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ISSN 1005-0302
CN 21-1315/TG
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      20 January 2018, Volume 34 Issue 1 Previous Issue    Next Issue
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    Orginal Article
    Friction stir based welding and processing technologies - processes, parameters, microstructures and applications: A review
    G.K. Padhy, C.S. Wu, S. Gao
    J. Mater. Sci. Technol., 2018, 34 (1): 1-38.  DOI: 10.1016/j.jmst.2017.11.029
    Abstract   HTML   PDF

    Friction stir welding (FSW) has achieved remarkable success in the joining and processing of aluminium alloys and other softer structural alloys. Conventional FSW, however, has not been entirely successful in the joining, processing and manufacturing of different desired materials essential to meet the sophisticated green globe requirements. Through the efforts of improving the process and transferring the existing friction stir knowledge base to other advanced applications, several friction stir based daughter technologies have emerged over the timeline. A few among these technologies are well developed while others are under the process of emergence. Beginning with a broad classification of the scattered frictions stir based technologies into two categories, welding and processing, it appears now time to know, compile and review these to enable their rapid access for reference and academia. In this review article, the friction stir based technologies classified under the category of welding are those applied for joining of materials while the remnant are labeled as friction stir processing (FSP) technologies. This review article presents an overview of four general aspects of both the developed and the developing friction stir based technologies, their associated process parameters, metallurgical features of their products and their feasibility and application to various materials. The lesser known and emerging technologies have been emphasized.

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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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    Friction-stir welding and processing of Ti-6Al-4V titanium alloy: A review
    S.Mironov, Y.S.Sato, H.Kokawa
    J. Mater. Sci. Technol., 2018, 34 (1): 58-72.  DOI: 10.1016/j.jmst.2017.10.018
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    In this work, the current understanding and development of friction-stir welding and processing of Ti-6Al-4V alloy are briefly reviewed. The critical issues of these processes are addressed, including welding tool materials and design, tool wear, processing temperature, material flow, processing window and residual stresses. A particular emphasis is given to microstructural aspects and microstructure-properties relationship. Potential engineering applications are highlighted.

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    Friction stir welding of high-strength aerospace aluminum alloy and application in rocket tank manufacturing
    Guoqing Wang, Yanhua Zhao, Yunfei Hao
    J. Mater. Sci. Technol., 2018, 34 (1): 73-91.  DOI: 10.1016/j.jmst.2017.11.041
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    Friction stir welding (FSW) has been widely adopted in aerospace industry for fabricating high-strength aluminum alloy structures, such as large volume fuel tanks, due to its exceptional advantages including low distortion, less defects and high mechanical properties of the joint. This article systematically reviews the key technical issues in producing large capacity aluminum alloy fuel tanks by using FSW, including tool design, FSW process optimization, nondestructive testing (NDT) techniques and defect repairing techniques, etc. To fulfill the requirements of Chinese aerospace industry, constant-force FSW, retractable tool FSW, lock joint FSW, on-line NDT and solid-state equal-strength FSW techniques, as well as a complete set of aerospace aluminum FSW equipment, have been successfully developed. All these techniques have been engineered and validated in rocket tanks, which enormously improved the fabrication ability of Chinese aerospace industry.

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    Maintaining nano-lamellar microstructure in friction stir welding (FSW) of accumulative roll bonded (ARB) Cu-Nb nano-lamellar composites (NLC)
    Judy Schneider, Josef Cobb, John S. Carpenter, Nathan A. Mara
    J. Mater. Sci. Technol., 2018, 34 (1): 92-101.  DOI: 10.1016/j.jmst.2017.10.016
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    Accumulative roll bonded (ARB) Copper Niobium (Cu-Nb) nano-lamellar composite (NLC) panels were friction stir welded (FSWed) to evaluate the ability to join panels while retaining the nano-lamellar structure. During a single pass of the friction stir welding (FSW) process, the nano-lamellar structure of the parent material (PM) was retained but was observed to fragment into equiaxed grains during the second pass. FSW has been modeled as a severe deformation process in which the material is subjected to an instantaneous high shear strain rate followed by extreme shear strains. The loss of the nano-lamellar layers was attributed to the increased strain and longer time at temperature resulting from the second pass of the FSW process. Kinematic modeling was used to predict the global average shear strain and shear strain rates experienced by the ARB material during the FSW process. The results of this study indicate that through careful selection of FSW parameters, the nano-lamellar structure and its associated higher strength can be maintained using FSW to join ARB NLC panels.

