## 1 Introduction  
Magnesium and aluminum alloys have the merits of low density and high specific strength. Thus, both alloys have been widely used in automobiles, aircraft, bicycles, computers and communication equipment. Magnesium alloys are the lightest structural alloys with high damping capacity and easy recyclability. However, their poor corrosion resistance seriously impedes wider application. Aluminum alloys have good surface decoration characteristics and excellent corrosion resistance. It would therefore be of practical significance to join magnesium alloy with aluminum alloy, as doing so could promote the application of these alloys and thereby further improve existing light-weight solutions [1; 2; 3]. Due to discrepancies in their thermal expansion coefficients, joining magnesium alloy and aluminum alloys by conventional welding methods is a challenge. Moreover, due to the low melting temperatures of both alloys and the high affinities of magnesium and aluminum to oxygen, fusion welding of both alloys is considered difficult. Spot-welding, friction-stir welding, tungsten-inert-gas welding and laser-welding processes result in the formation of brittle MgAl, Al\({}_{3}\)Mg\({}_{2}\) and Al\({}_{12}\)Mg\({}_{17}\) intermetallic compounds (IMCs) in the joint area [4; 5; 6]. Stress-induced cracks alongside the unexpected brittle intermetallic compounds results in considerably reduced bonding strength. Classical welding methods therefore prove unsuitable for bonding magnesium alloy and aluminum alloys. An effective method of joining such dissimilar materials is brazing. Commercial brage is used to join aluminum alloys such as Al-Si alloys. In order to carry out a high-quality welding of AA7020 aluminum alloy, the laser welding process with Al-5 Ti-B and Al-5 Mg filler metal was developed by [PERSON] et al. [7]. Unfortunately, these aluminum brazes have high melting temperatures in the range of 575-610 \({}^{\circ}\)C. Although zinc, copper, tin, magnesium and germanium have been identified as additive elements in Al-Si alloys to reduce the brazing temperature to around 500 \({}^{\circ}\)C, this temperature is also too high relative to the solidus temperatures of high-strength aluminum alloys [8; 9]. Magnesium alloys and aluminum alloys soften greatly at temperatures above 300 \({}^{\circ}\)C. Moreover, high-temperature joining processes lead to the problem of oxidation. [PERSON] et al. [10; 11] studied the bonding of A356 aluminum alloy/Al\({}_{2}\)O\({}_{3}\) using ultrasonic-assisted soldering with Sn-Ag-Ti solder containing trace rare earth element (Ce) and wetting promoter (Ga). Results of their studies demonstrated that the addition of rare earth element in solder can effectively enhance the solderability and a satisfactory joint of aluminum can thereby be obtained at 250 \({}^{\circ}\)C in air. Low-temperature soldering with Sn-Zn solders is suitable for joining temperature-sensitive aluminum and magnesium alloys. [PERSON] et al. [12] found that the average shear strength of magnesium/aluminum alloy joints bonded with Sn-Zn alloy increased with increasing Zn content, and a high joining strength of 70.73 MPa was achieved with Sn-30 Zn filler in argon protective atmosphere. One of the key issues hindering their application is the poor oxidation resistance of Sn-Zn filler metals [13; 14]. Some studies have indicated that adding trace amounts of rare-earth elements in solders significantly promotes the wetting of filler metals [15; 16; 17]. Furthermore, solders with rare-earth elements can prevent the formation of oxides on their surfaces. [PERSON] et al. [18] demonstrated that adding the rare earth (RE) elements Ce or La in concentrations of 0.05 wt.% and 0.1 wt.% into Sn-9 Zn alloy reduced the wetting angle and thereby increased the wetting force. [PERSON] et al. [19] reported that the oxidation resistance of Sn-9 Zn solder can also be effectively improved by adding trace amounts of rare earth elements (0.08% Er).