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Ultrasound enhanced friction stir welding of aluminum and steel: Process and properties of EN AW 6061/DC04-Joints
Thom? Marco, Guntram Wagner, Stra? Benjamin, Bernd Wolter, Sigrid Benfer, Wolfram Fürbeth
J. Mater. Sci. Technol.    2018, 34 (1): 163-172.   DOI: 10.1016/j.jmst.2017.10.022
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In this work, the joining of aluminum to steel was conducted by ultrasound enhanced friction stir welding (USE-FSW). The power ultrasound was introduced into one of the metal sheets by an ultrasonic roll seam module synchronously to the FSW-process. The effect of the ultrasound on the resulting welds, their microstructure and their corrosion properties was investigated by light and scanning electron microscopy and corrosion investigations. The USE-FSW-joints showed less and smaller steel particles in the nugget zone as well as a thinner continuous intermetallic phase of FeAl3 at the interface. The nondestructive testing method of computed laminography proved the observations made by optic microscopy due to non-porous joints for both techniques. Corrosion investigations showed only low corrosion current densities and no enhanced galvanic corrosion for the EN AW-6061/DC04-hybrid joints in sodium chloride solution.

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Inhomogeneity of microstructure and mechanical properties in the nugget of friction stir welded thick 7075 aluminum alloy joints
Yuqing Mao, Liming Ke, Yuhua Chen, Fencheng Liu, Li Xing
J. Mater. Sci. Technol.    2018, 34 (1): 228-236.   DOI: 10.1016/j.jmst.2017.11.039
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In this study, 20 mm thick AA7075-T6 alloy plates were joined by friction stir welding. The microstructure and mechanical properties of the nugget zone along the thickness direction from the top to the bottom was investigated. The results showed that the microstructure including the grain size, the degree of dynamic recrystallization, the misorientation angle distribution and the precipitation phase containing its size, type and content exhibited a gradient distribution along the thickness direction. The testing results of mechanical properties of the slices showed that the nugget was gradually weakened along the depth from the top to the bottom. The maximum ultimate tensile strength, yield strength and elongation of the slice in the nugget top-middle are obtained, which are 415 MPa, 255 MPa and 8.1%, respectively.

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A review of friction stir welding of steels: Tool, material flow, microstructure, and properties
F.C.Liu, Y.Hovanski, M.P.Miles, C.D.Sorensen, T.W.Nelson
J. Mater. Sci. Technol.    2018, 34 (1): 39-57.   DOI: 10.1016/j.jmst.2017.10.024
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Considerable progress has been achieved in friction stir welding (FSW) of steels in every aspect of tool fabrication, microstructure control and properties evaluation in the past two decades. With the development of reliable welding tools and precise control systems, FSW of steels has reached a new level of technical maturity. High-quality, long welds can be produced in many engineering steels. Compared to traditional fusion welding, FSW exhibits unique advantages producing joints with better properties. As a result of active control of the welding temperature and/or cooling rate, FSW has the capability of fabricating steel joints with excellent toughness and strength. For example, unfavorable phase transformations that usually occur during traditional welding can be avoided and favorable phase fractions in advanced steels can be maintained in the weld zone thus avoiding the typical property degradations associated with fusion welding. If phase transformations do occur during FSW of thick steels, optimization of microstructure and properties can be attained by controlling the heat input and post-weld cooling rate.

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Formability of friction stir processed low carbon steels used in shipbuilding
D.M. Sekban, S.M. Akterer, O. Saray, Z.Y. Ma, G. Purcek
J. Mater. Sci. Technol.    2018, 34 (1): 237-244.   DOI: 10.1016/j.jmst.2017.10.020
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The stretch formability of a low carbon steel processed by friction stir processing (FSP) was studied under biaxial loading condition applied by a miniaturized Erichsen test. One-pass FSP decreased the ferritic grain size in the processed zone from 25 μm to about 3 μm, which also caused a remarkable increase in strength values without considerable decrease in formability under uniaxial loading. A coarse-grained (CG) sample before FSP reflected a moderate formability with an Erichsen index (EI) of 2.73 mm. FSP slightly decreased the stretch formability of the sample to 2.66 mm. However, FSP increased the required punch load (FEI) due to the increased strength by grain refinement. FSP reduced considerably the roughness of the free surface of the biaxial stretched samples with reduced orange peel effect. The average roughness value (Ra) decreased from 2.90 in the CG sample down to about 0.65 μm in fine-grained (FG) sample after FSP. It can be concluded that the FG microstructure in low carbon steels sheets or plates used generally in shipbuilding provides a good balance between strength and formability.

