High-vacuum brazing of Mg-bearing aluminum alloys

In vacuum brazing, at a specific temperature, the middle layer of solder melts, wets, and spreads in the gap between the base metals, filling the gaps and completing the connection to the base metals through capillary action. During the vacuum brazing process, the original shape of the base metal is good and the temperature distribution is uniform. It realizes the connection of complex and large-sized structures and has the characteristics of high efficiency and low cost. Therefore, this technology is widely used in precision welding fields such as aerospace and electronic components. The vacuum brazing temperature depends on the melting temperature of the solder used. According to the brazing temperature, it can be divided into soldering (<450℃) and brazing (brazing, >450℃). The solder materials are also divided into soft solder materials and hard solder materials.

The quality of vacuum brazing joints mainly depends on the selected solder material. In order to obtain ideal joint performance, the designed solder material should meet the following requirements: have a suitable melting point, stable and uniform composition, good wettability, and good contact between the solder material and the parent material. There is a diffusion effect between materials, and the resulting joint should meet the service requirements under specific conditions. Finally, economy and practicality must be considered. When designing magnesium alloy brazing materials, the melting point of the solder, the diffusion of the solder and the base metal, and the service performance of the joint are the main issues to consider. According to the melting point of the solder, magnesium alloy solder can be divided into soft solder: Sn-Zn series, Zn-A1 series, Mg-Zn series, etc. Brazing material: Zn-Al series, Mg-Al series, AI-Mg series, Ag-Cu series, etc. Usually, there must be a certain solid solubility between the solder and the base metal to ensure diffusion between the two and achieve a strong bond. At a specific temperature, elements such as Sn, Zn, Al, and Ag can react with Mg and have a certain solid solubility in Mg. The connection strength and physical and chemical properties of joints are the main service requirements under specific conditions. Magnesium alloy soft and hard solders have different high and low temperature connection properties, and the potential difference between Sn, Zn, A1, Ag and other elements and Mg may affect Physical and chemical properties of the joint.

According to the binary or ternary phase diagram, in order to achieve sufficient metallurgical bonding between the solder and the base metal, the main element content of the solder is usually selected near the eutectic point or the maximum solid solubility point, and the brazing can be reduced by adding accessory elements. Temperature, regulating the brittle phase ratio in the joint, etc. For example, adding Zn and AI elements to Sn-based and Zn-based solder respectively can control the number of brittle phases in the weld. Adding Zn element to Mg-based solder can lower the melting point.

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