Research On Process And Numerical Simulation During Bobbin Tool Friction Stir Welding Of 6061-T6 Aluminum Alloy

As a solid-state joining process,Friction stir welding showed great advantage in the welding of aluminum alloy since it was been invented.The further research on the microstructure and properties of friction stir welding joint has important significance. As a new variant, BT-FSW solves problems of conventional friction stir welding, such as root defect of joint, high demand for fixtures. Therefore, more and more attention has been obtained, which has high research and application value..In this paper,6mm thick 6061-T6 Al alloy was welded by using BT-FSW and FSW. The influence of rotational speeds, welding speeds on micro-structure and properties of joints had been analyzed, then compared the similarities and differences between BT-FSW and FSW. Using the software of DEFORM-3D analyzed the temperature field and the material flow during the welding process. Finally, the comparison between the experiment results and the simulated results had also been validated.The experimental results showed that FSW welding joint was similar to"V" shape, and BT-FSW was similar to"dumbbell" shape. The joint was composed of four areas:base material(BM),heat affected zone (HAZ),thermo-mechanically affected zone(TMAZ) and nugget zone (NZ).During the welding process, the rotation speeds had a greater influence on the microstructure than the welding speeds. The HAZ grain size of BT-FSW was bigger than FSW, but the grain size in the middle NZ had no significant difference, in the thickness direction, the grain size of NZ did not exist the specific layer.The microhardness test showed that the hardness of welded joints were "W" shape distribution, the lowest hardness occured in HAZ near TMAZ, the lowest hardness of FSW valued higher than BT-FSW, but the average hardness in NZ varied little. Difference with FSW, the lowest hardness of BT-FSW occured in RS of the heat affected zone.The tensile tests showed that the fracture location of the non-defect samples occured at the lowest point of the hardness. The fracture mode was mainly ductile fracture, and the tensile property of BT-FSW was poorer than FSW.The corrosion test showed that corrosion resistance of NZ is the best. At the same time, when the rotating speeds and the welding speeds changed at the same time, the absolute value of n/v could not represent the welding heat input.But if only one was changed, the size of the heat input can be reflected sometimes, and rotation speeds had a more great influence on the microstructure and properties of the welded joints than the welding speeds.Simulation experiments showed that the temperature field distribution of cross-section is consistent well with the macro morphology.And the simulation results and the experimental results were in good agreement. For BT-FSW,the peak temperature in RS is higher than AS,but there is no significant difference between AS and RS during FSW. Both in the thickness direction, temperature distribution existed specific layer, the top and bottom temperature field of BT-FSW was symmetrically, the center temperature field distribution is the smallest.