Review on friction stir welding of dissimilar magnesium and aluminum alloys: Scientometric analysis and strategies for achieving high-quality joints

Magnesium and aluminum alloys continually attract interest as lightweight structural materials for transport applications. However, joining these dissimilar alloys is very challenging. The main obstacle that hinders progress in dissimilar Mg-Al joining is the formation of brittle intermetallic compounds(IMCs). As a solid-state joining technique, FSW is an excellent candidate to attenuate the deleterious IMC effects in dissimilar Al-Mg joining due to the inherent low heat inputs involved in the process. However, the IMCs, namely Al_(3)Mg_(2) and Al_(12)Mg_(17) phases, have also been reported to form during Al-Mg dissimilar FSW;their amount and thickness depend on the heat input involved;thus,the weld parameters used. Since the heat dissipated in the material during the welding process significantly affects the amount of IMCs,the heat input during FSW should be kept as low as possible to control and reduce the amount of IMCs. This review aims to critically discuss and evaluate the studies conducted in the dissimilar Al/Mg FSW through a scientometric analysis and also with a focus on the strategies recently applied to enhance joint quality. The scientometric analysis showed that the main research directions in Mg/Al FSW are the technological weldability of aluminum and magnesium during FSW, structural morphology, and mechanical properties of dissimilar welded joints. Considering the scope of application of the aforementioned joints, the low share of articles dealing with environmental degradation and operational cracking is surprising. This might be attributed to the need for well-developed strategies for obtaining high-quality and sustainable joints for applications. Thus, the second part of this review is conventional, focusing mainly on the new strategies for obtaining high-quality Mg/Al joints. It can be concluded that in addition to the necessity to optimum welding parameters to suppress the excessive heat to limit the amount and thickness of IMC formed and improve the overall joint quality, strategies such as using Zn interlayer, electric current assisted FSW(EAFSW), ultrasonic vibration FSW(UVa FSW), are considered effective in the elimination, reduction, and fragmentation of the brittle IMCs.

Kinematic Sensitivity Analysis and Dimensional Synthesis of a Redundantly Actuated Parallel Robot for Friction Stir Welding

Friction stir welding(FSW)has been widely applied in many fields as an alternative to traditional fusion welding.Although serial robots can provide the orientation capability required to weld along curved surfaces,they cannot adequately support the huge axial downward forces that FSW generates.Available parallel mechanism architectures,particularly redundantly actuated architectures for FSW,are still very limited.In this paper,a redundantly actuated 2 UPR-2 RPU parallel robot for FSW is proposed,where U denotes a universal joint,R denotes a revolute joint and P denotes a prismatic pair.First,its semi-symmetric structure is described.Next,inverse kinematics analysis involving an analytical representation of rotational axes is implemented.Velocity analysis is also conducted,which leads to the formation of a Jacobian matrix.Sensitivity performance is evaluated utilizing level set and convex optimization methods,where the local sensitivity indices are unit consistent,coordinate free,and of definite physical significance.Furthermore,global and hierarchical sensitivity indices are proposed for the design process.Finally,dimension synthesis is conducted based on the sensitivity indices and the optimal link parameters of the parallel robot are obtained.In summary,this paper proposes a dimensional synthesis method for a redundantly actuated parallel robot for FSW based on sensitivity indices.

Review: Progress and Trends in Ultrasonic Vibration Assisted Friction Stir Welding

This paper aims to reviewthe state-of-the-art of ultrasonic vibration assisted friction stir welding(UVAFSW) process. Particular attention has been paid on the modes of ultrasonic exertion,experimental results and effects of ultrasonic vibrations on process effectiveness and joint quality. The trends of various aspects with and without ultrasonic vibrations in FSW process are studied and presented. The influence of ultrasonic vibrations on welding loads, temperature history, weld morphology, material flow, weld microstructure and mechanical properties are revisited. Ultrasonic assisted FSW offers numerous advantages over the conventional FSW process. The superimposing of high-frequency vibrations improves various phenomena of the process and the physical,metallurgical,mechanical and tribological properties of the welded joint. The ultrasonic assisted FSW process has a potential to benefit the industry sector. A checklist listing the materials and process parameters used in the documented studies has been presented for quick reference.

