平面流铸快速凝固过程的数值模拟研究进展

本文对平面流铸工艺快速凝固过程的数值模拟研究现状作了介绍。主要内容包括 :(1)对溶潭的形成及特征、平面流铸工艺参数对条带形成及质量的影响的数值模拟 (2 )结合快速凝固过程的形核和晶体生长的动力学特征对平面流铸凝固过程的数值模拟 (3)平面流铸过程界面传热系数的数值模拟。文章最后 ,提出了平面流铸工艺快速凝固数值模拟研究的进一步发展方向     

动态布流平面凝固铸造AA1070/AA7050合金

提出了一种新的用于大尺寸复层铸锭平面凝固铸造的布流系统,并使用软件FLUENT对铸造过程中分流方式对温度场的影响进行了模拟分析,模拟结果表明,使用动态布流时熔体温度的均匀性显著优于定口布流,因而可实现凝固时固/液界面在上升过程中保持平坦。使用动态布流平面凝固铸造方法实现了1070/7050大尺寸双层铝合金的铸造复合,铸造结果表明,复合铸锭中两种铝合金的界面清晰平直,无气孔夹杂等缺陷,实现了冶金结合并存在元素扩散层;7050侧金相组织均匀,厚度方向上无明显宏观偏析。     

平面流铸快速凝固过程的数值模拟研究进展

本文对平面流铸工艺快速凝固过程的数值模拟研究现状作了介绍。主要内容包括：（1）对溶潭的形成及特征、平面流铸工艺参数对条带形成及质量的影响的数值模拟；（2）结合快速凝固过程的形核和晶体生长的动力学特征对平面流涛凝固过程的数值模拟；（3）平面流铸过程界面传热系数的数值模拟。文章最后，提出了平面流铸工艺快速凝固数值模拟研究的进一步发展方向。     

等轴晶移动条件下金属凝固温度场、流场及溶质分布数值模拟

本文利用作者提出的固相移动条件下金属凝固传热、传质及动量传输数学模型对砂型铸造 Al-4.5%Cu 合金铸锭凝固过程进行了数值模拟研究与试验验证。模拟冷却曲线及宏观偏析与试验结果基本相符。     

镁合金挤压铸造凝固过程数值模拟

利用有限元法对镁合金挤压铸造凝固过程进行数值模拟,分析了铸件的凝固收缩和凝固时间,预测铸件可能发生缺陷的位置,对比挤压压力对铸件缺陷的影响。 结果发现,与重力铸造相比,挤压铸造的凝固收缩小,铸件在挤压铸造的压力作用下可缩短凝固时间;缺陷模拟结果与实验相符,加大挤压铸造压力有助于减少缺陷的产生。     

微圆管内R1234ze(E)流动沸腾的环状流特性模拟研究

本文建立了制冷剂R1234ze(E)在微圆管内流动沸腾过程中的环状流模型,对传热和气液两相流动压降进行了模拟研究。综合考虑重力、表面张力及气液界面剪切力的影响,模拟分析了周向液膜不均匀分布特性及该特性对流动与换热的影响,经验证,计算结果与已有实验结果吻合较好。本文还研究了不同因素对环状流区域表面传热系数与压降的影响。模拟结果表明:在流动起始区域,截面液膜厚度的分布受重力作用影响,随着流动沸腾过程的进行,该影响作用开始减弱,且有重力作用时的环状流平均表面传热系数高于无重力作用时的环状流平均表面传热系数,随着重力加速度的增加,环状流的平均表面传热系数不断增大;随着质量流速的增大,表面传热系数与压降均随之增大;随着管径增大,表面传热系数与压降均随之减小。     

基于多元非线性回归分析的Cu-Ag合金偏析预测模型研究

目的研究Cu-Ag合金金属型铸造过程中,铸造工艺对宏观偏析的影响规律。方法通过数值模拟技术和实际浇注试验,对Cu-Ag二元合金金属型铸造过程中的宏观偏析缺陷进行分析,研究主要凝固参数,即冷却速率和温度梯度对宏观偏析程度的影响规律。在此基础上,利用多元非线性回归分析方法对所获得数据进行分析,明确主要凝固参数与宏观偏析程度之间的定量关系,建立Cu-Ag合金金属型铸造过程中的宏观偏析预测模型。结果采用研究获得的最佳浇注工艺方案,即浇注温度为1100℃、浇注时间为120 s、铸型温度200℃、涂料厚度为1.5 mm进行实际浇注,所获得铸件中的宏观偏析缺陷得到了明显的改善。结论降低浇注速度可以有效提高冷却速度和温度梯度,从而有效减小宏观偏析的倾向。     

