Study On Microstructures And Properties Of Bobbin Tool Friction Stir Welding Joints Of 6061-T6 Thick Plate Aluminum Alloy

Bobbin tool friction stir welding(BT-FSW)can avoid the defects of root produced by Friction stir welding(FSW),and is suitable for the welding of closed sections.It has been used gradually in the field of rail vehicle manufacturing applications in recent years.The current research and application of BT-FSW is mainly focused on the welding of 2-12 mm thin plate aluminum alloys,and is rarely reported in the literature for thick plate aluminum alloys above 12 mm.In this paper,the 16mm thick 6061-T6 aluminum alloy which is used in the traction beam of a certain type of automobile body was taken as the subject,and the thermal cycle temperature field along the thickness and horizontal directions of the weld during the welding process was measured by thermocouple temperature measurement technology.The macromorphology and microstructures of the welded joints and the mechanical properties of the joints were characterized by the Scanning Electron Microscopy(SEM)and Transmission Electron Microscopy(TEM).The influences of process parameters on welding thermal cycle temperature,macromorphology,microstructures and mechanical properties were discussed by changing the thermal influencing factors,and the BT-FSW plastic metal flow model of thick aluminum alloy was established to analyze the flow behavior of plastic metal during welding.The simulation software was used to investigate the heat generation mechanism,heat source model and the distribution of weld temperature field of BT-FSW of thick aluminum alloy.The research results show that:The thermal cycle temperature along the thickness direction of thick plate aluminum alloy of BT-FSW which on both sides of the temperature are higher than the middle,the peak thermal cycle temperatures of the joint obtained at process parameters of 300 r/min and 200 mm/min are 395°C,390°C and 397°C on the advancing side(AS)and 414°C,410°C and 416°C on the retreating side(RS)respectively,the temperature on both sides of the joint is approximately 6°C higher than the central layer,and the peak temperature at each temperature measurement point on the RS in the horizontal direction is approximately 20°C higher than that on the AS,with the cooling rate at each temperature measurement point increasing as the peak welding temperature rises.Under the mechanical action of the stirring head and the influence of thermal cycle temperature,the grain shape and size change significantly of the stir zone(SZ)and the Thermal-Mechanically Affected Zone(TMAZ).The electron backscattered diffraction(EBSD)shows that the SZ is characteristic by fine equiaxed grains,and the average grain sizes of SZ₁,SZ₂ and SZ₃ are 19.6?m,15.2?m and 21.3?m in the direction of thickness.The TEM analysis shows that:the precipitated phase of the BM is the needle-shaped?"phase,the precipitated phase of the SZ is GP zone and slatted?phase,as far from the center of the weld the lower peak temperature of welding,the TMAZ thermal cycle temperature is reduced to 300°C?320°C and the precipitated phases are?'phase and?phase,the precipitated phases of the HAZ are?"and?'phases.The microhardness of the joint along horizontally distributed is a"W"shape,the hardness of the SZ is only lower than that of the BM,with an average microhardness of 88 HV and the lowest hardness at the transition zone of the TMAZ and the HAZ,which is about 60 HV;the microhardness of the thickness direction is approximately symmetrical along the central layer of the joint,and the hardness on both sides of the upper and lower shoulder area is slightly higher than the hardness in the central area.The tensile strength of the joint is 225 MPa,reaching 73.1%of that of the BM,and the fracture mode is ductile fracture.As the heat impact factor increases,the welding heat input and thermal cycle temperature increases.When the heat impact factor is 1.5,the welded joint is well formed and free of internal defects;When the heat impact factor is 2,the joint appears convex in the SZ;when the heat impact factor reaches 2.5,the overflow of plastic metal on the RS of the joint increased and the hole-type defects appears in the SZ.The grain size in each region of the joint become larger,the mechanical properties is reduced,the tensile strength drops to199 MPa,and the microscopic morphology of the fracture changes from an equiaxed tough nest to a parabolic tough nest,and the second phase particles Al₈Fe₂Si phase consisting of Al,Fe and Si elements are found at the bottom of the tough nest.Combined with metallographic analysis,the flow model of the plastic metal of the thick plate aluminum alloy is established and the formation mechanism of the joint is revealed,the plastic metal on the AS,the plastic metal on the RS and the plastic metal in the influence zone of the shoulder converge at the back of the stirring head near the AS,and forming a welded joint,and under the stirring action of the stirring pin,transient cavities are created at the thread,which"draws"a small amount of plastic metal into the cavity and move around the stirring pin in a circular motion,thus forming an"onion ring"shape.Based on the aforementioned study,a model for BT-FSW heat production in thick plate aluminum alloys was developed and the experimental data in this paper are calculated by simulation software,and the results shows that:The thermal cycle temperatures of the three groups of the joints are elliptically distributed along the surface of the weld plate,with peak thermal cycle temperatures of 426.85°C,452.85°C and 467.85°C in that order,differing from the actual measured peak temperature by only 10°C,verifying the accuracy of the weld temperature field model.