Research On The Microstructure And Formability Of Friction Stir Welded Joint Of Aluminum Alloy In Automobile

Automobile lightweight has recently become an important technology for both traditional and new energy vehicles because of its evident advantages of energy saving and emission reduction.The forming technology of aluminum tailor-welded blanks has advantages in both material and structure,and it can satisfy the lightweight and collision safety of the body well.Friction stir welding(FSW)can effectively weld lightweight materials such as aluminum alloy,and this process meets the strength requirement of aluminum alloy components.However,there are some problems in the FSW joints of aluminum alloy,such as inhomogeneous deformation and instability of the joints after heat treatment,which affects the formability and application of the components.Therefore,it is necessary to study the microstructure,mechanical properties,heat treatment process and the deformation law of aluminum alloy FSW joints.The microstructure and mechanical properties of 2024-O and 2024-T6 aluminum alloy FSW joints were investigated by experiments,it was found that 2024-O aluminum FSW joints had better formability,the strength of the weld area was obviously higher than that of the base metal.And the strength of the joints can be improved through solid solution and aging process.Therefore,the FSW joints of 2024-O aluminum alloy were determined as the study objects for subsequent heat treatment and formability.This research explored pre-annealing treatment,short time solid solution and low temperature solid-solution treatment,to inhibit the abnormal grain growth(AGG)of the FSW joints during the solid-solution process.It is found that low temperature solid-solution treatment can significantly inhibit the occurrence of AGG.Although AGG does not occur in T6 treatment joints after reducing the solid solution temperature,its mechanical properties are lower than that of normal T6 treatment joints.After 450 ? solid-solution treatment followed by cold stretching and artificial aging,the hardness and strength of the joints increase as the amount of cold stretching increases.When the joints have a 10% cold stretching,both the hardness and ultimate strength reach the maximum values.Based on the Erichsen cupping test and the hot tensile test,the bulging capability and hot forming property of the joints were obtained.The bulging capability of the joints can be improved by proper heat treatment,after 450 ? solid-solution treatment,and the cupping value of joints increased by 49%.During the hot tensile test,the flow stress and elongation of the joint are mainly affected by the deformation temperature.With the increase of the deformation temperature,the flow stress of the joints decreases significantly,and the elongation increases correspondingly.The strength of the welded area is still higher than that of the parent material at 450 ? for 1 min,and the elongation of the joints is above 20%,which meets the forming requirements.The characteristics of microstructure and mechanical properties of high-strength aluminum FSW joints were revealed before and after heat treatment,and the effect on formability of joints was investigated,aiming to provide technological support and theoretical foundation to the application of the forming technology for aluminum tailor-welded blanks in vehicles.