Study On Stress And Distortion Of Friction Stir Welding By Stationary Shoulder

Since friction stir welding(FSW)process was invented in the 1990 s,it has been widely used in the manufacture field of aluminum alloy,such as aerospace,shipbuilding and railway.Due to the aluminum alloy has some characteristics of low melting point,large coefficient of linear expansion and high thermal conductivity,the welding joint produced the large residual stress and instability distortion after welding,which seriously affected the quality and mechanical properties of the welded joints.On this basis,this thesis applied a stationary shoulder friction stir welding(SSFSW)system to control welding residual stress and distortion,and analyzed the change law of welding stress combining with numerical simulation and experiment.In this thesis,6005A-T6 aluminum alloy plate with thickness of 3 mm was taken as the research object.Firstly,the finite element software ABAQUS was applied to investigate the temperature field of conventional FSW and SSFSW and researched the influence of different welding speeds on the welded joint.The results showed that the weld temperature decreases with increasing the welding speed.The external stationary shoulder absorbed a large amount of friction heat during welding,which reduced the peak temperature and high temperature distribution.Secondly,the heat source model was transformed into a structural model,and the simulation results of temperature field are superimposed into the structural model as thermal loads.The results showed that the thermally expanded material in the front of tool was limited by the cold material around the weld during welding,and compressive stress was generated.However,the softened material behind the tool was constrained by the material surrounding the weld due to the cooling shrinkage,and generated the tensile stress.Finally,the welding residual stress and deformation of conventional FSW and SSFSW were compared and investigated in this thesis.The results showed that the longitudinal residual stress perpendicular to the weld ing direction was a “M” type distribution and the tensile stress region of SSFSW was smaller than that of conventional FSW.SSFSW applied an additional tensile strain to the weld which made the weld generated,and offset the compressive plastic strain caused by thermal input during welding.Thus,the SSFSW process could reduce the tensile residual stress by approximately 50% compared with the conventional FSW.The external stationary shoulder not only absorbed the thermal input in the weld,but also applied a synchronous rolling pressure to the weld.Therefore,the distortion of SSFSW was smaller than that of conventional FSW under the same welding parameters.