Effect Of Aging On Friction Stir Welded Joints Of Al-Zn-Mg Alloy

After natural aging,the microstructure and mechanical properties of the friction stir welded joints of Al-Zn-Mg alloy have changed obviously,but the time of natural aging is relatively long.In order to accelerate the aging,the artificial aging could be used.It is significant to take the natural aging and artificial aging process for the friction stir welded joints of Al-Zn-Mg T4 alloy.The changes of the microstructure and mechanical properties on the aged joints are researched,and also the effects of the precipitation on the joints are analyzed.In this paper,a series of natural aging tests and artificial aging tests were carried out on friction stir welded joints of Al-Zn-Mg T4 alloy.The effects of aging treatment on the joints was analyzed by the microstructure and mechanical properties tests.With the natural aging,the microhardness and tensile properties of the aged joints improved remarkably.After natural aging for one month,the average microhardness has increased by 15 HV,compared with the original state.The nominal yield strength(R_P0.2)and the ultimate tensile strength of the aged joints increased by 20.7 MPa and 12.8 MPa respectively.The hardness of the aged joints for three months natural aging increased by 5 HV and the yield strength(RP_0.2)and the ultimate tensile strength were increased by 13.9 MPa and 10.1MPa respectively than the joints of one months.There was no obvious change of the hardness and the tensile properties of aged joints after six months.When the natural aging time reached 6 months,the yield strength?RP0.2?and the ultimate tensile strength raised8.7 MPa and 18.5 MPa respectively.When the natural aging time changed from 9 months to 12 months,the hardness of joints decreased by 5 HV,but the yield strength(RP_0.2)and the ultimate tensile strength unchanged.When the artificial aging were performed for joints,the mechanical properties of joint were changed more obviously.When the artificial aging temperature was 70?,the joints reached a higher aged condition at 15 days,the nominal yield strength and the ultimate tensile strength of the joints increased by 55.7 MPa and 23.2 MPa respectively.When the artificial aging temperature was 160?,the joints achieved the peak aging state obvious at3 hours.The nominal yield strength and the ultimate tensile strength of the joints increased by 63.9 MPa and 21.4 MPa respectively and the hardness of joint increased by 10 HV.As the aging time going on,the average microhardness of joints dropped more than 15 HV,the nominal yield strength and the ultimate tensile strength of the joints of 24 hours artificial aging decreased by 39.7 MPa and 81.1 MPa respectively compaired with the joints of 6 hours artificial aging.The overaging state appeared when the aging time was more than 6 hours.