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

Aim of this study is to analyse how the process parameters affect the mechanical properties and the corrosion behaviour of butt joints obtained by friction stir welding (FSW). The experimental study was performed by the friction stir welding of sheets having a thickness equal to 4 mm and made of three aluminum alloys, namely AA7075, AA6060 and AA2024, considering all the combinations among the three materials and varying the process parameters, namely rotational speed and feed rate. Tensile tests were performed orthogonally to the welding direction on specimens having the welding nugget placed in the middle of gage length, while micro-Vickers tests were carried out on each specimen moving from the joint axis until the hardness of the base material was reached. The best conditions in terms of mechanical strength were obtained using the "intermediate" values of rotational speed, and, in general, when the process parameters result in low values feed rate per unit revolution (F/S), that corresponds to the higher thermal contribution to the joint region. Since in many industrial applications the mechanical resistance is not sufficient for completely describing the joint reliability, further local corrosion potential measurements and four-point bending tests were performed to evaluate the corrosion behaviour and stress corrosion cracking susceptibility of FSW Joints. The tests were carried out on prismatic specimens obtained by FSW joints of the same alloy (7075-7075 and 2024-2024) and mixed joints (7075-2024). No specimens failed during the test. It was observed that the lower the hardness, the more anodic the corrosion potential. In these zones an intense localized attack takes place in the HAZ due to the presence of precipitates. No systematic correlations between the parameters and the resistance to corrosion were observed. The presence of preferential corrosion sites was confirmed also by means of long time immersion tests.