Study On The Regulation Mechanism Of Solidification Structure And Hot Rolling Process Of Au-Ge Eutectic Alloy

Au-Ge eutectic alloy(Au-12Ge)is a very important low-temperature gold-based brazing material,which has excellent electrical and thermal conductivity and dispersion characteristics,the brazing temperature of Au-Ge eutectic alloy is 380-400°C,it is widely used in high reliability and high accuracy microelectronic chip and optoelectronic device packaging.However,Au-Ge eutectic alloy is inherently brittle and very difficult to process.Therefore,it is of great significance for the application of Au-Ge solder to further explore the solidification structure regulation and thermal deformation mechanism of Au-Ge eutectic alloy.In this paper,the undercooled solidification behavior of Au-Ge alloy was studied in order to regulate the solidification structure of the alloy and prepare the microstructure for the subsequent hot processing of the alloy.The influence of purification process parameters on the undercooling of Au-Ge eutectic alloy was studied by the cladding cycle superheating method,and the solidification structure evolution and solidification mechanism of undercooled Au-Ge eutectic alloy were analyzed in detail to obtain the fine solidification structure.On this basis,the advanced thermostatic hot rolling technology was used to study the hot rolling process of Au-Ge eutectic alloy and the law of microstructure evolution in the process of hot rolling,and the high quality brazed foil was prepared.The main research results are as follows:(1)The maximum undercooling of 102 K(0.16 T_m)was obtained by using dimethicone as coating agent.The undercooling of Au-Ge eutectic alloy is affected by coating agent,superheating,holding time,cycle times and other parameters,choosing the appropriate viscosity of coating agent(silicone oil)is the necessary condition for the alloy to be effectively purified and obtain a large undercooling.With the increase of viscosity of silicone oil,the undercooling of the alloy increases first and then decreases.With the increase of superheating,the undercooling of the alloy increases first and then decreases.The undercooling of the alloy decreases gradually when the holding time is prolonged.With the increase of cycle times,the undercooling of the alloy increases first and then decreases.(2)The analysis of cooling curve shows that:The cooling curve of Au-Ge eutectic alloy consists of rapid solidification stage,recalescence stage and slow solidification stage.The recalescence degree of Au-Ge eutectic alloy increases with the increase of undercooling.(3)The undercooling of the melt has a significant influence on the solidification structure of Au-Ge eutectic alloy.Within the undercooling range of 11-102 K,there were two transitions in the solidified structure type.The coarse bulk and needle-shaped eutectic existed in the solidification structure when the undercooling was less than 34 K.When the undercooling was larger than 34 K and less than 56 K,the solidification structure transformed into cellular eutectic.The coarse primary Au phase precipitated from the undercooled alloy melt when the undercooling was larger than 56 K.(4)Combined with solidification theory analysis shows that:The Au and Ge phases grow in a coupled way when the undercooling is less than 34 K,and finally form regular bulk and needle-shaped eutectic structure.When the undercooling is larger than34 K and less than 56 K,due to the different growth directions of nucleation points in different parts of the alloy melt,the solidification structure with cellular shape appears during the solidification process.According to the classical nucleation theory,the relationship between the nucleation rate of Au and Ge phases and the undercooling was calculated.When the undercooling is greater than 56 K,the nucleation rate of Au phase is far greater than that of Ge phase.Au phase will nucleate preferentially and grow up as primary phase,and a large number of long rod-shaped dendrites of primary Au phase appear in the final solidification structure.(5)The Au-Ge eutectic alloy foil with a thickness of 20μm and a smooth surface can be prepared by constant temperature rolling technology.The optimized rolling process is:the rolling temperature is 275-280℃,the holding time between passes is less than 5 min,and the deformation of passes is 5%.The melting temperature of alloy foil is363℃,which can meet the actual brazing temperature demand.(6)During the hot rolling process of Au-Ge eutectic alloy,the evolution process of eutectic phases is:Firstly,the as-cast massive or acicular eutectic is broken and elongated under the action of high temperature and rolling force,and then the two phases gradually form equiaxed structure.With the increase of pass deformation,dynamic recrystallization occurs during hot rolling,the microstructure of the obtained alloy foil is recrystallization growth structure.Based on the above,this paper realized the structure regulation of Au-Ge eutectic alloy by undercooled solidification,deepened the understanding of the solidification behavior of Au-Ge eutectic alloy,which provided the theoretical and experimental basis for optimizing the processability of Au-Ge eutectic alloy,and the preparation of Au-Ge alloy brazing foil had been realized,which has solved the processing problem of Au-Ge alloy,it is of great significance to the application of Au-Ge alloy.