| China’s magnesium alloy reserves are relatively abundant,and the development of modern industry is becoming more and more demanding for light weight.Therefore,magnesium alloy has broad application prospects in the fields of traffic vehicles.However,the application of metal structure is inseparable from the support of connection technology and withstand complex working conditions.Therefore,it is necessary to study the high cycle fatigue properties and crack propagation behavior of magnesium alloy and its welded joints.The traditional fatigue test has the characteristics of long cycle and high cost.The infrared thermal imaging method has the characteristics of real-time and convenient,can characterize the temperature change information of the material surface during fatigue and crack propagation,and assist the fatigue behavior.Analysis,therefore,the infrared thermal imaging method has a certain practical significance for the study of fatigue fracture behavior of magnesium alloy FSW joints.In this test,AZ91 magnesium alloy sheet was welded by optimized welding parameters.The microstructure and fracture morphology of the base metal and its FSW joint were analyzed by scanning electron microscopy(SEM).The hardness of the welded joint was analyzed by microhardness tester.The distribution was analyzed,and the high temperature fatigue behavior of AZ91 magnesium alloy and its joints and the crack propagation behavior of different regions were analyzed in combination with the characterization of the surface temperature information of the material by infrared camera.The AZ91 magnesium alloy base material is a typical as-cast structure,and the coarse second phase is distributed in a network along the grain boundary.After FSW welding,dynamic recrystallization occurs in the NZ zone,the grains are fine,the second phase is almost completely integrated into the matrix,and the structure of the TMAZ zone is mechanically and thermally affected by the stirring head,and incomplete dynamic recrystallization occurs.In the elongated state,the grain size is not uniform,the HAZ zone is only subjected to heat,and the tissue change is not obvious.After FSW,the hardness distribution of the joint is W-shaped as a whole,and the hardness in the HAZ zone is slightly higher,while in TMAZ,it is slightly lower than that of the parent metal,and the hardness in the NZ zone is the lowest.When the base metal and the FSW joint sample are linearly fitted,the fatigue strength of the base metal is 84.4MPa,and the fatigue strength of the joint is 66.5MPa,which is only 78% of the fatigue strength of the base metal.Quality and welding caused by residual stress and the like.During the fatigue process,the sample temperature has gone through five stages: temperature rise,temperature drop,temperature balance,second temperature rise,and rapid drop.During the crack propagation test,the NZ zone has the strongest resistance to crack propagation,while the RS side is superior to the AS side,but both are higher than the parent metal.During the entire crack propagation process,the slow crack propagation phase takes up most of the time,while the fast expansion phase only accounts for a small portion.During the test loading process,the cyclic deformation has a good correlation with the surface temperature change.The temperature change can be divided into the following three stages: a slow expansion stage,a slow temperature increase,and a stable expansion stage,the temperature maintains a dynamic balance,and the three rapid expansion stages The crack tip has a large strain and the temperature rises rapidly.From the base metal,AS zone,RS zone,NZ zone from brittle fracture to plastic fracture.The NZ zone presents a relatively characteristic feature of plastic fracture. |
PDF Full Text Request