Influence of in-situ thermal conditions on the microstructure and mechanical anisotropy of WAAM-fabricated 316LSi structures

Abstract

Wire arc additive manufacturing (WAAM), a subset of additive manufacturing, is emerging as a viable method for fabricating metallic parts. Among the filler materials used, austenitic stainless steel 316LSi is notable for its corrosion resistance and mechanical properties, making it suitable for industries such as transport and oil and gas. However, WAAM-fabricated 316LSi components often exhibit anisotropic behavior at both micro and macro levels. This study investigates the influence of in-situ thermal conditions—specifically melt pool temperature and cooling rates—on the microstructure and mechanical properties of WAAM-produced structures. Walled structures were fabricated using the cold metal transfer (CMT) variant of WAAM. Melt pool behavior was monitored using a high dynamic range (HDR) camera, while thermocouples were employed to determine temperature gradients. Post-deposition surface features were analyzed using parameters such as R-profile and P-profile, along with waviness matrices, to reconstruct surface morphology under varying welding parameters. Microstructural analysis revealed the presence of lathy and skeletal ferrites in an austenitic matrix, with approximately 7% δ-ferrite formed due to rapid interlayer cooling. Mechanical testing indicated directional dependence: samples cut parallel to the deposition direction exhibited higher yield strength (Rp0.2) and tensile strength (Rm) than those cut perpendicularly, although toughness was lower compared to wrought samples. Hardness profiling across the wall thickness showed an increase from bottom to top, correlating with thermal gradients. These findings highlight the impact of thermal conditions on anisotropy in WAAM components and underscore the need for further investigation into optimizing processing parameters to improve structural performance.