Temperature evolution and mechanical properties of dissimilar fricztion stir weldments when joining AA2024 and AA7075 with an AA6056 alloy

Abstract

Three age and precipitation-hardening aerospace grade aluminium alloys (AA2024, AA7075 and AA6056) have been joined by the friction stir welding process to explore the potential of combining dissimilar aluminium alloys, particularly because they have proven difficult to weld using traditional fusion welding techniques. Four weld combinations were produced utilising the AA6056 alloy together with both the AA2024 and AA7075 alloys. In the case where the AA7075 and AA2024 alloys were placed in the advancing side of the join i.e. where tool rotation corresponds with weld travel direction it appeared that the weld nugget possessed a better flow/mixing pattern than when the AA6056 alloy was placed in the same location. Temperature measurements indicate that this is also the cooler weld configuration under constant processing parameters. In all cases, regardless of alloy combination, process temperatures could be observed to be higher for the side containing the AA6056 alloy. Hardness profiles made across the weldments correspond to material flow patterns with clear delineation existing between each alloy. Material flow patterns suggest that mixing improves in both the shoulder induced flow zone as well as in the region of the weld root in correspondence to increasing weld travel speed. Tensile properties were also observed to improve with increased weld travel speed. Contrary however to material flow and mixing behaviour the placement of the AA6056 alloy in the advancing side of the join led to improved tensile strength compared to when this alloy was placed in the retreating side of the join i.e. where tool rotation opposes weld travel direction.