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Direct joining of oxygen-free copper and carbon-fiber-reinforced plastic by friction lap joining
L.H. Wu, K. Nagatsuka, K. Nakata
J. Mater. Sci. Technol.    2018, 34 (1): 192-197.   DOI: 10.1016/j.jmst.2017.10.019
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Oxygen-free copper (Cu) was successfully joined to carbon-fiber-reinforced thermoplastic (CFRTP, polyamide 6 with 20 wt% carbon fiber addition) by friction lap joining (FLJ) at joining speeds of 200-1600 mm/min with a constant rotation rate of 1500 rpm and a nominal plunge depth of 0.9 mm. It is the first time to report the joining of CFRTP to Cu by FLJ. As the joining speed increased, the tensile shear force (TSF) of joints increased first, and decreased thereafter. The maximum TSF could reach 2.3 kN (15 mm in width). Hydrogen bonding formed between the amide group of CFRTP and the thin Cu2O layer on the Cu surface, which mainly contributed to the joint bonding. The influence factors of the TSF of the joints at different joining speeds were discussed. The TSF was mainly affected by the joining area, the degradation of the plastic matrix and the number and the size of bubbles. As the joining speed increased, the influence factors varied as follows: the joining area increased first and then decreased; the degradation of the plastic matrix and the number and the size of bubbles decreased. The maximum TSF was the comprehensive result of the relatively large joining area, small degradation of the plastic matrix and small number and sizes of bubbles.

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Local melting mechanism and its effects on mechanical properties of friction spot welded joint for Al-Zn-Mg-Cu alloy
Yunqiang Zhao, Chungui Wang, Jizhong Li, Jinhong Tan, Chunlin Dong
J. Mater. Sci. Technol.    2018, 34 (1): 185-191.   DOI: 10.1016/j.jmst.2017.11.014
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Local melting and the eutectic film and liquation crack formation mechanisms during friction spot welding (FSpW) of Al-Zn-Mg-Cu alloy were studied by both experiment and finite element simulation. Their effects on mechanical properties of the joint were examined. When the welding heat input was high, the peak temperature in the stir zone was higher than the incipient melting temperature of the Al-Zn-Mg-Cu alloy. This resulted in local melting along the grain boundaries in this zone. In the retreating stage of the welding process, the formed liquid phase was driven by the flowing plastic material and redistributed as a “U-shaped” line in the stir zone. In the following cooling stage, this liquid phase transformed into eutectic films and liquation cracks. As a result, a new characteristic of “U” line that consisted of eutectic films and liquation cracks is formed in the FSpW join. This “U” line was located in the high stress region when the FSpW joint was loaded, thus it was adverse to the mechanical properties of the FSpW joint. During tensile shear tests, the “U” line became a preferred crack propagation path, resulting in the occurrence of brittle fracture.

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Effects of welding parameters and post-heat treatment on mechanical properties of friction stir welded AA2195-T8 Al-Li alloy
J. Zhang, X.S. Feng, J.S. Gao, H. Huang, Z.Q. Ma, L.J. Guo
J. Mater. Sci. Technol.    2018, 34 (1): 219-227.   DOI: 10.1016/j.jmst.2017.11.033
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In this study, the effects of main welding parameters (rotation speed (ω) and welding speed (v)) on the microstructure, micro-hardness distribution and tensile properties of friction stir welded (FSW) 2195-T8 Al-Li alloy were investigated. The effects of T6 post-treatments at different solution and aging conditions on the mechanical properties and microstructure characteristics of the FSW joints were also investigated. The results show that with increasing ω and v, both strength and elongation of the joints increase first, and then decrease with further increase of ω and v. All the joints under varied welding parameters show significant strength loss, and the strength reaches only 65% of the base metal. The effect of T6 post-heat treatment on the mechanical properties of the joints depends on the solution and aging conditions. Two heat treatment processes (480 °C × 0.5 h quenching + 180 °C × 12 h, 520 °C × 0.5 h quenching + 180 °C × 12 h aging) are found to increase the joint strength. Furthermore, low temperature quenching (480 °C) is more beneficial to the joint strength. The joint strength can reach 85% of the base metal. Whereas both low temperature aging (140 °C × 56 h) and stepped aging (100 °C × 12 h + 180 °C × 3 h) processes decrease the joint strength. After heat treatment all the joints show decreased ductility due to the obvious grain coarsening in the nugget zone (NZ) and thermo-mechanically affected zone (TMAZ).

