液相线铸造法半固态金属组织特征的相场模拟研究

液相线铸造法是获取均匀、细小非枝晶组织半固态合金浆料的一种新工艺。凝固过程的相场模拟表明,金属凝固组织形成枝晶的趋向随过冷度加大而增强,长大速度随之显著加快。液相线铸造时结晶凝固过程在枝晶形成趋向很小、长大速度较慢的过冷度范围完成,故应得到细小非枝晶组织;液相线铸造法浇注前熔体在液相线温度附近保温,发生双相区等温结晶分解过程,该过程的相场模拟显示,这一分解作用促成大量形核,导致在随后的浇注熔体中生长成大量均匀、细小非枝晶组织;二次加热至双相区,引起部分熔化和重新分解结晶,同理会使组织进一步得到改善。     

相场法定量模拟强迫层流对微观偏析的影响(英文)

采用耦合流场的相场模型,模拟Ni-Cu合金过冷熔体流动对液固界面前沿浓度分布的影响;定量分析强迫层流对迎流方向、垂直流动方向与逆流方向枝晶尖端前沿浓度边界层厚度,浓度最大值和枝晶微观偏析的影响。结果表明:溶质场和流场二者相互影响;枝晶上游侧浓度边界层的厚度较薄,溶质浓度的最大值较小;而枝晶下游侧恰好与此相反。贫Cu区域在迎流方向枝晶臂沿枝晶轴线方向的宽度较大,但Cu原子的浓度较低,微观偏析最严重,枝晶下游侧缩松倾向加剧。     

相场法模拟凝固微观组织演化研究进展

相场法是凝固组织模拟中最有潜力的方法之一,近年来已成为凝固领域研究的热点.本文论述了相场法模拟凝固微观组织的原理,并分别介绍了无流动和流动条件下相场法模拟自由枝晶、定向凝固界面及小平面枝晶在国内外的研究进展,指出了进一步的研究方向.     

Numerical simulation of facet dendrite growth

Numerical simulation based on phase field method was performed to describe the solidification of silicon. The effect of anisotropy, undercooling and coupling parameter on dendrite growth shape was investigated. It is indicated that the entire facet dendrite shapes are obtained by using regularized phase field model. Steady state tip velocity of dendrite drives to a fixed value when γ≤0.13. With further increasing the anisotropy value, steady state tip velocity decreases and the size is smaller. With the increase in the undercooling and coupling parameter, crystal grows from facet to facet dendrite. In addition, with increasing coupling parameter, the facet part of facet dendrite decreases gradually, which is in good agreement with Wulfftheory.     

用相场方法模拟纯物质等轴枝晶生长

采用相场模型模拟了纯物质过程的等轴枝晶演化,并针对各向异性模数为４的金属镍进行了二维数值模拟,计算了正方形区域过冷熔液中的等轴枝昌基相场方程中采用３均匀网格的一般显示有限差分方法求解,通过数值计算显示了等轴枝晶的形态。     

NUMERICAL SIMULATION OF SUCCINONITRITE DENDRITIC GROWTH IN A FORCED FLOW

Numerical simulation based on phase field method is performed to describe solidification process of pure material in a free or forced flow. The evolution of the interface is showed, and the effects of mesh grid and flow velocity on succinonitrite shape are studied. These results indicate that crystal grows into an equiaxial dendrite in a free flow and into an asymmetrical dendritic in a forced flow. With increasing flow velocity, the upstream dendritic arm tip grows faster and the downstream arm grows slower. However, the evolution of the perpendicular tip has no significant change. In addition, mesh grid has no influence on dendritic growth shape when mesh grid is above 300×300.     

铸铝枝晶生长的相场法模拟

简介了相场方法,利用相场法对纯铝进行了数值模拟,研究了晶核半径、界面厚度等参数对晶粒形貌的影响。模拟结果与实验结果基本吻合。表明相场方法符合枝晶生长的物理机制。     

铝合金枝晶生长形貌数值模拟研究

采用宏微观耦合方法对铝合金枝晶生长形貌进行模拟计算。由于对整个试样进行微观计算存在困难 ,所以采取的方案是整体计算温度场 ,而微观组织计算则是在试样中心区域选取其中一个单元进行 ,该单元的微观组织模拟被简化为二维计算。采用较大的网格和时间步长Δt计算宏观温度场 ,当选取的单元温度降到液相线以下时 ,在一个宏观时间步长Δt内 ,采用小网格和小时间步长δt对该单元进行微观组织计算 ,宏观、微观计算交替进行。形核过程采用了高斯分布模型 ,生长过程由相场方程来控制。微观区域内的初始温度分布由周围的宏观单元温度插值得到 ,界面过冷包含热过冷、成分过冷和曲率过冷。溶质分布则用混合溶质守恒方程来描述。为了描述二次枝晶的产生 ,在相场模型中引入了噪声。模拟结果与实际浇注的试样的金相结果以及其他学者的结果进行了比较。     

Simulation of fluctuation effect on dendrite growth by phase field method

The dendrite growth process was simulated with the phase field model coupling with the fluctuation.The effect of fluctuation intensity on the dendrite morphology and that of the thermal fluctuation together with the phase field fluctuation on the forming of side branches were investigated.The results indicate that with the decrease of thermal fluctuation amplitude,the furcation of dendrite tip also decreases,transverse dendrites become stronger,longitudinal dendrites become degenerated,Doublon structure ...     

