Dissimilar friction stir lap welding of AA2198 and AA7075 sheets: forces, microstructure and mechanical properties

This paper investigates the dissimilar friction stir lap welding of AA2198 and AA7075 sheets. The influence of processing parameters, namely welding speed and tool rotational speed on joint features, microstructure, and mechanical properties were studied by implementing a full factorial design of experiments. Axial and transversal forces were continuously measured during the welding process using a sensed fixture aiming at the correlation of processing parameters, forces, and quality of the achieved joints. Obtained outcomes showed hook formation for all the combination of parameters and the existence of a very narrow processing window in which it is possible to avoid the formation of internal defects, such as grooves and tunnels. The influence of the weld bead morphology on the lap shear strength was elucidated proving that the strength is ruled by the hook morphology. The microstructure of the joints was studied and discussed considering also the microhardness distribution.