Microstructure And Properties Of Stationary Shoulder Friction Stir Welded T-Joints Of 2A14 Aluminum Alloy

Friction Stir Welding(FSW)technology has the advantages of beautiful appearance,energy saving,environmental protection and no pollution in the field of aluminum alloy structure connection.It is difficult to obtain joints with excellent performance in the application of T-joints which is commonly used in aircraft tail and box manufacturing because of the limitation of the shoulder structure.Stationary Shoulder Friction Stir Welding(SSFSW)is a new type of friction stir welding technology that separates the shoulder and the stirring pin.The shoulder does not rotate during the welding process,and the shoulder could be welding inside the T-joints by processing at right angle.It has a unique advantage which avoids the lack of penetration and internal defects that often caused by traditional welding.In this paper,SSFSW-T joints tests were carried out on the8.5mm thick 2A14 aluminum alloy.The welding tools and fixtures are designed and optimized.The effects of different process parameters on the joint microstructure and mechanical properties were explored.The results of numerical simulation reveal the connection mechanisms of T-joints,and provide theoretical guidance for the connection of aluminum alloy T-joints.In view of the structural characteristics and material properties of the T-joints,based on the high-quality connection of the T-joints and reduce the damage of the stirring tool in the welding processing,the T-joints welding tool and fixture were independently designed.After using the optimized design of welding tools to test,we successfully obtained a weld with low defect rates and beautiful appearance.The temperature and torque during the welding process were monitored,and the temperature field simulation results under different process parameters were analyzed.The results show that the welding temperature field is divided into three stages: preheating,welding stability,and heat dissipation.The temperature field extends elliptically from the weld to the two sides.Using the single control variable method,the macro forming and microstructure changes of joints at different welding speeds(25-100mm/min)and tool rotation speeds(1500-2250rpm)were studied in detail.As the welding speed and the rotation speed increased,the phenomenon of groove on the surface of the welding seam is aggravated.The T-joints could be divided into weld nugget zone,thermo-mechanically affected zone and heat affected zone,and the weld nugget zone could be divided into first weld nugget zone,second weld nugget zone and weld nugget overlap zone.The weld nuggets are all equiaxed grains of uniform size,mainly with weak shear texture,and the recrystallization form is mainly geometric dynamic recrystallization accompanied by continuous dynamic recrystallization.The thermo-mechanically affected zone is mainly composed of long curved grains.The heat affected zone consists of rolled grains of different sizes,as the welding speed increases and the rotation speed decreases,the grain size in this area tends to increase first and then decrease.The hardness distribution in different areas of the joints is related to the grain size and the distribution of the precipitated phases.The results show that the hardness distribution of the bottom plate and the ribbed plate of T-joints show "W" type and "N" type respectively,and the lowest hardness values are located in the thermo-mechanically affected zone.As the welding speed and tool rotation speed increase,the overall hardness distribution of the joints has no obvious change.Tensile and fracture tests show that with the increase of welding speed and rotation speed,the tensile strength of the bottom plate and the ribbed plate show a trend of increasing first and then decreasing.The highest values are 71.21% and 50.93% of the base material.The fracture modes are ductile fracture and ductile-brittle mixed fracture.