激光深熔焊中匙孔形成过程的动态模拟

为了准确的模拟出激光深熔焊接中匙孔变化的动态过程,根据匙孔内激光的能量吸收机制,采用射线跟踪法产生热源,建立了激光深熔焊接过程中气、液、固三相统一的数学模型,并采用了VOF方法追踪自由液面,模型中考虑了表面张力、热浮力和反冲压力的作用,通过数值计算得到了匙孔变化的动态过程与相应的温度分布.结果表明,匙孔形成的主要驱动力是反冲压力,在加热初期,激光主要用来加热金属基体,随着激光辐照时间的增加,熔池表面温度急剧升高,金属进而汽化,产生匙孔.针对304不锈钢的工艺试验表明计算结果与实际相符合.     

A coupled model on fluid flow,heat transfer and solidification in continuous casting mold

Fluid flow, heat transfer and solidification of steel in the mold are so complex but crucial, determining the surface quality of the continuous casting slab. In the current study, a 2D numerical model was established by Fluent software to simulate the fluid flow, heat transfer and solidification of the steel in the mold. The VOF model and k-ε model were applied to simulate the flow field of the three phases(steel, slag and air), and solidification model was used to simulate the solidification process. The phenomena at the meniscus were also explored through interfacial tension between the liquid steel and slag as well as the mold oscillation. The model included a 20 mm thick mold to clarify the heat transfer and the temperature distribution of the mold. The simulation results show that the liquid steel flows as upper backflow and lower backflow in the mold, and that a small circulation forms at the meniscus. The liquid slag flows away from the corner at the meniscus or infiltrates into the gap between the mold and the shell with the mold oscillating at the negative strip stage or at the positive strip stage. The simulated pitch and the depth of oscillation marks approximate to the theoretical pitch and measured depth on the slab.     

等离子多层熔积混相瞬态流场与温度场的模拟

建立了一种二维等离子熔积成形瞬态模型,该模型描述了熔积成形过程中多层熔积层的自由表面发展,并模拟了熔池内流体流动和传热.通过水平集(Level-Set)方法处理熔积轨迹、液/气界面等因素,考虑了熔体流动的主要驱动力-表面张力梯度、表面曲率以及浮力等.并采用SIMPLEC算法求解控制方程.以K163合金为例,对多层熔积成形过程混相瞬态场进行了模拟,所得到的计算结果与试验结果基本一致;并分析了熔积工艺参数(熔积电流、熔积扫描速度和送粉速度)对熔积层表面形貌及熔积质量的作用规律.     

基于非正交结构网格有限体积法的铝型材挤压过程数值模拟关键技术

文章基于非正交结构网格,建立了大变形铝型材非稳态挤压过程有限体积法数值模拟的数学模型。给出了非正交结构网格上有限体积法的基本控制方程,研究了非正交网格上有限体积法模拟铝型材挤压过程的关键技术。将流体体积法(VOF)应用于三维非正交网格来捕捉材料流动前沿,采用塑性剪切模型施加了摩擦边界,结合本构方程和粘度迭代,开发了铝型材挤压过程数值模拟程序。针对一大挤压比的典型薄壁铝型材挤压过程进行了数值模拟,得出了速度场、等效应力、等效应变速率等。将模拟结果与有限元软件DEFORM-3D的模拟结果进行对比,验证了该文所建立数学模型的可行性和正确性。     

A three-dimensional sharp interface model for the quantitative simulation of solutal dendritic growth

A three-dimensional (3-D) sharp interface model is developed to simulate the solutal dendritic growth in the low Péclet number regime. The model adopts a previously proposed solutal equilibrium approach to calculate the evolution of the solid/liquid interface. To describe specific crystallographic orientations of 3-D dendritic growth, a weighted mean curvature algorithm incorporated with the anisotropy of surface energy is proposed, allowing the simulation of 3-D dendrites with various orientations in a straightforward manner. The model validation is performed by comparing the simulations with the analytical predictions and experimental data for both single and multi-dendritic growth, which demonstrates the quantitative capabilities of the proposed model. The model efficiently reproduces realistic 3-D multi-equiaxed and columnar dendrites with various orientations and well-developed side branches.     

