等轴球晶凝固多相体系内热溶质对流、补缩流及晶粒运动的数值模拟Ⅰ.三相流模型

基于Eulerian-Eulerian方法和体积元平均技术，建立了模拟等轴球品凝固过程的液、固、气三相流模型．液、固两相处理成相互分离、相互扩散的介质，气相与液、固两相只存在热量及动量的相互作用，三相（凝固前）作为自由流体共享一个统一的压力场．分别求解三相的质量、动量、溶质、热焓守恒方程；相间的热量交换和摩擦拖拽以及液／固界面上的溶质再分配和凝固潜热释放，通过定义对应守恒方程的源项和交换项而加以考虑；另外，单独求解一个晶粒密度守恒方程．晶粒的形核生长（相变）也加以模型化并体现在对应的源项中，模型中所用的密度定义为溶质与温度的函数，因此可综合考虑热溶质对流、晶粒运动及凝固收缩所引起的补缩流动．凝固过程的体积收缩及补缩流动将体现在气／液自由表面的波动上．     

NUMERICAL SIMULATION OF ALLOY SOLIDIFICATION BY A TWO－PHASE MODEL

本文介绍了合金凝固传输过程中的二相流数学模型．该模型描述了固－液界面的传输现象和液相对流对宏观偏析的影响，并考虑了生核过程，热和溶质的过冷效应，及固相颗粒的宏观运动，从而把宏观传输过程和微观结构结合起来．采用该模型，对Ａｌ－４％Ｃｕ等轴晶合金的凝固进行了完整的数值模拟．     

Al－4％CU铸锭凝固中自由等轴晶的迁移行为及对宏观偏析的影响

研究了Ａｌ—４％ＣＵ合金铸锭在砂型中的缓冷凝固行为，测定了凝固铸锭中的晶粒度和宏观成分分布。研究结果表明，由于Ａｌ—Ｃｕ合金的非平衡凝固行为和固相密度与晶间液相密度差的逆转特性，以及液相成分的重力偏析，引起铸锭中上部等轴晶密度增加并产生明显的逆偏析。     

Al—4%Cu铸锭凝固中自由等轴晶的迁移行为及对宏观偏析的影响

研究了Ａｌ－４％Ｃｕ合金铸锭在砂型中的缓冷凝固行为，测定了凝固铸锭中的晶粒度和宏观成分分布。研究结果表明，由于Ａｌ－Ｃｕ合金的非平衡凝固行为和固相密度与晶间液相密度差的逆转特性，以及液相成分的重力偏析，引起铸锭中上部等轴晶密度增加并产生明显的逆偏析。     

合金凝固的二相流数值模拟

本文介绍了合金凝固传输过程中的二相流数学模型，该模型描述了固－液界面的传输现象和液相对流对宏观偏析的影响，并考虑了生核过程，热和溶质的过效应，及固相颗粒的宏观运动，从而把观传输过程和微观结构结合起来；采用该模型，对Ａｌ－４％Ｃｕ等轴晶合金的凝固进行了完整的数值模拟。     

冷却速率对Mg-Gd-Y-Zr合金凝固组织的影响

通过改变凝固过程中的冷却速率,研究了冷却速率对Mg-Gd-Y-Zr合金凝固组织与成分微观偏析的影响.随冷却速率提高,合金组织明显细化,初生相形貌由粗大等轴枝晶逐渐向细小树枝晶转变,合金凝固过程中形核率增加,合金晶粒尺寸逐渐减小;冷却速率的提高可以降低溶质元素的扩散速率,从而增加合金元素在枝晶干中的固溶度,减轻凝固过程中合金元素Gd与Y的微观偏析,同时使凝固过程中形成的共晶减少,共晶组织分布更加弥散、均匀.     