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    Correlation between microstructures and mechanical properties of high-speed friction stir welded aluminum hollow extrusions subjected to axial forces
    Xiangqian Liu, Huijie Liu, Tianhao Wang, Xiangguo Wang, Si Yang
    J. Mater. Sci. Technol., 2018, 34 (1): 102-111.  DOI: 10.1016/j.jmst.2017.11.015
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    The AA6005A-T6 aluminum hollow extrusions were friction stir welded at a high welding speed of 2000 mm/min and various axial forces. The results show that the nugget zone (NZ) is characterized by fine equiaxed grains, in which a low density of equilibrium phase β is observed. The grains in the thermo-mechanically affected zone (TMAZ) are elongated, and the highest density of dislocations and a low density of β' precipitates can be found in grains. The heat affected zone (HAZ) only experiences a low thermal cycle, and a high density of β” precipitates and a low density of β' precipitates remain in the coarsened grains. The microhardness evolutions in the NZ, TMAZ and HAZ are governed by the grain refinement and dislocation strengthening, the dislocation and precipitation strengthening, and the precipitation and solid solution strengthening, respectively. When increasing the axial force, the changing trend of one strengthening mechanism is contrary to the other in each zone, and the microhardness increases in different zones. As a result, the tensile strength roughly increases with raising the axial force, and all joints show good tensile properties as the high welding speed inhibits the coarsening and dissolution of strengthening precipitates significantly.

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    Fabrication of large-bulk ultrafine grained 6061 aluminum alloy by rolling and low-heat-input friction stir welding
    C.Y. Liu, B. Qu, P. Xue, Z.Y. Ma, K. Luo, M.Z. Ma, R.P. Liu
    J. Mater. Sci. Technol., 2018, 34 (1): 112-118.  DOI: 10.1016/j.jmst.2017.02.008
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    In this study, the ultrafine grained (UFG) 6061 Al alloys fabricated by cold rolling were friction stir welded (FSW) with different rotation rates under both air cooling and rapid cooling in water. Low-heat-input parameters of 400 rpm rotation rate in water (400-Water) could effectively inhibit the coarsening of recrystallized grains, reduce the precipitation rate, and retain more dislocations of the UFG 6061 Al parent metal. 400-Water joint showed high lowest-hardness value, narrow low-hardness zone, and high tensile strength, attributing to the effect of dislocation, grain boundary, solid-solution, and precipitation hardening. This work provides an effective strategy to fabricate large-sized bulk UFG Al alloy by cold rolling with large deformation and low-heat-input FSW.

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    Influence of a non-rotating shoulder on heat generation, microstructure and mechanical properties of dissimilar AA2024/AA7050 FSW joints
    Alessandro Barbini, Jan Carstensen, Jorge F. dos Santos
    J. Mater. Sci. Technol., 2018, 34 (1): 119-127.  DOI: 10.1016/j.jmst.2017.10.017
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    Friction stir welding (FSW) and stationary shoulder friction stir welding (SSFSW) were carried out for the butt joining of dissimilar AA2024-T3 and AA7050-T7651 aluminium alloys with thicknesses of 2 mm. A comparison between the two processes was performed by varying the welding speed while keeping the rotational speed constant. Through the analysis of the force and torque produced during welding and a simple analytical model, it was possible to show that in SSFSW there is more effective coupling with the tool and the heat produced is more efficiently distributed. This process decreases both the welding area and the diffusion at the interface of the two alloys compared with FSW. The minimum microhardness occurred at the advancing side (AS) at the interface between the thermo-mechanically affected zone (TMAZ) and the stir zone (SZ) in both processes, although the decrease was more gradual in SSFSW. This interface is also where all specimens failed for both welding technologies. An increase in tensile strength was measured in SSFSW compared with standard FSW. Furthermore, it was possible to establish the mechanical performance of the material in the fracture zone using digital image correlation.

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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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    Improving weld formability by a novel dual-rotation bobbin tool friction stir welding
    F.F. Wang, W.Y. Li, J. Shen, Q. Wen, J.F. dos Santos
    J. Mater. Sci. Technol., 2018, 34 (1): 135-139.  DOI: 10.1016/j.jmst.2017.11.001
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    A novel dual-rotation bobbin tool friction stir welding (DBT-FSW) was developed, in which the upper shoulder (US) and lower shoulder (LS) have different rotational speeds. This process was tried to weld 3.2 mm thick aluminum-lithium alloy sheets. The metallographic analysis and torque measurement were carried out to characterize the weld formability. Experimental results show that compared to conventional bobbin tool friction stir welding, the DBT-FSW has an excellent process stability, and can produce the defect-free joints in a wider range of welding parameters. These can be attributed to the significant improvement of material flow caused by the formation of a staggered layer structure and the unbalanced force between the US and LS during the DBT-FSW process.

