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Effects of the Reversely Rotating Assisted Shoulder on Microstructures During the Reverse Dual-rotation Friction Stir Welding
J.Q. Li, H.J. Liu
J. Mater. Sci. Technol.    2015, 31 (4): 375-383.   DOI: 10.1016/j.jmst.2014.07.020
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The reverse dual-rotation friction stir welding (RDR-FSW) has the capability to adjust the heat generation because of the separately designed tool shoulder and tool pin. The welding torque exerted on the workpiece by the reversely rotating shoulder is opposite to that exerted by the rotating tool pin, so the total welding torque is reduced, which is beneficial to reducing the clamping requirement of workpieces. In the present paper, a RDR-FSW joint was welded in a condition similar to the optimal welding condition of conventional FSW, and microstructures in various zones were investigated by comparison, aiming to highlight effects of the reversely rotating assisted shoulder. Due to the heat conduction of the middle cylinder and the bottom end cover on which the assisted shoulder was machined, the thermal effect of RDR-FSW was smaller than that of the conventional FSW. Moreover, the effect of assisted shoulder on the plastic flow or deformation of material or was constrained in a thin layer near the weld top surface, and thus the flow of material especially along the thickness direction was clearly decreased in the RDR-FSW. In the heat-affected zone (HAZ), the precipitate coarsening was the main evolution and was completed through the dissolution of small precipitates and the continuous growth of large precipitates. By contrast, the dissolution degree of precipitates increased significantly in the thermo-mechanically affected zone (TMAZ), and a small amount of original meta-stable precipitates transformed to block-shaped stable precipitates. Precipitate evolutions in the shoulder affected zone (SAZ) and the weld nugget zone were similar, i.e. the majority of original meta-stable precipitates dissolved into the matrix and the remainder transformed to stable precipitates, though the dissolution degree was greater in the SAZ. Compared with the conventional FSW joint, the coarsening degrees of precipitates in the HAZ and TMAZ of RDR-FSW joint were much smaller, as well as the dissolution degrees of precipitates in all four specified zones.
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Influence of Test Temperature on the Tensile Properties along the Thickness in a Friction Stir Welded Aluminum Alloy
Weifeng Xu, Jinhe Liu, Daolun Chen
J. Mater. Sci. Technol.    2015, 31 (9): 953-961.   DOI: 10.1016/j.jmst.2015.07.005
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The aim of this study was to evaluate microstructures and the influence of test temperature on the tensile properties, strain hardening behavior and fracture characteristics of friction stir welded (FSWed) 2219-T62 aluminum alloy thick plate joints. A fine and equiaxed recrystallized grain structure had no significant change in grains at the top of weld nugget zone (WNZ) at a rotational rate of 500 r/min compared with 300 r/min, but the grains and second-phase particles at the middle of WNZ exhibited obvious coarsening. The yield strength, ultimate tensile strength and joint efficiency were observed to decrease with increasing test temperatures. However, the elongation presented a contrast trend. Compared with the middle and bottom slices, the top slice (216 and 342?MPa) had a higher strength and a lower elongation (8.5%) at different test temperatures. Hardening capacity and strain hardening exponent of bottom slices were higher than those of the top and middle slices. Both of them at room temperature (RT) were bigger than those at higher temperature (HT) and lower temperature (LT). The FSWed joints basically failed in the border area between the thermo-mechanical affected zone (TMAZ) and heat-affected zone (HAZ) of the top slice, and in the HAZ of the middle or bottom slices, while the fracture surfaces exhibited dimple fracture characteristics at different test temperatures.
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Improving Joint Features and Mechanical Properties of Pinless Fiction Stir Welding of Alcald 2A12-T4 Aluminum Alloy
Liu Zhenlei,Cui Hutao,Ji Shude,Xu Minqiang,Meng Xiangchen
J. Mater. Sci. Technol.    2016, 32 (12): 1372-1377.   DOI: 10.1016/j.jmst.2016.07.003
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As a new solid state welding, pinless friction stir welding (PFSW) can be used to join thin-wall structures. In this study, four new pinless tools with different groove distributions were designed and manufactured in order to enrich technological storage of PFSW and obtain sound joint with high quality of alclad 2A12-T4 alloy. The results show that the small-obliquity tool is detrimental to the transfer of plasticized materials, resulting in the formation of kissing bond defect. For the through-groove tool or the large-curvature tool, bigger flashes form on the joint surface and alclad layer is observed in the nugget zone (NZ), deteriorating mechanical properties. Compared with the above-mentioned three tools, using the six-groove tool with rational curvature and obliquity can not only yield sound joint with small flashes and thickness reduction, but also prevent alclad from flowing into NZ, which has potential to weld thin alclad aluminum alloys. Meanwhile, the tensile strength and elongation of joint using the six-groove tool reach the maximum values of 362 MPa and 8.3%, up to 85.1% and 64% of BM.

