X-ray tomography and characterization of dissimilar interface revealing the interplay of intermetallics, interlocking, and voids on joint strength of Al6061 and AZ31Mg dissimilar friction stir welds

Abstract

This work focuses on the analysis of the combined effect of intermetallic, interlocking, and voids on the joint strength of Al6061 and AZ31 Mg dissimilar friction stir welds. The FSW of Al6061 and AZ31 Mg is performed with different tool rotation speeds and feed rates. Tensile test specimens are prepared from these joints and subjected to X-ray tomography, which reveals the serrations and voids at the Al6061/AZ31 interface. Here, Serrations interlock the materials and increase the interface area. Serrations pitch is higher in shoulder driven region (1.4 – 0.6 mm) compared to pin driven region (0.5 – 0.2 mm). The Al6061 and AZ31 Mg interface is also accompanied by voids, where void fraction is around 0 % near the top and 2–7 % near the bottom. SEM, EDS mapping, and XRD 2-theta scans of the Al6061/AZ31 Mg interface are utilized to investigate the intermetallic compounds. Al3Mg2 and Al12Mg17 intermetallic compounds are observed at Al6061/AZ31 Mg interface. Further, the intermetallic compound thickness increases from 2.8μm to 5.3 μm with increase tool rotational speed from 500 rpm to 800 rpm. Based on the variation of joint strength against these parameters, the intermetallic compound thickness is the most detrimental contributor to the joint strength, followed by void volume and interface area, for the Al6061 and AZ31 Mg dissimilar joints. The maximum joint strength of 112.1 MPa is achieved, which is 40 % and 33 % of AZ31 Mg and Al6061, respectively.