Study On Static Shoulder Friction Stir Welding Technology Of Angle Joint Of Medium Thickness High Strength Aluminum Alloy

Stational Shoulder Friction Stir Welding(SSFSW)is a new solid phase Welding technology,which can effectively improve the uniformity of weld microstructure and plays an important role in corner joint Welding.Due to the characteristics of high strength and hardness,7 series high strength aluminum alloy is prone to groove and mixing head fracture when welding Angle joint,and it is more difficult to weld Angle joint of medium thickness plate.Therefore,through the optimization design of SSFSW mixing tool,combined with numerical simulation and process test,the welding of high strength aluminum alloy Angle joint of medium thickness plate is completed,which is of great significance to its welding law and popularization and application.In this paper,6mm,8mm and 12 mm thick aluminum alloy as typical medium thickness plate,using numerical simulation method to analyze the characteristics of SSFSW Angle joint temperature field,and design three types of Angle joint(SSFSW Angle jointⅠ,SSFSW Angle jointⅡ,FSW Angle joint).The morphology,microhardness distribution,tensile properties and bending properties of SSFSW fillet weld were studied.The simulation results show that the temperature field of SSFSW corner joint of aluminum alloy with medium thickness plate presents a fusiform distribution as a whole.The high temperature zone of weld center continuously transfers heat to both sides of the plate,and the overall temperature of the forward side is higher than that of the backward side.The longitudinal temperature gradient increases with the increase of plate thickness.In the microstructure of SSFSW corner jointⅠ,pure white aluminum strip appeared at the boundary between the retreating side of weld and NZ zone,SSFSW corner jointⅡ and FSW corner joint no obvious boundary appeared between the retreating side of weld and NZ zone,and the grain bending deformation of SSFSW corner jointⅠweld TMAZ zone was the most severe.The distribution of microhardness values of the three types of corner joints is "W" shaped,and the overall hardness value of SSFSW corner jointⅡ is the largest.The tensile properties of SSFSW corner joints were evaluated by tensile test of SSFSW butt joints.The maximum tensile strength of 8mm corner joints was 321 MPa,which was 78%of the strength of base metal,and the maximum tensile strength of 12 mm corner joints was354 MPa,which was 84% of the strength of base metal.The bending strength of SSFSW corner jointⅡis the largest,while the difference between SSFSW corner jointⅠ and FSW corner joint is small.Through the analysis of bending fracture,it is found that the fracture mode of SSFSW corner jointⅠ is ductile fracture to brittle fracture,the fracture mode of SSFSW corner jointⅡ is ductile fracture,and the fracture mode of FSW corner joint is ductile fracture to laminar tear.