Study On Formation Mechanism,microstructure And Mechanical Properties Of Friction Plug Welded Joints For High-Strength Aluminum Alloy

Friction plug welding is a new and solid-state repairing technology,which exhibits significant application in welding manufacturing process of rocket propellant tanks.In this paper,friction plug welding technology are comprehensively and systematically studied through processing experiments and numerical simulations for structural material of rocket propellant tanks AA2219-T87.The main contents and conclusions are as follows:The optimized welding processes and defect-free joints were obtained for AA2219-T87 base metal and different welds based on plenty of friction plug welding experiments.The results show that material flow and normal force of friction interface could be improved by adding a supporting plate with a taper hole and optimizing welding parameters,and the root defect of the joint is successfully eliminated.The defect-free joints and optimized welding processes provide an important basis for subsequent studies of microstructure and mechanical property,as well as industrial application of this technology.The finite element model of friction plug welding process was established for AA2219-T87,then the validity and rationality of the established model were verified by weld formation and thermal cycle test results.The effects of welding parameters on material flow,temperature field,effective stress and effective strain field during friction plug welding process were analyzed.The results show that the temperature field,effective stress and effective strain field indicate obvious inhomogeneity along thickness direction.Unbonding and weak-bonding are mainly caused by poor material flow and insufficient interface normal force.The calculational results could provide an important theoretical basis for deep understanding the interaction of various factors,revealing the formation mechanism and guiding the process optimization.The microstructure and its inhomogeneity of friction plug welded joint for base metal and different welds were detailed studied,and the effects of post-weld heat treatment(PWHT)on microstructure and defect were also discussed.The results indicate that the joint can be divided into six zones,they are plug metal(PM),plug thermo-mechanically affected zone(PTMAZ),recrystallized zone(RZ),thermo-mechanically affected zone(TMAZ),heat affected zone(HAZ),and base metal(BM).Metallurgical connection and recrystallization are realized at bonding interface,with obvious original interface bonding line observed at upper and lower part of the joint.RZ is characterized by fine and equiaxed grain,with grain size of lower than 10 μm.In areas near bonding interface,a large amount of θ’ phases dissolved,and big-size θ phases formed.After PWHT,the abnormal grain growth in RZ and TMAZ,the re-precipitation of θ’ phases,and the improvement of original interface bonding line occurred.The mechanical property and fracture toughness of AA2219-T87 friction plug welded joints were comprehensively and systematically evaluated.The results show that the hardness of local softening zone is 85-95 HV.The tensile property of the joint indicates obvious inhomogeneity along thickness direction.At room temperature,the ultimate tensile strength(UTS)and elongation of the joint are 310-335 MPa and4.5-7%.The cryogenic UTS and elongation is higher than that of room temperature.After PWHT,the UTS indicates obvious increase at both room temperature and cryogenic temperature,while the elongation is seriously reduced.RZ and plug center present better fracture toughness at cryogenic temperature than room temperature.After PWHT,the fracture toughness of RZ and plug center is seriously deteriorated at both room temperature and cryogenic temperature.