after 5 hours of aging. Moreover, co-precipitation of Cu and M₂C precipitates were also observed. The mechanical properties of the builds printed with optimized parameters and those printed with factory-default parameters for SS316L (porosity~3%) were compared. Improved tensile properties such as yield strength and ductility were obtained for the builds printed with optimized parameters. The low-temperature toughness was found to be anisotropic, although Charpy toughness reaches the design target. The ductile-to-brittle transition temperature was below -40^oC for samples with a notch in the XZ plane, while it was between -20 and -40^oC for samples with a notch in the XY plane.