高拉速薄板坯连铸结晶器钢渣界面波动分析

为应对提高拉速薄板坯结晶器内钢液不稳定行为,以1 520 mm×90 mm薄板坯结晶器为研究对象,利用液面追踪技术VOF方法建模计算,对薄板坯钢渣界面进行了深入研究,实现了对薄板坯连铸结晶器内流体流动及钢/渣界面行为的模拟计算。并结合实际生产工艺,采用1∶1物理模型和数值模拟相互验证,分析了拉坯速度、浸入深度和保护渣黏度种类对结晶器流场及钢渣界面的影响。结果表明,当结晶器钢液面流速为0.20~0.25 m/s,且界面较平稳时,保护渣黏度高于0.237 Pa·s可以适用;当钢液流速为0.25~0.30 m/s,保护渣黏度为0.382 Pa·s时,现场低碳钢卷渣率小于0.5%,表现出良好的抗卷渣能力。     

基于实测小孔的激光深熔焊接三维传热模型

激光深熔焊接的传热过程包括热传导和焊接熔池内熔融材料的对流流动.考虑了熔池的对流和材料热物性参数的温度依存性,建立了激光深熔焊接的三维传热模型,以试验所得小孔为原型;考虑了熔池中流质的轴向流动及轴向的温度变化;采用了"固液同一法",让温度低于液相线的材料粘性系数趋于无限大,而高于液相线的材料则采用实际值.采用有限单元法对模型数值求解的结果表明,激光焊接温度中的等温线为一组类似椭圆的曲线;小孔前沿的温度梯度、速度梯度大;后沿的温度梯度、速度梯度小;焊接熔池中的对流流动是在表面张力驱动下产生的.     

低压铸造铝合金轮毂充型与凝固模拟

在低压铸造过程温度场及流场实验的基础上,开发了基于ＳＯＬＡＶＯＦ算法的充型与凝固过程数值模拟软件,对７２－４６轮毂铸件进行了模拟分析。结果表明,低压铸造充型过程中降温明显,准确的凝固模拟必须首先考虑充型。针对轮毂铸件的充型特点,开发了简化充型模拟软件。简化模拟得到的初始温度场与采用ＳＯＬＡＶＯＦ算法的模拟结果基本吻合,实现了为后续凝固模拟提供正确的初始温度场的设计目标。     

Modeling the first instant of the interaction between a liquid and a plasma jet with a compressible approach

In this work, the numerical simulation of the interaction between a liquid jet and a plasma flow is investigated for understanding and predicting the physical parameters involved in the liquid plasma spraying process. This process is used for the production of high performance coatings to obtain thinner deposits (< 100 μm) than in conventional plasma spraying. The up-to-date goal is to describe the dispersion of liquid in order to understand the effect of injection conditions on the surface coating quality. This work proposes an original model for dealing with three-dimensional interactions between a plasma flow and a liquid phase with Volume Of Fluid (VOF) methods. A compressible model, capable of managing incompressible two-phase flows as well as compressible motions, is used. First comparisons with experimental photography show rather good agreement during the first moments of the injection.     

考虑自由表面的定点A-TIG焊数值分析

根据流体力学基本原理,并用VOF方法追踪自由表面,建立了更接近实际情况的三维定点A-TIG焊熔池数学模型,综合考虑了熔池液态金属所受的浮力、电磁力、电弧压力和表面张力,同时考虑了熔池内液态金属的对流、辐射和热传导,运用FLOW-3D软件求解得到了定点A-TIG焊熔池自由表面、温度场及流场,并重点分析了有、无活性剂时熔池自由表面的变化情况. 结果表明,不使用活性剂时,熔池自由表面呈中心下凹、两边凸起的形状;使用活性剂时,表面张力温度系数由负变正,使熔池自由表面呈中心凸起、两边下凹的形状;在焊接电流I≤150 A的条件下,有、无活性剂时考虑自由表面变形情况下温度场、流场及熔池形貌的数值模拟结果与试验结果和不考虑自由表面变形时现有的数值模拟结果基本一致.     