A Study on Friction Stir Welding of 12mm Thick Aluminum Alloy Plates

Most of the investigations regarding friction stir welding(FSW)of aluminum alloy plates have been limited to about 5 to 6mm thick plates.In prior work conducted the various aspects concerning the process parameters and the FSW tool geometry were studied utilizing friction stir welding of 12 mm thick commercial grade aluminum alloy.Two different simple-tomanufacture tool geometries were used.The effect of varying welding parameters and dwell time of FSW tool on mechanical properties and weld quality was examined.It was observed that in order to achieve a defect free welding on such thick aluminum alloy plates,tool having trapezoidal pin geometry was suitable.Adequate tensile strength and ductility can be achieved utilizing a combination of high tool rotational speed of about 2000 r/min and low speed of welding around 28 mm/min.At very low and high dwell time the ductility of welded joints are reduced significantly.

Influence of welding parameters on material flow,mechanical property and intermetallic characterization of friction stir welded AA6063 to HCP copper dissimilar butt joint without offset

Joining of dissimilar aluminium-copper is an emerging area of interest for both research and industry due to its complex nature.Friction stir welding was attempted to evaluate the joint strength without offset at the butt line between AA6063 to HCP copper sheet under different combination of rotational speed of 800 and 1000 r/min and travel speed of 20 and 40 mm/min.Material flow was studied in detail for different combinations of parameters with optical microscopy and elemental mapping by energy dispersive X-ray spectroscopy（EDS）.The results were correlated with the microstructural characteristics and formation of intermetallics at the bond interface using microhardness test and X-ray diffraction（XRD） technique.Material flow clearly suggests that energy input at 800 r/min and 20 mm/min is sufficient to plasticize both the materials with formation of higher amount of thermodynamically stable and hard intermetallic phases Al4Cu9 and Al Cu4（slower cooling rate of 88 K/s） than that at 800 r/min and 40 mm/min（faster cooling rate of 154 K/s）,attributed maximum joint strength（~78.6% of aluminium base metal）.

Optimum traveling and rotation speeds in friction stir welding for dissimilar Al alloys

Aluminum alloys are subjected to large deformation and decreased strength due to the high expansion modulus caused by heat effects during friction stir welding （FSW）.The optimum conditions for friction stir welding of 5052-O and 6061-T6 Al alloys were determined.The optimum traveling and rotation speeds were identified to be 61mm/min and 1600r/min using various mechanical characteristic evaluation methods.

Experimental Investigations of Axial Force Monitoring and Control for Friction Stir Welding Process

Friction stir welding( FSW) is a solid-state welding process that utilizes a rotating tool to induce gross material plastic deformation and join two parts together. A large number of studies have indicated that axial force control can be used to achieve good welding quality. However,in the welding process,due to workpiece's geometry error,improper clamping and other process variations,the axial force can vary significantly and produce welding defects.The control of force in the process of FSW is investigated. At first,the development and evaluation of a closed-loop control system is described,which is equipped with a custom real-time wireless force dynamometer for FSW. Then,an axial force controller is designed based on nonlinear force controllers for FSW. Experimental validations are carried out on an FSW platform. The experimental results demonstrate that the controller maintains the constant axial force and shows desirable dynamic behavior, even when the disturbance is encountered during the welding process.

A bibliometric analysis on friction stir welding

Friction stir welding(FSW),as a growing welding technology,from its genesis in 1991,was attended by many researchers in interdisciplinary fields of science and engineering.The current paper presents a bibliometric analysis of the subject and can be addressed as guidance for the researchers who want to explore this field of welding technology.The research growth of FSW,most productive and influential authors,disciplines,leader journals,countries,and institutions are investigated.The highly influential papers in two historical periods and top keywords are also introduced.This paper summarizes the growth structure of FSW during the last 26 years and provides concise bibliometry.

