Three-dimensional heat and material flow during friction stir welding of mild steel

Three-dimensional viscoplastic flow and heat transfer during friction stir welding of mild steel were investigated both experimentally and theoretically. The equations of conservation of mass, momentum and energy were solved in three dimensions using spatially variable thermo-physical properties and a methodology adapted from well-established previous work in fusion welding. Non-Newtonian viscosity for the metal flow was calculated considering temperature and strain rate dependent flow stress. The computed results showed significant viscoplastic flow near the tool surface, and convection was found to be the primary mechanism of heat transfer in this region. Also, the results demonstrated the strong three-dimensional nature of the transport of heat and mass, reaffirming the need for three-dimensional calculations. The streamlines of plastic flow indicated that material was transported mainly along the retreating side. The computed temperatures were in good agreement with the corresponding experimentally determined values.     

A three-dimensional finite element method for a nonisothermal aluminum flat strip rolling process

The purpose of this study is to develop a three-dimensional coupled thermo-elastic-plastic finite element model of nonisothermal rolling and analyze the strip curvature caused by the difference in the heat transfer boundary conditions of the upper and lower rollers. The difference in the rotation speed between the upper and lower rollers was utilized in an attempt to correct the aforementioned curvature in hot rolling due to unsymmetrical cooling conditions. In addition, the changes in shape, temperature field, and strain field of the strip during the various stages were analyzed and can be used to obtain the lateral plastic flow of the strip. As for the aspect of heat transfer, the various possible boundary conditions in the actual hot rolling were considered, which include the convection boiling of the air and water, and the radiation loss. Then, the three-dimensional finite difference heat transfer equation is derived according to the concept of heat balance. As for the determination of the direction of tangential friction force, this study also developed a modification algorithm to adjust to the three-dimensional rolling process. After a comparison with the experimental data in Ref 8 and 15, and the simulated temperature distribution in Ref 17, the partial results obtained from the computation by the numerical analytical model verify that the theoretical model and computer programs established in this study are reasonable. This study shows that hot rolling can greatly reduce the rolling force and strain rate with the early appearance of plastic deformation, and the distribution of temperature field is basically affected by the heat transfer boundary conditions. However, unsymmetrical heat transfer boundary conditions will cause unsymmetrical rolling forces of the upper and lower rollers and cause strip curvature; this condition can be corrected by the difference in the rotation speed of the rollers.     

A moving boundary formulation for recursive plastic heat release during friction stir welding

In this paper, a model for the growth of third region during friction stir welding is presented. The domain of interest is the heat and deformation affected zone under the tool shoulder. The model integrates a moving boundary framework for the growth of a stirring induced plastic flow region within the domain and formulates the heat generated by recursive plastic work as an enthalpy jump condition across the moving interface. From global energy balance considerations, the approach provides the means to estimate the time required to develop the stirring induced plastic flow region as a function of tool geometry (shoulder radius and pin radius).     

中间包传热边界对钢液流动及传热计算的影响

以某厂T型中间包为例,建立了中间包内钢液流动、传热数学模型。针对数值模拟中存在的传热边界条件的准确性问题进行探讨,引入了中间包包衬散热计算模型,得出关于中间包包衬结构的热流散失与内壁温度的关系,作为中间包内钢液流动、传热计算的边界条件。利用商用Fluent软件模拟计算,并比较分析了不同边界条件对中间包内钢液流动、传热的影响,结果表明提出的边界条件模型法使计算模拟结果更符合实际。     

Numerical Simulation of Heat Generation,Heat Transfer and Material Flow in Friction Stir WeldingEI北大核心CSCD

The heat generation, heat transfer and plasticized material flow in friction stir welding determine directly the microstructure evolution and mechanical properties of weld joints. Numerical simulation of these thermo-physical phenomena is of great significance for getting a deep insight into the underlying mechanisms and optimizing the process parameters of friction stir welding. This article reviews the progress status in numerical simulation of heat generation, heat transfer and plasticized material flow behaviors in friction stir welding, and outlines the unsolved problems. The research work targeting these issues, which has been conducted by the authors' group, is introduced. According to the stress characteristics at the tool-workpiece interface, the expressions of sticking rate and friction coefficient are developed, and this measurement-calculation method lays foundation for improving the accuracy of numerical analysis. Through synthetically considering the characteristics of complex-shaped tools, a three dimensional model of friction stir welding process is established. Three types of tools are taken into consideration, i.e., normal CT (conical-pin tool), ST (conical-pin with 4 flats tool) and TT (conical-pin with 3 flats tool). For the cases in application of these tools, the heat generation, temperature profile, and material flow velocity are analyzed quantitatively. A mathematical model for the whole friction stir welding process including plunge stage, dwell stage, welding stage, and cooling stage is established for numerical analysis of transient development in heat generation rate, temperature and material flow fields in each stages. Based on the status review, the trend in numerical simulation of frictions stir welding is outlooked, and the research focus for next step is proposed.     

