Understanding the interfacial bonding mechanism and its impact on mechanical performance in SS316L/Cu functionally graded materials produced by laser powder bed fusion

The microstructural evolution, interfacial bonding mechanism and mechanical properties of the stainless steel 316 L/Cu (SS316L/Cu) functionally graded material (FGM) fabricated by depositing SS316L on Cu via powder bed fusion - laser beam (PBF-LB) were investigated. A combination of experimental characterization and computational modelling were employed for a comprehensive study. Existing studies have mainly focused on depositing Cu on steel . However, in this study, the inverse deposition sequence, i.e. steel on Cu is investigated to fabricate the FGM with fewer defects and desirable elongation and strength. Microscopic analyses revealed that vortex-like intermixing zone can be generated during PBF-LB process, which effectively locked Cu and SS316L phases at the interfacial fusion region (IFR). This intermixing zone enabled a coordinated deformation between Cu and SS316L, and delayed the occurrence of necking and improved the ductility of Cu during the tensile loading of the FGM. However, during the tensile test, the crack propagation from Cu into SS316L reduced the latter’s effective elongation, resulting in an overall FGM elongation ranging from 19 % to 21 %, and a fracture fusion region (FFR) spanning 150–200μm, which consisted of approximately 70–80 % SS316L and 30–20 % Cu. Further analysis identified a dual embedding mechanism at the SS316L/Cu IFR which is featured with nano-structural intermixing, voids embedded within Cu and mutual embedding of Fe and Cu nanoparticles. Apart from the nano-scale mutual embedding, the IFR was also characterized with refined grain structures, high dislocation densities, intermetallic, solid solution, and dominant high-angle grain boundaries and corresponding strengthening and failure mechanisms are discussed. These factors worked together and eventually contributed to the good mechanical performance of the FGM. The findings in this study provided critical insight into the design of dissimilar metal FGMs and validate the steel/Cu FGM for structural applications requiring both strength and ductility.