Structure-Property Correlation of AA2014 Friction Stir Welds: Role of Tool Pin Profile

The influence of rapid plastic deformation in the generation of welding heat during friction stir welding (FSW), supplementing the frictional heat generation by the tool shoulder, forms the thrust of the present investigation. Several researchers have highlighted the role of tool shoulder in the generation of frictional heat and suggested that the tool-material interface friction as the sole mechanism for heating. The configuration of tool pin profile is seldom studied for its contribution to welding heat through rapid plastic deformation at high strain rates (10³/s), especially while welding thick plates. An attempt has been made to understand the dependence of deformation heat generation with different tool pin profiles in welding 5 mm thick AA2014-T6 aluminum alloy, maintaining the same swept volume during the tool rotation. An attempt has also been made to correlate the influence of process response variables such as force and torque acting on the tool pin. To quantify the physical influence of tool pin profile, temperature measurements were made in the region adjacent to the rotating pin, close to nugget in the thermo-mechanically affected zone (TMAZ). It has been observed that the temperature rises at a relatively rapid rate in the case of hexagonal tool pin compared to the welds produced employing other tool pin profiles. It is observed that during FSW, extensive deformation experienced at the nugget zone and the evolved microstructure strongly influences the mechanical properties of the joint. The present study is also aimed at understanding the influence of tool profile on the microstructural changes and the associated mechanical properties. Transverse tensile samples failed at the nugget/TMAZ boundary due to localized softening. Hexagonal tool pin profile welds have shown higher tensile strength, low TMAZ width, and high nugget hardness compared to other tool pin profile welds.     

Development of Trivex friction stir welding tool Part 1 – two-dimensional flow modelling and experimental validation

Abstract

This paper describes the application of the computational fluid dynamics (CFD) code, FLUENT, to modelling the metal flow in friction stir welding (FSW). Through the use of a novel slip model, a two-dimensional pin profile was optimised to minimise the traversing force. Two versions of a tool based on the optimised profile were developed: the plain 'Trivex' and the threaded 'MX-Trivex'. These were tested against one version of the 'MX-Triflute' tool that had comparable pin and shoulder diameters. Experiments showed that the traversing force was reduced by between 18 and 25% and the down force was reduced by approximately 12% when compared with the MX-Triflute tool. The different pin designs had little effect on the heat input and the tensile strengths of the welds were comparable. The development and validation of the full three-dimensional model for both the Trivex and Triflute tools is described in Part 2.     

Numerical Simulation for the Optimization of Polygonal Pin Profiles in Friction Stir Welding of Aluminum

The tool with polygonal pin profile has been widely employed in friction stir welding(FSW) of aluminum, but there is hardly an effective optimization methodology existed as the thermomechanical characteristics affected by pins with various flats number have not been understood comprehensively. Therefore, the present work employs a 3-dimensional computational fluid dynamics(CFD) model to have an integrated observation of the FSW process with the effect of polygonal pin profiles. Both the heat generation modes due to contact friction at the tool–workpiece interface and volumetric viscous dissipation in the vicinity of the tool are considered. The model is utilized to give a quantitative analysis of the heat generation, temperature distribution, plastic material flow and welding loads during the FSW process for various tools with polygonal pin profiles, as well as a variety of shoulder diameters, welding speeds and tool rotation speeds. The calculated results of thermal cycles, tool torques and joint cross sections for some typical polygonal pins and welding parameters are all found to be compared well with the experimental ones, which demonstrates the feasibility and applicability of the present numerical model. Particularly, a methodology is developed for the optimization of the flats number by identifying the torque components in both parallel and vertical direction of the pin-side flat region. The results show that the optimized pin flats number increases with increasing tool rotation speed, while the influence of both welding speed and shoulder diameter can be supposed to be insignificant. Moreover, the dependability of the optimized results is also discussed by considering wear tendency and service life of the pin for multiple welding conditions.     

