Effects Of PWHT On The Mechanical Behaviors Of 7050-T7451 Aluminum Friction Stir Welding Joint

The 7050-aluminum alloy has been widely used in the fields of aerospace and rail transit due to its excellent comprehensive performance,and it's often welded by Friction Stir Welding.However,due to the interaction of heat and mechanical stirring,the microstructure of different regions of friction stir welded joint of aluminum alloy is different,which leads to the difference of properties.This inhomogeneity has a great influence on the safety and reliability of the joint in service.Therefore,it is necessary to study the microstructure inhomogeneity of welded joints and explore the appropriate solution.The friction stir welding experiments were proposed with six welding parameters,the microstructure of the joints was observed by the optical microscope,scanning electron microscopy(SEM)and transmission electron microscopy(TEM),and the hardness and tensile tests were carried,we found that:with the increase of welding heat input,the grain size of stir zone(SZ)increases,the tensile properties of the joints got worse.All the tensile specimens were fractured in the heat affected zone(HAZ),and the fracture surface contained many thick second phase.The hardness value of each regions is different,and the hardness value of HAZ is the lowest.The precipitated phase is coarsening in HAZ,and the width of precipitate free zone at grain boundary is 25times larger than that of the base metal.The heat treatment was proposed on the six welded joints,the microstructure observation and mechanical performance tests were carried out by the same means,the results show that:the precipitated phase in each regions of the joint were dissolved and re-precipitated,the hardness of each regions of the joints reached more than 165 HV3,the hardness inhomogeneity were eliminated.Compared with the welded joints,the yield strength and tensile strength of the heat-treated joints increased by 51.2%and31.4%respectively.the thick second phase dissolved,and the proportion of the thick second phase decreased from higher than 1.91%to less than 0.67%.After heat treatment,abnormal grain growth(AGG)will occur in the equiaxed grain of SZ,the area of AGG is related to the welding heat input,the larger the heat input,the smaller the AGG area.What's more,the region of AGG will become the weak link of the whole joint and the tensile specimens of heat-treated joints were fractured in SZ.Very high cycle fatigue tests were conducted on the welded and heat-treated joints with ultrasonic fatigue test device(frequency:20 k Hz),and the fatigue fracture surfaces were observed with SEM.The results showed that,the fatigue strength of the joints was improved after heat treatment.When the fatigue lives are 10~7,10~8 and 10~9,the fatigue strength of the heat-treated joint increases by 11.2%,16.7%and 25%respectively compared with the welded joint.The fracture positions of fatigue specimens were calculated,most of the welded fatigue specimens fractured from HAZ,the fatigue crack initiated from the location of the thick second phase in HAZ.However,most of the heat-treated fatigue specimens fractured from SZ,the abnormally grown grain in SZ is a potential source of fatigue crack.