The Microstructure And Mechanical Properties Of Friction Stir-welded AZ31 Magnesium Alloy Joints Subjected To Post-weld Heat Treatment

Magnesium alloy because of its many advantages have been widely used in the automotive,aviation,ship and electronic communications and other fields.In practical applications,the use of magnesium alloy can not be separated from the welding technology to achieve the material connection.Friction stir welding is a new type of solid-phase connection technology,resulting in high-performance welded joints.However,due to friction stir welding also has its own shortcomings,and thus difficult to obtain the performance of the welding joints to match the base metal.The heat treatment can improve the mechanical properties of the joint by homogenizing the joint structure and eliminating the residual stress.It provides a theoretical basis for the better application of magnesium alloy in industrial production and engineering practice.The research has certain practical significance.In this paper,5.3 mm thick magnesium alloy sheet was welding by friction stir welding with different welding parameters to obtain the best welding joints,and the best joints are heat treated,the heat treatment temperature is 150-450℃,the holding time is 1 hour.The microstructure and microhardness of the base metal and the joints were analyzed by optical microscope and hardness tester.The influence of different heat treatment conditions on the tensile properties of the base metal and the joints were analyzed by using the universal testing machine.The tensile fracture morphology and the fracture mechanism were analyzed.The residual stress was measured analyzed.The fatigue test was carried out on the untreated joint and the heat treated joint.The influence of theheat treatment on the fatigue performance was analyzed.The results of microstructure observation show that the grains on the two sides of AZ31 magnesium alloy friction stir welding were coarse and uneven,and the welding area was fine equiaxed crystal.When the heat treatment temperature was low,the change of the organization in the welding nucleus was not obvious,and 300 ℃-1 h treatment conditions made the organization was more uniform,when the temperature continues to rise,the grain begins to grow,and the grain was more coarse at 450 ℃-1h.It was found that the grain size was first refined and then grow,the heat treatment condition was 300 ℃-1h reached the smallest grain size,indicating that a complete static recrystallization process occurred under the heat treatment conditions.The tensile strength,yield strength and elongation of the joint were 199.1MPa,92.5 MPa and 7.3%,respectively,when the welding speed was 300mm/min and the rotational speed was 1200 rpm,which reached to 82.2%,65.9%and 34.4% of the parent metal,respectively.Heat treatment cannot effectively improve the tensile properties.Heat treatment can effectively improve the tensile properties of joints,when the heat treatment conditions was 300 ℃-1h,the tensile properties of the best.The tensile properties,tensile strength,yield strength and elongation of the joints were 238.4 MPa,139.9 MPa and 12.4%,respectively,which reached to 97.9%,99.7% and 58.5% of the parent metal,respectively.And there were a large number of dimples in the tensile fracture under the heat treatment condition.The residual stress and fatigue properties of the joints were analyzed.The maximum value of the longitudinal residual stress.The maximum residual stress was 97.6 MPa,69.6% of the yield strength of the base metal,the maximum transverse residual stress of 39.2 MPa,27.9% of the yield strength of the base metal,and are tensile stress.The minimum residual stress of the joint was located in the heat affected zone of the retreating side and the heat affected zone,which is 19.1 MPa and 23.7 MPa,respectively,which are compressive stress.After the heat treatment,the residual stress of the joint is eliminated,and the average longitudinal residual stress of the joint after heat treatment is 5.9 MPa.The average transverse residual stress of the joint after heat treatment is 2.3 MPa.In the untreated condition,the fatigue strength is 58.77 MPa,which is 69.12% of the fatigue strength of the base metal.The fatigue strength of the joint after heat treatment is 78.94 MPa,which is 92.8% of the fatigue strength of the base metal,which is 34.3% higher than that of the untreated joint %.The results show that the heat treatment can effectively improve the tensile properties of the joints,300℃ for 1h was the best heat treatment process,the microstructure and microhardness of the joints are homogenized.Under the heat treatment condition,the residual stress of the joint is effectively eliminated,so that the fatigue performance of the joint is provided,and the mechanical properties of the joint are improved comprehensively.