定向凝固微观组织相场法模拟的研究进展

定向凝固过程中的界面形态对凝固组织有着决定性作用,相场法可以很好地展示凝固过程固液界面形态的演变。阐述了定向凝固相场方法数值模拟的基本原理及国内外研究现状,并指出了该领域目前面临的问题及未来发展趋势。     

凝固原理的前沿进展及其应用

凝固原理是揭示液固相变过程基本规律的学科领域。基于凝固原理的材料制备与成形加工技术即是凝固技术。凝固技术的应用包括铸件、铸锭的铸造,以及各种金属与非金属材料的熔体法晶体生长。在综合分析凝固技术应用背景的基础上,讨论了凝固原理和凝固技术的发展现状。归纳了不同材料加工技术所对应的凝固条件,指出不同加工工艺中凝固过程的差异仅在于对应的温度梯度和冷却速率的不同。进而从工程实践的角度分析了凝固技术研究的前沿方向,指出多组元合金凝固原理,近快速凝固原理与技术,以及多层次凝固分析及其跨尺度耦合是凝固原理与技术研究的发展前沿。最后,分别探讨了铸件、铸锭铸造以及熔体法晶体生长过程中的核心凝固问题与过程控制的原则。     

基于数值模拟的K418B高温合金精密铸件组织与性能研究

目的 探究高温合金调压铸造的充型凝固过程,研究调压铸造工艺对铸件组织缺陷和力学性能的影响规律,并验证数值模拟对实际生产指导的可靠性。方法 以某精密构件为研究对象,借助ProCAST数值模拟软件模拟了铸件的调压铸造充型凝固过程并对组织缺陷的形成进行了预测。对成形铸件的特征关键部位进行了取样,通过金相显微镜和扫描电子显微镜对铸态试样的微观组织进行了观察,借助准静态万能拉伸试验机测试了特征试样的室温和高温(750 ℃)拉伸性能,并对断口形貌进行了观察和分析。结果 数值模拟结果表明,金属液充型平稳,凝固过程基本符合自上而下的顺序凝固,铸件缺陷较少,缩孔体积分数仅为0.22%。实验结果表明,铸件的铸态组织为典型的树枝晶组织,晶粒尺寸细小均匀;二次枝晶间距较小,组织致密,缩松缩孔缺陷较少,这与数值模拟的结果吻合较好;铸件的平均抗拉强度超过900 MPa,最大伸长率为15%,该铸件具备较好的综合力学性能。结论 通过数值模拟方法指导铸造生产具有一定的可靠性,同时,通过调压铸造工艺可以生产出具有较好组织和力学性能的高温合金薄壁铸件。     

Estimation of air gap and heat transfer coefficient at different faces of Al and Al–Si castings solidifying in permanent mould

Abstract

In the casting processes, the heat transfer coefficient at the metal/mould interface is an important controlling factor for the solidification rate and the resulting structure and mechanical properties. Several factors interact to determine its value, among which are the type of metal/alloy, the mould material and surface conditions, the mould and pouring temperatures, casting configuration, and the type of gases at the interfacial air gap formed. It is also time dependent. In this work, the air gap formation was computed using a numerical model of solidification, taking into consideration the shrinkage and expansion of the metal and mould, gas film formation, and the metallostatic pressure. The variation of the air gap formation and heat transfer coefficient at the metal mould interface are studied at the top, bottom, and side surfaces of Al and Al–Si castings in a permanent mould in the form of a simple rectangular parallelepiped. The results show that the air gap formation and the heat transfer coefficient are different for the different casting surfaces. The bottom surface where the metallostatic pressure makes for good contact between the metal and the mould exhibits the highest heat transfer coefficient. For the sidewalls, the air gap was found to depend on the casting thickness as the larger the thickness the larger the air gap. The air gap and heat transfer coefficient also depend on the surface roughness of the mould, the alloy type, and the melt superheat. The air gap is relatively large for low values of melt superheat. The better the surface finish, the higher the heat transfer coefficient in the first few seconds after pouring. For Al–Si alloys, the heat transfer coefficient increases with increasing Si content.     

