| The brazing of Cu-Al is the hotspot in welding study. Using aluminum instead of copper in some areas is a trend of the development, which is in accordance with the requirement of sustainable development strategy. Zn-Al solder is widely used in Cu-Al connection. However, the disadvantages of Zn-Al solder brought some difficulties to welding. The brittle appeared in Zn-Al solder is one of main obstacles in its development.In this paper, the samples were characterized by optical microscope, scanning electron microscopy (SEM), X-ray diffraction (XRD), microhardness tests and tensile strength tests and other analysis methods. The changes of microstructure and properties of Zn-Al solder aging in natural and high temperature, high humidity environment were studied. Then, the effects of Al content and trace alkaline earth elements on the change of microstructure and properties were analyzed, which is providing the theoretical and practical basis to solve the problem of brittle fracture appeared in Zn-Al solder.The results indicated that when ZnA115 solder placed in the natural environment, with the increase of aging time, the tensile strength and the elongation after fracture decreased gradually. After 7 months, the tensile strength decrease from 236N/mm2 to 168 N/mm2, the elongation after fracture decreased from 80% to 30%, On the contrary, the microhardness and the volume percentage ofα-phase in the microstructure increased.Al segregation occurred in the structure of solder when the content of Al increased to 20%. Compare with the ZnA115, the tensile strength increase from 236N/mm2 to 264 N/mm2, the elongation decreased from 83% to 70%, and hardness increased from 41.3 HV0.1 to 48 HV0.1. When ZnA120 was placed in the natural environment, the microstructure and properties were consistent with ZnA115.When add trace alkaline earth elements in ZnA115 solder, the tensile strength increases from 236N/mm to 358 N/mm, the elongation after fracture decreased from 83% to 44%, and hardness of solder were increased from 41.3HV0.1 to 72.6HV0.1. The hardness of solder increases gradually when solder was placed in the natural environment, while the elongation after fracture obviously decreases.When the solder was placed in dry environment at 95℃for 10 days, the crystal grain of solder grow up, while the tensile strength and hardness has little changed. When the solder was placed in steam environment at 115℃for 66 hours, the structure of solder has obviously changed. The striation structure has largely appeared. The tensile strength and hardness of solder were significantly improved. However, the brittle fracture obviously appeared in the solder.Brittle failure of solder in storage period results possibly from a combined action of precipitation of oversaturatedα,βphase and intercrystalline corrosion. |
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