Design Of Stirring Head With Stationary Shoulder And Temperature Field Analysis For Friction Stir Welding With Non-rigid Support

Stationary shoulder friction stir welding(SSFSW)is an improved welding technology derived from friction stir welding(FSW).In the welding process,only the rotating friction of the stirring head generates heat,while the external stationary shoulder only performs friction sliding along the welding direction.Under the action of axial force,the weld is extruded to obtain a dense weld.Taking the on-line welding repair of aluminum alloy truck as the specific application scenario,this paper uses SSFSW to carry out the on-site welding repair test.According to the actual welding effect,tensile test,fracture SEM analysis and other mechanical strength detection and microstructure analysis,and design the main structural dimensions of the stirring head with stationary shoulder by heat generation model analysis,mathematical fitting calculation and temperature field analysisn and verification.Aiming at the repair of aluminum alloy truck body damage,a mobile friction stir welding repair equipment with non-rigid support is designed.In order to solve the problems of insufficient support force and large deformation in the welding repair process of non-rigid support,the structures of rotary telescopic arm and petal cutting board are designed,and the deformation of aluminum plate and the contact state between petal cutting board and weld under different support conditions are simulated and analyzed.The results show that when petal cutting board structure is used at the back of weld to realize non-rigid support welding under the spindle pressure of 200-350 kgf,the deformation of the aluminum plate is small,and the petal cutting board with a slope angle of 3° can effectively concentrate the bracing force in the weld area and obtain a relatively flat welding plane.In order to solve the problem of welding failure caused by the traditional stirring head with single shoulder easily plunge into the welded plate under the condition of non-rigid support,the stationary shoulder welding structure is designed and developed,and the field test research is carried out.The tensile test and fracture SEM analysis are carried out on the weld specimens.The test and analysis results show that the tensile strength of the weld specimen by the existing stirring head with stationary shoulder structure is about 71% of the base metal,The elongation is about 30% of the base metal;Through the SEM analysis of the fracture,the fracture of the weld has obvious delamination.The upper layer near the shoulder of the stirring head shows brittle fracture characteristics,and the lower layer shows ductile fracture characteristics.By the establishment and analysis of the heat input model of SSFSW welding process,it is known that the friction heat production by the shoulder of the stirring head is the most in the welding process,accounting for about 77% of the total heat production.Like the traditional stirring head with single shoulder,due to the continuous friction heat generated by the rotation of the stirring head shoulder,the area near the upper surface of the weld is gravely affected by the heat-mechanical cycle,and the uneven strain of the welded joint occurs in the thickness direction of the weld,resulting in the non-uniformity of its mechanical properties,which further affects the overall strength of the welded joint.According to the welding defects and forming causes of the existing stirring head with stationary shoulder,the main structural dimensions are redesigned.In the steady-state welding process,the heat production of the stirring head with stationary shoulder is equal to the heat dissipation of the approximate critical heat dissipation surface for mathematical fitting calculation and analysis,the main structural dimensions of the redesigned stirring head with stationary shoulder are determined,and the heat production model of the welding process is analyzed.It is found that the friction heat production of the side of the stirring needle is the most in the welding process,under the welding process parameters of spindle pressure 250-300 kgf,spindle speed 800r/min and c-axis speed 0.08r/mm,its heat generation accounts for about 86% of the total heat generation.The finite element simulation analysis and comparison of the welding steady-state temperature field of the stirring head with stationary shoulder before and after redesign are carried out,and the results show that: in the welding process of the existing stirring head with stationary shoulder,the temperature field distribution in the thickness direction of the weldment is uneven,the temperature near the shoulder of the stirring head is the highest,and the temperature gradient of the material on the advancing side(AS)is slightly greater than that of the material on the retreating side(RS);In the welding process of the redisigned,the temperature field in the thickness direction of the weldment is evenly distributed,showing a round table shape similar to the shape of the stirring needle;The temperature gradients on the advancing and retreating sides are evenly distributed,and the other areas are symmetrical about the weld center,which is conducive to obtain good joint quality;Spindle speed and feed speed are the main welding process parameters that affect the steady-state temperature field of welding.The research contents and results of this paper broaden the application scope of friction stir welding,provide a reference design method for the structural redesign of the stirring head with stationary shoulder,and provide a certain engineering reference value for the formation of high-quality weld.