Analysis On Embrittlement Mechanism Of ZnAl15Solder

Copper is wildly used in the field of power transfer, heat-exchange and daily life, due to its good electric conductivity and heat conductivity. However, copper is strategic resources and its price is too high in recent years, which restricts its application. Nevertheless, aluminum is a suitable material to replace the copper based on satisfying the use requirement in some fields, due to its good electric conductivity, heat conductivity, and corrosion resistance. Joining of copper and aluminum is essential for application of replacing copper with aluminum. Comparing with fusion welding and pressure welding, brazing is one of the most promising methods for joining of copper to aluminum, due to its good adaptability and low price. Zn-Al solder are very appropriate for Al/Cu brazing, which not only solves the environmental problem of Sn-Pb solder and Zn-Cd solder, but also has better corrosion resistance and higher temperature mechanical properties than Zn-Sn solder. In addition, the melting point of Zn-Al solder is much lower than that of Al-Si solder. Unfortunately,"embrittlement phenomenon" of Zn-Al solder during storage restricts its application to Al/Cu brazing, due to its harm to the wettability and spreadability of Zn-Al solder.In this paper, the microstructure evolution of ZnAl15solder during storage and the effect of homogenization treatment, relief annealing and stabilizing treatment on microstructure and properties of ZnAl15solder were investigated, by OM, SEM, EDX, XRD, tension test, reverse bend test, microhardness measurement, damp heat test and immersion test, in order to find out the cause of embrittlement phenomenon and the corresponding refinement measurements.The results showed that the embrittlement of ZnA115solder was caused by electrochemical corrosion and the phase transformation stress from decomposition of supersaturated solid solution and residual stress from cold-drawing accelerated the corrosion. Additionally, homogeneity of microstructure affected the embrittlement. The microhardness increased and the number of bends decreased after the embrittlement of ZnAl15solder.The homogenization treatment was beneficial for refinement of microstructure, and improved the homogeneity and stability of microstructure of ZnA115solder, which has apparent benefit to comprehensive mechanical properties and corrosion resistance. The relief annealing has little effect on the microstructure and mechanical properties of ZnA115solder. However, the relief annealing eliminated the residual stress from cold-drawing process and decreased the corrosion degree. During process of stabilizing treatment, the supersaturation ηE phase in ZnAl15solder decomposed, which led to increase of microhardness and decrease of tensile strength and number of reverse bends. Moreover, the stabilizing treatment not only released the residual stress from cold-drawing, but also eliminated the transformation stress from decomposition of η phase, which contribute to the improvement of corrosion resistance of ZnAl15solder.In sum, the electrochemical corrosion and stress caused the embrittlement of Zn-Al solder together. The heat treatment has a certain beneficial effect for preventativing the embrittlement of Zn-Al solder.