Characterization of multilayered multipass friction stir weld on ASTM A572 G50 steel

A multilayered multipass friction stir weld (MM-FSW) on ASTM A572 Grade 50 steel was characterized to understand its potential application for thick-section structures. The 15-mm-thick section was fabricated by stacking three steel plates and then friction stir welding the plates together in a total of five passes. The unique butt/lap joint configuration encountered in the multilayer weld was examined to understand the effect of tool rotation direction on the joint quality, especially the formation of hooking defect. Charpy V-notch impact toughness tests showed generally higher impact toughness energy for the stir zone than the base metal with a ductile fracture mode. The microhardness value was measured from 195 to 220 HV in the stir zone, while the base metal showed an average value of 170 HV. The microstructure in the stir zone and the adjacent heat-affected zone was quantified using optical and scanning electron microscopy (SEM) including electron backscatter diffraction (EBSD). The increased toughness and hardness were correlated with the refined microstructure in the stir zone, resulting from severe plastic deformation and subsequent dynamic recrystallization during friction stir welding.