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    Solid state crack repair by friction stir processing in 304L stainless steel
    C. Gunter, M.P. Miles, F.C. Liu, T.W Nelson
    J. Mater. Sci. Technol., 2018, 34 (1): 140-147.  DOI: 10.1016/j.jmst.2017.10.023
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    Friction stir processing (FSP) was investigated as a method of repairing cracks in 12 mm thick 304L stainless steel plate. Healing feasibility was demonstrated by processing a tapered crack using a PCBN/W-Re tool with a 25 mm diameter shoulder and a pin length of 6.4 mm. The experiment showed that it was possible to heal a crack that begins narrow and then progressively grows up to a width of 2 mm. Bead on plate experiments were used to find the best parameters for creating a consolidated stir zone with the least amount of hardness difference compared to the base metal. Grain refinement in some specimens resulted in much higher stir zone hardness, compared to base metal. A plot of grain size versus microhardness showed a very strong inverse correlation between grain size and hardness, as expected from the Hall-Petch relationship. Corrosion testing was carried out in order to evaluate the effect of FSP on potential sensitization of the stir zone. After 1000 h of intermittent immersion in 3.5% saline solution at room temperature it was found that no corrosion products formed on the base material controls or on any of the friction stir processed specimens.

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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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    Tensile properties and fracture behavior of friction stir welded joints of Fe-32Mn-7Cr-1Mo-0.3N steel at cryogenic temperature
    Yi-jun Li, Rui-dong Fu, Yan Li, Yan Peng, Hui-jie Liu
    J. Mater. Sci. Technol., 2018, 34 (1): 157-162.  DOI: 10.1016/j.jmst.2017.11.034
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    This study investigates the cryogenic tensile properties and fracture behavior of fiction stir welded and post-weld heat-treated joints of 32Mn-7Cr-1Mo-0.3N steel. Cryogenic brittle fracture, which occurred in the as-welded joint, is related to the residual particles that contain tungsten in the joint band structure. Post-weld water toughening resulted in the cryogenic intergranular brittleness of the joint, which is related to the non-equilibrium segregation of solute atoms during the post-weld water toughening. Annealing at 550 °C for 30 min can effectively inhibit the cryogenic intergranular brittleness of the post-weld water-toughened joint. The yield strength, ultimate tensile strength, and uniform elongation of the annealed joint are approximately 95%, 87%, and 94% of the corresponding data of the base metal.

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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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    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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    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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    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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    Variant selection in stationary shoulder friction stir welded Ti-6Al-4V alloy
    Xiaoqing Jiang, Bradley P. Wynne, Jonathan Martin
    J. Mater. Sci. Technol., 2018, 34 (1): 198-208.  DOI: 10.1016/j.jmst.2017.11.024
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    Stationary shoulder friction stir welding of Ti-6Al-4V of 7 mm thickness was conducted with varying welding speeds and rotation speeds. Variant selection analysis was carried out based on the inherited α phase texture and the reconstructed β phase texture. The weld surfaces became significantly smoother with increasing welding speed and decreasing rotation speed. Heat input decreased greatly with increased welding speed and it decreased slightly with decreased rotation speed. The orientation relationship between the prior β grains was measured based on the reconstructed electron backscattered diffraction (EBSD) data. Weak Variant selection has occurred in all the welds because most of the prior β grains did not share {110} poles. Strong links between crystal orientation of the prior β grains and hardness have been found.

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    Impact of friction stir welding on recrystallization of oxide dispersion strengthened ferritic steel
    Wentuo Han, Pingping Liu, Xiaoou Yi, Qian Zhan, Farong Wan, Kiyohiro Yabuuchi, Hisashi Serizawa, Akihiko Kimura
    J. Mater. Sci. Technol., 2018, 34 (1): 209-213.  DOI: 10.1016/j.jmst.2017.11.032
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    Oxide dispersion strengthened (ODS) steels can be used as the structural materials in the future fusion reactors and the fuel cladding materials in the advanced fission reactors. However, the weldability of ODS steels is a severe problem. In the present study, defect-free joints of the 15Cr-ODS ferritic steel were achieved by friction stir welding at different rotation speeds. The recrystallization, hardness and tensile properties are highly related with the rotation speed of the stir tool. The higher rotation speed results in coarser grains in the top SZ, while the grain size exhibits more complicated relation with the rotation speed in the SZ center. The joint welded at 250 rpm exhibits a maximum tensile strength of 974 MPa that reaches about 84% of that of the base metal.

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    Effect of friction stir processed microstructure on tensile properties of an Al-Zn-Mg-Sc alloy upon subsequent aging heat treatment
    Indrajit Charit, Rajiv S. Mishra
    J. Mater. Sci. Technol., 2018, 34 (1): 214-218.  DOI: 10.1016/j.jmst.2017.10.021
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    An as-cast Al-Zn-Mg-Sc alloy was friction stir processed varying tool related parameters, yielding microstructures with different grain sizes (0.68, 1.8 and 5.5 μm). Significant increases in room temperature ductility were obtained in these materials with reasonable enhancement in strength. It is demonstrated that the type of microstructure produced by friction stir processing (FSP) has a significant influence on the choice of post-FSP heat treatment design for achieving improved tensile properties. It is also found that the ultrafine grained FSP material could not achieve the desired high strength during the post-FSP heat treatment without grain coarsening, whereas the micro-grained FSP materials could reach such strength levels (>560 MPa) under conventional age hardening heat treatment conditions.

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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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    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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    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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