| A new type of bobbin tool friction stir welding (BT-FSW), employing two rotating shoulder connected by a probe, is a complement to the conventional friction stir welding, which can achieve the high quality of joint for the hollow profiles, such as ship sheet with rib, storage tank, aircraft panels,et al. However, Little study on the BT-FSWed thin-sheets has been reported in the open literature, expecially 2024-T3 high-strength aluminum alloy. For the BT-FSW process, the defects and other problems are difficult to avoid due to the particularity design and the process of BT-FSW equipment. It is greatly restricted the application of BT-FSW in the aerospace, high-speed trains, ship warships and other fields. Moreover, the establishment and improvement of evaluation criteria for BT-FSW is seriously restricted. So the BT-FSW butt joints were produced by commercially as-rolled 2024-T3 aluminum alloy thin sheets with a thickness of 3mm. The influence of BT-FSW welding tool design on the welding force, temperature distribution, microstructure evolution and mechanical properties were investigated. Meanwhile, The characteristics and process optimization of BT-FSWed thin sheet high strength aluminum alloy are also discussing deeply. Results in this study could lay the foundation of theoretical guidance and data support for controlling the quality, evaluating the performance and optimizing the structure of welding tool of BT-FSW joint.The main conclusions are as follows.The floating BT-FSW special equipment is performed to produce the joints according to its characteristics. We also find that the bobbin tool, having concentric rings on both shoulders and three flates feature on the cylindrical pin, is contributed to the plastic material flow and the weld formation in FSW process by comparing six different types of welding tools. The optimized BT-FSW tool with an asymmetric shoulder, significantly improves the welding stability and strength compared to symmetrical shoulder. The untimate tensile strength of joint is 394 MPa, and joint efficiency can reach 81% of the base metal.The relationship of forward resistance, torque force and temperature distribution on the both sides (advancing side and retreating side) are analyzed during BT-FSW process. Meanwhile, effect of the symmetrical or asymmetrical shoulder on them has also been investigated. The testing results shows the torque force for the symmetrical or asymmetric shoulder welding tool, having almost the same peak with a relatively stable oscillation amplitude, is no increase to with the increase of welding speed The torque force for the welding tool increases with the increase of welding speed at the same rotation speed. The peak temperature gradually decreases with the increase of welding speed. The temperature of lower shoulder is higher than that of the upper shoulder. The temperature in the retreating side is obviously higher than that in the advancing side. The temperature of lower shoulder temperature is also higher than that of upper shoulder in the retreating side. In comparision with the symmetric shoulder BT-FSWing tool, both the peak of torque and temperature by asymmetric shoulder BT-FSWing tool decrease. It will lay the foundation for the further study of BT-FSWed 2024 -T3 high-strength aluminum alloy joint.Effect of welding speed on the tensile and fracture behavior of the BT-FSW joints are performed at the canstant of rotation speed. Both of the yield strength (YS) and ultimate tensile strength (UTS) of the joints first increase to peak and then decrease with the increase of welding speed. The maximum joint efficiency of BB-FSW joints by the asymmetric shoulder of welding tool can be obtained at the welding speed to 200mm/min, the corresponding value is above 81%. It has been proved that welding speed is important factor and significantly affects the mechanical properties of the joint. This result also lays the foundation for the better description of BT-FSW joint features as follows.The relationship between the microstructure, hardness, and the tensile strains of BT-FSW joint are revealed by the theory and experiment methods.The rule for asymmetric shoulder BT-FSW joint in the process of tensile strain distribution is revealed by comparing with the symmetrical shoulder. The flow of weld metal material flow in both vertical and horizontal directions can be changed using the asymmetric shoulder welding tool, results in the asymmetrical structure formation and improving the tensile properties.Meanwhile, the results of strain distribution in the gauge cross-section of BT-FSW joint by asymmetric shoulder presents the inverted 'W' shape. . The peak position of the specimen is mainly appearing in the advancing side and the peak spacing of joint by the asymmetric shoulder increases obviously than that of symmetrical shoulder BT-FSW. The joint efficiency reaches up- to 81%of the base material for the asymmetric shoulder and it is significantly higher than that of symmetrical shoulder BT-FSW joint. The fracture location of high qualityjoint locates in the the heat affected zone at the advancing side. |
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