Study On Processes And Properties Of Mg/Al Dissimilar Metal Joints By Using Low Temperature Brazing

Magnesium and its alloys are extensively applied in automotive, electronics and aerospace industries, due to their outstanding physical and mechanical properties such as high strength-to-weight ratio, excellent electrical and thermal conductivity, high damping capacity, ease of machinability and a large recycling potential. Aluminium and its alloys have a series of advantages including high strength-to-weight ratio, excellent electrical and thermal conductivity, ease of machinability and high corrosion resistance, which shows a wide application prospects. It is necessary to bond Mg and Al to form complex structure because of the universal and intersecting application of Mg and Al alloys. Unfortunately, traditional fusion welding processes and solid-state joining processes are normally not suitable for joining of Mg and Al alloys due to the formation of Mg-Al intermetallic compounds, which seriously deteriorates the performance of Mg/Al joint. Brazing, especially low temperature brazing, is considered as a proper method for joining dissimilar metals, because it can achieve a reliable join in the relatively low temperature and maximize to avoid the formation of detrimental structure, and subsequently the microstructure and mechanical property of the joint are optimized.In this article, AZ31B Mg alloy and6061Al alloy are joined by using low temperature brazing with Sn-xZn and Sn-30Zn-xCe solder alloys. The effect of technological parameter and solder composition on microstructure evolution and mechanical properties of the different brazed joints are investigated. By experimental study, an optimized solder alloy, which is the most favorable filler alloy for joining Mg/Al dissimilar metals, is obtained.Results show that When the solder of Sn-30Zn is selected to join Mg/Al dissimilar metals, an optimized microstructure and mechanical property of the joint can be obtained with using the optimized brazing parameters. These parameters are brazing temperature of330℃, holding time of5s, operating pressure of2MPa and heating rate of about115℃/min. For Mg/Sn-30Zn/Al brazed joint, Mg2Sn IMC transition layer and dispersive distribution of Al-Sn-Zn solid solutions present in the brazing zone of Mg side, blocky Mg2Sn IMCs and Al-Sn-Zn solid solutions are both generated and dispersedly distributed in brazing zone, while Al-Sn-Zn solid solution transition layer is formed in the brazing zone of Al side. The Al-Sn-Zn solid solutions, forming and dispersing in the brazing zone of the Mg/Sn-30Zn/Al joint in the brazing process, reduce the embrittlement of the brazed joint, and thus the mechanical property is greatly improved. However, the coarse Mg2Sn IMCs form in the center of the brazing zone, which makes there to be the weak area, and the joint fails at this area. On this basis, Ce is added into Sn-30Zn solder alloy in order to further optimize the microstructure and mechanical property of the Mg/Al joint. Results show that comparing with the solder of Sn-30Zn, adding appropriate amount of Ce into Sn-30Zn solder is conducive to decreasing the amount of Mg2Sn intermetallic compounds and increasing the amount of Al-Sn-Zn solid solutions in the brazing zone of the brazed joint, which enhances the mechanical property of soldered joint. Unfortunately, the excessive content of Ce leads to the formation of some Ce-Zn and Ce-Sn intermetallic compounds in brazing zone and subsequently decreases the pull force of soldered joint.After a series of technology parameters and solder compositions optimization, Sn-30Zn-0.05Ce solder is considered as the most favorable filler alloy for joining Mg/Al dissimilar metals in this article, and the Mg/Sn-30Zn-0.05Ce/Al joint can offer the highest average shear strength of77.48MPa.