2025 Volume 74 Issue 9 Pages 551-557
In this study, we systematically investigated the effects of number of laser scans per layer (once and twice) combined with two different laser powers (140 W and 400 W) on the porosity, microstructure, hardness, and residual stress values of martensitic stainless steel SUS420J2 fabricated by the laser powder bed fusion (LPBF) method. As a result, there was a significant difference in porosity depending on the laser power when laser-scanned once per layer. However, laser-scanned twice per layer, the porosity was decreased by the progress of melting. There were no significant differences in the constituent phases of each material, because the cooling rate during solidification was high regardless of the laser power or the number of laser scans per layer. On the other hand, in all materials, the volume fraction of the retained γ phase in the middle and lower area were higher due to the thermal effect from the upper layers during build up layers, and the hardness values became lower comparing with the upper area. When the laser power for the first scan was low, the porosity remained high even though laser-scanned twice per layer, so the residual stress value also remained low. On the other hand, when the laser power for the first scan was high, the very low porosity caused the generation of high residual stress. Furthermore, when laser-scanned twice per layer, although the stress was redistributed by remelting, the residual stress value increased because the strain caused by resolidification was added.
