Achieving high-performance SS304 joints through dynamic recrystallization in friction stir welding

Abstract

The remarkable mechanical and corrosion resistance of SS304 stainless steel makes it a popular choice for structural and industrial applications. However, achieving joint integrity in its weldments remains challenging due to microstructural instability and thermal distortions. This study investigates the effect of heat input and dynamic recrystallization (DRX) on the microstructure and mechanical properties of SS304 during Friction stir welding (FSW). Optimized parameters (50 mm/min feed rate, 1120 rpm tool rotation, 1.5° tilt angle, conical tungsten carbide tool) were used to produce defect-free welds. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) analyses revealed substantial grain refinement in the stir zone, with the average grain size reduced from 13 µm to 10.1 µm, a 22.3% reduction attributed to DRX. Fine and ultrafine DRX grains dominated the stir zone, while partially recrystallized grains were observed in the transition zone. KAM and grain orientation spread (GOS) analyses indicated the presence of 8.2% fully recrystallized grains and confirmed DRX-induced misorientation. A 31.5% reduction in HAGB in the stir zone further supported subgrain formation. The welds retained 92.8% of the base material's tensile strength and showed a 4.29% increase in microhardness. Charpy impact testing revealed improved toughness over the base material. Fractography showed a shift from ductile fracture in the base metal to mixed-mode failure in the weld zone. These findings establish a strong correlation between heat input, material flow, and DRX in producing high-quality SS304 joints. FSW is demonstrated as a promising alternative to fusion and microwave welding, offering refined microstructures and enhanced mechanical performance. Further parameter optimization is suggested to maximize DRX and minimize localized brittleness.