Microstructure Evolution Of 6082-T6 Aluminum Alloy During Friction Stir Welding And Its Effect On Mechanical Properties Of Welded Joint

High speed is the only goal pursued by the transportation industry.They are also faced with energy problems and environmental pollution problems.It is very important to make a resource-saving and environment-friendly structure,and lightening the structural component is one of the most effective ways to solve the problems of energy and environmental pollution.The application of aluminum alloy instead of traditional steel in structural component will reduce the weight,and thus achieve the target of lightening.6082-T6 aluminum alloy has excellent weldability,corrosion resistance and formability,and has been successfully used in the manufacture of high-speed train and subway car body.Friction stir welding,as a new solid phase jointing method,has been recognized as the best welding method for aluminum alloy.In this paper,the microstructure evolution of 6082-T6 aluminum alloy during friction stir welding and its effect on the mechanical properties of welded joints were studied,providing theoretical guidance for the application of 6082-T6 aluminum alloy.(1)In order to observe the dynamic microstructure evolution during friction stir welding(FSW),the grain size,grain boundary characteristics,grain orientation,texture types and texture components of 6082-T6 aluminum alloy during FSW were studied by combining"Stop-action" technique with backscatter electron diffraction characterization technique.(2)During friction stir welding,due to the shear stress introduced by shoulder and pin,the coarse grains of base metal undergo thermoplastic deformation.Under the action of welding thermal cycle,the deformed grains undergo continuous dynamic recrystallization and(111)[110]shear texture occurs in the weld nugget zone.At the same time,the grains are refined.(3)The "geometric dynamic recrystallization" occurs in the retreating side of the weld nugget,and the lamellar structure is formed.As the welding process continues,the dislocations in the elongated grains also accumulate and recombine,and the "continuous dynamic recrystallization"occurs in the elongated grains.Finally,the grains in the retreating side of the weld nugget are arranged in a "necklace" shape.(4)Based on the Thompson tetrahedrons that were formed behind the pin,the(110)[001]Goss texture and(114)[2 21]texture were formed firstly in nugget zone,The aggregated grains with(011)[100]Goss orientation and with(114)[2 21]appear alternately.Thus,the lamellar structure was formed.(5)(112)[111]Copper texture,(110)[001]Goss texture,(111)[1 10]shear texture,(001)[100]cubic texture and(001)[110]rotating cubic texture have different Schmid factors along the welding direction(WD)and transverse(TD).Among them,(112)[111]Copper texture has smaller Schmid factor along the welding direction,which enhances the tensile strength of welded joints along the welding direction.And(111)[110]shear texture has also enhanced the transverse tensile properties of the joints.(6)Anisotropy exists in the welded joint due to the preferred orientation of grain.The tensile strength along the transverse direction is 275 MPa,reaching 83.3%of the base metal strength and 6%of the elongation after fracture.The tensile strength along the welding direction is 292 MPa,reaching 88.5%of the base metal strength and 9.8%of the elongation after fracture.(7)The cross-sectional microhardness curve of FSW welded joints is approximately "W"shape.The hardness of the upper layer is higher than that of the lower layer,and the minimum hardness appears in the forward heat-affected zone.(8)During the tensile process of welded joints,the plastic deformation mainly concentrates on the weld nugget zone,but there is no obvious plastic deformation in the thermo-mechanically affected zone of advancing side.Stress concentration occurs at the junction of the weld nugget zone and the hermo-mechanically affected zone of advancing side,and further cracks initiate.The cracks propagate along this junction boundary,eventually leading to the fracture of the welded joint.Thus,the advancing side becomes the weakest part of the mechanical properties in the welding joint.