A novel approach to dissimilar joining of AA7075 to AZ31B by friction stir soldering using Sn intermediate layer

The dissimilar joining of 7075 to AZ31 by the friction stir soldering (FSS) process using a Sn interlayer was performed for the first time. The most appropriate microstructure with the highest shear strength was obtained by the one-pass FSS process via a 120 mu m-thick intermediate layer. In the optimum situation, the 7075/solder interface was covered with an uninterrupted wavy layer of Sn solid solution, whereas the AZ31/Solder interface was covered with a thin intermetallic region (IMC) consisting of fine-Mg2Sn intermetallics distributed in an Mg2Sn + beta Sn eutectic matrix. The thickness of the IMC region increased with the decreasing thickness of the intermediate layer and/or the increasing FSS passes, leading to the deterioration of the joint strength. No Al-Mg intermetallics were detected on either the retreated or the advanced side of the one-pass joints; however, isolated Islands and a continuous network of Al-Mg intermetallics were identified at the advanced and retreated sides of the three-pass joints, respectively. In comparison with the advanced side, the retreated side contained a thinner tin-rich solid solution and a thicker IMC region. Moreover, the probability of crack formation or creation of brittle Al-Mg intermetallics on the retreated side was more than those on the advanced side.