Numerical simulation and experimental investigation of two filling methods in vertical centrifugal casting

1 Introduction Titanium alloys possesses a high potential for production of work pieces with high strength, mechani- cal properties and high corrosion resistance, which makes them widely applied in aviation, astronavigation and civil fields[1- 5]. However…     

Contribution to the modeling of the interaction between a plasma flow and a liquid jet

A numerical simulation of the interaction between a plasma flow and a liquid jet was investigated, leading to the proposal of a compressible model, based on augmented Lagrangian, Large Eddy Simulation (LES) turbulence modeling and Volume of Fluid (VOF) approaches, capable of managing incompressible two-phase flows as well as turbulent compressible motions. The VOF method utilized volume markers to advect the local concentration of gas and liquid in a Eulerian manner. The numerical model was validated on single-phase plasma configurations as well as two-phase cross flow liquid jet interactions. Finally, an example of the first simulations of the interactions between a liquid jet and a plasma is presented. However improvements should be realized as to increase the speed of the VOF-SM algorithm or add specific subgrid models related to jet fragmentation and phase change.     

基于图像处理及模式聚类的二次仿真方法及其在回转窑温度监控中的应用

提出一种基于图像处理和模式聚类的二次仿真方法。应用Fluent商业软件对回转窑内粉煤在27类设定工况的燃烧进行温度场的三维数值模拟,将仿真结果构建仿真结果数据库;采用模式聚类的方法判断实际工况属于哪一类标准工况,调用仿真结果数据库建立温度与该类标准工况的数学模型,求解模型并计算实际工况下的温度场;利用图像处理中的伪彩色变换得到二次仿真结果,用VC＋＋6.0来实现。结果表明：该方法能够较准确地给出仿真结果,极大缩短计算时间,为回转窑内温度监测提供一种新思路,具有较大的现场应用价值,并很容易推广到其他领域中。     

On the application of modelling to study the surface and interfacial phenomena in liquid alloy–ceramic substrate systems

The wetting phenomena of molten alloy/ceramic substrate depend on the bonding characteristics of liquid alloys and ceramics as well as on the magnitude of interactive forces at the interface. According to this, the first step of this investigation is to determine the surface properties of Ag–Cu, Ag–Ti and Cu–Ti liquid alloys. The energetics of mixing in liquid alloys has been analysed through the study of surface properties (surface tension and surface composition) and microscopic functions (concentration fluctuations in the long-wavelength limit and chemical short-range order parameter) in the frame of statistical mechanical theory in conjunction with the quasi-lattice theory (QLT). The results obtained for these binary systems have been extended to the ternary Ag–Cu–Ti system. Combining the Young and the Dupré equations, the computed results of surface tension together with contact angle data have been used to calculate the work of adhesion and, in the case of non-reactive wetting, the interfacial tension between the solid substrate and the liquid alloys over the whole concentration range. The evaluation of the interfacial tension values is determined from calculated and measured data using solid surface tension data from literature. These results provide more information on the characteristics of metal–ceramic systems, and are therefore useful in guiding experiments, or in predicting the surface properties of metallic systems with similar characteristics as well as their wetting behaviour in contact with ceramic materials.     

Numerical simulation of heat transfer and fluid flow of molten metal in MMA–St copolymer lost foam casting process

In this study, a computational fluid dynamics (CFD) code was developed to calculate the filling pattern using volume of fluid (VOF) algorithm with donor–acceptor method for free surface simulation. This algorithm has been modified to include the pressure of the gas produced from foam degradation. For this purpose a heat transfer model and 2D foam degradation model were developed. In heat transfer model, radiation and conduction between foam and molten metal; and convection between gas and molten metal were considered. In order to evaluate the results of simulation, a bench scale casting apparatus was assembled and the casting was conducted in a transparent mold. The effect of several parameters such as coating thickness, foam density and vacuum level on the gap temperature, gap pressure and filling speed was studied with the developed software. It was found that the simulated results are in good agreement with experimental results.     

