Effect of In-Situ Post Heating on Repairing STS316L Built by Laser Powder Bed Fusion Using Direct Energy Deposition

Abstract

The repairing parts were fabricated by PBF (powder bed fusion) using direct energy deposition (DED), interfacial cracks can occur at the interface between the substrate and the area to be repaired. Such interfacial defects are due to the thermal stress induced by the temperature gradient which results from repeated melting and solidification during the powder deposition by laser. These cracks degrade the mechanical properties of the repaired parts. Therefore, in this study, in-situ post heating is proposed to lower the cooling rate of deposited layers directly after repairing, in which several layers are additionally deposited onto the repair zone. To investigate the effect of the in-situ post heating, we have studied the microhardness, microstructure, and tensile properties according to the post heating, as well as the occurrence of cracks. The experimental results showed the formation of macro-scale cracks in the absence of post heating, whereas only micro-scale cracks (10 μm or less) were observed in the repaired sample with a low repair depth in the presence of post heating. Meanwhile, regardless of the use of in-situ post heating, complex dendritic structures (columnar and cellular shapes) were found on the deposited layer, which also appeared in the substrate built by PBF. Similar to the microstructure, no hardness changes were observed in the deposited layer; however, the hardness of the DED repaired zone tended to be slightly lower compared to that of the PBF substrate. For a specimen with a repair depth 1 mm, the tensile strength and elongation of the specimen repaired with the in-situ post heating increased by 8 and 13%, respectively, compared to the specimen repaired without the in-situ post heating. However, in the specimen with a large repair depth (2mm), macro-scale cracks occurred, which led to degradation of tensile properties. Key words: direct energy deposition (DED), crack, post heating, microhardness, tensile test