| Friction stir welding(FSW),a new solid-state connection technology,can effectively solve the traditional weldability problems of aluminum alloy.However,the welded joint of aluminum alloy welded by the FSW technique has an inherent defect-root“kissing bonding”due to its process characteristics.The root“kissing bonding”defect has a negative effect on the two common failure modes of the high-speed rail train body:fatigue failure and stress corrosion cracking failure,which significantly limits the application of the FSW technique in key positions such as the bearing structure in train carbody.To improve the fatigue performance of 6082-T6 aluminum alloy FSW joints with the root“kissing bonding”under the premise of ensuring the stress corrosion resistance of the joint.The following research was carried out for root“kissing bonding”defect:firstly,FSW joint with 0.5 mm“kissing bonding”defect was designed and prepared,and Ultrasonic impact treatment(UIT),ultrasonic peening(UP),UIT+UP were carried out on the weld root of the joint in batches for the first time,compared with the traditional treatment.For the as-welded FSW joints with 0.5 mm root“kissing bonding”defect and the above three types of joints treated by different processes,the differences in mechanical properties,fatigue performance,and corrosion resistance of joints under different conditions were explored.Furthermore,the fracture morphologies of the corresponding joint samples were compared.The improvement mechanism of fatigue performance of the joint was investigated using microstructure observation,microhardness test,residual stress measurement,and digital image correlation technique characterization.The test results show that compared with the as-welded joint,the performance of the three types of treated joints was improved.Among them,the tensile strength of the UP joint was the most significantly improved,from 189 to 206 MPa.The bending strength of the UIT+UP joint was increased from 354 to 377 MPa.In addition,the pitting corrosion resistance of the treated joints was also improved.Compared with that of the as-welded joints,an order of magnitude reduction in the corrosion current density of the UP joint existed.Under the Na Cl corrosion condition,the slow strain tensile strength of the as-welded joint was 173 MPa,and that of the UIT+UP joint was 188MPa.For the fatigue performance,the results show that UP and UIT+UP can significantly improve the fatigue performance,and the fatigue strength was increased by 13%and 40%,respectively,corresponding to 2×10~6 cycles.In contrast,the fatigue strength of the joint which underwent ultrasonic impact was improved only in the high applied stress level.The macroscopic fatigue surface shows that the fatigue cracks of the treated joints show a more obvious torn state in the initiation and propagation process.Based on the metallographic analysis,it was found that the morphology of the“kissing bonding”defect changed after treatment.The finite element analysis results show that the stress intensity factor of the defect tip was reduced due to the change in“kissing bonding”defect orientation.These factors were beneficial to improving the treatment joint’s fatigue performance.In addition,it could be seen that the root of the treated joint was strengthened,a large number of high-density dislocation areas appeared,and the microhardness was significantly improved.High compressive residual stress was introduced at the root of the FSW joint after strengthening treatment,which can improve the mechanical properties,fatigue properties,and corrosion resistance of the joints to varying degrees.DIC analysis shows that the bearing capacity of the“kissing bonding”region of the treated joint is improved,which significantly improves the joint performance. |
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