旋转电弧GMAW堆焊短路过渡熔池动态仿真

为探究旋转电弧GMAW堆焊短路过渡时熔池的温度和对流分布规律,利用Flow-3D软件建立三维数学模型,采用球形旋转热源模型,考虑重力、熔滴拖拽力、表面张力、浮力作用,模拟了堆焊状态下,工件材料为Q235的旋转电弧GMAW短路过渡的熔池成形规律. 采用流体体积法追踪熔滴过渡和熔池表面的自由变形,并分析熔滴进入熔池时熔池内部温度场和流场的变化. 结果表明,熔池形成过程中,旋转熔滴对熔池有搅拌作用,并使熔池内部液态金属活性增强,流速变快,熔池内部液态金属体积变大,熔池的宽度变大. 模拟预测的焊缝尺寸、形状与试验吻合良好. 为优化焊接工艺参数、改善旋转电弧GMAW堆焊焊缝质量提供参考依据.     

涡轮叶片切削参数研究与仿真

为解决涡轮叶片可选加工参数较多、加工质量与效率难以保证的难题,提出一种涡轮叶片的五轴加工工艺。利用解析分析的方法建立切削力理论模型,对比验证切削力经验公式的模型精度。结合工件受力变形有限元模型,选取优化后的切削参数,并利用可视化软件实现对叶片无偏摆点铣与侧铣程序的编制与仿真。可视化仿真结果表明：该加工工艺及参数下,可获得加工精度较高的叶片表面;点铣法加工精度较高,通用性强,与侧铣法相比效率较低。铣削试验结果表明：仿真表面结果与试验表面在变化规律上吻合良好,证明了所提工艺与参数的有效性,提升了涡轮叶片的制造精度与效率。     

一种常温间接测量液态高温合金表面张力的方法

由于液态合金与铸型界面间的相互作用,液态的自由表面与铸型表面接触处形成弯月形的空间曲面.基于此,提出了一种测量液相线温度下表面张力的方法.对于具有一定宽度结晶范围的高温合金,对定向凝固过程分析发现,凝固后的自由表面弯月形曲面在能够较为精确反映液相线温度下液态合金的弯月形曲面.在常温下测量弯月形曲面,根据Young-Laplace关系计算,就可以得到该合金液相线温度下的表面张力.通过对单晶高温合金DD6进行实验,得到液相线温度下DD6的表面张力约为1.34J·m~(-2).     

Prediction model of surface residual stress within a machined surface by combining two orthogonal plane models

The variation of surface residual stress within a machined surface layer caused by face turning was studied. The size of the tool’s corner radius and the feed rate affect residual stress. A process model using the finite element method is proposed and the mechanical effects of the corner radius and feed rate on a machined surface were discussed. When a tool with a small corner radius is used, surface residual stress perpendicular to the cutting direction becomes compression stress. As well, surface residual stress changes from tension to compression as the feed rate decreases. The process model consists of an orthogonal cutting simulation and an indentation-like simulation of a corner radius into a work piece surface. The simulated results show quantitative agreement with the residual stress measured experimentally. The integrity of the machined surface will be controlled more efficiently if the cutting conditions during finishing are determined with the proper consideration of the surface generating process.     

Dynamic surface tension measurements on molten metal-oxygen systems: model validation on molten tin

A theoretical model on oxygen transport at the surface of liquid metals has been validated by dynamic surface tension measurements performed on liquid tin as test metal. The oxygen contamination conditions have been obtained at different oxygen partial pressures under low total pressure conditions (Knudsen regime), confirming that an oxide removal regime occurs under an oxygen partial pressure much higher than the equilibrium one (the “Effective Oxidation Pressure”). Experimental results are reported which give a new insight on the relative importance of the various processes due to the oxygen mass transport between the liquid metal and the gas phase. The critical aspects involved in surface tension measurements of liquid metals, related to the problem of liquid metal–oxygen interactions, are also carefully underlined.