Research On The Undercooled Solidification Behavior Of Ag-Cu Binary Alloy

Ag-Cu alloy is a typical binary eutectic alloy. It is the perfect material in electric motor commutator and contract line which has plenty advantage, such as well conductive thermal conductivity, good mechanical property, high hardness, superior abrasion performance, perfect liquidity and wonderful wettability. It has a vast application in area of break joint with medium load or low load, for instance, the air circuit breaker, contractor, switch. Ag-Cu alloy could apply in micromotor slip ring and joint material like rectiblock. At the same time, Ag-Cu alloy has a important position in welding metals of aviation industry.Now, the majority study about Ag-Cu alloy with supercooling solidification remain keep focus on the point of eutectic, while hypereutectic or the side of rich Cu which has rare report. Therefore, exploration of the supercooling and evolution of solidification structure in different supercooling of Ag50Cu50 alloy and Cu9gAg2 alloys by measure of molten glass purification. It is also analyse the mechanism of supercooling base on nucleation kinetics and supercooling solidification kinetics theory model. The research on Ag-Cu alloy will have significant theoretical and practical benefit for further study. Main conclusion are drawn as follows:With molten glass purification, Ag50Cu50 alloy could get the peak supercooling as 135K and 4 critical supercooling. There’s close relation between the solidification structure of Ag50Cu50 alloy and recalescence, and it existed 5 kinds of different progresses. The progress of recalescence may be different for different undercooling. Meanwile, different recalescence curve is corresponding to different solidification structure. With increasing undercooling, the structrure evolving law of Ag50Cu50 alloy follow as P-Cu dendrite with (a-Ag+P-Cu) regular lamellar eutectic-small dendrite (globular crystal)-cellular lamellar eutectic-anomalous eutectic+dendrite after melt. The main formative reasons of regular lamellar eutectic microstructure is that the nucleation time of two phase is short in undercooling range of 76K to 115K, and growth rate is closer, so appearing coupling of eutectic growth. When above 115K, the nucleation time of two phases is closer and the growth rate of β-Cu has advantage to a-Ag, so P-Cu separate out firstly, then transform into dendrite. At last, releasing numerous crystal latent heat result in dendrite molten into anomalous eutectic with great undercooling.By method of molten glass purification, we acquired 3 critical undercooling and maximum undercooling as 253K in Cu98Ag2 alloy. Cu98Ag2 alloy also has close correlation with recalescence, which has 3 different recalescence courses and obvious differ undercooling to recalescence. With medium undercooling, two recalescence occur in chart.Meanwile, it has a long recalescence time. But when further undercooling take place, only one recalescence arise and the value of recalescence decline comparing with medium undercooling, the time of recalescence become smaller and smaller. So there’s definitely some relationship between solidification structure and recalescence rate.5 different solidification structures shaped with growing undercooling in Cu98Ag2, namely, dendrite with small undercooling -primary particle crystal-supercooling dendrite-second particle crystal -second particle crystal with metastable phase in grain boundary. Through analyze the recalescence rate, different rate correpond different solidification structure and the structure is similar when it has little margin of recalescence rate. Digging the samples in different undercooling, find the facts that with the undercooling enhancing, grain has been found exist two times refinement. BCT dendrite growth model has been used to explain the mechanism of grain refinement. When undercooling reach 253K, massive dark second phase were found in grain boundary of second particle crystal. Take this sample to anneal for 2h in 350℃,then the dark second phase vanished, so make a speculation that dark second phase was metastable in condition of enormous undercooling.