Determination of the traverse force in friction stir welding with different tool pin profiles

ABSTRACT

A methodology is developed for the estimation of the traverse force in friction stir welding (FSW) for various pin profiles by combining the results of numerical modelling and experimental monitoring. The effect of pin profiles on the traverse force is evaluated by introducing a modified ratio of the plastic deformation zone, which is obtained by numerical modelling. The formula is validated with the experimental data in the literature and indicates that the traverse force decreases exponentially with increasing ratio of the plastic deformation zone. The proposed methodology provides a concise approach for the estimation of the traverse force for various pin profiles in FSW and can be adopted for the design and assessment of the FSW tool.     

Influences of tool pin profile and welding speed on the formation of friction stir processing zone in AA2219 aluminium alloy

AA2219 aluminium alloy has gathered wide acceptance in the fabrication of light weight structures requiring a high strength to weight ratio. Compared to the fusion welding processes that are routinely used for joining structural aluminium alloys, friction stir welding (FSW) process is an emerging solid state joining process in which the material that is being welded does not melt and recast. This process uses a non-consumable tool to generate frictional heat in the abutting surfaces. The welding parameters and tool pin profile play major roles in deciding the weld quality. In this investigation, an attempt has been made to understand the effect of welding speed and tool pin profile on FSP zone formation in AA2219 aluminium alloy. Five different tool pin profiles (straight cylindrical, tapered cylindrical, threaded cylindrical, triangular and square) have been used to fabricate the joints at three different welding speeds. The formation of FSP zone has been analysed macroscopically. Tensile properties of the joints have been evaluated and correlated with the FSP zone formation. From this investigation it is found that the square pin profiled tool produces mechanically sound and metallurgically defect free welds compared to other tool pin profiles.     

Subshoulder formation during friction stir welding of steel using tungsten alloy tool

Abstract

A study was carried out on the friction stir welding of low alloy steel plates, to understand the influence of the tool profile on the tool wear rate, using a tungsten alloy tool. Three tool profiles were used for the wear study, namely, tool A with a concave shoulder and tapered pin of 2·8 mm, tool B with a flat shoulder and tapered pin of 2·6 mm and tool C with a concave shoulder and tapered pin of 2·6 mm. The tool wear characteristic was analysed by three methods, namely, the profile projector, weight loss and the image processing technique, and it was found that tool C gave reduced tool wear, when compared to the other tool profiles. In tool C, an impression of a step-like formation was generated automatically in the pin during welding and named as a subshoulder, which proved to be the optimised design.     

三维搅拌摩擦焊接过程数值模拟

将搅拌摩擦焊接过程中材料的流动看作是层流、粘性、非牛顿流体绕过旋转的圆柱体,并基于流体力学理论,建立了三维搅拌摩擦焊缝金属塑性流动的数值分析模型.计算结果表明,在焊缝上部表面附近,由于搅拌头轴肩的影响,材料流动比较混乱,发生多次绕流现象;焊缝下部材料流动规律性较明显:只有很少一部分靠近搅拌头探针的材料在焊接过程中受到探针的作用而发生变形和流动,在大多数的模拟条件下,探针直径范围内的材料仅仅在回撤边一侧沿旋转方向绕过探针.焊缝中部具有底部和上部材料的流动特点,是探针和轴肩共同影响的结果.采用"标记嵌入技术"对焊缝金属流动进行可视化研究,试验结果与模拟结果进行了验证,模拟结果能很好地预测塑性金属流动趋势.     

Three-dimensional modeling of the friction stir-welding process

This paper presents an attempt to model the stir-welding process using three-dimensional visco-plastic modeling. The scope of the project is focused on butt joints for aluminum thick plates. Parametric studies have been conducted to determine the effect of tool speeds on plate temperatures and to validate the model predictions with available measurements. In addition, forces acting on the tool have been computed for various welding and rotational speeds. It is found that pin forces increase with increasing welding speeds, but the opposite effect is observed for increasing rotational speeds. Numerical models such as the one presented here will be useful in designing welding tools which will yield desired thermal gradients and avoid tool breakage.     

