Influence of the Process Temperature on the Properties of Friction Stir Welded Blanks Made of Mild Steel and Aluminum

Abstract

Hybrid components made of steel and aluminum sheet metal are a promising approach for weight reduction for automotive applications. However, lightweight components made of steel and aluminum require suitable joining technologies, particularly if forming operations follow after the welding process. Friction Stir Welding (FSW) is a promising solid-state welding technology for producing dissimilar joints of steel and aluminum. Within this work dissimilar butt joints were produced using sheet metals of mild steel DC04 and the aluminum alloy AA6016 with a thickness of about 1 mm. The FSW joints show approximately 85 % of the tensile strength of the aluminum base material. In metallographic investigations of the produced FSW blanks it was found that the microstructure in the area of the weld seam changes in the aluminum alloy due to the process temperature and plastic deformation. Due to temperature dependent changes of precipitations of the aluminum alloy, temperature measurements have been carried out during the welding process. To find an explanation of the reduction in tensile strength of the FSW joints, short time heat treatment experiments in the temperature range between 250 °C and 450 °C were performed using the aluminum base material. Heat treatments in the temperature range of the measured process temperature result in a reduction of the tensile strength of about 20 % regardless the annealing time.