Microstructure and Mechanical Properties of Low-Carbon Q235 Steel Welded Using Friction Stir Welding

This study focuses on the microstructure and mechanical properties of the joints of Q235 mild steel, which was formed by the friction stir welding (FSW). The results indicated that, after the FSW, the heat-affected zone (HAZ) of the retreating side (HAZ_RS) and the HAZ of the advancing side (HAZ_AS) recovered under the influence of the heating cycle. The transformation of the phases in the thermo-mechanically affected zone (TMAZ) of the retreating side (TMAZ_RS), the stir zone (SZ) and the TMAZ of the advancing side (TMAZ_AS) generated the pearlite and acicular ferrite. The continuous dynamic recrystallization occurred in all the three zones, whereas the grains were refined. The SZ mainly consisted of D₁, D₂ and F shear textures, while the TMAZ_AS was made up of only the F shear texture. The fine-grained structure, pearlite and the acicular ferrite improved the hardness and tensile strength of the joint. Its ultimate tensile strength was 479 MPa, which was 1.3% higher than that of the base metal. However, the uniform elongation was 16%, which showed a decrease of 33%. The fracture was a ductile fracture with the appearance of dimples. Besides, the joints of the FSW showed an excellent bending performance.