搅拌摩擦焊接过程的有限元模拟

对摩擦搅拌焊接工艺进行数值仿真，建立了搅拌摩擦焊接的二维数值计算模型，通过数值手段进行焊接参数研究。并研究了焊接工艺过程中焊件材料的流动情况以及在焊接过程中材料的应力和应变情况，且与已有结果进行比较。证实了所建立模型的正确性与可靠性。     

A simple Eulerian thermomechanical modeling of friction stir welding

A simple three-dimensional thermomechanical model for friction stir welding (FSW) is presented. It is developed from the model proposed by Heurtier et al. (2006) based on a combination of fluid mechanics numerical and analytical velocity fields. Those velocity fields are introduced in a steady state thermal calculation to compute the temperature field during welding. They allow partial sliding between the shoulder and the workpiece, the amount of which is provided as an additional result of the model. The thermal calculation accounts for conduction and convection effects by means of the particular derivative. The complete thermomechanical history of the material during the process can then be accessed by temperature and strain rate contours.The numerical results are compared with a set of experimental test cases carried out on an instrumented laboratory device. The choices for modeling assumptions, especially tribological aspects, are discussed according to agreements or deviations observed between experimental and numerical results. The amount of sliding appears to be significantly influenced by the welding conditions (welding and tool rotational velocities), and physical interpretations are proposed for its evolution.     

On the thermo-mechanical loads and the resultant residual stresses in friction stir processing operations

In friction stir welding and processing both a thermal flux and a mechanical action are exerted on the material determining metallurgical evolutions, changes in the mechanical behaviour and a complex residual stress state. In the paper, the metallurgical changes are examined through numerical simulation and experiments to highlight and distinguish the effects of thermal and mechanical loadings. A particular focus is made on the residual stresses generated during the stir processing of AA7075-T6 aluminium blanks. The predictions of FE model are validated by experimental measurements. Lastly, this paper presents an in-process quenching of the processed blanks for improved mechanical properties and microstructure.     

Predictions of tool forces in friction stir welding of AZ91 magnesium alloy

Abstract

Adaptive remeshing technique based finite element model of friction stir welding was used for the investigation of tool forces in the welding processes. Results indicate that the maximum tool forces occur at the initial time in the translational stage. In the tool forces in the three directions, the axial force is the maximum. The tool force in the direction perpendicular to the welding line is the minimum. The tool forces in the three directions are all increased with the increase in transverse speed and decreased with the increase in angular velocity.     

Experimental and thermomechanical analysis of friction stir welding of poly(methyl methacrylate) sheets

In the present work, the feasibility of friction stir welding (FSW) of poly(methyl methacrylate) sheets was studied experimentally and theoretically by employing thermomechanical simulations. The effect of processing parameters including tool plunge depth, tilt angle, tool rotational speed (w), and transverse velocity (v) was investigated to determine suitable conditions to attain sound and defect-free joints. It is shown that a low tool plunge depth of 0.2?mm and a tilt angle of 2° provide suitable material flow to gain sound joints. By controlling the heat input into stir zone by increasing the tool rotational speed and decreasing linear velocity, the formation of defects can be minimised. Mechanical examinations in both longitudinal (LS) and transverse (TS) directions indicate that the highest joint strength of 57?MPa (for LS) and 42?MPa (for TS) are obtained at processing conditions of w?=?25?mm?min^?1 and v?=?1600?rev?min^?1. The measured tensile strengths are 81 and 60% of the base polymer, respectively.     

A thermal model of friction stir welding in aluminum alloys

A thermal model of friction stir welding was developed that utilizes a new slip factor based on the energy per unit length of weld. The slip factor is derived from an empirical, linear relationship observed between the ratio of the maximum welding temperature to the solidus temperature and the welding energy. The thermal model successfully predicts the maximum welding temperature over a wide range of energy levels but under predicts the temperature for low energy levels for which heat from plastic deformation dominates. The thermal model supports the hypothesis that the relationship between the temperature ratio and energy level is characteristic of aluminum alloys that share similar thermal diffusivities. The thermal model can be used to generate characteristic temperature curves from which the maximum welding temperature in an alloy may be estimated if the thermal diffusivity, welding parameters and tool geometry are known.     

