镍基高温合金定向凝固斑点偏析的数值模拟研究

为了研究铸造高温合金中的斑点偏析，建立了描述多元合金凝固过程传输行为的数学模型。基于伪二元相图方法，模型给出了液相线温度与固相分数及液相多元溶质浓度的耦合关系式。利用该模型对Ni-5．8Al-15．2Ta（质量分数，％）合金铸锭的垂直定向凝固过程进行了模拟。结果表明：该模型能够反映多组元镍基高温合金凝固过程中斑点偏析的形成及发展过程。凝固界面前沿附近热-溶质双扩散对流引起的密度倒置，是诱发斑点偏析的主要原因。在糊状区中形成的偏析通道中，富集溶质从糊状区流向液相区，通道周围局部流动可以通过糊状区从液相区补充通道中的流动。凝固初期形成的通道不能稳定存在，多个通道合并促使局部凝固前沿优先生长，最终形成稳定的偏析通道。     

Experimental study of freezing in a rectangular enclosure

An experimental investigation into the freezing of n-paraffin solutions in a rectangular enclosure is reported. One vertical side of the enclosure was isothermal while the other walls were adiabatic. Using n-hexadecane and a binary mixture of 50% n-hexadecane and 50% n-octadecane, the effects of aspects ratio and wall temperature on the shape and advancement of the solid-liquid interface and the rate of freezing were investigated. For both the pure material and the binary mixture, heat transfer during freezing for the early stages was conduction-dominated. The effect of natural convection is evident for longer times, with convection currents being confined mainly to the top of the enclosure. However, for enclosures of small aspect ratio, this effect may extend to the bottom of the enclosure. It was also found that the aspect ratio has no effect on the solidification rate. Frozen-mass fractions were found to be greater for the binary mixture as compared to the pure material. For both materials, the freezing rate was greater for lower wall temperatures.     

自然对流条件下非稳态相变过程数值模拟

利用ＰＨＯＥＮＩＣＳ软件计算流场与温度场的功能,针对金属熔化与凝固过程的特点,依据单相统一控制方程,采用焓孔隙度方法,模拟了自然对流条件下相变的过程;并用经典的金属镓熔化过程基准实验进行了验证,良好的一致性表明,该程序能可靠地用于模拟金属相变过程的温度场、流场及确定相变前沿的位置。     

数值模拟和集中参数分析法确定熔模铸造工艺凝固时间的对比分析

分析了用求解基于有限体积的数值计算得出的简单形状的铸型凝固时间和由集中参数模型预测的同一简单形状铸型的凝固时间的关系。液态金属假定浇注于一个初始的过热体中,在大气条件下冷却。铸型壁的热损失同时有对流和辐射。能量方程求解采用固定网焓方法。可以获得糊状区的液体部分与温度成线性的关系。用A356合金为材料得到的数值模拟结果与相关物体模数的集中参数分析法做比较。实际凝固时间与用集中解法求出的凝固时间的比率,与模数值高达约0.0125相关。在模数为零的极限情况下,这一比率接近了2.36的值。     

Zn-5%Al合金定向凝固液固界面形态分析

对Zn-5%Al合金在不同液淬下拉速率和过热状态下的定向凝固液固界面形态进行了研究。结果表明,当合金熔体的过热温度分别为较低和较高时,随着液淬下拉速率的增加,定向凝固界面形态变化与一般的界面形态演化相比各自表现为相符和相异;定向凝固界面稳定性变化存在着若干个由液淬下拉速率和过热状态互相制约决定的分界点。     

Competition of the primary and peritectic phases in hypoperitectic Cu–Sn alloys solidified at low speed in a diffusive regime

Directional solidification experiments on hypoperitectic Cu–Sn alloys have been performed at very low velocity in a high thermal gradient to ensure planar front growth of both phases. The diameter of the sample has been reduced to 500 μm to strongly reduce convection. Lamellar and fibrous peritectic cooperative growth of the primary α- and peritectic β-phases has been observed on length spanning several millimeters. For the first time in a high solidification interval peritectic alloy, a quenched interface of both phases in contact with the liquid has been obtained. An unexpectedly high volume fraction of the primary phase, which furthermore fluctuates over time, has been observed. This is attributed to the transient state of the (α + β) growth front to a steady state and the associated evolution of the large diffusion layer ahead of the solid–liquid interface.     