新型圆坯结晶器铜管结构模拟优化设计

为优化设计新型圆坯结晶器铜管结构,提高结晶器寿命和铸坯质量,利用商业有限元软件ANSYSTM建立二维连铸结晶器内钢水凝固传热及弹塑性应力有限元瞬态分析模型,铸坯传热边界采用与气隙相关的热流密度修正函数,建立与温度相关的热物性参数、力学性能和屈服函数,并利用多场间接耦合的方式对不同工况下的4种圆坯在结晶器内的凝固收缩变形过程进行数值模拟.结果表明,钢种和工况条件对铸坯边界凝固收缩过程影响显著,6瓣模型为4种计算模型的最优形状,依据铸坯边界凝固收缩曲线和新型结晶器设计准则,新型结晶器铜管结构设计采用凸型花瓣状内腔和大锥度设计.     

Effect of casting/mould interfacial heat transfer during solidification of aluminium alloys cast in CO2-sand mould

The ability of heat to flow across the casting and through the interface from the casting to the mold directly affects the evolution of solidification and plays a notable role in determining the freezing conditions within the casting, mainly in foundry systems of high thermal diffusivity such as chill castings. An experimental procedure has been utilized to measure the formation process of an interfacial gap and metal-mould interfacial movement during solidification of hollow cylindrical castings of Al-4.5 % Cu alloy cast in CO₂-sand mould. Heat flow between the casting and the mould during solidification of Al-4.5 % Cu alloy in CO₂-sand mould was assessed using an inverse modeling technique. The analysis yielded the interfacial heat flux (q), heat transfer coefficient (h) and the surface temperatures of the casting and the mould during solidification of the casting. The peak heat flux was incorporated as a dimensionless number and modeled as a function of the thermal diffusivities of the casting and the mould materials. Heat flux transients were normalized with respect to the peak heat flux and modeled as a function of time. The heat flux model proposed was to estimate the heat flux transients during solidification of Al-4.5 % Cu alloy cast in CO₂-sand moulds.     

Determination and verification of the gap dependent heat transfer coefficient during permanent mold casting of A356 aluminum alloy

In complex castings, the heat transfer across the casting / mold interface depends on the local gap size and contact pressure. Thus, an experimental setup is constructed to measure and evaluate the air‐gap dependent heat transfer coefficient during solidification of an A356 permanent mold casting. In order to evaluate the heat transfer coefficient, the temperature gradient and air gap development is measured at the casting / mold interface. This allows the interface temperatures and the time‐dependent heat flux across the gap to be calculated as a function of the measured gap size. Furthermore, the heat transfer coefficient and gap size are correlated to the interface temperature of the casting. The experimental setup and the evaluation procedure provide consistent and reproducible results. The heat transfer coefficient thus evaluated is employed to simulate the experimental setup. The temperatures measured are well reproduced. The results of the present work are compared to simulations using two heat transfer coefficient functions found in literature. This comparison shows a substantial improvement over the state of the art. This improvement is due to the exact knowledge of gap formation and the corresponding values of the heat transfer coefficient.     

Effect of magnesium content on thermal and structural parameters of Al–Mg alloys directionally solidified

In this study, the influence of magnesium content on thermal and structural parameters during the unsteady-state unidirectional solidification of Al–Mg alloys is analyzed. Using a special device, Al–Mg alloys containing 5, 10, and 15 wt% Mg were submitted to unidirectional solidification. Using a data acquisition system, the temperature variations along the casting during solidification were measured. From these results, the variations of solidification parameters as growth rate of dendrite tips, thermal gradient, cooling rate, and local solidification time were determined. The variation of global heat transfer coefficient at metal/mould interface was estimated through the adjustment of experimental temperature variation close to the interface and numerical predictions. Primary and secondary dendrite arms spacing variations during solidification were measured by optical microscopy. From these results, comparative analysis were developed to determine the influence of magnesium content.     

