Study On Heat Treatment Strengthening And Fracture Toughness Of AZ91 Magnesium Alloy Friction Stir Welded Joint

Based on the performance and resource advantages,magnesium alloys have been hailed as green engineering materials in the 21st century,received extensive attention and continuous research by scholars.Its extensive use is inseparable from the support of welding technology,friction stir welding method is a solid phase connection,welding defects such as pores and inclusions generated during welding by conventional fusion welding methods are avoided effectively,thus suitable for welding light alloys such as aluminum and magnesium,and obtain sound welded joint.AZ91 magnesium alloy has excellent casting performance,however,the microstructure of the joint is complicated,which has a great influence on its mechanical properties and fracture behavior.By solution treatment and artificial aging treatment of AZ91magnesium alloy welded joints can receive homogenized joints,which is of great significance for ensuring the service life of the welded joints.In this paper,the as-cast AZ91 magnesium alloy and its friction stir welded joint were solution treated and followed by artificial aging treatment.The microstructure of the base metal and its welded joint after heat treatment was analyzed by optical microscope and scanning electron microscope,and the vickers hardness tester was used to test the microhardness of the material,acoustic emission technology was used to monitor the acoustic signal generated by the crack tip during the fracture process.The effects of solution treatment and artificial aging on the mechanical properties and fracture toughness of the material were investigated.The fracture morphology was observed by scanning electron microscope and the fracture mechanism was analyzed.The fracture process of the welded joint was characterized by the acoustic emission detection signal at the same time.It has been found that the brittle hard secondary phase?-Mg₁₇Al₁₂ phase was distributed on the coarse?-Mg matrix of as-cast AZ91 magnesium alloy,which lead to poor mechanical properties.After solution treatment,the coarse network-shaped phase which distributed at the grain boundary and the short rod-shaped phase which scattered in the grain were disappeared completely,and the elongation and fracture toughness were significantly improved,while the microhardness value of the material decreased.The performance of the base metal after 7 hours of solution treatment at 425°C was improved:its fracture toughness was 21.63 MPa·m^1/2,doubled,the elongation was 6.25%,nearly tripled,the yield strength was 102 MPa,and the tensile strength was212 MPa.When the artificial aging treatment was continued,the discontinuous secondary phase was precipitated from the grain boundary at first,and the fine rod-like?phase was precipitated in the crystal as the aging time was prolonged.The dissolution of the?phase during solid solution treatment increases the toughness of magnesium alloy while decreases the microhardness value,while the re-precipitation of the?phase increases the microhardness value of the aged magnesium alloy,but the toughness is basically restored to the original state.After solution treatment of AZ91 magnesium alloy friction stir welded joint,the elongation was increased by more than 170%,the yield strength and fracture toughness were reduced by about 20%compared with untreated welded joints;After artificial aging treatment,the elongation,yield strength and fracture toughness were restored slightly higher than the original state,however,the solid solution treatment and aging had no significant effect on the tensile strength.After the material was solid solution treated at 425°C for 7 h,the performance was obviously improved:yield strength 103 MPa,tensile strength 209 MPa,elongation 6.4%,weld center fracture toughness 17.93MPa·m^1/2.The solution treatment can improve the toughness of the base metal and appropriately reduce the toughness of the joint seam,thereby achieving the purpose of homogenizing the structure and performance of the welded joint.