Tool-workpiece interaction and shear layer flow during friction stir welding of aluminium alloys

Material flow phenomena during friction stir welding(FSW) and the forming mechanism of "onion rings" are complicated and currently not fully understood. In the present FSW study aluminium alloys 5083 and A356,with the latter more readily welded than the former,were used. The experiments were conducted to obtain samples of tool and workpiece "frozen" together during FSW for analysis. Two deposition modes for forming the weld nugget zone were observed,one for each alloy with the present FSW parameters. The first is the deposition of shear layers forming the "onion rings" in 5083 alloy nugget. The tool-workpiece interaction leading to the layer thickness being equal to the ratio of welding speed and tool rotation speed is suggested. The second mode is the combination of depositing the rotational shear material in the lower part on the advancing side and the drag flow of material from the retreating site forming the rest of the A356 nugget. The latter mode resulted in the absence of a clear ring structure.

Numerical analysis of reverse dual-rotation friction stir welding process

A three-dimensional model of reverse dual-rotation friction stir welding （RDR-FSW） is developed to conduct the numerical simulation of heat generation and material flow during the process. The reverse rotation of the assisted shoulder and the tool pin is considered to model the heat generation rate. The predicted temperature difference between the advancing side and the retreating side in RDR-FSW is less than that in conventional FSW. There are two reverse flows during the RDR-FSW which is beneficial to the uniformity of the temperature profile. Due to the reverse rotation effects of the assisted shoulder, the predicted shape and size of thermal-mechanically affected zone （TMAZ） based on the iso-viscosity line are decreased greatly compared to the conventional FSW. It lays solid foundation for optimizing the process parameters in RDR-FSW.

STUDY ON THE TEXTURE OF A FRICTION STIR WELDED Mg-Al-Ca ALLOY

Macro-texture of an Mg-Al-Ca alloy prepared by friction stir welding （FSW） was investigated through pole figure measurement and X-ray diffraction （XRD） pattern analysis. It was found that at the top and bottom surfaces of friction stir zone （FSZ）, （0002） basal planes of magnesium tend to be arranged parallel to the plate surface. In the cross section of FSZ, no obvious texture had evolved and （0002） basal planes showed a random distribution.

High-cycle fatigue behavior of friction stir butt welded 6061 aluminium alloy

Friction stir welding （FSW） of 6061 aluminium alloy butt joint was carried out at each rotation speed of 600, 800, 1000, 1200 r/min for two different travel speeds, 80 and 100 mm/min, at a constant probe depth of 1.85 mm. The calculated energy input based on the FSW parameters studied shows that the ultimate tensile strength （UTS） of the butt joint is obtained within a certain range of energy input of 297 kJ to 354 kJ out of total range of energy input studied from 196 kJ to 405 kJ. The fatigue behaviors of high-strength and low-strength joints performed at different stress ratios, i.e., 0.5, 0.3, 0.1, -0.3, -0.5, indicate that the fatigue behaviors of both the welds are sensitive to the microstructural features, such as stir zone （SZ）, thermo mechanically affected zone （TMAZ） and heat affected zone （HAZ）. The observed fatigue strengths were discussed in terms of the microstructure, crack path behavior and fracture surface.

Effect of Post-weld Heat Treatment on Microstructure and Mechanical Properties of Friction Stir Welded SSM7075 Aluminium Alloy

7XXX series aluminium alloys generally present low weldability by fusion welding methods because of the sensitivity to weld solidification cracking, vaporization of strengthening alloys and other defects in the fusion zone. Friction stir welding（FSW） can be deployed successfully with aluminium alloys. We presented the effect of post-weld heat treatment（PWHT） on the microstructure and mechanical properties of SSM7075 joints. Semi solid plates were butt-welded by FSW at a rotation speed of 1110 r/min, welding speeds of 70 and 110 mm/min. Solution treatment, artificial aging, and T6（solution treatment and artificial aging combined） were applied to the welded joints, each with three samples. It was found that the T6 joints at the speed of 70 mm/min yielded the highest tensile strength of 459.23 MPa. This condition best enhanced the mechanical properties of FSW SSM7075 aluminium alloy joints.

