Research On Formation Of Anomalous Eutectic Microstructure In AuSn20Alloy Prepared By Melt Mixing

AuSn20(mass fraction,%) alloy solders are widely used in the electronics industry due to its low melting point and high reliability. As the existence of brittle intermetallic compounds in AuSn20phase structure, the alloy is hard to deform and process. Many researchers performance a lot of research work to solve this problem, while few focus on the solidification microstructure, which is closely related to its processing performance. Therefore, the research work on AuSn20solidification structure is particularly necessary.In this thesis, high-low temperature melt mixing method was used to study the the formation of anomalous eutectic in AuSn20solidification microstructure. First, the optimal temperature of low-temperature melt is identified by investigating its effect on the the melt mixed alloy microstructure. Then, the influence of the high-temperature melt’temperature and the mass ratio of two mixing melts on the anomalous eutectic formation. Optical microscopy, SEM and EDS used to observe and analysis of the microstructure, phase composition and morphology of AuSn20alloy. Combing with the experimental results and theoretical analysis, prepared by the formation mechanism of anomalous eutectic in AuSn20alloy under high-low temperature melt mixing was explored. The main conclusions are as follows:(1) In the high-low temperature melt mixing experiment, the primary condition for the formation of anomalous eutectic is to find the appropriate low-temperature liquid temperature in the system. Experimental studies showed that the best low-temperature melt’s temperature is274℃, which is the actual solidification temperature of alloy.(2) The study on the influence of the high-temperature melt showed that:under the condition of low-temperature melt274℃and mass ratio of1:3.4(high temperature melt:low temperature melt), it was found that when temperature of high-temperature melt was388℃,386℃or384℃, the anomalous eutectic structure and its transition structure (grid-like eutectic structure) were obtained, and they were mainly concentrated on the top part of the ingots. When the temperature was382℃or377℃, the grid-like eutectic structure and anomalous eutectic structure were found in the bottom of ingot, and a large lump or spherical island anomalous eutectic were formed there. When the temperature was higher than390℃or below370℃, the anomalous eutectic structure was suppressed and the lamellar eutectic structure increased gradually until the fully lamellar structure formed. Thus, the optimum temperature of high-temperature melt is from377℃to388℃.(3) The experiment about the effect of mass ratio on anomalous eutectic structure showed that:the optimal mass ratio was1:3.4(high temperature melt:low temperature melt). When the ratio was higher or lower than this value, it is not conducive to the formation of anomalous eutectic and the volume fraction of lamellar structure increased gradually. And when the mass ratio increased to1:2, the anomalous eutectic structure disappeared completely. While when the mass ratio value decreased to a certain extent, the solidified structure stratified obviously.(4) By melt mixing,the desired anomalous eutectic were obtained in solidification structure, which is similar the structure through deep undercooling. The optimal conditions is:low-temperature melt274℃, high-temperature melt temperature382℃and a mass ratio of1:3.4.(5) One of the formation mechanism of anomalous eutectic can be shown as lamellar disturbedâ†'brokenâ†'fusingâ†'equiaxed for lamellar eutectic. That is:during solidification, a large amount of heat that bring from hot high temperature melt cause thermal disturbance and shearing, as well as the impact force to the regular lamellar eutectic, and then the lamellar becomes to be disrupted, broken, fusing and equiaxed. Finally, the anomalous eutectic is formed.(6) The other anomalous eutectic formation mechanism, come from AusSn primary phase fusingâ†'AusSn phase nearby establishing the eutectic frameworkâ†'AuSn phase nucleation and growth:when the high-temperature melt was poured into the low one, the primary AusSn phase is fused and remelted partially, then the tiny remained primary AusSn phase act as a eutectic leading phase to nucleation and growth, which establish the interpenetrate eutectic frameworks, and then the AuSn phase nucleate and grow separately in these frameworks. Finally, the anomalous eutectic structure is formed completely.