Mechanical Properties and Welding Power of Friction Stirred AA2024-T35 Joints

This study is concerned with the effect of friction stir welding (FSW) parameters on the mechanical properties and the consumed welding power for AA2024-T35 joints. AA2024-T35 is friction stir welded at different welding speeds (16, 40, and 80 mm/min), rotation speed (900, 1120, and 1400 rpm), and two tool profiles (triangular and square). The welding power is measured and evaluated with two previously established models (O. Frigaad, O. Grong, and O.T. Midling, A Process Model for Friction Stir Welding of Age Hardening Aluminum Alloys, Metall. Mater. Trans. A, 2001, 32A, p 1189–1200; O.P. Heurtier, M.J. Jones, C. Desrayaud, J.H. Driver, F. Montheillet, and D. Allehaux, Mechanical and Thermal Modelling of Friction Stir Welding, J. Mater. Process. Technol., 2006, 171, p 348–357). The tool profile as well as the welding speed show significant effect on the microstructure especially at lower welding speeds. The increase of the welding speed improves the mechanical properties for both tool profiles whereas it has an insignificant effect on the welding power. The square profile produces better mechanical properties and consumed more power, at 40 mm/min, than the triangular one. Moreover, the welding speed showed a weak effect on the welding power, but the need of power increased with the increase of the rotation speed. The measured power is found to be in agreement with the computed one through a theoretical work established by Heurtier et al. (Mechanical and Thermal Modelling of Friction Stir Welding, J. Mater. Process. Technol., 2006, 171, p 348–357).     

Two-dimensional CFD modelling of flow round profiled FSW tooling

Abstract

This paper describes the application of the computational fluid dynamics (CFD) code, FLUENT, to modelling the two-dimensional metal flow in friction stir welding (FSW). The primary goal is to assist in the development of new welding tools, though the models also improve the understanding of the deformation mechanism in FSW, and enable a first order visualisation of the flow round the probe. The paper describes a quantitative method of comparing the flow round different tool shapes. A novel 'slip' model was developed, where the interface conditions were governed by the local shear stresses. This revealed significant differences in behaviour when compared with the common assumption of material stick. The two-dimensional model has demonstrated the viability of the approach for investigating the flow round practical tool shapes, and gives confidence that the development of more computer intensive three-dimensional models will be justified.     

搅拌工具尺寸和工艺参数对塑料搅拌摩擦焊焊缝质量的影响

用不同尺寸的搅拌工具对聚氯乙烯(PVC)板材进行了搅拌摩擦对接焊工艺试验.试验证明,在搅拌工具肩部直径为30 mm,搅拌头直径为10 mm,搅拌头旋转速度为1 660 r/min,焊接速度为25 mm/min的情况下,可以得到焊缝饱满、成形美观的焊接接头.提高搅拌头的旋转速度可以成比例地提高焊接温度;焊接速度的影响较复杂,增大焊接速度一方面会降低焊接热输入,一方面又会间接地增大搅拌头的进给阻力,从而增大摩擦发热功率,提高焊接温度;搅拌工具肩部直径直接影响肩部与被焊材料表面的摩擦发热功率,增大肩部直径可以提高焊接温度,还有利于阻止焊缝材料的飞溅和外溢;而搅拌头直径的影响较复杂,增大它既可以提高搅拌头侧面与被焊材料之间的相对运动线速度,从而提高焊接温度,又会增加被焊材料的吸热功率和传热面积,从而降低焊接温度.     

Influence of tool material on mechanical and microstructural properties of friction stir welded 316L austenitic stainless steel butt joints

In the present research, the influence of friction stir welding (FSW) tool material on the mechanical and microstructural properties of friction stir (FS) welded 316L stainless steel butt joints is investigated. FS welds were produced using two different tungsten based FSW tools having identical tool shoulder and pin profiles. In both the cases, the FSW experimental runs were carried out using tool rotational speed of 600 rpm, welding speed of 45 mm/min, axial force of 11 kN and tool tilt angle of 1.5°. The results of the study show that the joints produced using the tungsten lanthanum oxide tool are having superior mechanical and microstructural properties when compared to the joints produced using tungsten heavy alloy tool. Furthermore, the tool degradation study by mass loss and photographic techniques suggests that the tungsten lanthanum oxide tool is more prone to degradation by plastic deformation, whereas the tungsten heavy alloy tool is more prone to degradation by wear.     

