Study On The Influence Of Non-isothermal Aging Heat Treatment On The Properties Of 7050 Aluminum Alloy

Al-Zn-Mg-Cu alloy is one of the important structural materials in modern industry,and the future research direction will focus on the direction of ultra-high-strength and corrosion resistance,large-scale high-performance materials.The traditional process often sacrifices part of the mechanical strength while improving the corrosion resistance of the alloy.This paper aims to study the non-isothermal aging,a new aging process,to control the heating/cooling rate to control the size and distribution of the precipitates and,obtain good comprehensive properties.And analyze the law of influence.The research work is as follows:（1）A combination experiment with solution temperatures of 465℃,475℃,and485℃and times of 2 h and 3 h was designed,and a combination with better performance was obtained for the aging experiment.The micro-structure shows that the residual second phases in the alloy are mainly the S phase（Al₂MgCu）,T phase（AlZnMgCu）,fine needle-likeηphase,and insoluble Si-containing phase and Al₇Cu₂Fe phase.With the increase of solution temperature and the prolongation of solution time,the residual second phase content in the matrix gradually decreased.The residual second phase content of the alloy treated at 475℃/3 h is 0.36%,the highest hardness value is 160.6 HV,the residual second phase content of the alloy treated at485℃/3 h is 0.293%,and the lowest conductivity is 38.2%IACS.（2）Non-isothermal aging experiment:the initial/end temperature is 100℃,the heating rate is 20℃/h and 40℃/h,the cooling rate is 20℃/h and 40℃/h,and the maximum heating temperature is 200℃.The results of mechanical and corrosion resistance showed that the relationship between the two was negatively correlated in the later stage of aging,but the overall level of both properties improved.S4-（HR40/CR20）obtained hardness close to T6（162.46 HV）,higher conductivity（41.9%IACS）than T74,the lowest corrosion current density(2.9240×10^-6 A·cm^-2),and Intergranular corrosion depth（86.79μm）and the highest thermal conductivity（126.528 W/mK）.（3）Transmission electron microscopy（TEM）was used to analyze the changes in intragranular micro-structure and morphology during aging and the morphology of each group of grain boundaries at the end of aging.During the aging process of S4-HR40/CR20,in the heating stage and the early stage of cooling,the range,and spacing of the average size and distribution of the precipitates increased with time;In the middle and late cooling stage,some solute atoms dissolve back,and fine GP regions andη’phases are re-precipitated in the matrix.As the heating rate increases,the aging is terminated,and the precipitates at the grain boundaries tend to be coarsened and arranged intermittently.The HR20/CR20 sample has no obvious grain boundary precipitates and no grain boundary precipitation zone,and the corrosion resistance of the alloy is poor.The grain boundary precipitates of HR40/CR20 samples are distributed at intervals,the width of the non-precipitation zone at the grain boundary is about 87.2 nm,and the corrosion resistance is obviously improved.