Study on the interfacial characteristics and crack propagation of 630 stainless steel fabricated by hybrid additive manufacturing (additional DED building on L-PBFed substrate)

In this study, the interfacial characteristics of materials deposited by hybrid metal additive manufacturing (AM) processes (laser powder bed fusion (L-PBFed) and directed energy deposition (DEDed)) were investigated. The deposition characteristics, microstructure, bonding properties, and crack propagation characteristics at the interface were analyzed with different relative building directions of L-PBFed and DEDed. The additional layers by DED were deposited on the top surface (L-PBFed-Tracked surface) and on the side (L-PBFed-Layered surface) of the substrate built by L-PBFed. Defects occurred at the interface between PBFed and DEDed during the DED process due to the oxidized powder and non-uniform distribution of powders on the rougher surface of L-PBFed-Layered compared to L-PBFed-Tracked. Moreover, owing to the difference in the laser absorption rate and cooling rate caused by the difference in surface roughness, the retained austenite fraction of the DEDed region appeared different. In particular, the DEDed region exhibited a higher retained austenite fraction because it had a higher content of austenite-stabilizing elements than the L-PBFed region; thus, the hardness decreased signifi-cantly. In some parts of the heat-affected zone, the hardness increased significantly because of aging effect. An impact test was performed to investigate the crack-propagation characteristics at the interface according to the relative building directions of DEDed and L-PBFed. Consequently, despite the interfacial defect at the interface of the DEDed and L-PBFed depositions, the resistance to cracking increased when the crack-propagation direction was perpendicular to the L-PBFed layer plane. However, even if there were no defects at the interface of the DEDed and L-PBFed parts, cracks progressed rapidly after a low plastic deformation in the specimen whose crack propagation direction was parallel to interlayer plane. This suggests that the characteristics at the DEDed/L-PBFed interface vary considerably depending on the relative building directions of the two metal AM processes.