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Computational fluid dynamics simulation of friction stir welding: A comparative study on different frictional boundary conditions
Gaoqiang Chen, Qingxian Ma, Shuai Zhang, Jianjun Wu, Gong Zhang, Qingyu Shi
J. Mater. Sci. Technol.    2018, 34 (1): 128-134.   DOI: 10.1016/j.jmst.2017.10.015
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Numerical simulation based on computational fluid dynamics (CFD) is a useful approach for quantitatively investigating the underlying thermal-mechanical conditions during FSW, such as temperature field and material deformation field. One of the critical issues in CFD simulation of FSW is the use of the frictional boundary condition, which represents the friction between the welding tool and the workpiece in the numerical models. In this study, three-dimensional numerical simulation is conducted to analyze the heat transfer and plastic deformation behaviors during the FSW of AA2024. For comparison purposes, both the boundary velocity (BV) models and the boundary shear stress (BSS) models are employed in order to assess their performances in predicting the temperature and material deformation in FSW. It is interesting to note that different boundary conditions yield similar predictions on temperature, but quite different predictions on material deformation. The numerical predictions are compared with the experimental results. The predicted deformation zone geometry by the BSS model is consistent with the experimental results while there is large difference between the predictions by the BV models and the experimental measurements. The fact that the BSS model yields more reasonable predictions on the deformation zone geometry is attributed to its capacity to automatically adjust the contact state at the tool/workpiece interface. Based on the favorable predictions on both the temperature field and the material deformation field, the BSS model is suggested to have a better performance in numerical simulation of FSW than the BV model.

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Corrosion fatigue behavior of friction stir processed interstitial free steel
Wen Wang, Ruiqi Xu, Yaxin Hao, Qiang Wang, Liangliang Yu, Qianying Che, Jun Cai, Kuaishe Wang, Zongyi Ma
J. Mater. Sci. Technol.    2018, 34 (1): 148-156.   DOI: 10.1016/j.jmst.2017.11.013
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In this study, interstitial free (IF) steel plates were subjected to double-sided friction stir processing (FSP). The fine-grained structure with an average grain size of about 12 μm was obtained in the processed zone (PZ) with a thickness of about 2.5 mm. The yield strength (325 MPa) and ultimate tensile strength (451 MPa) of FSP IF steel were significantly higher than those of base material (BM) (192 and 314 MPa), while the elongation (67.5%) almost remained unchanged compared with the BM (66.2%). The average microhardness value of the PZ was about 130 HV, 1.3 times higher than that of the BM. In addition, the FSP IF steel showed a more positive corrosion potential and lower corrosion current density than the BM, exhibiting lower corrosion tendency and corrosion rates in a 3.5 wt% NaCl solution. Furthermore, FSP IF steel exhibited higher fatigue life than the BM both in air and NaCl solution. Corrosion fatigue fracture surfaces of FSP IF steel mainly exhibited a typical transgranular fracture with fatigue striations, while the BM predominantly presented an intergranular fracture. Enhanced corrosion fatigue performance was mainly attributed to the increased resistance of nucleation and growth of fatigue cracks. The corrosion fatigue mechanism was primarily controlled by anodic dissolution under the combined effect of cyclic stress and corrosive solution.

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Comparative study on local and global mechanical properties of bobbin tool and conventional friction stir welded 7085-T7452 aluminum thick plate
Weifeng Xu, Yuxuan Luo, Wei Zhang, Mingwang Fu
J. Mater. Sci. Technol.    2018, 34 (1): 173-184.   DOI: 10.1016/j.jmst.2017.05.015
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7085-T7452 plates with a thickness of 12 mm were welded by conventional single side and bobbin tool friction stir welding (SS-FSW and BB-FSW, respectively) at different welding parameters. The temperature distribution, microstructure evolution and mechanical properties of joints along the thickness direction were investigated, and digital image correlation (DIC) was utilized to evaluate quantitatively the deformation of different zones during tensile tests. The results indicated that heat-affected zone (HAZ), the local softening region, was responsible for the early plastic deformation and also the fracture location for SS-FSW samples, while a rapid fracture was observed in weld nugget zone (WNZ) before yield behavior for all BB-FSW specimens. The ultimate tensile strength (UTS) of SS-FSW joints presented the highest value of 410 MPa, 82% of the base material, at a rotational speed of 300 rpm and welding speed of 60 mm/min, much higher than that of BB-FSW joints, with a joint efficiency of only 47%. This should be attributed to the Lazy S defect produced by a larger extent of heat input during the BB-FSW process. The whole joint exhibited a much higher elongation than the slices. Scanning electron microscopic (SEM) analysis of the fracture morphologies showed that joints failed through ductile fracture for SS-FSW and brittle fracture for BB-FSW.

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