Minimizing residual stress and fatigue crack propagation rate of FSW joints of AA2024-T3 by transient thermal tensioning: Effect of heater distance

Abstract

Friction stir welding (FSW) is a solid-state welding process that is suitable for joining hardly weldable metals such as aircraft AA2024-T3 aluminum alloy. Despite FSW owns many advantages, however, some problems still arise, especially welding residual stress which influences the weld fatigue performance. In the present work, in-process transient thermal tensioning (TTT) treatment was applied to the FSW process of AA2024-T3 aluminum alloy by putting two symmetrical heaters at the sides of the weld line at distances of 25 mm, 40 mm, and 55 mm. The FSW was conducted at the tool rotational speed of 1500 rpm and tool traveling speed of 30 mm/min whereas the heating temperature was set at 200ºC. Subsequently, changes in microstructure, strength, hardness, residual stress, and fatigue crack propagation rate under TTT treatment were evaluated. The results showed that the use of TTT generated peaks of tensile residual stress along the heater passage which changed the residual stress distributions. It was found that the best fatigue crack propagation resistance of the weld occurred at the heater distance of 25 mm which was attributable to the compressive residual stresses present in the weld region induced by thermal tensioning combined with re-precipitation during welding.