Effect of welding speed on the microstructure and texture development in the individual weld zone of friction stir welded DP780 steel

Abstract

In this study, microstructural and texture development in the individual weld zone during friction stir welding (FSW) of DP780 steel at similar tool rotation speed (200 rpm) different welding speeds (200 mm/min and 400 mm/min) was investigated. Phase transformations and competition between recovery and recrystallization mechanisms in the distinct weld zones were explored by electron back scattered diffraction (EBSD). More pronounced grain refinement was observed in stir zone (SZ) of sample welded at higher speed (0.62 μm) as compared to lower welding speed (0.51 μm) which is attributed to the enhanced Zener pinning effect at higher welding speeds. Continuous dynamic recrystallization (cDRX) was found to be the dominating recrystallization mechanism in SZ of both welded samples. Orientation distribution function (ODF) based texture investigations of SZ revealed the development of D1 {1¯1¯2}<111> and D2 {112¯}<111> shear components along with the cube {001}<100> texture. The lower texture intensities witnessed in SZ of higher welding speed sample was due to the accelerated recrystallization kinetics at higher welding speeds. The microstructural investigations of thermo-mechanically affected zone (TMAZ) of both welded samples revealed that cDRX was predominant recrystallization mechanism while microshear bands, grain boundary bulging and geometric DRX (gDRX) also assisted the grain refinement. Texture analysis showed the development of D2 {112¯}<111>, J{110}<11¯2> and J¯{1¯1¯0}<1¯12¯> shear components in the TMAZ lower welding speed sample and D1 {1¯1¯2}<111>, D2 {112¯}<111>, J{110}<11¯2> and J¯{1¯1¯0}<1¯12¯> shear components in the TMAZ of higher welding speed sample. Moreover, heat affected zone (HAZ) of both samples showed higher evolution of low angle grain boundaries (LAGBs) indicating that the grains are in the dynamic recovery stage due to lower temperatures and strain.