Numerical Simulation of Columnar to Equiaxed Transition for Directionally Solidified Ti-44Al Alloy

Solute diffusion controlled solidification model was applied to simulate the columnar to equiaxed transition （CET） during directional solidification of Ti-44Al alloy. The simulation results show that the solutal interactions from growing equiaxed grains play an important role on CET. The effects of the applied thermal gradient and pulling velocity, the equiaxed seed spacing and nucleation undercooling on the CET are investigated in the present simulation. The simulated results indicated that the columnar branch spacing depends not only on the thermal gradient and the pulling velocity, but also on number of the seeds. A spacing adjustment can occur through initiation of seeds that develop into new columnar grains. The dependence of the CET on the thermal gradient and pulling velocity, qualitatively agrees with the analytical CET model of Hunt,     

NUMERICAL SIMULATION OF COLUMNAR TO EQUIAXED TRANSITION FOR DIRECTIONALLY SOLIDIFIED Ti-44Al ALLOY

Solute diffusion controlled solidification model was applied to simulate the columnar to equiaxed transition (CET) during directional solidification of Ti-44Al alloy. The simulation results show that the solutal interactions from growing equiaxed grains play an important role on CET. The effects of the applied thermal gradient and pulling velocity, the equiaxed seed spacing and nucleation undercooling on the CET are investigated in the present simulation. The simulated results indicated that the columnar branch spacing depends not only on the thermal gradient and the pulling velocity, but also on number of the seeds. A spacing adjustment can occur through initiation of seeds that develop into new columnar grains. The dependence of the CET on the thermal gradient and pulling velocity, qualitatively agrees with the analytical CET model of Hunt.     

Directional solidification processing on CET in Al-based alloys

The control of the transition from columnar to equiaxed (CET) dendrite microstructure is an important point to obtain desired final properties of industrial products. The objective is to understand how the formation and the evolution of the CET are influenced by the processing parameters and natural convection with Al - 3.5 wt.% Ni and Al - 7.0 wt.% Si alloys. Various experiments are carried out in a Bridgman furnace for which the thermal gradient and pulling velocity can be independently controlled. We concentrate our interest on the CET tendency, added particle effects and the evolution of dendrite grain structures under different test conditions. On the other hand, in-situ and real time observation of the solid-liquid interface is used to reveal the dynamics of the phenomena that occur, thus deepening our understanding. To achieve this objective, Synchrotron X-ray Radiography has been designed and performed at the European Synchrotron Radiation Facility.     

From constrained to unconstrained growth during directional solidification

A one-dimensional solidification model has been developed to study the directional solidification of dendritic alloys. It is based on the resolution of the heat flow equation using a two-interface front tracking technique. The two interfaces are defined by imaginary limits, assumed to be macroscopically flat, which correspond to the positions of the growing dendritic and eutectic interfaces. These delimit the three regions that are considered: liquid, mushy zone and solid. Growth kinetics laws are applied to the interfaces by velocity vs temperature relationships. It was found that, if complete solidification was carried out directionally up to the top of the ingot (i.e. formation of a fully columnar structure), then the velocity of the dendrite tips first increased during the stage of the superheat loss, then decreased when no substantial thermal gradient remained in the liquid ahead of the growing dendritic interface. Applied to directional solidification experiments carried out with aluminium–silicon alloys, the model shows that this maximum velocity was reached when the top position of the mushy zone (i.e. the dendritic interface) reached about two-thirds the length of the ingots. This position being in the vicinity of the columnar-to-equiaxed transition (CET) observed in the longitudinal section of the ingots, a CET scenario is proposed based on a constrained-to-unconstrained growth transition, leading to breakdown of the columnar dendritic front.     

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.     

Columnar and Equiaxed Solidification of Al-7 wt.% Si Alloys in Reduced Gravity in the Framework of the CETSOL Project

During casting, often a dendritic microstructure is formed, resulting in a columnar or an equiaxed grain structure, or leading to a transition from columnar to equiaxed growth (CET). The detailed knowledge of the critical parameters for the CET is important because the microstructure affects materials properties. To provide unique data for testing of fundamental theories of grain and microstructure formation, solidification experiments in microgravity environment were performed within the European Space Agency Microgravity Application Promotion (ESA MAP) project Columnar-to-Equiaxed Transition in SOLidification Processing (CETSOL). Reduced gravity allows for purely diffusive solidification conditions, i.e., suppressing melt flow and sedimentation and floatation effects. On-board the International Space Station, Al-7 wt.% Si alloys with and without grain refiners were solidified in different temperature gradients and with different cooling conditions. Detailed analysis of the microstructure and the grain structure showed purely columnar growth for nonrefined alloys. The CET was detected only for refined alloys, either as a sharp CET in the case of a sudden increase in the solidification velocity or as a progressive CET in the case of a continuous decrease of the temperature gradient. The present experimental data were used for numerical modeling of the CET with three different approaches: (1) a front tracking model using an equiaxed growth model, (2) a three-dimensional (3D) cellular automaton–finite element model, and (3) a 3D dendrite needle network method. Each model allows for predicting the columnar dendrite tip undercooling and the growth rate with respect to time. Furthermore, the positions of CET and the spatial extent of the CET, being sharp or progressive, are in reasonably good quantitative agreement with experimental measurements.     

