Friction stir welding of advanced high strength dual phase steel: Microstructure, mechanical properties and fracture behavior

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.