Highest welding efficiency of dissimilar Al/Fe friction stir welding joints achieved in the transition region of fracture mode

The dissimilar deformed pure Al and pure Fe friction stir welding (FSW) joints with a fixed welding speed were investigated using tensile test, microhardness test, scanning electron microscopy (SEM) and electron back-scattered diffraction (EBSD) techniques. With increasing rotation speeds, the welding efficiency increased first and then decreased, corresponding fracture location changed from interface to Al-side. The highest welding efficiency, 98±4%, with a mixed fracture mode was achieved at 360 rpm. The SEM results of the interface showed no intermetallic compound, and both Al and Fe interface fractures showed mutual adherence. The EBSD and microhardness results of the Al-side joint showed an obvious grain refinement in the weld zone, but the microhardness values were low. For the dissimilar FSW, the enhanced heat input made an obvious strengthening of interfacial bonding and softening of Al-side joint simultaneously. When the two strengths are approximately equal, the highest welding efficiency is achieved in the transition region.