Numerical modelling of 3D plastic flow and heat transfer during friction stir welding of stainless steel

Abstract

Three-dimensional (3D) viscoplastic flow and temperature field during friction stir welding (FSW) of 304 austenitic stainless steel were mathematically modelled. The equations of conservation of mass, momentum and energy were solved in three dimensions using spatially variable thermophysical properties using a methodology adapted from well established previous work in fusion welding. Non-Newtonian viscosity for the metal flow was calculated considering strain rate and temperature dependent flow stress. The computed profiles of strain rate and viscosity were examined in light of the existing literature on thermomechanical processing of alloys. The computed results showed significant viscoplastic flow near the tool surface, and convective transport of heat was found to be an important mechanism of heat transfer. The computed temperature and velocity fields demonstrated strongly 3D nature of the transport of heat and mass indicating the need for 3D calculations. The computed temperature profiles agreed well with the corresponding experimentally measured values. The non-Newtonian viscosity for FSW of stainless steel was found to be of the same order of magnitude as that for the FSW of aluminium. Like FSW of aluminium, the viscosity was found to be a strong function of both strain rate and temperature, while strain rate was found to be the most dominant factor. A small region of recirculating plasticised material was found to be present near the tool pin. The size of this region was larger near the shoulder and smaller further away from it. Streamlines around the pin were influenced by the presence of the rotating shoulder, especially at higher elevations. Stream lines indicated that material was transported mainly around the pin in the retreating side.     

搅拌摩擦焊接材料流动模型及在缺陷预测中的应用

为了研究搅拌头倾角对搅拌摩擦焊接过程的影响机理,基于DEFORM-3D软件建立了带倾角的FSW三维热-力耦合模型,模拟了搅拌摩擦焊接过程中焊缝区材料的三维运动轨迹,对比分析了有无倾角时FSW过程中材料流动行为的差异. 结果表明,前进侧材料绕搅拌针旋转后大部分沉积于搅拌头后方前进侧区域,返回侧的材料大部分被搅拌头旋推至后方而沉积;采用倾角可以增强搅拌头后方材料从返回侧运动至前进侧区间的流动性,同时还有利于增强材料在厚度方向的运动能力. 根据模拟的材料流动行为对接头缺陷进行了趋势预测,预测结果与试验结果吻合良好.     

Modelling of electrically enhanced friction stir welding process using finite element method

Abstract

Conventional friction stir welding (FSW) of high strength and high melting point materials, such as steel and titanium, has the disadvantages of a serious tool wear problem and slow welding speed. A new friction stir welding process for such materials called 'electrically enhanced friction stir welding process (EHFSW)' has been suggested and analysed using finite element modelling. The basic idea of EHFSW is that electric current passes from the welding tool into the workpiece through the contact area in the welding region. Thus it results in more localised heating while welding is in progress and is not simply a preheating process. The temperature distribution in the workpiece during the pin plunge stage and the welding stage of the EHFSW process has been determined. The results show that EHFSW can reduce the plunge force significantly with the help of localised electrical heating during the pin plunge stage, which may imply lower tool wear when compared with conventional FSW. At the same time, in the welding stage, the simulation results indicate that the welding speed of the EHFSW process can be at least two times faster than that of the conventional FSW process. Thus, finite element analysis shows that EHFSW is a promising process and could reduce tool wear while improving the welding speed, especially for high melting/O point materials.     