连续凝固偏晶合金薄带凝固组织进化过程模拟

分析了偏晶合金薄带垂直连续凝固过程,发展了描述该条件下偏晶合金凝固组织演变的数学模型,将计算的温度场和浓度场与凝固组织演变的控制方程相耦合,模拟了Al-5Pb(质量分数．%)合金的凝固过程．结果表明．在固/液界面前存在一过冷区、弥散相液滴在此区间内形核,这些液滴在移向凝固界面的过程中进行扩散长大,随着凝固速度的提高,形核率升高,弥散相液滴的数量密度增大．平均半径减小。     

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。     

稀土钇对Mg-Zn-Y-Zr合金热裂敏感性的影响(英文)

通过Clyne-Davies模型对MgZn2.5YxZr0.5(x=0.5,1,2,4,6)系合金的热裂敏感性进行预测;采用X射线衍射和扫描电子显微镜分别对MgZn2.5YxZr0.5系合金进行显微组织和热裂区域组织形貌观察,并用自制的"T"形热裂模具,通过A/D转换,用计算机对MgZn2.5YxZr0.5系合金凝固过程中的温度、收缩应力信号数据进行采集和进一步的处理,并描绘其曲线。研究MgZn2.5YxZr0.5合金的凝固温度区间、脆弱区域的凝固温度变化、凝固最后阶段剩余液相分数以及合金中第二相种类等因素对MgZn2.5YxZr0.5系合金热裂倾向的影响:合金热裂倾向从大到小顺序为MgZn2.5Y2Zr0.5,MgZn2.5Y0.5Zr0.5,MgZn2.5Y4Zr0.5,MgZn2.5Y6Zr0.5,MgZn2.5Y1Zr0.5。由于MgZn2.5Y2Zr0.5合金的凝固温度区间最宽,脆弱区域的凝固温度变化最大,凝固最后阶段形成的液膜最少,枝晶干涉点后析出的第二相阻碍枝晶间的补缩等多种原因而造成合金的热裂倾向最大。     

Effect of pouring temperature on cooling slope casting of semi-solid Al-Si-Mg alloy

Present trend of semi-solid processing is directed towards rheocasting route which allows manufacturing of near-net-shape cast components directly from the prepared semi-solid slurry. Generation of globular equi-axed grains during solidification of rheocast components, compared to the columnar dendritic structure of conventional casting routes, facilitates the manufacturing of components with improved mechanical properties and structural integrity. In the present investigation, a cooling slope has been designed and indigenously fabricated to produce semi solid slurry of Al-Si-Mg (A356) alloy and successively cast in a metallic mould. The scope of the present work discusses about development of a numerical model to simulate the liquid metal flow through cooling slope using Eulerian two-phase flow approach and to investigate the effect of pouring temperature on cooling slope semi-solid slurry generation process. The two phases considered in the present model are liquid metal and air. Solid fraction evolution of the solidifying melt is tracked at different locations of the cooling slope, following Schiel s equation. The continuity equation, momentum equation and energy equation are solved considering thin wall boundary condition approach. During solidification of the liquid metal, a modified temperature recovery scheme has been employed taking care of the latent heat release and change of fraction of liquid. The results obtained from simulations are compared with experimental findings and good agreement has been found.     

连铸Ag-28Cu温度场非稳态过程的数值模拟

采用Procast软件中的Mile算法对Ag-28Cu合金连铸凝固过程中温度场的非稳态变化进行了模拟,研究了不同拉速、过热度和换热系数对温度场分布、凝固速率和凝固前沿温度梯度变化的影响。模拟结果表明:随拉速的增大,铸件中心区域的凝固速率加快,凝固前沿温度梯度变化范围减小。换热系数的改变在靠近铸件表面区域对凝固速率基本没有影响,但靠近铸件中心区域后,其凝固速率随换热系数开始大幅度增大。在整个凝固过程中,换热系数越大,凝固前沿温度梯度越大。随浇注温度的提高,其凝固速率呈振荡式增大,凝固前沿温度梯度呈振荡式减小。     

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.     

含钛微合金钢凝固过程中TiN的析出行为分析

分别从热力学和动力学方面研究了低碳含钛微合金钢凝固过程选分结晶对TiN夹杂物析出的影响。热力学分析表明,液相线温度以上不会有TiN析出;由于凝固过程凝固前沿Ti、N元素富集,凝固分数达到0.377时,凝固前沿固相中开始析出TiN;凝固末期,Ti和N的富集程度进一步增大,固液相中均有TiN析出。动力学分析表明,随着冷却速度的降低,凝固过程TiN夹杂物的尺寸显著增加,当冷速高于50 K/s时,TiN的理论半径为5.5 μm,当冷速低于5 K/s时,TiN的理论半径在17.5 μm以上;固相中析出的TiN为纳米级别。铸坯中TiN析出物主要尺寸为1~5 μm,且大尺寸夹杂主要在铸坯厚度方向的1/4处和中心处析出,这表明铸坯中的大尺寸夹杂物是在凝固过程中析出的。     

试样尺寸对Al-4.5%Cu合金定向凝固组织和界面稳定性的影响

在传统Bridgeman方法下,利用Al-4.5%Cu合金内外同心嵌套试样,研究了自然对流对凝固过程及组织的影响。试验结果显示,温度梯度和抽拉速率均相同的定向凝固试样,尺寸较大试样的组织呈规则胞状或树枝状时,对应尺寸较小试样组织胞晶尖端发生分叉或杂乱树枝状,表明尺寸较小的试样液-固界面稳定性较差。较小试样尺寸限制了液相中自然对流致使溶质原子液-固界面处大量富集是凝固组织不规则与液-固界面不稳定的主要原因。     

