Influence of dwell time on mechanical and microstructural anisotropy of additively manufactured SS 316 L: Experimental and numerical investigation

Abstract

Wire arc additive manufacturing (WAAM) has several benefits, such as high deposition rates, the capability to print large sizes of objects, and less environmental impact. Higher deposition rates come at the cost of high heat accumulation, which drastically affects the part performance. An appropriate interpass dwell time is necessary to balance deposition rates and heat accumulation. In this study, WAAM deposits of SS316L are made with four different dwell times (0 s, 15 s, 30s, and 45 s), and the tensile performance was evaluated along three different directions (0°, 45°, and 90°). A three-dimensional transient finite element-based numerical model was also developed to understand the effect of dwell time on temperature evolution and heat accumulation during the process. Thermal gradient in the build direction indicates heat conduction towards the substrate. This resulted in directional solidification, which is responsible for the epitaxial grain growth in the build direction. Microstructural analysis revealed the presence of austenite (γ) and residual ferrite (δ). The microstructure obtained from electron backscattered diffraction maps indicates fiber texture, i.e., crystallographic orientation and presence of second-phase particles, which leads to anisotropy with mechanical properties. The tensile test results reveal significant anisotropy with the best mechanical performance along the 45° direction. The texture becomes stronger with dwell time and affects the anisotropy. The fractography results show the presence of dimples and conformed to the ductile failure mode. Some facet surfaces and shear fracture marks were seen in the samples oriented at 90^o (build) direction. The presence of facet and shear failure marks suggests localized brittle failure. Based on the tensile properties and microstructural features, a dwell time of 15 s can be considered as the optimum dwell time.