Fatigue Resistance Of A7N01P-T4 Alloy Joints Repaired By Laser-MIG Hybrid Heat Source

Light weight,long service life,and high reliability are the key technical indicators and directions for the development of high-speed trains with a speed of 600 km/h and above.Aluminum alloy has become the main application material in the structural design of modern high-speed train due to its advantages of low-density and high-strength.However,various defects and damages inevitably exist in the process of manufacturing and application of aluminum alloy welded structure due to its characteristics.Therefore,repair and remanufacturing has become an important measure to reduce the cost of vehicle operation and maintenance.As the main welding process of high-speed train,laser–MIG hybrid welding technology has attracted much attention because of the good forming quality and high welding efficiency.If this technology can be developed into an advanced one,the vehicle health management system will be optimized and the vehicle operation and maintenance level will be improved.In this thesis,the welding defects of A7N01P-T4 aluminum alloy weld for the original high-speed train body were determined using surface inspection and internal flaw detection methods,and then the repair scheme was formulated.The optimal repair process parameters were determined through the optimization test of laser-MIG composite heat source welding repair process.Then the optimal process was selected to repair the actual weld defects.The repair results showed that the surface and internal quality of the repaired weld had been greatly improved in comparison with that of the original one,and the quality is up to the relevant NDT acceptance level I standard.By utilization of the microstructure analysis,tensile test,and hardness test,the microstructure features and mechanical properties of the joint were obtained before and after welding repair,respectively.Next,the competitive influence of the improvement of macro morphology and the transformation of microstructure on its mechanical properties can be analyzed.The test results showed that the burning loss of elements and the aggregation and growth of precipitates occured due to welding repair.The yield strength of the joint decreased from 295 MPa to 232 MPa,and the average hardness of the weld center decreased from HV105 to HV77.However,the elongation of the joint increased from 2.8% to 7.1% since the welding repair alleviated the stress concentration effect at the original defect position.According to above test conclusions,fracture mechanics test,low cycle fatigue test and high cycle fatigue test were further carried out to analyze the fatigue and fracture properties of the joint before and after repair,respectively.The fracture process was reproduced using the high cycle fatigue test,and the analysis conclusion was validated.The test results showed that the crack propagation rate of the repaired joint was smaller than that of the original one,which was due to the reduction of the proportion of large angle grain boundaries and the reduction of crack propagation resistance.However,due to the increase in crack initiation time of the repaired joint,its overall life was improved,and the fatigue strength was increased from54 MPa to 72 MPa.The fracture positions of high cycle fatigue specimens were transferred from the incomplete penetration defect of the original joint to the fusion line of the repaired joint.