Modeling transient fluid flow and heat transfer phenothena in stationary pulsed current TIG weld pool

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激光焊接瞬态小孔与运动熔池行为模拟

提出了一种三维瞬态小孔与瞬态熔池相结合的激光小孔焊接耦合数学模型.该模型综合考虑了多重反射菲尼尔吸收、小孔界面上的间断边界条件、金属蒸气/等离子体的辐射传热、Marangoni力、固-液相间的摩擦力、浮力、粘性力、蒸发潜热、熔化/凝固潜热,以及热传导、对流、辐射等多种因素对激光小孔焊接过程的耦合作用.采用水平集方法和快速扫描方法对小孔演化方程进行求解,得到了三维瞬态小孔形貌;并采用SOLA方法求解了瞬态熔池的三维传热和流动过程.结果表明,该模型能够较好地模拟激光小孔焊接中瞬态小孔和熔池的动态演化行为.     

Hybrid modelling of 7449-T7 aluminium alloy friction stir welded joints

Abstract

Many finite element models use adjustable parameters that control the heat loss to the backing bar, as well as the heat input to the weld. In this paper, we describe a method for determining these parameters with a hybrid artificial neural network (ANN) coupled thermal flow process model of the friction stir welding process. The method successfully determined temperature dependent boundary condition parameters for a series of friction stir welds in 3·2 mm thick 7449 aluminium alloy. The success of the technique depended on the method used to input thermal data into the ANN and the ANN topology. Using this technique to obtain the adjustable parameters of a model is more efficient than the conventional trial and error approach, especially where complex boundary conditions are implemented.     

铝合金点焊熔核流场及热场的有限元分析

根据计算流体力学与传热学原理 ,建立了描述铝合金电阻点焊液态熔核流动行为和传热过程的轴对称有限元模型。模型中考虑了移动边界层内部液态金属的对流传热和层外固体导热、材料热物理性能参数和接触电阻随温度的变化、焊件表面通过对流和辐射向周围环境的散热、球面电极传热以及熔化 /凝固相变潜热对熔核形成热过程的影响 ,并采用有限元法对铝合金点焊熔核形成过程温度场和流场分布进行了数值计算。计算结果表明 ,强烈的对流位于熔核中心沿轴线附近区域 ,其流速最大值数量级为1× 10 -1mm/s;在直流焊接条件下 ,5ms时间内开始形成液态熔核 ,并迅速沿轴向和径向扩展 ;回流环速度矢量将能量从熔核中心通过对流传热方式传递到熔核边缘 ,降低熔核内部温度梯度 ,促进熔核生长。试验表明 ,计算结果与实测值吻合良好     

17CrNiMo6钢斜齿轮热处理仿真分析中的位移边界条件

基于17CrNiMo6钢热处理过程传热、组织场转变和热弹塑性数学模型,建立了斜齿轮零件热处理有限元模型。对17CrNiMo6钢斜齿轮的热处理过程进行了模拟,并分析了固定约束、对称约束、摩擦约束3种位移边界条件对热处理仿真畸变的影响。结果表明:17CrNiMo6钢斜齿轮淬火后齿轮表面受压应力,而心部受拉应力,齿根处压应力大于齿顶处。与目前普遍采用的固定约束或对称约束边界条件相比,采用摩擦约束的仿真结果更加符合实际情况。     

Performance evaluation of Ti–6Al–4V grinding using chip formation and coefficient of friction under the influence of nanofluids

Nanofluid, fluid suspensions of nanometer sized particles are revolutionizing the field of heat transfer area. Addition of nano-particles to the base fluid also alters the lubricating properties by reducing the friction. In grinding process, friction between the abrasive grains and the workpiece is a key issue governing the main grinding output. It has a direct influence on grinding force, power, specific energy and wheel wear. Moreover, high friction force increases the heat generation and lead to thermal damage in the surface layer of the ground work. Hence, any effort towards the friction control will enhance the component quality significantly. In this study, nanofluid as metal working fluid (MWF) is made by adding 0.05, 0.1, 0.5 and 1% volume concentration of Al₂O₃ and CuO nano-particles to the water during the surface grinding of Ti–6Al–4V in minimum quantity lubrication (MQL) mode. Surface integrity of ground surface, morphology of the wheel, and chip formation characteristics are studied using surface profilometer, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and stereo zoom microscopy (SZM). Coefficient of friction was estimated On-Machine using the measured forces. The results showed that the type of nanoparticle and its concentration in base fluid and the MQL flow rate play a significant role in reducing friction. Application of nanofluid leads to the reduction of tangential forces and grinding zone temperature. The cooling effect is also evident from the short C-type chip formation. MQL application with Al₂O₃ nanofluid helps in effective flushing of chip material from the grinding zone, thereby solving the main problem during the grinding of Ti–6Al–4V.     

