The Effects of Process Parameters on Mechanical Properties and Corrosion Behavior in Friction Stir Welding of Aluminum Alloys

The present study was carried out to evaluate how the process parameters affect the mechanical properties and the corrosion behavior of joints obtained by friction stir welding (FSW). The experimental study was performed by means of a CNC machine tool for the friction stir welding of two aluminum alloys, namely AA7075 and AA2024, taking also into account the combination between the two materials. The joints were executed varying the process parameters, namely rotational speed and feed rate. Tensile tests and hardness tests were carried out to evaluate the mechanical properties of the joints. The corrosion behavior of welded specimens was analyzed by means of local free corrosion potential measurements to determine anodic and cathodic areas of welds. The results evidenced that the low hardness areas have the free corrosion potential more anodic than the nearest zones. The differences of potential between the different areas of the welding have the consequence of galvanic corrosion of the less noble area. The location and the extension of the anodic areas depend both on the alloy and on the welding parameters. The preferentially corrosion of these areas were confirmed by means of long time immersion tests. The attacks morphology depends on the alloy: in AA2024 a severe crevice and pitting attack takes place, whereas the AA7075 shows exfoliation corrosion along the rolling bands. Coupling the two different alloys, a severe galvanic attack takes place on the AA7075, in the correspondence of the lower hardness areas. The decreasing of hardness and the different electrochemical behavior in the correspondence of the welding were due to the microstructural alteration of the alloys during the FSW. The correlation between process parameters and joints properties allowed to identify the most suitable welding conditions.