Effect of in-situ heat treatments on deposition characteristics and mechanical properties for repairs using laser melting deposition

This study explores the effects of different in-situ heat treatments on the deposition characteristics and mechanical properties of austenitic stainless steel samples repaired using direct energy deposition (DED). In the DED repair process, defects such as pores and cracks can occur at the interface between the substrate and deposited material. Such interfacial defects are caused by thermal stresses created by differences in thermal expansion between the substrate and powder material, as well as the temperature gradient that results from repeated melting and solidification cycles during deposition. In order to reduce thermal stresses and therefore mitigate crack generation, we implemented in-situ substrate heating and in-situ post heating treatments during the repair process. The in-situ post heating treatment delays the cooling rate by depositing additional material on top of the repaired area immediately after the repair process. Despite the cooling rates being similar with the two in-situ heat treatments, macro-scale cracks were observed in the in-situ post heat-treated samples as well as in the samples repaired without heat treatment (i.e., untreated) owing to a lack of fusion between the substrate and powder material, which led to low tensile properties. In-situ substrate heating provided the highest tensile strength and elongation (increases of 145 % and 767 % compared to those of the untreated specimen, respectively), but the lowest hardness due to grain coarsening from extended exposure to high temperature.