Scaling of coupled heat transfer and plastic deformation around the pin in friction stir welding

This paper presents a coupled phenomenological model of heat transfer and plastic flow around the pin in friction stir welding (FSW). The approach is analogous to the boundary layer analysis in fluid mechanics, and is based on the methodology of scaling. The results are a set of novel closed-form expressions for the maximum temperature reached in the process, the thickness of the shear layer, the shear stress around the pin, the torque and the thermal effect of the shoulder. The model presented focuses on the most common conditions encountered in the practice of FSW, which involve relatively slow translation velocities, relatively high rotation velocities and thin shear layers. The ultimate purpose of this model is to provide simple and accurate expressions useful for providing a temperature and strain rate context for metallurgical analysis of FSW and for the selection of process parameters when using FSW to join novel alloys.     

Numerical simulation of semisolid continuous casting process

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Analysis of submerged arc welding process by three-dimensional computational fluid dynamics simulations

The effect of torch angle and current polarities on the convection heat transfer in single wire submerged arc welding is analyzed. To develop arc models such as arc heat flux, arc pressure and electromagnetic force, this study adopts the Abel inversion method with CCD camera images for direct and alternating current polarities. The heat transfer by molten slag from the flux consumption is considered as an additional boundary heat source in the numerical simulation. The variation of arc forces, the direction of droplet flight with polarity and the torch angle significantly affect the molten pool flow and the resultant weld beads. The simulated weld pool profiles are validated with corresponding experimental results and found to be in good agreement.     

Internal Friction and Grain Boundary Viscosity of Copper and of Binary Copper Solid Solutions

Measurements have been made of the variation of internal friction with temperature for OFHC copper, and for a series of binary solid solutions of high purity copper with zinc, gallium, germanium, arsenic, and silicon, to investigate the effect of alloying elements in substitutional solid solution on grain boundary viscosity. The apparatus used was of the torsional pendulum type developed by Ke,¹ modified so that the specimen and vibrating system could be maintained in a high vacuum. The activation energy for grain boundary relaxation in copper is 33,000 cal per mol. This is considerably less than the activation energy for self-diffusion, in disagreement with Ke’s theory² that the two activation energies should be the same. All the alloying elements increase the activation energy for grain boundary relaxation to approximately 44,000 cal per mol, increasing the grain boundary viscosity at a given temperature. The results are discussed in terms of theories of grain boundary slip put forward by Mott³ and by Ke.⁴ Experiments have also been made to investigate the effect of small amounts of oxygen on the variation of internal friction with temperature for copper.     

单孔圆形喷嘴射流冷却带钢的数值模拟研究

利用计算流体力学软件FLUENT,采用不可压缩流体流动基本原理和流热固耦合有限体积方法,研究了单孔圆形喷嘴射流冷却带钢的冲击换热,得到了在不同参数和边界条件下冲击射流冷却带钢的传热特性及喷射距离、喷射介质的压力和温度及喷射介质中氢气含量对传热系数的影响,为提高带钢冷却速度提供理论参考。     

管内扭带传热与流动阻力的实验研究

为了研究管内强化换热技术,对3根不同结构参数的扭带插入光管的换热特性和流体动力学特性进行了实验研究.以空气为工质,Re在8 000～100 000之间,管外被水冷却.在大量实验数据的基础上,用多元线性回归法得到了具有较高精度的扭带管的传热系数和摩擦系数的统计关联式,分析了扭带管的传热与流阻性能,为换热器的设计及改造提供了理论依据.     

汽车万向节叉热挤压成形的数值模拟分析

采用刚粘塑性有限元法对万向节叉热挤压成形过程进行数值模拟分析,综合考虑了变形、热传导、变形生热、摩擦生热等多个因素,得出了成形过程中金属流动变化的3个阶段。研究了摩擦条件和终锻温度对成形力的影响,得出良好的润滑条件和给模具进行预热,能够有效的控制成形力,有利于金属的流动及提高模具寿命。     

油气润滑条件下电主轴轴承腔内多相流特性分析

基于油气润滑气液两相流理论,采用COMSOL Multiphysics仿真软件建立B7003CY/P4角接触球轴承腔内的油气两相流模型,分析轴承腔内气相流速、压力以及润滑油的分布。通过计算轴承的摩擦生热量以及腔内关键点的换热系数,分析轴承腔内温度场的分布。试验与仿真结果表明:润滑油沿着进油管道进入轴承腔内,大量的润滑油聚集在轴承腔内的前端,少许润滑油会随着空气进入轴承腔内,而进气速度影响润滑油的分布;轴承腔内的温度受电主轴的转速和进气压力影响,随着电主轴转速提高,摩擦产生的热量增多,轴承腔内的温度升高;进气压力越大,空气流速越大,轴承腔内的温度越低,且轴承腔内换热系数细化后得出的温度场更接近真实值。