| Friction stir welding (FSW), as a solid-state joining process, has been utilized to weld nearly all serious of aluminum alloys, especially the high high-strength aluminum alloys wich are difficult to be welded by conventional fusion welding processes. Therefore, it has been applied in industries such as aerospace and aviation, railway vehicle, ship manufacturing. In regard to the FSW of high-strength auluminum alloys, the local softening always occurs when affected by the welding thermal cycle, leading to the deterioriation of joint properties. At present, the post-weld heat treatment and the forced cooling are two main measures to improve the joint property further, but they are constrained for those large-scale aluminum components. Therefore, the dual-rotation FSW is proposed in the present thesis, aiming to reduce the thermal softening caused by the welding thermal cycle, and thus, to improve the microstructures and joint properties.According to the characteristics of the dual-rotation FSW of high-strength aluminum alloys, a tool system for the dual-rotation FSW is reasonably designed, which lays the foundation for comprehensive researches. As for the dual-rotation FSW, it is the non-rotational shoulder assisted FSW (NRSA-FSW) when the rotation speed of assisted shoulder is zero. Through adding driving components to the NRSA-FSW tool system, the dual-rotation FSW in which the rotation speed of assisted shoulder is not zero can be realized in the manner of gear engagement. Furthermore, the dual-rotation FSW at different relative plunge depths can be realized when the bottom end covers with different thicknesses are chosen, on which the assisted shoulder is machined. Defect-free joints with good appearances are obtained utilizing the designed NRSA-FSW tool system and the tool system for the dual-rotation in which the rotation speed of assisted shoulder is not zero.In the NRSA-FSW, the non-rotational shoulder helps to avoid the flash defect and the cavity defect, thus the process window in which good weld formation can be obtained is broadened greatly when compared with the FSW without the assistance of non-rotational shoulder. As for the2219-T6joints welded by NRSA-FSW with the sub-size concave shoulder plunging deeper than the assisted shoulder, the scraping characteristics are obvious only at two sides of the weld, thus the assisted shoulder does not exert on the whole weld width. The maximum tensile strength only reaches307MPa, which is much lower than the optimal tensile strength obtained by convetional FSW. When the plunge depth of assisted shoulder is larger than that of the sub-size concave shoulder, the NRSA-FSW generates a much smoother weld appearance. The tensile strength reaches354MPa (79.6%of the BM), showing an obvious improvement when compared with the optimal tensile strength obtained by convetional FSW. However, the translational force increases greatly due to the increased plunge depth of assisted shoulder, leading to the friction between the sub-size concave shoulder and the assisted shoulder. The increased friction is disadvantageous for both the welding process and the tool system.To obtain excellent joint property, the plunge depth of the assisted shoulder should be larger than that of the sub-size concave shoulder. Therefore, the dual-rotation FSW, in which the rotation speed of assisted shoulder is not zero, is proposed to reduce welding process loads such as the translational force, aiming to reduce the adverse effects on the welding process and the tool system. In the dual-rotation FSW, the plastic flow and the plastic deformation of materials caused by the assisted shoulder are constrained in the near surface layer, thus differences of both the width of softened zones and the microstructures in stirred zones along the thickness direction are decreased obviously. Meanwhile, the high-strength joints, whose tensile strengthes are higher than the optimal tensile strength of conventional FSW joints, are produced by both the co-rotating and the reverse dual-rotation FSW (RDR-FSW). According to the microhardness distributions, it can be seen that the dual-rotation FSW is beneficial to reduce the asymmetry of joints about the weld centerline. Based on the finite element simulation, the temperature fields of the conventional FSW and the dual-rotation FSW are investigated and verified by experimental results. It is showed that the high-temperature area in the temperature field of dual-rotation FSW is decreased obviously, thus both the peak temperature and the dwelling time at elevated temperature are reduced in various zones of the welded joints. Among various welding parameters of the RDR-FSW, rotation speeds of the tool pin and the assisted shoulder have great effects on the peak temperature, while the welding speed has great effects on the dweeling time at the elevated temperature. Based on numerical results of the temperature field, the process loads can be caculated through the discretization method according to the meshing. When compared with the NRSA-FSW, the axial force, the translational force and the spindle torque in the RDR-FSW are decreased obviously. On the other hand, the axial force and the translational force are a little larger than those in the conventional FSW, but the spindle torque and the total torque exerted on the worlpiece are decreased greatly. The rotation speed of assisted shoulder has great effects on the welding process loads in RDR-FSW, thus it is efficient and feasible to reduce the welding process loads through the RDR-FSW.Microstructures and properties of the RDR-FSW joints are investigated in detail, as well as the effect laws of welding parameters. The optimization of welding parameters is also conducted. When compared with the conventional FSW and the water submerged FSW, the process window in which good weld formation and high-strength joints can be obtained is broadened obviously utilizing the RDR-FSW. Both the joint property and the tensile fracture location are determined by microstructural evolutions in various zones in the welded joint. In the heat affected zone, the majority of original metastable precipitates coarsen and some others are transferred to stable precipitates. Moreover, the coarsening process of metastable precipitates is accompanied by the dissolution of small precipitates. In the thermo-mechanically affected zone, the transferring to stable precipitates and the dissolution into the matrix are two main evolutions of those original metastable precipitates, thus the stable precipitates are the primary. However, some metastable precipitates still residue in this zone in the welding condition with low heat input. Affected by the interaction between the high temperature and the svere plastic deformation, original metastable precipitates in the shoulder affected zone (SAZ) and the weld nugget zone (WNZ) are almost transferred to stable precipitates or dissolved into the matrix, but the solution degree is larger in the SAZ and thus the distribution density is larger. In the welding condition with high heat input (the spindle rotation speed is rather high or the welding speed is rather low), some metastable precipitates also repricipitate in the SAZ during the cooling process after welding. This process is not only associated with the dwelling time at elevated temperature but also the peak temperature and the degree of plastic deformation. However, there is no metastable precipitates reprecipitating in the WNZ during the the cooling process after welding even in the welding condition with high heat input. According to the effects of welding parameters on the tensile properties, the response surface methodology based on Box-Behnken design is applied, and a second-order response model between the tensile strength and welding parameters is developed. It is showed that R1075W225r550is the optimal welding parameters for the RDR-FSW of2219-T6high-strength aluminum alloy. The optimal tensile strength357MPa (80.2%of the BM) is much higher than that of the joints welded by convetional FSW, and is also equivalent to that obtained by the submerged FSW. |
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