The Static Softening Behavior And Microstructure Evolution Of High-strength Aluminum Alloy During Hot Working

The microstructure evolution of new type of A1-Zn-Mg-Cu high strength aluminum alloy during hot working is not only affected by deformation temperature,strain rate and strain,but also inevitably influenced by holding time between each pass.Because the complex mirostructure evolution,such as static recrystallization,static recovery and static precipitation is possible to occur during the holding time between each pass in unstable state microstructure of aluminum alloy after hot deformation.This paper mainly studied flow stress softening behavior and the corresponding microstructure evolution of the alloy through double-pass compression tests by means of optical microscopy（OM）,transmission electron microscopy（TEM）.Microhardness tests were conducted and analyzed to reveal the hardness transformation law during hot deformation of the alloy.1.Isothermal double-pass compression tests of as-cast Al-Zn-Mg-Cu high strength aluminum alloy were carried out on Thermecmastor-Z,300KN thermal-mechanical simulator at temperature of 300℃,330℃,360℃and 400℃,strain rates of 0.01s^-1,0.1s^-11 and 1s^-1,strains of 33%,40%and 50%with holding time after frist pass ranging from 0 to 900 s.Flow stress softening behavior and microstructure evolution of the alloy after forging were investigated.（1）The comparison experiments at different deformation temperatures were conducted under the same strain rate of 0.1s^-11 and the first pass deformation of 33%.The results indicate that the stress softening ratio in double-pass compression test of the alloy decreases with increasing temperature,and increases with increasing holding time.At the condition of 300℃,stress softening ratio between passes is the most remarkable.At 400℃,stress softening between passes is not obvious and it does not vary with changing of strain rate and holding time.And at the condition of 360℃and 400℃,stress level of the second pass is higher than that of the first pass,which is the result of stress hardening phenomenon.The comparison experiments at different strain rates were conducted under the same temperatures of 330℃and the first pass deformation of 33%.The results indicate that the stress softening ratio in double-pass compression of the alloy increases with increasing strain rate and holding time.The comparison experiments at different deformations were conducted under the same strain rate of 1s^-11 and the same temperature of 330℃.The results indicate that the stress softening ratio during double-pass compression of the alloy increases with increasing holding time.But no obvious trend is found with increasing strain rate.（2）Twin boundaries are observed in metallographs after deformation at temperatures of 330℃and 330℃.Yet,they are not found after deformation at temperatures of 360℃and 400℃.It is revealed that precipitation gradually increases with increasing holding time,and grain boundaries becomes obscure at deformation temperatures of 330℃and 360℃;but precipitation phase is not observed at deformation temperature of 400℃.The average grain size first increased and then decreased with the increasing holding time.（3）By analysis of TEM bright field image observed in metallographs,precipitation phase is the most remarkable and the granularity of precipitation is the largest at deformation temperature of 330℃,including circular and elongated shape of precipitation particles.At deformation temperatures of 360℃and 400℃,precipitation particles are only containing Al3Zr.Dislocation density at deformation temperature of 400℃is lower than that of 360℃.Comprehensive study on stress softening infers:The main reason of softening are caused by precipitation phase and recovery.With increasing deformation temperature,the type and quantity of precipitated phase which play an effect on softening gradually decrease.On the other hand,the storage energy decreases due to gradually increase of the dynamic recovery level.（4）Through hardness tests,it is concluded that hardness decreases with increasing holding time,and increases with increasing holding temperature in300℃³60℃.At the temperature of 400℃,hardness does not vary with strain rate and holding time.Hardness decreases the stress softening degree during double-pass compression of the alloy.2.Isothermal double-pass compression tests of as-cast Al-Zn-Mg-Cu high strength aluminum alloy were carried out on Gleeble-1500D thermal-mechanical simulator at 300℃and 400℃,strain rates of 0.01s-1 and 1s-1,strain of 0.4for each pass and holding time in the range of 10-900 s.Flow stress softening behavior and microstructure evolution of the alloy after forging were investigated.The results indicate that the stress softening degree during double-pass compression of the alloy decreases with temperature increasing,and increases with strain rate and holding time increasing.At 400℃,stress softening between passes is not obvious due to complete recovery and recrystallization during deformation and releasing the most of deformation stored energy,and it does not vary with strain rate and holding time.Under the condition of 300℃and 1 s-1,stress softening degree between passes is the most remarkable.Stress softening during holding period of 0-240 s is caused by the growth of recrystallization grains;and stress softening during holding period of 240-900s is mainly influenced by precipitation phase.