Numerical Simulation of Titanium Alloy Ingot Solidification Structure during VAR Process Based on Three-Dimensional CAFE Method

建立三维多尺度数学模型计算Ti-6Al-4V合金铸锭真空自耗电弧熔炼(VAR)过程中的温度场、流场及凝固组织的形成。该模型包括宏观质量、动量及能量守恒方程和介观晶粒形核生长模型。在传热与流动计算的基础上,模拟铸锭VAR过程中的三维凝固组织的形成。对比计算结果与实验观察可知,两者在晶粒结构与晶粒生长方式方面吻合较好。当考虑VAR过程中熔池表面的辐射换热后,铸锭顶部的柱状晶被很好地呈现。最后,考察了自然对流对铸锭凝固组织的影响,计算结果表明自然对流对柱状晶-等轴晶转变(CET)及晶粒尺寸影响较大,表现为促进CET及细化晶粒。     

选择性激光烧结金属粉末瞬态温度场模拟

局部输入的集中移动热源造成了选择性激光烧结过程中温度场分布不均衡且不稳定,因此研究其温度场对掌握烧结过程中温度动态分布规律具有重要意义。在考虑了热传导、热辐射和热对流,材料的非线性热物性参数和相变潜热的作用下,建立了水雾化Fe多道烧结的三维有限元模型,采用ANSYS参数化设计语言(APDL)实现移动的高斯热源的加载。模拟结果表明:激光烧结过程中,在光斑中心前端存在着较大的温度梯度;光斑中心的温度高于金属粉末的熔点,烧结过程存在液相;粉床内部温度场在深度方向呈漏斗状阶梯分布,随烧结深度的增加,粉床内部的温度和温度梯度迅速衰减;同一烧结道各点的最高温度相对稳定,但随着烧结道的增加,各点最高温度都有小幅度增加的趋势,这是温度累加的结果。     

关于连铸传热数学模型中等效导热系数的讨论

讨论了连铸传热数学模型中等效导热系数对模拟结果的影响,结论表明,采用等效导热系数法可以较为准确的预测铸坯表面温度及凝固终点,但等效导热系数越大,凝固前期的铸坯坯壳厚度越薄;等效导热系数的取值对铸坯两相区将产生较大影响,等效导热系数越大,某一厚度处钢液越早进入两相区,相应地,两相区厚度越大、两相区内温度梯度越小,局部凝固时间越大;采用计算获得的二次枝晶间距与实测值强制拟合的方法可以获得对流等效因子[（m）]的合理取值;对于铸机,在液相穴中强制对流区取[m＝7、]强制对流与自然对流的过渡区取[m＝5、]自然对流区取[m＝3、]静滞区取[m＝1]计算,可以获得较为准确的模拟结果。     

激光熔覆反应合成TiCP／Al复合材料过程的模拟研究

针对激光熔覆反应过程中的熔化和凝固问题，采用对流／扩散单相统一控制方程组，统一处理固相、固液两相和液相中的传热和流动问题；对运动引起的动量传输和能量传输，采用源项法分别在动量方程和能量方程中加以处理；在商用软件PHOENICS基础上增添相应模块，开发了激光熔覆三维瞬态温度场数值模拟软件。并以ZL104合金激光熔覆原位合成TiCp／Al复合材料的工艺为实例，进行数值模拟，模拟结果与实验结果基本一致。     

Solidification behaviour of TiAl-based alloys studied by directional solidification technique

Solidification behaviour of TiAl-based alloys with the nominal chemical composition Ti–46Al–8Ta and Ti–46Al–8Nb (at.%) was studied at steady- and unsteady-state conditions in Y₂O₃ moulds using Bridgman solidification technique. The effect of growth parameters (growth rate V and temperature gradient G_L) on primary solidification phase, solidification path and microstructure parameters were determined. Directional solidification at unsteady-state conditions was applied to determine columnar to equiaxed transition (CET) in the studied systems.     

Phase and Microstructure Selection During Directional Solidification of Peritectic Alloy Under Convection Condition

A phase and microstructure selection map used for peritectic alloy directionally solidified under convection condition was presented,which is based on the nucleation,constitutional undercooling criterion(NCU criterion),and the highest interface temperature criterion.This selection map shows the relationships between the phase/microstructure,the G/V ratio(G is the temperature gradient,V is the growth velocity),and the alloy composition under different convection intensities and nucleation undercoolings.Comparing with the results from directional solidification experiments of Sn–Cd peritectic alloys,this selection map was generally in agreement with the experimental results.     

