Effects Of Environmental On Microstructure And Mechanical Properties Of Zn-Al Solder

Braze welding is widely applied in the connection of Cu-Al. The Zn-Al solder is the most commonly used to weld Cu-Al. The Zn-Al solder is going to be brittle in the environment with time passing, which makes a lot of economic loss and brings inconvenience in use. The object of this essay is to find out why the Zn-Al solder becomes brittle and avoid it.The microstructure and mechanical properties of Zn-Al solder in 118℃water vapour,118℃dry condition, humid and cool condition and saline-fog environment are investigated through metallographic observation, SEM, EDAX, hardness measurement and toughness test.Many circumferential cracks and radial cracks appear on the periphery of ZnAl2 and ZnAl15 solder when treated by high temperature steam for 10 hours. The crack number of ZnAl15 is much more than that of ZnAl2. Intergranular corrosion was found in ZnAl2 and ZnAl15, the oxygen content of which is increased. The solder became harder. The micro-hardness of ZnA12 is 92.5HV10, which of ZnAl15 is 98.6HV10. The toughness turns bad. Flex the ZnAl2 solder thrice, it broke up. ZnAl15 was worse than ZnAl2. Both ZnAl2 and ZnAl15 became brittle.The microstructure became more homogeneous and the grain size became larger at 118℃dry condition for 10 hours. The aluminium-rich a phase of ZnAl15 solder was decomposed and a lamellar microstructure was found. The mechanical properties did not change much. The hardness of ZnAl2 increased from 55.6HV10 to 65.7HV and from 64.9HV to 77HV, respectively. Both of them were not brittle.The microstructure and mechanical properties of the same Zn-Al solder didn’t change much in 5℃癈and -18℃environment after 4 months. Intergranular corrosion of ZnAl2 at 5℃environment was more serious than that at -18℃environment, when the time was 12 months.Moreover,βphase decomposition was obvious. The mechanical properties of ZnAl2 decreased. It became harder and it’s toughness decreased. The situation of ZnAl15 was similar to ZnAl2.In neutral salt spray, standing time-64 hours, intergranular corrosion was found in ZnAl2. The dendritic a phase became thick and big. There were corrosion products at the grain boundaries. The hardness of ZnAl2 increased from 53.5HV10 to 62.3HV10. The toughness turned bad. Comparing with ZnAl2 which is already brittle, the mechanical properties of ZnAl2 in neutral salt spray was a little better. It did not btittle.The conclusions are as follows:Intergranular corrosion and phase transition cause the solder become brittle. Water vapor is the main cause of intergranular corrosion. Besides, the higher the temperature is, the greater the phase transition. Salt spray accelerates intergranular corrosion and phase transition.