Research On Optimization Of Bobbin Tool Friction Stir Welding Process And Joint Microstructure-properties For 2219 Aluminum Alloy

Bobbin tool friction stir welding(BT-FSW)is a newly developed technique,which not only keeps the merits of conventional friction stir welding,but also overcomes the common limitations of conventional friction stir welding,e.g.,the demand for strong fixtures.In this thesis,the 8 mm thick 2219 aluminum alloy was bobbin tool friction stir welded using the specially designed tool.The influence of process parameters on microstructure characteristics and mechanical properties of joints were investigated in detail and the optimization of process parameters was obtained based on this.Meanwhile,ANSYS workbench 17.0 software was employed to simulate the temperature field evolution during the welding process and the formation mechanism of joint was analyzed based on this,which lays the foundation for guiding welding process and engineering application.Cross-sections of the BT-FSWed 2219 aluminium alloy joints are hourglass-shaped and can be divided into four distinct metallurgical zones,i.e.,the weld nugget zone(WNZ),the heat-affected zone(HAZ),the thermo-mechanically affected zone(TMAZ)and base metal(BM).Among them,the WNZ consists of fine equiaxed grains without significant difference in the thickness direction.The TMAZ is composed of distorted and elongated grains.The HAZ consists of coarse grains due to the heat exposure.It was found that the average grain size in WNZ,TMAZ and HAZ all become coarser with increase of heat input.The precipitation phase in the WNZ is broken and dissolved into the matrix.What’s more,most of the precipitated phases in the TMAZ have been dissolved.Whereas the precipitated phase in HAZ retained but becomes coarser.The precipitation strengthening effect in the welded region is compromised.It’s obvious that the hardness of the welded region is lower than that of the BM.The hardness profile exhibits a W shape.The hardness value of the HAZ is the lowest due to the coarse grains,and the hardness value is slightly recovered in the WNZ due to the refined grains.When the joint temperature is too high or too low,the tensile performance is poor due to the defect in the joint with the fracture surface exhibiting a mixed fracture feature.When a proper heat input is generated by using welding speed of 300 mm/min and the rotation speed of 400 rpm,the tensile strength of the joint reaches a maximum value of 304 MPa,which corresponds 66.1% of that of the BM.The simulation results indicates that the peak temperature during BT-FSW is always below the melting point of the base metal,indicating significant characteristics of solid state joining;The temperature distribution maps in the cross-section of joint indicates that the contour of the highest temperature region is consistent with the WNZ contour,both of which exhibit a dumbbell-shape.In the WNZ,the peak temperature reaches 500 ℃ and thus dynamic recrystallization happens;Meanwhile,it is found that the temperature in the TMAZ and HAZ is lower than that of the WNZ,which is insufficient to induce fully dynamic recrystallization.The experimentally measured temperatures of each characteristic point coincide well with the simulated temperatures,indicating that the simulation results are quite accurate.