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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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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 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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Pinless Friction Stir Spot Welding of Mg?3Al?1Zn Alloy with Zn Interlayer
R.Z. Xu, D.R. Ni, Q. Yang, C.Z. Liu, Z.Y. Ma
J. Mater. Sci. Technol.    2016, 32 (1): 76-88.   DOI: 10.1016/j.jmst.2015.08.012
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With the addition of a thin Zn interlayer, 2.4 mm thick Mg?3Al?1Zn alloy sheets were friction stir spot welded (FSSW) using a pinless tool with flat, convex and concave shoulder shapes. The results showed that an alloying reaction took place between the Mg substrate and Zn interlayer during FSSW, forming a discontinuous intermetallics layer composed of dispersive (α-Mg + MgZn) eutectic structure underneath the shoulder and a Mg?Zn intermetallics bonding zone at the outside of the joints. This alloying reaction increased the bonded area and eliminated the hook defects, thereby producing sound FSSW joints with a shallow keyhole without hook defects. The increase of plunge depth was beneficial to the Mg?Zn diffusion, thereby increasing the tensile-shear load of the joints. However, excessive plunge depths resulted in a decrease of the effective sheet thickness, reducing the strength of the joints. At a small plunge depth, the convex and concave shoulders were more beneficial to the interface reaction than the flat shoulder. The maximum joint load of 6.6 kN was achieved by using the concave shoulder at a plunge depth of 1.0 mm. A post-welding heat treatment promoted the dissolution of the discontinuous reaction layer in the joints; however, it led to the occurrence of void defects, influencing the bonding strength.
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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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Intermetallic Compounds in the Banded Structure and Their Effect on Mechanical Properties of Al/Mg Dissimilar Friction Stir Welding Joints
Shi Hui, Chen Ke, Liang Zhiyuan, Dong Fengbo, Yu Taiwu, Dong Xianping, Zhang Lanting, Shan Aidang
J. Mater. Sci. Technol.    2017, 33 (4): 359-366.   DOI: 10.1016/j.jmst.2016.05.006
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Dissimilar friction stir welding (FSW) between aluminum and magnesium alloy was performed, using various tool rotational speed (TRS) at a fixed travel speed, with tool offset to aluminum to investigate the formation of intermetallic compounds (IMCs) in the banded structure (BS) zone and their effect on mechanical properties. Large quantities of IMCs, in the form of alternating bands of particles or lamellae, were found in the BS zone, where drastic material intermixing occurred during FSW. The BS microstructural characters in terms of the morphology of the bands and the quantity and distribution of IMC particles varied with TRS. All welds exhibited brittle fracture mode with their fracture paths propagating mainly in/along the IMCs in the BS. It is shown that these BS microstructural characters have significant effect on the mechanical properties of the joints. Suggestions on tailoring the BS microstructure were proposed for improving the strength of the BS zone and the final mechanical properties of the Al/Mg FSW joints.

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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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Enhanced Mechanical Properties of Friction Stir Welded 5083Al-H19 Joints with Additional Water Cooling
Wang B.B., Chen F.F., Liu F., Wang W.G., Xue P., Ma Z.Y.
J. Mater. Sci. Technol.    2017, 33 (9): 1009-1014.   DOI: 10.1016/j.jmst.2017.01.016
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3-mm-thick 5083Al-H19 rolled plates were friction stir welded (FSW) at tool rotation rates of 800 and 200 rpm with and without additional water cooling. With decreasing the rotation rate and applying water cooling, softening in the FSW joint was significantly reduced. At a low rotation rate of 200 rpm with additional water cooling, almost no obvious softening was observed in the FSW joint, and therefore a FSW 5083Al-H19 joint with nearly equal strength to the base material (BM) was obtained. Furthermore, the grains in the nugget zone were considerably refined with reducing the heat input and ultrafine equiaxed grains of about 800 nm were obtained in the lowest heat input condition. This work provides an effective method to achieve high property FSW joints of precipitate-hardened and work-hardened Al alloys.

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Microstructures and mechanical properties of friction stir welds on 9% Cr reduced activation ferritic/martensitic steel
Chao Zhang, Lei Cui, Yongchang Liu, Chenxi Liu, Huijun Li
J. Mater. Sci. Technol.    2018, 34 (5): 756-766.   DOI: 10.1016/j.jmst.2017.11.049
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In this study, the microstructures and mechanical properties of 9%Cr reduced activation ferritic/martensitic (RAFM) steel friction stir welded joints were investigated. When a W-Re tool is used, the recommended welding parameters are 300 rpm rotational speed, 60 mm/min welding speed and 10 kn axial force. In stir zone (SZ), austenite dynamic recrystallization induced by plastic deformation and the high cooling rates lead to an obvious refinement of prior austenite grains and martensite laths. The microstructure in SZ contains lath martensite with high dislocation density, a lot of nano-sized MX and M3C phase particles, but almost no M23C6 precipitates. In thermal mechanically affect zone (TMAZ) and heat affect zone (HAZ), refinement of prior austenite and martensitic laths and partial dissolution of M23C6 precipitates are obtained at relatively low rotational speed. However, with the increase of heat input, coarsening of martensitic laths, prior austenite grains, and complete dissolution of M23C6 precipitates are achieved. Impact toughness of SZ at -20 °C is slightly lower than that of base material (BM), and exhibits a decreasing trend with the increase of rotational speed.

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Friction stir welding of carbon nanotubes reinforced Al-Cu-Mg alloy composite plates
Zhao Ke, Liu Zhenyu, Xiao Bolyu, Ma Zongyi
J. Mater. Sci. Technol.    2017, 33 (9): 1004-1008.   DOI: 10.1016/j.jmst.2017.01.033
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Carbon nanotubes (CNTs) reinforced Al-Cu-Mg composite plates of 2.2 mm in thickness after extrusion and T4 treatment were joined by friction stir welding (FSW) and the joint efficiency reaches 87%. There was no precipitate in both heat-affected zone (HAZ) and nugget zone (NZ) as a medium rotation rate of 800 rpm and a relative high travel speed of 100 mm min-1 were used. In the NZ, FSW disarranged the alignment of CNTs to random orientation and dispersed CNT uniformly. The orientation of CNTs perpendicular to the tensile direction and the possible dissolution of solute clusters made the HAZ become the weakest zone in the joint leading to the failure in the HAZ.

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