The influence of process parameters on mechanical properties and corrosion behavior of friction stir welded aluminum joints

Aim of the present paper is to analyze if and how the process parameters (namely rotational speed S and feed rate f) affect both the mechanical properties and the corrosion behavior of friction stir welded (FSWed) butt joints made of AA7075 and AA2024 alloys and their combination. Tensile tests were performed orthogonally to the welding direction on specimens having the welding nugget placed in the middle of gage length. Rockwell tests were carried out on each specimen moving from the joint axis until the hardness of the base material was reached. A clear dependence of the Ultimate Tensile Strength (UTS) from the process parameters were observed when the joints are obtained using the same material while the effect is not so evident when two different materials are concerned. In almost all cases, the best conditions, in terms of mechanical resistance, were obtained for intermediate values of rotational speed and feed rate. Further tests were performed for evaluating the corrosion behavior and the stress corrosion cracking susceptibility of FSWed joints. In particular, local corrosion potential measurements were executed and longtime immersion tests were carried on four points bending specimens loaded up to 80% of the Yield Strength (TYS), in 35 g/L sodium chloride solution. The same tests were replicated on unloaded specimens. The potential trends are always well defined and much higher for the AA2024 alloy, since it is more noble than the AA7075 alloy. Mixed welding allows to identify the AA7075 alloy as an anode part, having a lower copper concentration and a high concentration of zinc. A correlation of some corrosion behaviors to the mechanical characteristics was evidenced, in particular for the AA2024 alloy.