航空铝合金板材搅拌摩擦连接有限元分析

根据搅拌摩擦连接特点,充分考虑搅拌摩擦连接过程中搅拌针各部位与连接板材之间的摩擦生热及在连接过程中连接板材摩擦系数随温度变化规律,建立了适合搅拌摩擦连接自身特点的动态热元模型,并利用此模型进行了航空铝合金板材搅拌摩擦连接过程的有限元分析模拟.通过模拟结果和试验结果对比验证了所建立的动态热源模型和有限元分析过程是合理的.模拟结果显示搅拌摩擦连接残余拉应力主要集中在连接区,且在连接区中间位置出现最大残余拉应力区,连接区两端及其它部位出现残余压应力.     

Finite element simulation of material flow in friction stir welding

Abstract

A brief summary of recent finite element simulation results for the friction stir welding process is presented. The focus of the present study is on the characterisation of material flow around the rotating tool pin. Material flow patterns predicted by finite element simulations are found to compare favourably with experimental observations. The simulation results also reveal that material particles tend to pass and move behind the rotating pin from the trailing side, rather than both sides, of the pin. Possible variations in the material flow pattern due to variations in process parameters are discussed.     

Thermal history in UNS S32205 duplex stainless steel friction stir welds

Consolidated UNS S32205 duplex stainless steel joints welds were performed using a friction stir welding (FSW) process. An experimental set-up was used to record the thermal history of duplex stainless steel FSW joint. For points at equal distance from the weld centreline, temperature measured near the beginning of the weld was lower than that measured in the middle of the welded joint. This was attributed to a non-stationary transfer condition. FSW thermal cycle showed shorter time spent at elevated temperature compared that presented by fusion welding, indicating less propensity to detrimental second phase precipitation. To support temperature measurements with thermocouples, a three-dimensional finite element thermal model of FSW was implemented, which provided a good agreement with experimental data.     

Thermo-mechanical model with adaptive boundary conditions for friction stir welding of Al 6061

Thermo-mechanical simulation of friction stir welding can predict the transient temperature field, active stresses developed, forces in all the three dimensions and may be extended to determine the residual stress. The thermal stresses constitute a major portion of the total stress developed during the process. Boundary conditions in the thermal modeling of process play a vital role in the final temperature profile. The heating and cooling rates with the peak temperature attained by the workpiece determine the thermal stress. Also, predicting realistic peak temperature becomes important as the operating temperature at the interface of tool-workpiece is very close to the solidus temperature of the aluminum workpiece.The convection heat-transfer coefficients of the surfaces exposed to air can be theoretically determined using Newton's law of cooling. Contact conductance depends on the pressure at the interface and has a non-uniform variation. The actual pressure distribution along the interface is dependent on the thermal stress from local temperature and non-linear stress–strain state. Therefore, applying an adaptive contact conductance can make the model more robust for process parameter variations.A finite element thermo-mechanical model with mechanical tool loading was developed considering a uniform value for contact conductance and used for predicting the stress at the workpiece and backplate interface. This pressure distribution contours are used for defining the non-uniform adaptive contact conductance used in the thermal model for predicting the thermal history in the workpiece. The thermo-mechanical model was then used in predict stress development in friction stir welding.     

Thermomechanical deformation behaviour of DH36 steel during friction stir welding by experimental validation and modelling

Friction stir welding is a solid state thermomechanical deformation process from which the plasticisation behaviour of the stirred material can be evaluated through the study of flow stress evolution. Flow stress data also supporting the development of a local microstructural numerical model have been generated. Hot compression testing of DH36 steel has been performed at a temperature range of 700–1100°C and strain rates from 10^?3 to 10² s^?1 to study the alloy’s thermomechanical deformation behaviour in conditions that simulate the actual friction stir welding process. It has been found that the evolution of flow stress is significantly affected by the test temperature and deformation rate. The material’s constitutive equation and constants have been calculated after analysis of these data. Preliminary numerical analysis results are in good agreement with experimental observations.     

Effects of the friction stir welding process variants on residual stress

A high-strength aerospace aluminium alloy, 25?mm thick plate, has been friction stir welded using four different process variants. The process variants used were stationary shoulder single pass, conventional shoulder single pass, stationary shoulder dual pass and conventional shoulder dual pass. The goal of the reported work was to investigate effects of different process variants on residual stress. Defect-free welds were produced by all process variants. Through thickness average, longitudinal, residual stress in both as-welded and post-weld heat-treated conditions is reported.     

