## 3 Results and Discussion  
Cross-sectional SEM micrographs of the AZ31 magnesium/6061 aluminum alloy joints soldered with Sn-xZn (x = 5, 10, 20 and 30 wt.%) filler metals containing rare-earth elements are presented in Figure 2a-d, respectively. Satisfactory bonding interfaces formed in the AZ31 magnesium alloy and the 6061 aluminum alloy joints. No major defects were observed at the joint interfaces. During the soldering process, Mg dissolved irregularly in the solder and formed a bond layer of Mg-Sn \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Alloy**} & \multicolumn{6}{c}{**Chemical Composition (wt.\%)**} \\ \cline{2-7}  & **Mg** & **Al** & **Zn** & **Cr** & **Cu** & **Si** & **Mn** \\ \hline AZ31 magnesium alloy & Bal. & 3 & 1 & & 0.005 & 0.02 & 0.2 \\ \hline
6061 aluminum alloy & 1.1 & Bal. & & 0.12 & 0.25 & 0.61 & 0.01 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical compositions of the AZ31 magnesium alloy and 6061 aluminum alloys. Figure 1: Schematic representation of the soldered specimens used for shear testing in this study. intermetallic compounds between the AZ31 magnesium alloy and the solder. The chemical composition of the reaction layer was identified by EDS as Mg:Sn = 63.5:36.5 (at.%), which corresponds to the Mg\({}_{2}\)Sn phase. The thicknesses of all of the Mg\({}_{2}\)Sn layers were about 3-5 \(\upmu\)m. No oxide film was observed at the interfaces between the 6061 aluminum alloy and solder. According to the binary Sn-Al phase diagram [33], aluminum and tin are completely miscible when the temperature is above the liquidus line. Hence, the dissolution of aluminum into the solder alloy during soldering was violent. When the melted solder was cooled to below the eutectic temperature (228.3 \({}^{\circ}\)C) of Sn-Al alloy, the aluminum-rich phase segregated out due to the immiscible binary Sn-Al alloy system. Thus, some Al-rich clusters were found in the solder matrix. The EDS results showed that the Al-rich clusters contained about 97.3 at.% Al and 2.7 at.% Zn. Higher Zn compositions in the solder facilitated the dissolution of aluminum and increased the number of Al-rich clusters in the solder. A large number of Al-rich and coarse Zn-rich phase particles were observed to be embedded in the filler metal of the AZ31 magnesium and 6061 aluminum alloy joints bonded with the Sn-30 Zn-0.1 RE filler metal. Figure 2: _Cont._  
The bond shear strengths of the 6061 aluminum alloy/AZ31 magnesium alloy joints bonded with Sn-xZn (x = 5, 10, 20 and 30 wt.%) filler metals containing trace rare earth elements were determined to be 14.84, 17.08, 19.39 and 20.86 MPa, respectively. The shear strengths increased significantly with increasing Zn content, as shown in Figure 3.  
Figure 3: The shear strengths of 6061 aluminum alloy/AZ31 magnesium alloy joints bonded with Sn-xZn-0.1 RE filler metals with various Zn contents. Figure 2: Cross-sectional SEM micrographs of the AZ31 magnesium/6061 aluminum alloy joints bonded with Sn-xZn-0.1 RE filler metals. (**a**) Sn-5 Zn-0.1 RE, (**b**) Sn-10 Zn-0.1 RE, (**c**) Sn-20 Zn-0.1 RE, and (**d**) Sn-30 Zn-0.1 RE. Figures 4-7 show the fractography of the AZ31 magnesium alloy/6061 aluminum alloy joints soldered with Sn-5 Zn0.1 RE, Sn-10 Zn0.1 RE, Sn-20 Zn0.1 RE, and Sn-30 Zn0.1 RE after shear tests, respectively. The fractographs of joints soldered with Sn-5 Zn-0.1 RE and Sn-10 Zn-0.1 RE filler metals indicated that all the fractured surfaces of both the AZ31 magnesium alloy and 6061 aluminum alloy were covered with Mg\({}_{2}\)Sn intermetallic compound, suggesting that the fractures of the joints occurred mainly in the Mg\({}_{2}\)Sn intermetallic compound layer. Results indicated that the strength of the interface of 6061 aluminum alloy/solder was higher than that of the Mg\({}_{2}\)Sn intermetallic compound. In contrast, the fractured surfaces of both the AZ31 magnesium alloy and the 6061 aluminum alloy joints soldered with the Sn-20 Zn-0.1 RE and Sn-30 Zn-0.1 RE filler metals were identified as Sn with Mg\({}_{2}\)Sn particles.