任意固相反扩散条件下枝晶凝固溶质传输微观／宏观模型化

通过对合金枝晶凝固中有任意固相反扩散条件下微观溶质再分布的模型化,扩展了作者提出的合金凝固传输及宏观偏析连续介质模型。本文微观／宏观统一模型提出的归一化固相反扩散集合参数除了包括合金的固相扩散系数外,还包含了生长枝晶尺度及几何形态参数,凝固速率和溶质分配系数等影响因素。模型的数值计算结果表明,在相同的凝固条件下不同的合金固相扩散能力可对其铸锭的宏观偏析行为产生明显的影响。     

电磁搅拌对Incoloy825合金凝固组织和偏析的影响

研究了电磁搅拌对Incoloy825高温合金凝固组织及凝固过程中溶质元素偏析的影响。结果表明,在合金凝固过程中,施加电磁搅拌能够有效细化晶粒组织,同时增加等轴晶比例。当电磁搅拌电流强度由150A增加到300A时,铸坯中晶粒尺寸由300μm减小至210μm,同时等轴晶含量由66.7%增至77.5%。未施加电磁搅拌的Incoloy825高温合金铸坯内部的宏观偏析和微观偏析均比较严重,通过施加电磁搅拌,引起熔体强制对流,使熔体内部传热和传质得到改善,同时增大了冷却速率使二次枝晶臂间距减小,并促进液穴内熔体成分的均匀化,大幅度降低了合金元素的偏析程度。     

电磁振荡频率对上引连铸Cu-15Ni-8Sn合金组织及逆偏析的影响

通过在上引连铸Cu-15Ni-8Sn(质量分数,%)合金过程中施加电磁振荡,研究了电磁振荡频率对合金凝固组织及逆偏析的影响。结果表明:当施加的电磁振荡频率为10或30Hz时,合金的宏观凝固组织发生了柱状晶到等轴晶的转变,微观凝固组织由树枝晶转变为蔷薇状组织;当电磁振荡频率从30 Hz增加到50 Hz时,宏观凝固组织又逐渐转变为柱状晶,微观凝固组织也随之转变为发达的树枝晶。电磁振荡频率为10和30 Hz时形成的等轴晶组织有利于改善合金中Sn元素的逆偏析。电磁振荡使合金熔体中产生往复强制对流,10或30 Hz频率的电磁振荡引起的宽幅往复流动使固液界面前沿温度场和溶质场均匀,有利于形成等轴晶,并促使枝晶折断,形成游离晶,消除定向枝晶通道,从而抑制了逆偏析。而在50或100 Hz频率下,电磁振荡频率过高形成了窄幅往复流动,不能有效折断枝晶、达到细化晶粒的目的,从而减弱了抑制逆偏析的作用。     

轻压下枝晶变形及溶质分布的试验模拟研究

通过丁二腈-水合金试验类比研究了连铸过程中的轻压下工艺。水冷强度1.2、0.6、0.3mL/s下,分别研究了不同固相分数和不同变形量对枝晶变形及溶质再分布的影响,并用示踪剂进行示踪。结果表明：实施轻压下时,大部分柱状晶发生弯曲变形,不再继续生长,此时二次枝晶和部分等轴晶快速长大;在溶液凝固早期,会形成凝固中心上升,凝固前沿下沉的对流,对凝固前沿进行充分的溶质交换,当这种对流减弱之后才能进行有效的轻压下操作;变形量对溶质分布的影响比较复杂,在变形量逐渐增大的过程中,首先形成负偏析,经过一个偏析为零的阶段后再形成正偏析,且偏析越来越严重,并且位于压下区间不同的区域,影响程度不一样。试验结果对研究轻压下对偏析的改善机理有意义。     

Preparation of Al-Zn-Mg-Cu alloy semisolid slurry through a water-cooled serpentine pouring channel

