Material Flow Behavior and Mechanical Properties of Dissimilar Friction Stir Welded Al 7075 and Mg AZ31 Alloys Using Cd Interlayer

Abstract

Friction Stir Welding (FSW) has great potential over fusion welding for joining dissimilar alloys. However, formation of intermetallic compounds (IMCs) Al12Mg17 and Al3Mg2 in FSW of Al/Mg alloys is almost unavoidable and problematic for the research community. Such IMCs in the form of continuous layer provide easy crack propagation path which increases brittleness and reduces the joint strength of the weld. To improve the material flow behavior and reducing brittleness, accurate and precise attempts were taken to join two dissimilar alloys i.e. Al 7075 and Mg AZ31 with the incorporation of cadmium interlayer first time. Tool rotational speed of 1300 rpm and traverse speed of 20 mm/min were maintained throughout the welding process. Various phase formation and their distribution in the nugget zone has been studied in details by using optical microscopy, SEM–EDS and XRD. The supersaturated solid solution (SSSS) of Mg in Al, and a thin layer of Al3Mg2 and small particles (~\u20091 µm) of Al12Mg17 have been observed in the stir zone. During processing Cd reacted with Mg and formed CdMg and CdMg3 which have been distributed in the upper-middle portion of Mg region as intercalated lamellar structure. Formation of these compound provided micro-mechanical interlocking. Further, Al material has been found to be deposited into the Mg region in the form of elongated balloon which provided anchoring effect due to mechanical interlocking. The maximum tensile strength achieved is 129 MPa which is higher than the previously reported dissimilar FSW of Al 7xxx series alloy with Mg alloy. Fractographic study of the dissimilar FSW tensile sample was carried out to understand crack propagation path and scope for further improvement in the weld joint strength.