Microstructure Transformation And Precipitation Kinetics Of Rolled Al-6Zn-2Mg-2Cu Alloy During Aging Process

Al-Zn-Mg-Cu high strength aluminum alloy has good specific strength,processing performance and high cost performance,it is widely used in industry and other fields,especially in the aerospace industry and national defense construction.Nowadays,with the increasing demand of production,it is necessary to improve the microstructure of Al-Zn-Mg-Cu high strength aluminum alloy to improve the mechanical properties of the material.Wrought aluminum alloy after rolling has better performance than that of cast aluminum alloy.In this paper,Al-6Zn-2Mg-2Cu high strength aluminum alloy was used as the experimental object.Through the combination of heat treatment technology and rolling deformation process,the homogenized Al-6Zn-2Mg-2Cu aluminum alloy was rolled by different deformation,followed by T6 aging treatment.The microstructure of the alloy was observed by optical microscope,scanning electron microscope and X-ray diffractometer.Tensile properties of the materials under different states were tested by tensile test.The macro and micro hardness of Al-6Zn-2Mg-2Cu aluminum alloy were analyzed by Brinell hardness tester and digital display microhardness tester,respectively.The effect of aging on the precipitation kinetics of MgZn₂ phase after rolling was studied.The results show that when the deformation of Al-6Zn-2Mg-2Cu aluminum alloy is 20%,the grain boundary is flat,the grain begins to appear recrystallization,and some of them remain granular.When the deformation of Al-6Zn-2Mg-2Cu aluminum alloy is 40%,the grains are elongated along the rolling direction,fiber like,and the grains are recrystallized.After 20%and 40%rolling,and then single-stage aging treated at 180?,the characteristic of "bimodal distribution" exists between the strength and hardness of the alloy and the aging time.For the alloy with 20%rolling deformation,the first aging peak appears in 24h,the yield strength is 559MPa,the tensile strength is 570MPa,and the hardness is 146.97HB.The second aging peak appears in 104h,the yield strength,tensile strength and hardness of the material are 569MPa,576MPa and 139.88HB,respectively.For the alloy with 40%rolling deformation,the first aging peak appears in 16h,the yield strength is 579MPa,the tensile strength is 587MPa,and the hardness is 151.87HB.The second aging peak appears in 104h,the yield strength,tensile strength and hardness of the material are 592MPa,597MPa and 130.03HB,respectively.In the first peak aging time,the grain size of MgZn₂ phase increases with the extension of aging time,and its maximum is no more than 32nm.The size of second phase in alloy rolled with 40%deformation is larger than that of the 20%at every point of aging time.After aging for 8h,the growth rate of the second phase is decreased,and the size of the second phase tends to be stable.After the rolling deformation of 20%and 40%,the fitting equation of the MgZn₂ phase precipitation kinetics is y=1-exp(-0.00060t^0.5306)and y=1-exp(-0.00063t^0.5276)in the aging process at 180?,respectively.The fitting degree of precipitation kinetics curve of MgZn₂ phase in alloy with 40%rolling deformation is higher than that of 20%.