Modeling the Effects of Tool Shoulder and Probe Profile Geometries on Friction Stirred Aluminum Welds Using Response Surface Methodology

The present paper discusses the modeling of tool geometry effects on the friction stir aluminum welds using response surface methodology. The friction stir welding tools were designed with different shoulder and tool probe geometries based on a design matrix. The matrix for the tool designing was made for three types of tools, based on three types of probes, with three levels each for defining the shoulder surface type and probe profile geometries. Then, the effects of tool shoulder and probe geometries on friction stirred aluminum welds were experimentally investigated with respect to weld strength, weld cross section area, grain size of weld and grain size of thermo-mechanically affected zone. These effects were modeled using multiple and response surface regression analysis. The response surface regression modeling were found to be appropriate for defining the friction stir weldment characteristics.

冷喷涂对7050铝合金FSW接头耐应力腐蚀性能的影响

采用冷喷涂技术在7050铝合金搅拌摩擦焊(FSW)接头表面进行纯Al涂层的制备,研究冷喷涂对FSW接头耐应力腐蚀性能的改善作用,并对其改善因素进行探索。结果表明,冷喷涂后的FSW接头的耐应力腐蚀性能有明显的提高,应力腐蚀断裂时间从24 h内延长至480 h以上,断裂位置也从前进侧热影响区变为焊核区。纯Al涂层沉积厚度约500μm,孔隙率较低,仅0.74%,均匀分布于接头上表面,无氧化和粉末粒子堆积现象,可在海水腐蚀环境中为接头提供较好的物理防护作用,有效改善接头的应力腐蚀敏感性及腐蚀环境对应力腐蚀的促进作用。冷喷涂层对接头产生压应力,可显著降低接头的残余拉应力,减少内外拉应力的交互作用,延缓应力腐蚀开裂速度。

Welding of nickel free high nitrogen stainless steel: Microstructure and mechanical properties

High nitrogen stainless steel(HNS) is a nickel free austenitic stainless steel that is used as a structural component in defence applications for manufacturing battle tanks as a replacement of the existing armour grade steel owing to its low cost, excellent mechanical properties and better corrosion resistance.Conventional fusion welding causes problems like nitrogen desorption, solidification cracking in weld zone, liquation cracking in heat affected zone, nitrogen induced porosity and poor mechanical properties.The above problems can be overcome by proper selection and procedure of joining process. In the present work, an attempt has been made to correlate the microstructural changes with mechanical properties of fusion and solid state welds of high nitrogen steel. Shielded metal arc welding(SMAW), gas tungsten arc welding(GTAW), electron beam welding(EBW) and friction stir welding(FSW) processes were used in the present work. Optical microscopy, scanning electron microscopy and electron backscatter diffraction were used to characterize microstructural changes. Hardness, tensile and bend tests were performed to evaluate the mechanical properties of welds. The results of the present investigation established that fully austenitic dendritic structure was found in welds of SMAW. Reverted austenite pools in the martensite matrix in weld zone and unmixed zones near the fusion boundary were observed in GTA welds. Discontinuous ferrite network in austenite matrix was observed in electron beam welds.Fine recrystallized austenite grain structure was observed in the nugget zone of friction stir welds.Improved mechanical properties are obtained in friction stir welds when compared to fusion welds. This is attributed to the refined microstructure consisting of equiaxed and homogenous austenite grains.

Quality assessment of friction-stir-welded aluminum alloy welds via three-dimensional force signals

An online detection technology must be developed for realizing the real-time control of friction stir welding.In this study,the three-dimensional force exerted on a material during friction stir welding was collected synchronously and the relationship between the forces and welding quality was investigated.The results indicated that the fluctuation period of the traverse force was equal to that of the lateral force during the stable welding stage.The phase difference between two horizontal forces wasπ/2.The values of the horizontal forces increased with welding speed,whereas their amplitudes remained the same.The proposed force model showed that the traverse and lateral forces conformed to an elliptical curve,and this result was consistent with the behavior of the measured data.The variational mode decomposition was used to process the plunge force.The intrinsic mode function that represented the real fluctuation in the plunge force varied at the same frequency as the spindle rotational speed.When tunnel defects occurred,the fluctuation period features were consistent with those obtained during normal welding,whereas the ratio parameter defined in this study increased significantly.

