Influence of Processing Conditions on Unmelted Defect Generation in Maraging Steel Made with Selective Laser Melting

Abstract

The unmelted regions in a material made by additive manufacturing degrade its mechanical properties. First, the effect of processing conditions in additive manufacturing with selective laser melting (SLM) on the mechanical properties of maraging steel was investigated under constant productivity, keeping the volume energy density constant. Various combinations of hatching pitch and scan rate of the laser were considered. Four blocks were fabricated under different processing conditions, and micro specimens cut out from the blocks were used in tensile tests. The microstructure of the blocks was observed and the void area fraction was obtained. The fractured equivalent strain was the largest in the case of the lowest void area fraction. The effect of the void area fraction on the ductility was confirmed. Second, single bead experiments under each processing condition were conducted, and the cross-sectional shapes of beads and molten pools were observed. These cross-sectional shapes were approximated by elliptic arcs and the conditions required for reducing unmelted defects were investigated geometrically. In this study, the molten overlap height in the middle of the neighboring scanning lines was a function of hatching pitch, and the optimal processing conditions for maximizing the molten overlap height were found.