Computer model of unidirectional solidification of single crystals of high temperature alloys

Abstract

I t has been common practice to use mould withdrawal unidirectional solidification to produce single crystal castings. To grow single crystals successfully, it is important to control several solidification parameters, such as the morphology of the solidification front (solid/liquid interface), thermal gradient, and growth rate during solidification. It is the aim of this study to develop a solidification model that can predict such solidification parameters for various design and operating conditions. The solidification phenomena in the process modelled are basically controlled by two heat transfer mechanisms: conduction and radiation. A set of heat transfer equations and boundary conditions were employed to describe mathematically the heat transfer phenomena. Then the finite difference method was used numerically to solve these equations for specified boundary conditions to obtain the temperature distribution and temperature variation in the casting. The solidification parameters can subsequently be deduced from these temperature data. Several thin plate castings were tested using the model developed. The following design and operating conditions were evaluated: susceptor temperature (power input), withdrawal speed, changes of cross-sectional area in the casting, and geometrical arrangement of the casting tree.

MST/1422     

铝合金压铸工艺数值模拟分析

采用压铸工艺成形铝合金薄壁长轴类零件。首先根据压铸模具浇注系统的设计原则,对铝合金件压铸模的浇注系统进行了设计计算;其次运用procast软件对铝合金压铸成形工艺进行数值模拟,根据压铸过程中的温度场云图,进行了压铸模具的热平衡分析和压铸件的充型凝固分析;最后针对模拟的结果进行了压铸模具的设计。     

基于CAFE模拟双辊连铸纯铝凝固微观组织

鉴于双辊薄带连铸等轴晶区半固态铸轧组织对产品性能的重要性,基于热传输、流动传输、溶质传输等基本传输过程以及晶粒生长物理过程,建立了双辊连续铸轧纯铝凝固过程的宏观温度场、浓度场、微观组织及枝晶形貌演化的三维数学模型.采用CA-FE法对双辊连续铸轧纯铝在水冷钢辊连续铸轧中的凝固过程进行了模拟,采用光学显微镜研究了铸轧工艺参数对凝固组织的影响.数值模拟结果表明,所建立的数学模型能够合理描述晶粒沿任意角度生长的过程,温度场、溶质场和微观组织形貌的模拟计算结果合理.模拟结果与实验结果基本吻合,检验了模型的正确性.     

异型坯结晶器与铸坯热力耦合数值模拟

为研究异型坯连铸结晶器内铸坯凝固和变形过程,采用ABAQUS软件建立了异型坯连铸结晶器和铸坯的二维瞬态热力耦合有限元模型,编制ABAQUS用户自定义子程序GAPCON实现结晶器内壁和铸坯之间的传热模拟.计算模型中考虑了铸坯的凝固和热变形、气隙对传热的影响以及铸坯与结晶器之间的接触应力.分别对2种不同水缝设计结晶器进行了数值模拟,数值分析结果表明,小孔水缝设计的结晶器温度峰值较低、热面温度分布更均匀,因而比大孔水缝设计结晶器具有更长的使用寿命和更优的铸坯质量,这一结论与现场试验结果一致.     

Modelling of Mould Filling and Solidification of Castings

An experimental casting for validation has been designed. The casting is composed of two 50×600×2.5 (width×length×thick) thin-wall pieces. One downsprue is located in the middle. A pouring cup with a stopper is used. This design allows to using two different types of moulds simultaneously. An Al-10%Si alloy has been poured at different temperatures. Two effects have been studied: one is the pouring temperature and the other is the moulding method (namely by machine or manually). The filling length is proportional to the pouring temperature. The influence of different moulding methods on mould filling is more complicated. The filling length in the manual-made mould is 1.5 times as long as the one in the machine-made mould due to the different thermal conductivities. Vents have little influence. A finite volume based computer code which can simulate fluid flow during mould filling coupled with heat transfer as well as solidification has been developed in WTCM Foundry Center.. The code can predict cold shut during mould filling and shrinkage defects during solidification. The simulated results are in good agreement with the experiments.In the second part of the paper, an example is given which illustrates how to use computer simulation to aid designing the casting system. The final computational result is compared with the industrial casting. The process of designing castings by using simulation is completely different from the traditional way. The computer aided casting design offers the possibility to obtain a sound casting from the first time.