In this study, butt welds of an advanced high strength dual phase steel were fabricated using friction stir welding at a constant rotational speed of 800 rpm and different transverse speeds of 100, 150 and 200 mm/min. It was observed that sound welds can be obtained at transverse speeds of 100 and 150 mm/min. At transverse speed of 100 mm/min, the entire stir zone processed at temperatures higher than Ac3. By increasing transverse speed to 150 mm/min, temperature in the bottom region of the stir zone lied between Ac1 and Ac3, while the middle and the top regions experienced temperatures higher than Ac3. The lower peak temperature in the bottom region of the stir zone resulted in finer and harder microstructure. Transmission electron microscopy analysis revealed that the martensite phase partially tempers during non-isothermal tempering of the outer part of the heat affected zone, which gives rise to the softening phenomenon. It was also found that increasing strength of the softened HAZ, decreasing width of the softened HAZ as well as increasing the strength of the stir zone, owing to increasing tool transverse speed, results in higher overall strength and ductility of the joint.@en

The heat-affected zone (HAZ) softening is considered one of the most significant challenges during welding of ferrite–martensite dual-phase (DP) steels. In fact, the strain localization in the softened area results in a premature fracture that degrades the mechanical properties of the joint. Herein, the objective is to investigate the effectiveness of improved cooling using a high thermal diffusivity backing plate (BP) to reduce HAZ softening and enhance the mechanical properties of friction stir-welded DP700 steel. Accordingly, friction stir butt welding of DP700 steel was conducted using copper and mild steel BPs. The findings show that the replacement of steel BP with copper significantly reduces the weld temperature. It is also observed that the thermal diffusivity of the BP has a substantial effect on the microstructure development and mechanical properties of the welded joint. Increasing the thermal diffusivity of the BP results in a reduction of HAZ softening from 20 to 7 HV and improving tensile strength and total elongation of the joint by 40 MPa and 6.2%, respectively. Work hardening behavior and absorbed energy before the fracture are significantly improved by the replacement of steel BP with a copper.@en