Microstructure and mechanical properties of weathering mild steel joined by friction stir welding

Cracks can occur in the welded joints of weathering mild steel during the conventional welding process, due to the high phosphorus content. The present work studies the microstructure and mechanical properties of weathering mild steel by friction stir welding (FSW), which makes welding possible at lower temperature. The weathering steels SMA490-AW and SPA-H with different chemical compositions were joined by FSW below the A1 temperature. The microstructure of the welded joints was observed by scanning electron microscope (SEM) and electron backscatter diffraction (EBSD), and the micro-hardness was assessed corresponding to the various weld regions. The mechanical properties were then assessed by digital image correlation (DIC). The results suggest that the joints with no defects were obtained for both materials (SMA490-AW and SPA-H) and no softening was observed in HAZ. The FSW joints experienced two steps of yield due to the high micro-hardness and strength of weldment. The joint efficiency of SMA490-AW and SPA-H by FSW could be as high as 100%. A sound ductile behavior was observed for both materials, while the SPA-H, also characterized with enhanced anti-corrosion properties, illustrated a more favorable strength and ductile property than SMA490-AW. •Weathering steel with high phosphorus contents is welded by FSW.•The weld is joined below A1 temperature without any defects or HAZ hardening.•The DIC is used for strain measurement at different regions of the weld.•The influence of micro-hardness on fracture positions is discussed by FEM.