Strain rate-sensitive behavior and failure mechanisms of additively manufactured 309LSi stainless steel for next-generation structures

This work explores how different crosshead speeds affect the tensile properties and fracture behaviour of ER309LSi austenitic stainless steel (ASS) fabricated using Cold Metal Transfer-based Wire Arc Additive Manufacturing (CMT-WAAM). To understand how strain rate influences mechanical behavior, tensile tests were performed on circular walls built by CMT-WAAM, with test speeds set at 0.1 mm/min, 2 mm/min, and 5 mm/min. As the testing speed increased, both yield and ultimate tensile strength improved by 15–20%, which is consistent with strain rate-sensitive behavior were limited time for dislocation motion increases resistance to deformation. However, the increased strength came with reduced ductility, evident from the drop in elongation at break. At lower test speeds, the fracture surfaces showed clear signs of necking along with plenty of dimples like typical features of ductile failure. As the speed increased, those ductile features started to fade. The fracture surfaces became noticeably flatter, with less plastic deformation. The microstructure showed mostly columnar austenitic dendrites, along with traces of ferrite scattered through the matrix. Microhardness was around 220 HV 0.5 across the sample. Specimens tested along the 90° direction showed slightly higher strength, hinting that the material's response doesn’t vary much with orientation. Taken together, the results provide a clear understanding of how strength and ductility evolve with testing speed, offering critical insights for the development and performance optimization of next-generation WAAM-built ER309LSi stainless steel components subjected to varying strain rate conditions.