Effect of welding current on the mechanical performance and microstructural evolution of GTAW-welded AISI 304 stainless steel butt joints

Precise control of heat input is critical in Gas Tungsten Arc Welding (GTAW) of thin-section stainless steels to ensure sound fusion, microstructural stability, and reliable mechanical performance. This study systematically investigates the influence of welding current (15–40 A) on bead morphology, microstructural evolution, and mechanical behavior of autogenously welded AISI 304 stainless steel. To isolate the effect of welding current, all experiments were conducted under constant shielding gas flow rate and travel speed. Microstructural characterization was performed using optical microscopy, while mechanical performance was evaluated through microhardness profiling and tensile testing across the weld metal (WM), heat-affected zone (HAZ), and base metal (BM). The results indicate that welding current strongly governs grain morphology, HAZ width, and overall joint integrity. The base metal exhibited the highest hardness (∼202 HV), whereas the HAZ showed the lowest hardness as a result of thermal softening. Although the maximum ultimate tensile strength (∼350 MPa) was obtained at 40 A, excessive heat input promoted grain coarsening and non-uniform HAZ development, adversely affecting metallurgical stability. In contrast, insufficient heat input at low currents (15–20 A) resulted in incomplete fusion and reduced joint efficiency. A favorable balance between fusion quality and microstructural refinement was consistently achieved within the 25–30 A range, producing fine austenitic grains, smooth hardness gradients, and stable tensile performance. Taken together, these results clarify the structure-property relationships governed by welding current and highlight the importance of heat-input optimization in GTAW. The findings provide practical guidance for parameter selection and quality control in the fabrication of thin-walled AISI 304 stainless-steel components requiring high structural reliability.