Numerical simulation for the comparison of thermal and plastic material flow behavior between symmetrical and asymmetrical boundary conditions during friction stir welding

An accurate description of the contact condition between the tool and the workpiece material is one of the most important issues for expounding the underlying multi-physics coupled mechanism during friction stir welding(FSW)process.In the present study,a novel asymmetrical boundary condition around the tool-workpiece contact interface is proposed for the FSW of AA2024-T4 alloy.A three-dimensional computational fluid dynamics model is employed for the comparison of the coupled thermal and plastic material flow behavior between asymmetrical and symmetrical boundary conditions.Numerical results of heat generation,temperature distribution,tunnel defect formation and material flow streamline during the welding process are quantitatively analyzed.Besides,various experimental measuring methods are utilized to obtain information about temperature,thermal cycle,tool torque and horizontal cross-section around the exiting keyhole.It is revealed that the modeling results of heat flux density and temperature distribution around the pin,as well as material flow characteristics all change significantly for the two models with different boundary conditions.Particularly,the asymmetrical boundary condition is more capable of predicting temperature fluctuation,plastic material flow along the vertical direction,as well as tunnel defect formation during FSW.Therefore,the superiority of the model with asymmetrical boundary condition over the symmetrical one during the numerical simulation of FSW is elucidated.

双机镜像搅拌摩擦焊顶锻力变化规律与影响因素研究

随着搅拌摩擦焊板厚增大,设备承载能力无法满足单侧搅拌摩擦焊的高顶锻力要求,因而难以形成质量良好的焊缝。为了降低设备所受顶锻力,本文提出了一种双机镜像搅拌摩擦焊工艺方法,研究了工艺过程中顶锻力的变化规律和影响因素,并验证了该工艺的可行性和优越性。首先,建立热力耦合的有限元仿真模型,得到顶锻力随板厚的变化规律,同时得到了双机镜像搅拌摩擦焊工艺能够减小顶锻力的结论;之后,建立顶锻力估计模型,并基于数据驱动对模型进行试验修正和验证,实现无传感器的顶锻力监测;最后,在搭建好的试验平台样机上开展了镜像焊接试验、3组工艺参数的多因素试验及单侧搅拌摩擦焊对比试验,得到顶锻力随时间的变化规律和工艺参数的影响,验证了双机镜像搅拌摩擦焊工艺可以减小搅拌头所受顶锻力、降低设备承载能力需求的作用。

6082–T6/7075–T6异种铝合金搅拌摩擦焊接接头应力腐蚀行为

为提高航空、航天运载工具异种高强度铝合金零部件搅拌摩擦焊接接头在腐蚀环境下的服役可靠性,采用金相分析、电子显微分析和四点弯曲应力腐蚀法,对经过T6处理的6 mm厚6082–T6/7075–T6异种铝合金搅拌摩擦焊接接头应力腐蚀行为进行了研究。结果表明,搅拌头转速1200 r/min、焊接速度80 mm/min条件下,6082–T6/7075–T6母材获得了具有“洋葱环”结构的致密、成形良好的搅拌摩擦焊接接头组织。6082–T6和7075–T6母材搅拌摩擦焊接接头四点弯曲应力腐蚀主要发生在后退侧的7075–T6铝合金一侧,而前进侧6082–T6铝合金一侧表现出较好的耐蚀性。这主要是由于异种铝合金母材电极电位差异导致焊接接头发生应力作用下的电偶腐蚀所致。此外,α–Al基体与第二相形成微区电偶对,以及焊接接头热机械影响区(TMAZ)、热影响区(HAZ)和母材区(BM)存在的大角度晶界也是造成应力腐蚀的重要原因。