Laser powder bed fusion of high-strength low-alloy steels with post-heat treatment design for enhanced mechanical performance

A commonly used structural steel, namely high-strength low-alloy (HSLA) steel, HSLA-100, was fabricated using the laser powder bed fusion process in this work. Critical processing parameters such as laser power and scanning speed for achieving the least porosity were identified. The designed post-heat treatment differs from the traditionally manufactured HSLA steels. The optimum homogenization time was 80 min at 950 degrees C with the lowest prior austenite grain size. The peak hardness was achieved after tempering at 550 degrees C for 5 h, and atom probe tomography showed that the fraction of Cu and M2C (M: Mo, Cr) was the highest along with the co-precipitation of these strengthening phases at this condition. The yield strength (YS) and elongation (%El) of the builds printed with optimized parameters (porosity similar to 0.5%, YS = 875 MPa, and %El = 23%) were superior to those printed with factory-default parameters (porosity similar to 3%, YS = 772 MPa, and %El = 15%) after applying the designed post-heat treatment. Despite the anisotropy observed in the low-temperature toughness, the ductile-to-brittle transition temperature is below -40 degrees C for samples with a notch in the XZ plane and between -20 and -40 degrees C for samples with a notch in the XY plane. This work demonstrates the successful adaptation of HSLA-100 steel in additive manufacturing, and thus is critical for scaling up engineering applications at a higher technology readiness level.