Dual phase (DP) steels are the most widely used type of advanced high strength steels which have allocated great attention from the automobile manufacturers. Application of these steels in tailored welded blanks needs welding. In this research, microstructure and tensile properties of DP600 steel sheets welded by friction stir welding (FSW) using different heat inputs were investigated. Microstructural and texture studies were performed by optical microscopy, scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Mechanical properties of weldments were investigated by microhardness measurements, tensile test and digital image correlation technique. In order to study plastic deformation and damage mechanisms, the cross section of tensile tested specimens was analyzed using SEM and EBSD. The results showed that with increasing the FSW heat input, the microstructure of stir zone (SZ) become coarser and the volume fraction of displacive transformation products such as Widmanestatten ferrite and bainite increases. The texture of primary austenite in the SZ of sample welded by the lowest heat input is included strong B component and weak R, while for the sample welded with the highest heat input the texture consists of strong R and weak B components. The microhardness profiles showed that tempering of martensite leads to the formation of a softened zone in the heat affected zone (HAZ). The ultimate tensile strength of welded samples decreases with an increase in the heat input, while the yield strength is nearly unaffected. However, a joint efficiency of 87% can be achieved by FSW. Study of the strain distribution at different stages of tensile test showed that after the initiation of plastic deformation, the elongation is locally accumulated in the softened zones until the fracture occur in one of them. All the samples exhibited typical ductile fracture behavior with dimples on the fractured surfaces. Analysis of damage mechanisms for different specimens showed that the dominant void formation mechanism in the DP600 steel BM is ferrite-martensite interface decohesion. By tempering of martensite in the subcritical area of HAZ, the above mechanism become less important and voids start to nucleate at higher strains. EBSD investigations indicated that softening of martensite due to tempering leads to the more uniform strain distribution within the ferrite grains, resulting in the reduction of work hardening rate and enhancement of uniform plastic deformation and fracture strain of subcritical HAZ compared to the BM.