铝合金连续铸造过程CET位置判据的研究

柱状晶向等轴晶转变(CET)是在一定的凝固过程中必须控制的一种显微组织转变.本文针对铝合金连铸坯凝固过程中柱状晶向等轴晶的转变,综合运用了正交试验研究、数值模拟计算、数学拟合等方法,分析了连铸过程主要工艺因素拉坯速度、一冷水量、二冷水量和浇注温度对连铸坯凝固过程中柱状晶向等轴晶转变位置的影响,提出了柱状晶向等轴晶转变的转变位置判据,即当连铸坯某一位置处的固相率等于0.3时,温度梯度G与冷却速度R满足函数关系G0.8072/R=0.469时,在该位置处将发生柱状晶向等轴晶的转变.     

5056铝合金稳恒磁控激光深熔焊接过程熔池流动与传热行为分析

针对外加稳恒磁场条件下6 mm厚度5056铝合金激光深熔焊接过程,建立了热场—流场—电磁场耦合熔池瞬态动力学数值模型,求解了特定时刻温度场、速度场与电磁场分布,建立了熔池沿不同方向的Peclet数模型,分析了不同磁场感应强度对熔池流动与传热行为的影响.结果表明,稳恒磁场条件下熔池中产生显著的哈特曼效应,表现为液态金属M...     

纵向磁场对不同尺寸定向凝固高温合金DZ417G组织的影响

进行了外加纵向静磁场下高温合金DZ417G的定向凝固实验,考察了纵向磁场对不同尺寸试样凝固组织形貌的影响.结果显示,在温度悌度为70℃/cm,抽拉速率为5μm/s时,施加磁场后一次枝晶间距减小,并在试样边缘出现等轴晶组织;随着试样尺寸的增大,在试样边缘和中心的柱状枝晶组织遭到破坏,形成等轴晶组织,且出现"斑状"偏析.这些现象可归结为磁场在固/液界面前沿合金熔体中诱发的热电磁对流(TEMC)所致.     

Microstructure transition from stable to metastable eutectic growth in Ni–25%Al alloy

The microstructural evolution during directional solidification of the Ni–25%Al (mole fraction) alloy was investigated in the range of growth velocity from 10 to 100 μm/s under a given thermal gradient of 10 K/mm. The solidification microstructures reveal a transition from γ‘–β equilibrium eutectic to γ–β metastable eutectic plus β dendrites. A mixed microstructure of γ‘–β and γ–β eutectics produced at a growth velocity of 25 μm/s illustrates that the transition occurs during the competitive growth between γ and γ' phases. The growth temperature for each phase was considered to understand the microstructure selection during solidification. The experimental results show that a phase or a microstructure solidifying with the highest temperature under a given growth condition is preferentially selected upon solidification. In addition, both stable eutectic and metastable eutectic are shown to coexist and simultaneously grow in the velocity range between 25 and 60 μm/s due to their similar growth temperatures.     

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     

Microstructure analysis of AZ31 magnesium alloy welds using phase-field models

We use phase-field models to characterize the microstructure present in magnesium AZ31 alloy solidified under welding conditions. We focus our attention on the study of the conditions under which a columnar-to-equiaxed transition (CET) is observed in resistance spot welds. Our simulations show how the size and shape of the columnar and equiaxed regions depend on factors such as cooling rate, temperature gradient and the nature of inoculant particles. Our results are compared with experimental observations. In addition, we contrast our findings with predictions from a previously developed steady-state model for the CET.     

金属熔滴碰撞、铺张数值分析

基于金属熔滴沉积成形的3D打印技术,其成形质量与金属熔滴及基板参数有很大关系。本文对金属铝熔滴在不同参数下,比如：熔滴直径、滴落速度、比热、导热系数、潜热以及基板温度等,其凝固过程中的最大铺展因子关系进行了研究。采用VOF对自由表面进行追踪的方法,建立包括对流和相变的计算方程和能量方程的仿真模型,耦合水平集函数,跟踪熔融粒子与周围空气之间的接口,获得的最大传播因子之间的关系,与在文献中出现的实验数据吻合。     

Zn—2.75%Cu合金定向凝固组织及对流的影响

研究了Zn-2.75%Cu合金在传统Bridgeman法和ACRT－B法下定向凝固组织的差别。着重讨论了生长速度和液相强制对流对Zn-Cu包晶定向凝固组织的影响。实验发现,在Bridgeman法定向凝固过程中,生长速度的增加使得液相中的温度梯度减小,一次枝晶间距减小,一次枝晶间距与生长速度和温度梯度的关系式为：λ1=0.6064R^-0.25GL^-0.5。强制对流使得一次枝晶发生分叉和偏转。并且随着对流强度的加大,一次枝晶间距降低,二次枝晶的生长被抑制。包晶反应的存在,使得枝晶的尖端和二次枝晶熔解,造成了固相中枝晶和初生枝晶的差别。