Coupled thermomechanical analysis of friction stir welding process using simplified models

Abstract

It is widely recognised that the fundamental mechanisms associated with the weld formation process and their relationships with welding parameters are complex and remain to be fully understood. The present paper reports a series of general findings based on a set of simplified numerical models that were designed to elucidate various aspects of the complex thermomechanical phenomena associated with friction stir welding. The following phenomena were investigated in separate numerical models: (i) coupled friction heat generation; (ii) plastic flow slip zone development; and (iii) three-dimensional heat and material flow. The friction induced heat generation model was used to quantify the contributions of coupled thermomechanical friction heating, including non-linear interfacial phenomena between the tooling (e.g. stir pin) and material being welded. The plastic work induced heating effects were also examined. The plastic slip formation mechanisms were then investigated by considering contributions from various heating mechanisms. Finally, a simplified three-dimensional heat and material flow model, based on the observations from the coupled friction heat generation model, was used to establish some initial insight regarding the heat and material flow. The results from the three subproblem areas were then generalised in the form of a simple parametric relationship between welding variables (i.e. travel and rotating speeds) and weld formation conditions. A series of assumptions were made in constructing these individual models since there exists little information on actual material behaviour under friction stir welding conditions. However, the findings from the present study not only illuminate some of the important weld formation mechanisms in friction stir welding, but also provide an effective framework for more focused investigations into some of the fundamental phenomena identified in the three subproblem areas: such investigations will be reported separately in a future publication.     

2024铝合金板搅拌摩擦焊接塑性材料流动的可视化检测与数值模拟(英文)

将薄铜片作为标示材料镶嵌于2024铝合金板中,经搅拌摩擦焊接焊后,用金相法观察其最终位置。参考材料流动的可视化实验结果,建立搅拌摩擦焊传热与材料流动的三维数值分析模型。搅拌针附近塑性材料流动速度分布模式的计算结果与可视化实验结果基本一致。当焊接速度一定时,随搅拌针旋转速度的提高,搅拌针附近塑性材料流动加剧。焊核区形状与尺寸的计算结果与实测数据吻合。     

基于MORFEO的采用不同搅拌针形状时沿板厚方向流动性的数值分析

基于专业搅拌摩擦焊有限元分析软件MORFEO的局部分析模块,对5 mm厚2024-T3铝合金薄板对接焊缝不同厚度处的材料流动性进行分析. 焊接速度为80 mm/min,转速为600 r/min时,对圆柱形和圆锥形搅拌针分别建模,后处理提取不同厚度水平面准稳态流线,得到材料流动特点:靠近轴肩质点运动最快;搅拌针前方质点绕过后退侧,最终沉积在搅拌针后方的后退侧,类似于回填作用;搅拌针为圆锥形时,质点可在搅拌针后方两侧均匀沉积. 在该焊接参数下,使用圆锥形搅拌针,铜箔作为标记材料进行试验,模拟结果得到了验证.     

搅拌头受力模型及应用

针对搅拌摩擦焊实际过程,建立了搅拌针扎入阶段和稳定焊接阶段搅拌头的受力模型,通过与实测数据对比分析对模型进行了验证.由受力模型可知无论扎入阶段或稳定焊接阶段搅拌头的薄弱部位均为搅拌针根部,若设计不当易从搅拌针根部破坏.在此基础上,针对Ti-6Al-4V的搅拌摩擦焊,将模型用于指导搅拌头设计.结果表明,合理的搅拌头设计是实现钛合金搅拌摩擦焊的关键,在合适的焊接工艺参数下可以获得成形良好的焊缝.     

A flow-partitioned deformation zone model for defect formation during friction stir welding

A friction stir welding (FSW) metalworking model is proposed which partitions flow through distinct deformation zone geometries around the pin probe and beneath the shoulder. These geometries form under the effects of the temperature field, constitutive properties, strain rate and pin tool geometry in these zones. Inadequacies in this flow are related to specific FSW defect types. An initial volume of material equal to the projected area of the pin tool probe is assumed to enter the FSW processing zone. An excess material function is presented in terms of controllable process parameters to describe flowing material added to this initial volume due to temperature and pressure conditions around the pin tool. A forcing function defines the state of applied stresses which partitions the flow through these distinct zones to fill the cavity behind the pin tool. These are equated using the equations of motion for a multi-body dynamic system. Only the concept of a flow-partitioned deformation zone model is presented in this paper. Experimental validation of the model is in progress and the results will be made available in the future.