Study on microstructure and properties of friction stir welding in mild steel

The microstructure and properties in friction stir welds of Q235 steel are studied. The weld surface is well formed, without grooves, pits and other defects. The microstructure of joint is made up of the stir zone, the heat-affected zone, the thermo-mechanically affected zone and basic material zone. The microstructures of the stir zone are fine grain pearlite and ferrite, which with good mechanical properties. However, the average grain size in the whole region of the heat-affected zone is larger than that of basic material zone, and also the macroscopic mechanical property is reduced,which is the weakness area of the joint performance. The basic material zone is composed of a long strip of pearlite and a fine ferrite which is evenly distributed on the matrix. Tensile fracture surface is located near the heat-affected zone. Fracture mode is mainly brittle fracture, local toughness fracture exists, and the tensile strength of joint can reach about 68% of basic metal; the microhardness of the joint is unevenly distributed in all regions, and the overall distribution is approximately "W" shape. (paper)