| Magnesium alloys are known as "green engineering material" in the 21st century. Because Magnesium alloys have a series of excellent performances, such as small specific gravity, high specific strength and stiffness, good abilities of shock resistance, thermal conductivity, recyclability. Also the content of Mg reserve is rich in the world. Nowadays magnesium alloys are increasingly used in automotive, aerospace, electronics and other fields. Due to the weldabilities of magnesium alloys, welding ways of magnesium alloys have become the factors that restrict the development of magnesium alloys. Fusion welding is easy to generate defects such as cracks, porosity, the performance of heat affected zone may decrease. Compared with the fusion welding, soldering is a method that the base metal does not melt and solder melt when heated, the soldering joint is formed relying on mutual diffusion of the base metal and liquid solder, In the process of soldering, impact of physical and chemical properties of the base metal is small, welding stress and deformation is small.So soldering has become an important welding method of magnesium alloys.Based on the perspective of the magnesium alloy soldering materials, this paper mainly studied low melting point solder using to soldering magnesium alloy AZ31B, got the low melting point, good wettability, excellent performance, and economic non-toxic solder alloy. By the analysis of the role of various alloying elements, combined with the phase diagram, determined the solder alloy system with Sn-Zn solder as the base, added right amount of Al and Mg, to improve the wettability and mechanical properties of solders. Through designing the solder formulas, conducted solder spreading test and optimized the solder formulas.Test results showed that a small amount of Al and Mg element can improve the wettability of Sn-Zn eutectic solder, when the content of Al and Mg is 1.7% and 3.4%, solders of Sn-Zn-Al and Sn-Zn-Mg have good wettability.After optimization, the wettability of Sn-Zn-Al-Mg got further improved.The microstructure, phase, and performance of solders have been studied by optical microscope, scanning electron microscopy, X-ray diffraction, and the performance test respectively. In Sn-Zn-Al solder, element Al refine the matrix organization, number of needle-like Zn-rich decreased obvious, Al-Zn-Sn solid solution distribute in the matrix organization with dendritic shape or petals; gray block organization of (Mg2Sn+β-Sn) phase distribute in Sn-Zn-Mg solder; in Sn-Zn-Al-Mg solder, round block groups are (Sn-Zn+Mg2Al3+Mg32(Al, Zn)49) organization, quantity of petal or treeing organizations which are supposedly (Al-Zn-Sn) solid solution decreased compared with Sn-Zn-Al solder. Melting point of Sn-Zn-Al-Mg solder is the lowest, the melting temperature range from 181.3℃ to 191.8℃, the solid-liquid temperature range is 10.5℃; tensile strength of Sn-Zn-Al-Mg solder is highest,it is 122MPa. Through comprehensive analysis, Sn-Zn-Al-Mg solder is the best solder,the content of Al is 1.8% and the content of Mg is 1.5%.The solder meet the design requirements, and it can be used for soldering AZ31B magnesium alloy.Using the best solder to soldering base metal, organization of the soldered joint is analyzed. In soldering seam interface area, bright transition layer is found, maybe it is (Mg2Sn phase+Al-Zn solid solution+Sn-Zn eutectic organization), substrate of center area in soldering seam is Sn-Zn hypereutectic organization, with distribution of the Al-Zn-Sn solid solution phase, by surface scanning,found that solder elements and base metal elements diffused apparently. |
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