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Effect of Sheet Configuration on Microstructure and Mechanical Behaviors of Dissimilar Al-Mg-Si/Al-Zn-Mg Aluminum Alloys Friction Stir Welding Joints
Yan Zhongjie,Liu Xuesong,Fang Hongyuan
J. Mater. Sci. Technol.    2016, 32 (12): 1378-1385.   DOI: 10.1016/j.jmst.2016.10.011
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Friction stir welding (FSW) was used to weld dissimilar Al-Mg-Si/Al-Zn-Mg aluminum alloys in this work. Influences of sheet configuration on microstructure and mechanical properties of the joints were mainly discussed. Results showed that rather different joint cross sections were obtained when using different sheet configurations. Coarser β' phases can be observed at the heat affected zone (HAZ) of the Al-Mg-Si alloy side, which was the main factor affecting the tensile properties and the fatigue properties. Tensile strengths of the dissimilar Al-Mg-Si/Al-Zn-Mg joints using both configurations were higher than that of the Al-Mg-Si FSW joint. When the Al-Zn-Mg alloy was located at the advancing side (AS), the joints owned better fatigue properties due to the bridging effect of the big secondary phase particles.

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Shape Memory Effect, Thermal Expansion and Damping Property of Friction Stir Processed NiTip/Al Composite
D.R. Ni, J.J. Wang, Z.Y. Ma
J. Mater. Sci. Technol.    2016, 32 (2): 162-166.   DOI: 10.1016/j.jmst.2015.12.013
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NiTi particles reinforced aluminum (NiTip/Al) composite was prepared via friction stir processing, eliminating interfacial reaction and/or elemental diffusion. The NiTip in the composite maintained the intrinsic characteristic of a reversible thermoelastic phase transformation even after heat-treatment. The shape memory characteristic of the NiTip decreased the coefficient of thermal expansion of the Al matrix, and an apparent two-way shape memory effect was observed in the composite. The composite owned a good combination of adjustable damping and thermal physical properties.
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Effect of Rotation Rate on Microstructure and Mechanical Properties of Friction Stir Spot Welded DP780 Steel
G.M. Xie, H.B. Cui, Z.A. Luo, W. Yu, J. Ma, G.D. Wang
J. Mater. Sci. Technol.    2016, 32 (4): 326-333.   DOI: 10.1016/j.jmst.2015.10.009
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DP780 steel sheets consisting of ferrite and martensite were successfully friction stir spot welded (FSSW) at the rotation rates of 500 to 1500 r/min using a W-Re alloy tool. The effect of rotation rate on microstructure and mechanical properties of the FSSW DP780 was investigated. The peak temperatures in the welds at various rotation rates were identified to be above A3 temperature. FSSW caused the dynamic recrystallization in the stir zone (SZ), thereby producing the fine equiaxed grain structures. At the higher rotation rates of ≥1000 r/min, a full martensitic structure was observed throughout the SZs, whereas at the lower rotation rate of 500 r/min, the SZ consisted of a fine dual phase structure of ferrite and martensite due to the action of deformation induced ferrite transformation. The maximum average failure load as high as 18.2 kN was obtained at the rotation rate of 1000 r/min and the fracture occurred at the thinned upper sheet.
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Effect of Reverse-threaded Pin on Mechanical Properties of Friction Stir Lap Welded Alclad 2024 Aluminum Alloy
Yumei Yue, Zhengwei Li, Shude Ji, Yongxian Huang, Zhenlu Zhou
J. Mater. Sci. Technol.    2016, 32 (7): 671-675.   DOI: 10.1016/j.jmst.2016.03.005
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In friction stir welding (FSW), tool geometry plays an important role in joint quality. In order to improve mechanical properties of friction stir lap welding (FSLW) joint, a tool with a reverse-threaded pin was designed in the present study. Using 2024-T4 aluminum alloy as the research object, tools with the full-threaded pin and reverse-threaded pin were used in FSLW. Results showed that, when using the same parameter combination, FSLW joint using the reverse-threaded pin owned bigger effective sheet thickness (EST), bigger lap width and better lap shear failure strength. Compared with the full-threaded pin, fracture mode of the FSLW joint changed from shear fracture mode to tensile fracture mode when the reverse-threaded pin was used. Fracture morphologies presented ductile fracture.
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Effect of Friction Stir Processing on Pitting Corrosion and Intergranular Attack of 7075 Aluminum Alloy
Navaser M,Atapour* M
J. Mater. Sci. Technol.    2017, 33 (2): 155-165.  
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The effect of friction stir processing (FSP) on the pitting corrosion and the intergranular attack of 7075 aluminum alloy was investigated. Three friction stir processed samples were produced by employing a constant tool travel speed of 100?mm/min at the rotating speeds of 630, 1000 and 1600?rpm. It was demonstrated that the processed samples suffered from both pitting and intergranular corrosion. Also, the sample processed at 1600?rpm exhibited the best pitting corrosion resistance. For all FS processed samples, the corrosion attack in the heat affected zone was pitting corrosion, whereas no intergranular corrosion was detected in this area.

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ISSN: 1005-0302
CN: 21-1315/TG
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