| Due to the own strength and energy advantage,lightweight alloy such as magnesium alloy is widely used in automobiles,high-speed train,aerospace and other traffic transport structures.These structures under cyclic loading are most likely to fatigue failure.Fatigue failure of metal structural component is a main form of structure collapse,so improving fatigue behavior of structures is very important for their usage and safety.However,when magnesium alloy is welded by the conventional fusion weld,it is difficult to get excellent mechanical and metallurgical properties of joints.Friction stir welding(FSW)is a new solid state joining method,which improves the mechanical properties of magnesium alloy welded joints.And since its energy consumption is less,no protection gas,no need to fill the welding wire,no splash,no smoke and dust,which makes this technology more environmental protection and was applied to today ’s more than one area.The infrared thermography as a real-time,intuitive display,nondestructive method to investigate the distribution of temperature on sample surface during failure process,besides,it can analyze the reason of the fatigue fracture by the relationship between the distribution of temperature and the deformation in the process of the fatigue process.This behavior of fatigue fracture was studied by infrared thermography,which has important theoretical significance and practical value.The material used in this project is AZ31 magnesium alloy,and the optimization parameters are used for butt welding.And then,the high-cycle fatigue fracture behavior and thermographic analysis of friction stir welded(FSW)AZ31 were studied.Finally,an effective method to improve the fatigue life of welded joints is put forward.The experiment results show fatigue limit of friction stir welded AZ31 is about 55.11 MPa,and fatigue specimens fractured along the HAZ of the advancing side(AS).The infrared camera was used to investigate the distribution of the welding temperature field,through analysising we find that the frictional heating of AS was higher than the RS during the friction stir welding.Based on the metallographic analysis of the joint,Heataffected zone(HAZ)contained a greater fraction of coarse grains and a small amount of twins.The grains in the thermo-mechanically affected zone(TMAZ)are smaller,indicating that grains may originate from incompletely dynamic recrystallization during the FSW.The nugget zone(NZ)is composed of equiaxed grains caused by the dynamic recrystallization.The micro-hardness of friction stir welding joint showed that the micro-hardness of the HAZ region of AS was the lowest,which further explains the poor mechanical properties of the region.During cyclic deformation,hysteresis loops from the AS of the FSW joints with the stain amplitude were higher than the retreating side(RS)of the FSW joints.The scanning electron microscopy is used to study fatigue fracture appearance,the results showed that: the fracture surfaces of AZ31 magnesium alloy FSW joints show mainly cleavage fracture with white tear edge and second-crack,which is brittle fracture mechanism.Through the infrared thermal imaging system,the test results show that the surface temperature of AZ31 magnesium alloy during the whole process of fatigue deformation can be divided into five stages: initial increase(stage1),steep reduction(stage2),steady stage(stage3),abrupt increase(stage4),and final drop(stage 5).The temperature of HAZ at AS is higher than other regions,the result further explain the maximum strain in the region under cyclic loading.The fatigue limit of the two-sided FSW joint is increased by 67% compare with that of the single-sided FSW joint.The heat input is equal on both sides of two-sided FSW,and the micro-structure of the welded joint is symmetrical,which avoids the phenomenon that coarse grains of AS of single-sided welding weaken the strength of welded joint.Under the same cyclic loading,the strain ranges of single-side FSW joint were 2.5%-5.25%(at AS)and 1.5%-3.75%(at RS),respectively.One strain range of double-side FSW joint is approximately from 1.25% to 3.75%,which is very close to the RS strain of single-side FSW joint,and also shows that the double-side FSW has high fatigue strength.The fatigue fracture appearance of the welded joint was analyzed by SEM,which results show that the fatigue crack of AZ31 magnesium alloy two-sided FSW joints originated from the surface or near surface of AS,showing a steady herringbone pattern,and eventually fracture at the other side of the welded joint.Its morphology is characterized by brittle cleavage morphology. |
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