Effects Of Electric Current On Microstructures And Properties Of 7b04 Alloy During Two-stage Aging And Regression Re-aging Treatments

The 7××× series aluminum alloy has low density,high strength and good processing performance.It is an indispensable material for the aerospace industry.In this paper,effects of electric current on microstructures and properties of 7B04 alloy during two-stage aging and regression re-aging treatments are investigated by means of conductivity testing,electrochemical experiments,tensile testing,scanning electron microscopy(SEM),differential scanning calorimetry(DSC),and transmission electron microscopy(TEM).The main results are as follows:(1)The two-stage aging without applying current field is 110?/8 h+160?/12 h,and the conductivity is 33.6%IACS.When 200 A current intensity is applied in the pre-aging stage,the maximum conductivity is 34.6%IACS.The conductivity of RRA with no current intensity at the regression stage is 33.1%IACS,however,the maximum value of 33.4%IACS is obtained when 20 A current intensity is applied to the regression stage of RRA;(2)Only when 200 A direct current is applied to the pre-aging stage of the two-stage aging and the 60 A direct current is applied only at the high temperature aging stage,and 20 A direct current is applied at regression stage of RRA,the corrosion potential of the alloy moves positively,and the corrosion current density decreases gradually with the extension of aging time of corresponding conditions;(3)The tensile strength of the two-stage aging without applying current field is 622.0 MPa,the elongation is 11.3%.When 200 A current intensity is applied to the pre-aging stage,the tensile strength increases first and then decreases with the pre-aging time prolonging.When the high temperature aging stage is applied 60 A direct current,the tensile strength also increases first and then decreases with the high temperature aging time prolonging.The optimum process of two-stage aging is 110?/8 h+160?/60 A/4 h,getting the tensile strength of 687.0 MPa,and the elongation of 9.5%.Besides,After RRA(120?/24 h+170?/1 h+120?/24 h)treatment with no current field applied,the tensile strength is 690.5 MPa,and the elongation is 10.0%.When 20 A is applied to the regression stage of RRA,the tensile strength increases to 698.9 MPa,and the elongation is 10.8%.The best RRA process is 120?/24 h+170?/20 A/20 min+120?/24 h;(4)The DSC results show that heat enthalpies of endothermic peak I and peak temperature increase with the increase of the current intensity and the extension of aging time,that is,the volume fractions of the GP zones and the ?' phases are increasing;(5)The TEM results indicate that under the conditions of two-stage and three-stage peak aging,the size of the GP zones and ?' phases are refined,and the precipitation density is relatively large.