Effect of Micro-Structure and Mechanical Properties on 309L Austenitic Stainless Steel Deposition by Cold Metal Transfer Wire-Arc Additive Manufacturing

Wire-arc additive manufacturing based on cold metal transfer (CMT-WAAM) is a high-precision forming technology suitable for thin-wall stainless steel parts, yet the unclear correlations between linear energy density, forming stability, microstructure evolution and mechanical anisotropy restrict the precision forming and performance control of 309L austenitic stainless steel components. Aiming at the scientific problems of end collapse defects and mechanical performance anisotropy in multi-layer thin-wall WAAM forming of 309L stainless steel, this study systematically explores the regulation mechanism of process parameters and deposition path on forming quality, microstructure characteristics and mechanical properties. The optimal single-track forming process window (wire-feeding speed of 5 m/min, welding speed of 3 mm/s) for 309L deposits is determined using single-track single-layer experiments. Compared with unidirectional deposition, the reciprocating deposition path effectively balances inter-layer temperature field distribution, solves the end collapse problem of multi-layer thin-wall structures, and controls the collapse height within 5 mm after 35 layers of continuous deposition. The variation of linear energy density is the core factor dominating microstructure evolution and performance difference: with an increase in linear energy density, the retained delta ferrite in the matrix transforms from a discrete vermicular to continuous skeletal structure, and austenite grains are significantly coarsened. In terms of mechanical properties, the longitudinal tensile performance of the deposits is stable and less affected by heat input, while the improvement of inter-layer metallurgical bonding quality significantly improves the transverse tensile strength and elongation, effectively weakening the mechanical anisotropy of the formed parts. Meanwhile, grain coarsening and the reduction of delta ferrite content led to a gradual decrease in the average Vickers hardness of the deposits. This study reveals the inherent process–structure–performance correlations of CMT-WAAM formed 309L austenitic stainless steel, clarifies the key mechanism for suppressing forming defects and regulating anisotropic performance, and provides theoretical and technical support for the high-quality forming and engineering application of stainless steel WAAM manufacturing and component repair.