The semisolid slurry of Al-Zn-Mg-Cu alloy was prepared through a self-designed water-cooled copper serpentine pouring channel(WSPC) machine. Influences of pouring temperature, the number of turns and the cooling water flow rate on the microstructure of the semisolid Al-Zn-Mg-Cu alloy slurry were investigated. The results show that the semisolid Al-Zn-Mg-Cu alloy slurry with satisfactory quality can be generated by the WSPC when the pouring temperature is in the range between 680 ℃ and 700 ℃. At a given pouring temperature, the average grain size of primary α-Al decreases and the shape factor increases with the increase of the number of turns. When the cooling water flow rate is 450 L·h~(-1), the obtained semisolid slurry is optimal. During the preparation of the semisolid Al-Zn-Mg-Cu alloy slurry with low superheat pouring, the alloy melt has mixed inhibition and convection flow characteristics by "self-stirring". When the alloy melt flows through the serpentine channel, the chilling effect of the inner wall of the channel, the convection and mixed inhibition of the alloy melt greatly promote the heterogeneous nucleation and grain segregation. This effect destroys the dendrite growth mode under traditional solidification conditions, and the primary nuclei gradually evolve into spherical or nearspherical grains.     

Simulation of cooling channel rheocasting process of A356 aluminum alloy using three-phase volume averaging model

The cooling channel process is a rehocasting method by which the prematerial with globular microstructure can be produced to fit the thixocasting process. A three-phase model based on volume averaging approach is proposed to simulate the cooling channel process of A356 Aluminum alloy. The three phases are liquid, solid and air respectively and treated as separated and interacting continua, sharing a single pressure field. The mass, momentum, enthalpy transport equations for each phase are solved. The developed model can predict the evolution of liquid, solid and air fraction as well as the distribution of grain density and grain size. The effect of pouring temperature on the grain density, grain size and solid fraction is analyzed in detail.     

Grain refinement behavior of an aluminum alloy by inoculation and dynamic nucleation

Grain refinement offers several benefits in aluminum casting applications. Two methods are normally used to achieve a grain-refined microstructure: inoculation and dynamic nucleation. Inoculation is widely applied in industry, but is not an efficient process. Dynamic nucleation, achieved by application of localized forced convection with rapid cooling, is an alternative process. However, a deeper understanding of dynamic nucleation is required if this process is to be used commercially. This study aims to understand the grain refinement behavior of an aluminum alloy under the influences of inoculation and dynamic nucleation. A rapid quenching method was used to investigate the combined effects of inoculation and dynamic nucleation on the solid fraction, particle density and particle size of the secondary nuclei. In addition, the effects of the particle density of the secondary nuclei on the final cast microstructure were studied. The rapid quenching results show that dynamic nucleation by application of forced convection with localized cooling to the melt yields an increased solid fraction and particle density of secondary nuclei. The solid fraction and particle density are further increased by inoculation. This study also shows that increasing the convection level in an inoculated melt held at a temperature slightly above the liquidus temperature increases the effectiveness of dynamic nucleation, which consequently yields a finer microstructure of the final cast samples. The findings suggest that grain refinement can be effectively achieved by applying forced convection with localized cooling to create a low fraction solid of secondary nuclei in the melt prior to pouring and casting.     

Modeling segregation and grain structure development in equiaxed solidification with convection

Modeling the development of micro- and macrosegregation patterns and grain structures in the equiaxed solidification of metal alloys under the combined influences of melt convection and the motion of free solid has been the subject of intense recent research efforts. This article presents a summary of selected experimental and theoretical studies aimed at understanding the convective transport processes both for a single grain and at the scale of a casting. The need for much additional research is emphasized. C. Beckermann earned his Ph.D. in mechanical engineering at Purdue University in 1987. He is currently a professor at the University of Iowa. He is also a member of TMS.     

2-Dimensional FEM modeling of macrosegregation in the directional solidification with mesh adaptation

In order to improve the prediction accuracy of macrosegregation channel, an algorithm for dynamic remeshing is proposed. The basic idea is to generate fine elements near the liquidus isotherm. The norm of the gradient of solid fraction is used for piloting the remeshing in the mushy zone; whereas, the objective mesh size in the liquid is considered as a function of the distance to the liquidus isotherm. The efficiency of mesh adaptation is demonstrated by prediction of macrosegregation channel in a case of unidirectional solidification.     

