Similar and dissimilar lap friction stir welding of titanium alloys: on the elimination of the hook defect

This work investigates similar and dissimilar lap FSW of commercially pure titanium (CP-Ti) and Ti6Al4V sheets welded with two different tool shapes, truncated conical and enlarged pin heads. It aims to assess the influence of the two tools on the formation of the hook defects, mechanical strength, and fracture mode. The spindle torque and vertical force have also been monitored during the experimental FSW tests to relate the welding practice to each joint configuration. The joint strength of the FSW joints has been compared with that of joints obtained with laser welding to compare them with the most competitive fusion welding technique. The welding data have shown that the spindle torque and vertical force are related to the upper sheet strength and thermal conductivity. The dissimilar joints were particularly affected from the hook defects when welded with the truncated conical pin tool. This phenomenon was highlighted in the AS for the Ti6Al4V/CP-Ti joints and in the RS for the CP-Ti/ Ti6Al4V joints due to asymmetrical material flow between AS and RS and to the different material properties of the two alloys. The hook defect is absent in the joints obtained with the enlarged pin tool due to the more vigorous metal stirring. However, these welds gave a higher variability during the tensile test which was traced back to the superior variability in the monitored vertical force (from 10 kN to 23 kN). This evidence suggests the difficulty of adopting complex tool features in FSW of high strength-to-mass ratio materials as noticed from the “built up edge’’ in the enlarged pin tool at the end of the weld.