垂直定向凝固条件下通道偏析形成过程的数值模拟

建立了描述合金凝固过程热－溶质对流和宏观偏析形成过程的数学模型。模型中耦合求解了凝固过程中质量、动量、能量和溶质守恒方程；同时，基于固液两相区中温度和成分的耦合关系建立了固相分数场的更新方法。利用该模型模拟了底部冷却的二维矩形区域内Fe-C合金凝固过程中通道偏析的形成和发展过程。模拟结果表明，垂直定向凝固条件下通道偏析形成于液相线前沿附近，而不是两相区内部。这一结果很好地支持了文献中基于实验观察提出的通道偏析形成机理。     

Direct thermal method: new process for development of globular alloy microstructure

Semisolid metal processing (SSMP) is of growing industrial significance particularly for magnesium and aluminium alloys. SSMP requires a binary micro-structure in which the primary phase is approaching a spheroidal (globular or non-dendritic) shape. Traditionally this is achieved by stirring the alloy in the mushy state. An alternative method, which is gaining popularity, is the so-called slurry-on-demand or new rheocasting process in which solidification conditions are controlled via active thermal management to yield non-dendritic solid in a liquid matrix. The authors present here a novel low-cost process, the direct thermal method (DTM), in which a globular microstructure, suitable for SSMP, is achieved via the naturally occurring thermal environment in a very simple casting experiment. Basically the DTM is a process in which liquid alloy of low superheat is poured into a cylindrical metallic mould of very low thermal mass but high thermal conductivity. Heat-matching between alloy and mould results in a pseudo-isothermal hold within the solidification range of the alloy, made possible by the very low rate of heat loss to the environment. Without the use of any special insulation or heating devices, the fraction solid during the experiment and the hold time can be modified by simple alterations to the process variables and geometry. The thin mould walls also make quenching easy. The resultant alloy morphology is characterised for an aluminium alloy designation A356. IJCMR/463     

Convective Flow and Its Effect on the Liquid-liquid Phase Transformation of Monotectic Alloys

建立了偏晶合金液--液相变过程中的两相流动模型, 发展了偏晶合金凝固理论模型; 以Al-Pb合金为例, 模拟研究了偏晶合金凝固时的组织演变进程, 考察了流动对凝固组织形成的影响. 结果表明, 对流影响液--液相变的形核特性, 提高凝固界面前沿最高弥散相液滴数量密度、最大液滴半径和最高弥散相 体积分数, 促进宏观偏析组织的形成.     

Theoretical analysis of diffusion of solutes during the solidification of alloys

When an alloy solidifies and the composition of the solid differs from the composition of the liquid, atoms of the alloying elements rejected at the solid-liquid interface have to diffuse toward the bulk liquid. Diffusion may be supported by convection. Theoretical calculations have been made for a liquid without convection, for a liquid involving natural convection, and for solidification of a liquid alloy at the surface of a rotating disk as an example of forced convection.     

Solidification microstructures and solid-state parallels: Recent developments,future directions

Rapid advances in atomistic and phase-field modeling techniques as well as new experiments have led to major progress in solidification science during the first years of this century. Here we review the most important findings in this technologically important area that impact our quantitative understanding of: (i) key anisotropic properties of the solid–liquid interface that govern solidification pattern evolution, including the solid–liquid interface free energy and the kinetic coefficient; (ii) dendritic solidification at small and large growth rates, with particular emphasis on orientation selection; (iii) regular and irregular eutectic and peritectic microstructures; (iv) effects of convection on microstructure formation; (v) solidification at a high volume fraction of solid and the related formation of pores and hot cracks; and (vi) solid-state transformations as far as they relate to solidification models and techniques. In light of this progress, critical issues that point to directions for future research in both solidification and solid-state transformations are identified.     

FH96高温合合金的凝固过程及元素平衡分配系数的研究

基于Scheil-Gulliver凝固模型,采用热力学计算软件Thermo-Calce及相应的Ni基数据库,对FGH96高温合金的凝固过程进行了热力学模拟计算,以揭示FGH96合金凝固过程中各相的析出规律,及凝固过程中各元素的偏析行为;并用Scheil-Gulliver方程对残余液相中各合金元素的质量分数随固相分数的变化进行回归,得到了各元素的平衡分配系数.结果表明,在FGH96合金凝固过程中,当固相分数达到0.7、0.94、0.98时由残余液相中分别开始析出MC型碳化物、TCP相、γ′相;B、C、Mo、Nb、Ti、Zr、Si的平衡分配系数小于1,而Co、Cr、W、Fe、Al的平衡分配系数大于1.