Research On Homogenization And Solution Treatment Of Ultra High-strength Al-8.2Zn-2.1Mg-2.3Cu Alloy

Domestic Al-8.2Zn-2.3Mg-2.1Cu ultra high aluminum alloy material cannot match international high quality products in quality stability.One reason is that this material contains high contents of Zn,Mg and Cu elements.The temperature ranges of large coarse second phase containing these elements in the casting process dissolving into matrix are narrow.Whether in the process of homogenizing or final solution treatment,second phase containing Mg,Zn,Cu elements is difficult to dissolve into matrix at low temperature,while alloy will overheat at higher ones.So the ideal process of homogenizing and the final solution treatment has not yet been developed till now.In this paper,semi-continuous casting ultra high-strength Al-8.2Zn-2.1Mg-2.3Cu-0.12Zr aluminum alloy was used as experimental materials,the dissolution of second phase and development of microstructure in the process of homogenization and the final solution treatment were investigated by optical microscopy,scanning electron microscopy,energy dispersive spectrometer and DSC in order to provide experimental evidence for develop best hot-rolled plate.The results show that:(1)The industrial semi-continuous casting Al-8.2Zn-2.1 Mg-2.3Cu-0.12Zr ultra high strength aluminum alloy ingots mainly contained ?-Al,non-equilibrium freezing eutectic phase containing Al,Zn,Mg,Cu elements and crystalline phase Al7Cu2Fe.Its over-burning temperature was 475?.Non-equilibrium freezing eutectic phase containing Al,Zn,Mg and Cu dissolved significantly during single stage homogenization at 470?.But after homogenization at 470? for 48h,this non-equilibrium freezing eutectic phase still existed.Then heating up to 480? in 20 minutes,the ingot overheated and many Al2CuMg and Al7Cu2Fe phases remained in the microstructure.(2)Overheating Al-8.2Zn-2.1 Mg-2.3Cu-0.12Zr alloy ingot during homogenization treatment appeared obvious ball-like eutectic phase obviously after hot roll.The ball-like eutectic phase was identified to be the mixed microstrcuture of phase containing Al,Zn,Mg,Cu elements,Al7Cu2Fe and Al2CuMg by EDS.Ball-like eutectic phase was not found in the direct hot roll of hot-rolled plate.This overheat phase was stable and changed little during hot roll.When the rolling reduction was 47%,the cast structure is still existed in the plates.When the rolling reduction reached 73%,the cast structure basically disappeared.(3)A lot of phase containing Al,Zn,Mg,Cu elements,Al2CuMg phase and Al7Cu2Fe phase with large size existed in this 30mm thick hot-rolled plate.The second phase were distributed unevenly in the thickness direction of the plate,and the thickness of the plate was more than the surface.Its over-burning temperature was 475?.(4)Hot-rolled plate After single stage solid solution treatment at 470? for 24h,hot-rolled plate still existed phase containing Al,Zn,Mg,Cu elements with low melting point.Then heating up to 480? for two-stage solution treatment in 20 minutes,ball-like eutectic phase appeared in hot-rolled plate and Al7Cu2Fe phase exited in isolation at the same time.Hot-rolled plates appeared quenching cracks when directly quenched after solution treatment at 480? in a short time.With the extending of solution treatment time,the probability of quenching cracks decreased.