## 3 Results and Discussion
The strengths and the melting points increased with increasing Zn content in the Sn-xZn-0.1 RE solders. More Al-rich phase clusters were generated in the solder due to the higher Zn content. [PERSON] et al. [12] studied the joining of magnesium alloy to aluminum alloy with Sn-Zn solders and pointed out that the formation of Al-Sn-Zn solid solution reduces the embrittlement of the AZ31 alloy/solder interface. Thus, the Al-rich solid solution phase inhibited the negative effects of the Mg\({}_{2}\)Sn. Figure 4: Fractography of the AZ31 magnesium alloy/6061 aluminum alloy joint bonded with the Sn-5 Zn0.1-RE solder. (**a**) 6061 aluminum alloy side; (**b**) AZ31 magnesium alloy side. Figure 5: Fractography of the AZ31 magnesium alloy/6061 aluminum alloy joint bonded with the Sn-10 Zn-0.1 RE solder. (**a**) 6061 aluminum alloy side; (**b**) AZ31 magnesium alloy side. Figure 6: _Cont._  
Figure 6: Fractography of the AZ31 magnesium alloy/6061 aluminum alloy joint bonded with the Sn-20 Zn-0.1 RE solder. (**a**) 6061 aluminum alloy side; (**b**) AZ31 magnesium alloy side. Figure 1:0 Fractography of the AZ31 magnesium alloy/6061 aluminum alloy joint bonded with the Sn-30 Zn-0.1 RE solder. (**a**) 6061 aluminum alloy side; (**b**) AZ31 magnesium alloy side. The shear strengths of the AZ31 magnesium alloy/6061 aluminum alloy joints soldered with Sn-xZn-0.1 RE (x = 5, 10, 20, and 30) filler metals after aging at 150 \({}^{\circ}\)C for various times are presented in Figure 1:1 These results indicated that increasing the aging time decreased the joint strength. The cross-sectional SEM micrograph 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 after aging at 150 \({}^{\circ}\)C are given in Figures 9-12, respectively. The lower shear strength of joints after aging could be attributed to the diffusion of Mg from the base metal and the formation of irregular blocky Mg\({}_{2}\)Sn intermetallic compounds. The fracture surfaces of AZ31 magnesium alloy/6061 aluminum alloy joints aged at 150 \({}^{\circ}\)C are shown in Figures 13-16. After aging at 150 \({}^{\circ}\)C, fracture occurred similarly to that in the Mg\({}_{2}\)Sn intermetallic layer of unaged specimens. Before aging, fracture of joints occurred in the interior of the Mg\({}_{2}\)Sn intermetallic layer. After aging, some failures occurred at the interface of the Mg\({}_{2}\)Sn intermetallic layer/AZ31 magnesium alloy. During aging, a certain amount of Mg reacted with Sn, causing an embrittlement of the Mg\({}_{2}\)Sn intermetallic layer. Figure 1:2 Cross-sectional SEM micrographs of the AZ31 magnesium/6061 aluminum alloy joints bonded with the Sn-10 Zn-0.1 RE filler metal after aging at 150 \({}^{\circ}\)C for (**a**) 50 h; (**b**) 100 h.  
Figure 1:3 Cross-sectional SEM micrographs of the AZ31 magnesium/6061 aluminum alloy joints bonded with the Sn-5 Zn-0.1 RE filler metal after aging at 150 \({}^{\circ}\)C for. (**a**) 50 h; (**b**) 100 h  
Figure 1:4 Cross-sectional SEM micrographs of the AZ31 magnesium/6061 aluminum alloy joints bonded with the Sn-20 Zn-0.1 RE filler metal after aging at 150 \({}^{\circ}\)C for **(a)** 50 h; **(b)** 100 h.  
Figure 1:5 Cont. Figure 1:6 Cross-sectional SEM micrographs of the AZ31 magnesium/6061 aluminum alloy joints bonded with the Sn-30 Zn-0.1 RE filler metal after aging at 150 \({}^{\circ}\)C for (**a**) 50 h; (**b**) 100 h.  
Figure 1:7 _Cont._  
Figure 1:8 _Cont_. Figure 1:9 Fractographs of the AZ31 magnesium alloy/6061 aluminum alloy joints bonded with the Sn-5 Zn-0.1 RE filler metal after aging at 150 \({}^{\circ}\)C. (**a**) the 6061 aluminum alloy side after 50 h of aging; (**b**) the AZ31 magnesium alloy side after 50 h of aging; (**c**) the 6061 aluminum alloy side after 100 h of aging; (**d**) the AZ31 magnesium alloy side after 100 h of aging.