薄板搅拌摩擦焊焊缝区应力集中的有限元分析

为研究经过搅拌摩擦焊处理的薄板在飞机上受拉应力时焊缝区的应力集中状况,建立飞机大型铝合金薄板搅拌摩擦焊焊缝区平齐、上凸、下凹三种典型几何形状的有限元模型。在有限元前处理中使模型右端面受拉,左端面设置对称约束条件以消除应力集中影响,使得模拟得到最大精度的数值结果。结果表明,在焊缝区假设为各向同性前提下,其形状决定应力集中程度。当焊缝区为平齐时,没有出现应力集中;当焊缝为上凸或下凹时,拉应力引起的偏心弯矩将影响焊缝区应力水平分布,特别是模型下凹时应力集中明显,且使薄板受力端产生显著挠度。     

Effect of trailing heat sink on residual stresses and welding distortion in friction stir welding Al sheets

Abstract

This paper investigates a trailing heat sink, which was designed and applied to friction stir welding (FSW) in order to control the residual stresses and welding distortion. Residual stresses, residual plastic strains and welding distortion of 2024-T3 and 5083-H321 Al sheets welded by FSW with and without the trailing heat sink were compared. The optimal placement of the heat sink was discussed. The results revealed that the reductions in peak tensile stresses were 66% for 2024-T3 and 58% for 5083-H321 by application of the trailing heat sink in FSW. In addition, the welding distortion could be reduced drastically by this method. The 5083-H321 sheet with a size of 1000×100×3·5 mm welded by this method was very flat and had almost no distortion. This method achieved in-process control of stresses and welding distortion, without additional complicated work before or after welding operation.     

Finite element modeling of ultrasonic surface rolling process

Ultrasonic surface rolling (USRP) is a newly developed process in which ultrasonic vibration and static force are applied on work-piece surface through the USRP operator to generate a nanostructured surface layer with mechanical behaviors highly improved. Compared with other surface severe plastic deformation (S²PD) methods, it can realize mechanized machining and be directly used for preparing final product. Notwithstanding the excellent performance of USRP, elaborate relation between process parameters and surface layer characteristics is still inadequacy due to inconvenient and costly experimental evaluation. Therefore, in this paper a three-dimensional finite element model (FEM) has been developed to predict the treatment conditions that lead to surface nanocrystallization. Simulated results of surface deformation, stress and strain are investigated to assess the formation of nanostructured layer. The numerical results from the FEM corresponds well with the values measured experimentally, indicating that this dynamic explicit FEM is a useful tool to predict the processing effects and to relate the treating parameters with the surface layer in terms of the size of nanostructured layer, residual stress and work hardening.     

Study of hybrid welding repair process of friction stir welding grove defect

Abstract

Gas tungsten arc welding (GTAW) repair process and GTAW+FSW (friction stir welding) hybrid repair process are studied to remove the large size groove defect formed during FSW. The experimental results indicate that the groove defect can be removed by both the repair processes. The tensile strength of the GTAW repair joint is only 55% of that of the base metal. The tensile fracture occurs at the transition zone between the weld zone and the heat affected zone, and the fracture surface of the repair joint is characterised by clear brittleness. In contrast, the GTAW+FSW hybrid repair joint has a high tensile strength equivalent to 70% of that of the base metal. The tensile fracture occurs at the overlap thermomechanically affected zone between the two FSW nuggets, and the fracture feature of the hybrid repair joint is partially plastic and partially brittle.     

搅拌摩擦焊接过程产热作用机制研究进展

搅拌摩擦焊接(FSW)的一个重要的研究领域就是过程的产热研究。对FSW热过程的良好理解可以预测搅拌摩擦焊接过程的温度分布及接头区域的硬度，从而可以评价FSW接头的性能。掌握FSW温度分布可以更好地计算与FSW过程塑流计算密切相关的粘度和残余应力。目前关于FSW产热作用机制一般都认为主要是轴肩与母材之间的摩擦生热。     

搅拌摩擦焊接中参数变化对温度场分布的影响

文章建立了搅拌摩擦焊接过程的三维有限元热分析模型,讨论了旋转速度、搅拌头半径、轴肩半径、搅拌针锥角以及螺纹角等焊接参数对工件温度场的影响。计算结果表明,在焊接过程中,工件的最高温度低于熔点,为固相连接。在焊接工艺参数许可的范围内,旋转速度和搅拌针半径的增加,会使搅拌摩擦焊接过程中的最高温度值增加,而锥角和螺纹角增加,会使最高温度值随之减小,轴肩半径的改变对工件温度场的影响相对较小。