Liquid Phase Separation And Solidification Structure Control Of Al-Bi-Sn Immiscible Alloy

The immiscible alloy is widely used in self-lubricating materials,electrical contact materials,superconducting materials,etc.It has received considerable attention due to its many unique physical and chemical properties and processing properties.However,the density difference of the two phases in such alloys is usually large,which often causes severe gravity segregation,limiting the development and application of immiscible alloys.If the minority phases are dispersed in the matrix by appropriate methods,the application prospect of the immiscible alloy will be broader.Aiming at the problem of segregation in immiscible alloys,the Al₇₀Bi₁₀Sn₂₀ immiscible alloy was solidified under different cooling conditions to study the liquid phase separation process of the alloy,and then the effect of cooling rate on the distribution of the minority phase and solidification structure of the alloy was clarified.Firstly,the liquid phase separation process and solidification structure of Al₇₀Bi₁₀Sn₂₀alloy under conventional solidification condition were investigated.The phase transition process of alloy melt during solidification was analyzed by the cooling curve and the DSC curve.It can be seen from the cooling curve that the melt undergoes a liquid phase separation process for about 50 s,and the minority phase droplets have ample time to move downward under the effect of Stokes motion to form a layered structure.After analyzing the morphology of the minority phase droplets in the Al-rich region,the relationship between the restoring force and the equivalent radius after deformation is summarized.The smaller is the size of the droplet,the easier is the recovery after the same degree of deformation.The liquid phase separation process of the Al₇₀Bi₁₀Sn₂₀ immiscible alloy under conventional solidification conditions was explained by the related dynamics principle.Secondly,in order to further explore the effect of cooling rate on the solidification structure and liquid phase separation of Al₇₀Bi₁₀Sn₂₀ alloy,the copper mold rapid cooling and water quenching experiments were conducted.In the copper mold rapid cooling experiment,it’s found that when the cooling rate increases from 400℃/s to 720℃/s,the alloy still exhibits a spatial phase separation structure,but the volume fraction of the Bi-Sn-rich region is significantly reduced.When the cooling rate reaches 1320℃/s,the spatial phase separation structure disappears,and some large Bi-Sn-rich droplets are distributed in the middle of the sample.In the water quenching experiment with a cooling rate of 2900℃/s,the Bi-Sn-rich droplets are distributed in the Al-rich matrix evenly.Studies have demonstrated that the cooling rate affects the solidification structure of Al₇₀Bi₁₀Sn₂₀ alloy by affecting the solidification duration and temperature gradient of the alloy.With the increase of the cooling rate,the movement,collision and aggregation duration of the minority phase droplets are shortened after liquid phase separation,and the droplets tend to be dispersed in the matrix,so that the gravity segregation of the alloy is gradually suppressed.By properly changing the cooling rate,the solidification structure of the immiscible alloy can be effectively regulated,and the segregation behavior of the minority phase can be suppressed,which is of great significance for optimizing the performance of the alloy in industrial applications.