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.     

导流器在AZ91D镁合金半固态组织形成中的作用

采用新型的自孕育法制备AZ91D镁合金半固态浆料,研究了导流器在半固态组织形成中的作用.结果表明,合金熔体加入孕育剂并经导流器浇注后,导流器吸收大量过热,使合金温度迅速降低至液相线附近;合金液在导流器表面流动时,导流器促进了熔体中非均质形核及游离晶粒的产生,使合金凝固时晶粒增殖.导流器表面凝固组织从上至下依次为树枝晶、细小树枝晶、等轴晶;倾斜角度影响合金流动方式及传热效果,角度过大和过小均不利于晶粒增殖,角度为45°时冷却效果和剪切作用最佳,此时合金有效形核数量最多,半固态浆料组织初生相细小且分布均匀.     

Numerical Simulation of Macrosegregation in Steel Ingot CastingEI北大核心CSCD

Many key forging components of heavy equipment are manufactured by large steel ingots. Macrosegregation in steel ingots is a key defect formed during the solidification process. Over the past few decades, numerical modeling has played a more and more important role in the study of macrosegregation. Various models have been developed and applied to different ingot casting processes. This paper focused on the application of macrosegregation models to the steel ingot. Firstly, the formation mechanism and influencing factors of macrosegregation were introduced. Then, the existing macrosegregation models and their recent development were summarized. Macrosegregation models accounting for such mechanisms as solidification shrinkage- induced flow and mushy zone deformation were analyzed, respectfully. To model macrosegregation due to solidification shrinkage, the key was to solve the free surface. A simple derivation showed that the multi-phase (including gas phase) models were equivalent to the VOF-based segregation models in dealing with the shrinkage-induced flow. Finally, our recent research work on numerical modeling of macrosegregation in steel ingots was illustrated, including application of the developed multi-component and multi-phase macrosegregation model to a 36 t steel ingot, and numerical simulation of multiple pouring process. The carbon and sulphur concentrations at about 1800 sampling points, covering the full section of a 36 t ingot, were measured. By detailed temperature recording, accurate heat transfer conditions between the ingot and mould were obtained. Typical macrosegregation patterns, including the bottom-located negative segregation and the pushpin-like positive segregation zone in the top riser, have been reproduced both in the measurements and the predictions, The carbon and sulphur concentrations predicted by the three dimensional multi-component and multi-phase macrosegregation models agreed well with the measurements, thus proving that the model can well predict the macrosegregation formation in steel ingots. As for the multi-pouring process simulation, the results show a high concentration of carbon at the bottom and a low concentration of carbon at the top of the mould after the multi-pouring process with carbon content high in the first ladle and low in the last ladle. Therefore, the multiple pouring process could get the initial solute distribution with the opposite form of segregation. Such carbon concentration distribution would reduce the negative segregation at the bottom and the positive segregation at the top of the solidified ingot, thus proving the ability of the multiple pouring process for the control of macrosegregation.     

Density and solidification feeding model of vacuum counter-pressure cast aluminum alloy under grade-pressuring conditions

The density of vacuum counter-pressure cast aluminum alloy samples under grade-pressuring condition was studied. The effect of grade pressure difference and time on the density of aluminum alloys was discussed, and the solidification feeding model under grade-pressuring condition was established. The results indicate the grade-pressured solidification feeding ability of vacuum counter-pressure casting mainly depends on grade pressure difference and time. With the increase of grade pressure difference, the density of all the aluminum alloy samples increases, and the trend of change in density from the pouring gate to the top location is first decreasing gradually and then increasing. In addition, in obtaining the maximum density, the optimal grade-pressuring time is different for samples with different wall thicknesses, and the solidification time when the solid volume fraction of aluminum alloy reaches about 0.65 appears to be the optimal beginning time for gradepressuring.