Residual stresses in friction stir welded Al sheets

Friction stir welding (FSW) is a relatively new joining technique particularly for aluminum alloys that are difficult to fusion weld. A potential field of application is aircraft structures where cost and weight can be reduced by using new joining techniques instead of riveting. Efforts are currently being made to qualify FSW for this purpose. In this study, the influence of a coolant applied during welding of Al sheets on the residual stress state of the FSW joint was investigated. Liquid CO2 coolant was applied near the weld seam for rapid cooling of the weld zone. The residual stresses across the weld were measured by neutron diffraction. Three sheets were produced, one without cooling, and two with cooling, where two different distances of the coolant nozzles from the FSW tool were chosen. The results show that, by applying a coolant, the magnitude of the tensile stress in the center of the weld can be reduced significantly.