Regulation Of Strength And Toughness Gradient Of Friction Stir Welding H13 Tool And Study Of Joint Defects

Friction stir welding technology is mainly used in the welding of low melting point metal materials such as high strength aluminum alloy and magnesium alloy.It has the dual advantages of high welding efficiency and good quality,and has a broad application prospect in the field of new energy vehicle manufacturing.In the process of friction stir welding,the performance of the stirring tool has a decisive influence on the welding efficiency,the fusion degree of the joint and the surface quality.However,hardness and toughness of stirring tools are two key indicators of its performance,and there is usually a contradictory relationship between them.In order to coordinate the relationship between hardness and toughness of the stirring tool,the stirring tool was prepared with H13 steel by the method of cyclic quenching and gradient control.The strength and toughness of the stirring tool changed in a gradient,and the mechanism of microstructure evolution was clarified.Then,6082 aluminum alloy was welded by stirring tool,and the microstructure and properties of joints under different aging states and welding parameters were studied,and the influence mechanism of tunnel defects and penetration defects on joint properties was revealed.Through experimental research and theoretical analysis,the effect of cyclic quenching+gradient control on the performance of H13 steel stirring tool was systematically studied,and the effect of aging state of aluminum alloy on the quality of friction stir welding joint was analyzed.The microstructure evolution of H13 steel was studied by cyclic quenching method.The results show that the austenite grains are refined by the recrystallization of phase change for many times during the cyclic quenching process.The initial austenite grain boundary limits the size of the tempered martensite,which makes the material have high strength and good toughness,and its tensile strength and yield strength are 1878 MPa and 1715 MPa,respectively.However,after quenching and tempering process(once tempering and twice tempering),H13 steel not only has martensite coarsening,but also leads to precipitated carbide aggregate and grow up.The tensile strength and yield strength are 1798 MPa and 1633 MPa,1735 MPa and 1607 MPa,respectively.After cyclic quenching,the H13 steel stirring tool is subjected to gradient tempering using end face heating,so that the material exhibits a gradient distribution of strength and toughness.The quantity and size of carbides and the content of V element increase gradually in the low temperature zone,the middle temperature zone and the high temperature zone,which makes the thermal stability of carbides improve.However,in the high temperature region,the width of martensite and the size of carbide increase,and the plug dislocations merge,resulting in the decrease of dislocation density,which affects the mechanical properties of martensite.The final hardness of low temperature zone,middle temperature zone and high temperature zone were 50.8 HRC,53 HRC and 47.4 HRC,respectively.This shows that the performance of the stirring tool can be regulated through the design of the strength and toughness gradient process,so that it can meet the requirements of high tip hardness and strong bottom toughness.6082 aluminum alloy plates with different initial aging states were welded by using the H13 steel stirring tool which was obtained by the combination of cyclic quenching and strength and toughness gradient control.The experimental results show that the strength of natural aging aluminum alloy sheet is lower than that of artificial aging.Therefore,the heat input required by the joint with natural aging initial state is slightly less than that of artificial aging initial state to eliminate tunnel defects.By changing the rotation speed and welding speed and adjusting the heat input of friction stir welding,the defection-free aluminum alloy joint was obtained.Under the optimized process parameters(compression depth:0.15 mm,rotation speed:1400 R/min,welding speed:1000 mm/min),the mechanical properties of aluminum alloy joints with different initial aging states are similar.After artificial aging(temperature of 170℃,holding time of 10 h),the β"phase in the 6082 aluminum alloy joints re-precipitates in the matrix and dislocation lines,which play the role of pinning dislocations and improve their mechanical properties.The tensile strength reaches 282 MPa,which is about 10%higher than that of the joint without post-weld aging treatment.By appling different kinds of stirring tools,three kinds of 6082 aluminum alloy joints with different incomplete penetration defects were prepared,and the microstructure and properties of the joints were studied.Through research,it is found that the incomplete penetration defects in the joint are mainly kissing defects,and the surface has the typical characteristics of ductile fracture.It is due to the opposite tensile stress generated at the weak connection between the base metal and the stirring zone interface.After the welding process,the tensile stress causes the root of the joint to tear,resulting in kissing bonding defects.The high-cycle fatigue test results show that the small-sized incomplete penetration defects(0.15 mm and 0.4 mm)have limited influence on the fatigue performance,and the fatigue strength can reach 80 MPa and 70 MPa when the number of cycles is 107;the large-sized incomplete penetration defects(0.65 mm)has a great influence on the fatigue performance,and the fatigue strength is lower than 45 MPa when the number of cycles is 107.