Study On Homogenization Of High Strength Aluminum Alloy In Current Field

This work is a part of the predictive project for national defense"Processing Technology of 750MPa Rank Superhigh Al Alloy".The aim is to work out the insufficient homogenization of the alloy by conventional process because of its very high content of the alloying elemnts.A new method was adopted in this paper.The dissolution of the nonequilibrium eutectics was promted by applying DC current in homogenization,so that the formability and properties of the alloy can be improved.The changes of microstructures and properties of the alloy after single stage and two-stage homogenization were studied by microstructural observations,SEM and TEM observations,EDS and DSC analysis,conductivity and hardness testing.The as-cast microstructure of high strength aluminum alloy is mainly composed of solid solution of α(Al).Besides,there are a large number of non-equilibrium eutectic phases,which are distributed continuously in grain boundary and among dendrite grains,which mainly includes T(AlZnMgCu)phase and S(Al2CuMg)phase.In single stage homogenization process,the residual amount of non-equilibrium eutectic phases depends on the homogenization time,homogenization temperature and current intensity.The longer homogenization time,the higher temperature and the higher the current intensity,the more the non-equilibrium eutectic dissolved in matrix and the smaller the area fraction of residual phase,the higher the hardness,the lower the conductivity of the alloy.When the alloy homogenized at 450℃ with 4A/mm2 current for 24h,the residual phase area fraction reduced from 10.1%in cast alloy to 2.76%,the hardness reaches HV 142.3,and the conductivity is 28.4%IACS.When homogenized at 470℃ with 4A/mm2 current for 4h,the residual phase area fraction reduced from 10.1%in cast alloy to 2.40%,the alloy hardness reaches HV 153.9,and the conductivity is 27.6%IACS.During the homogenization process,the T phase dissolved first and transformed into S phase continously.Then S phase dissolved in a low velocity because it melts at high temperature.When homogenized at 450℃with 4A/mm2 for 24h,most of residual phase is S phase,a few of which can not be dissolved in matrix and the enthalpy is 8.328J/g.When homogenized at 470℃ with 4A/mm2 for 4h,the enthalpy is 4.489J/g and the amount of residual S phase is lower than that at 450℃ with 4A/mm2 for 24h.In two-stage homogenization process,the application of current in the pretreatment stage can greatly promote the dissolution of non-equilibrium eutectic towards the matrix so that the dissolution rate increased.When the first stage homogenized at 300℃ for 4h then homogenized at 465℃ for 24h,the residual phase area fraction is 3.27%,the alloy hardness reaches HV 138.4,and the conductivity is 28.4%IACS.When the first stage homogenized at 350℃ with 2.5A/mm2 for 4h and then homogenized at 465℃ for 24h,the residual phase area fraction is 1.45%,the alloy hardness reaches HV 154.6,and the conductivity is 26.9%IACS.The application of current in the pretreatment stage can obviously accelerate the trasfomation of T to S phase and dissolution of S phase in matrix.After homogenization,most of the constituents are S phase.The two-stage homogenization can achieve preferable result when 2.5A/mm2 was applied at 350℃ in pretreatment process.