Phase field method simulation of faceted dendrite growth with arbitrary symmetries

A numerical simulation based on a regularized phase field model is developed to describe faceted dendrite growth morphology. The effects of mesh grid, anisotropy, supersaturation and fold symmetry on dendrite growth morphology were investigated, respectively. These results indicate that the nucleus grows into a hexagonal symmetry faceted dendrite. When the mesh grid is above 640×640, the size has no much effect on the shape. With the increase in the anisotropy value, the tip velocities of faceted dendrite increase and reach a balance value, and then decrease gradually. With the increase in the supersaturation value, crystal evolves from circle to the developed faceted dendrite morphology. Based on the Wulff theory and faceted symmetry morphology diagram, the proposed model was proved to be effective, and it can be generalized to arbitrary crystal symmetries.     

Phase field modeling of multiple dendrite growth of AI-Si binary alloy under isothermal solidification

Phase field method offers the prospect of being able to perform realistic numerical experiments on dendrite growth in metallic systems. In this study, the growth process of multiple dendrites in AI-2-mole-%-Si binary alloy under isothermal solidification was simulated using phase field model. The simulation results showed the impingement of arbitrarily oriented crystals and the competitive growth among the grains during solidification. With the increase of growing time, the grains begin to coalesce and impinge the adjacent grains. When the dendrites start to impinge, the dendrite growth is obviously inhibited.     

Fe-C-Mn三元合金等温凝固过程枝晶生长的相场法模拟

采用相场方法模拟了Fe-1.0%C-0.1%Mn三元合金等温凝固过程二维枝晶生长,预测了溶质浓度分布以及Mn元素对枝晶形貌的影响.结果表明:Mn和C浓度分布相似,枝晶主干溶质浓度最低,而在二次枝晶臂之间形成了溶质富集浓度最高.与Fe-1.0%C合金比较,Fe-1.0%C-0.1%Mn合金由于Mn元素的添加抑制了侧向分枝的生长,使二次枝晶不发达.     

半固态温度区间扰动条件和浇注温度对镁合金铸态组织的影响

研究了镁合金熔体在半固态温度区间进行不同扰动处理后铸件的组织特征。结果表明经变质精炼的镁合金熔体在半固态温度区间进行吹Ar处理可获得均匀的非枝晶组织，非枝晶组织的形成与晶粒细化及枝晶臂熔断有关。     

界面动力学系数影响枝晶生长的相场模拟

利用相场法对纯金属熔体中的枝晶生长进行了模拟;研究了界面动力学系数取值的影响因素及其取值对枝晶尖端生长速度和尖端半径的影响。结果表明：界面动力学系数的取值由相场和温度场的耦合系数、热扩散系数、界面厚度、界面原子运动时间变量等参数综合决定,随着界面动力学系数的增加,枝晶尖端生长速度减小,尖端半径增大。     

Phase field simulation of dendrite growth under convection

The phase-field model coupled with a flow field was used to simulate the solidification of pure materials by the finite difference method. The effects of initial crystal radius, the space step and the interface thickness on the dendrite growth were studied. Results indicate that the grain grows into an equiaxial dendrite during free flow and into a typical branched structure under forced flow. The radius of an initial crystal can affect the growth of side-branches but not the stability of the dendrite's tip when an appropriate value is assigned to it. With an increase in space steps, side-branches appear at the upstream of the longitudinal principal branch and they grow rapidly. With an increase in the interface thickness, the trunk of the longitudinal upstream and lateral principal branches grow longer and become more slender while the number of secondary branches increases.     

Effect of Transverse Static Magnetic Field on Microstructure of Al-12% Si Alloys Fabricated by Powder-Blow Additive ManufacturingEI北大核心CSCD

Due to the great advantage in manufacturing component with complex structures, additive manufacturing (3D print), essentially the rapid solidification of tiny metallic molten pool (hemisphere like with diameter ranging from dozens of microns to several millimeters) has become an important formation technique. Using powder laser melting, the effect of transverse static magnetic field on the solidified structure of additive manufactured Al-12% Si alloy was studied. The macrostructure was formed by white band (mainly primary alpha-Al phase) and dark grey area (mainly eutectic phase) and no obvious influence was presented with or without static transverse magnetic field of 0.35 T. However, for the microstructure, the primary alpha-Al in dark grey area formed as columnar structure without magnetic field was found to transform to dendritic like with developed dendrite arms when under a static transverse magnetic field. The analysis on thermoelectricity and dimensionless Hartman parameter which used to characterize the restriction of static magnetic field on molten flows show that under a static transverse magnetic field of 0.35 T, the thermoelectric magnetic force can be as high as a magnitude of 10(5) N/m(3), and Hartman values is far more than 10. The results indicate that the Marigoni and thermosolutal convection in laser melting pool was restricted. The transform from columnar to equiaxed dendrite of primary alpha-Al in dark grey area under static magnetic field was attributed to the fragmentation by thermoelectric magnetic force (10(5) N/m(3)) in solid phase. In addition, the formation of high order dendrite arms was supposed to be caused by the restriction of static magnetic field on the melt.     

CREM法半连续铸造Al合金非枝晶组织的形成机制

研究了低频电磁场作用下7075Al合金半连续铸坯的微观组织。结果表明：铸坯由近球形和蔷薇形晶粒构成。且随磁场强度增大,近球形组织增加,蔷微形组织减少,整体组织变得细小均匀,这种非晶组织的形成机制可解释为：电磁场作用下,从结晶器壁游离的晶粒数量增多,使熔体中的晶核心增加。此外,游离晶粒随对流熔体一同运动以及晶粒自身的旋转运动抑制了其按枝晶方式生长,从而导致最终形成细小均匀的非枝晶组织。     

CREM法抑制7075铝合金半连铸坯中溶质元素宏观偏析的机制

实验研究了CREM法半连铸7075铝合金过程中，低频交变电磁场对凝固前沿形态、微观组织结构、非平衡凝固液固相线位置、结晶区间间隔以及温度场和流动场的影响规律。并据此对CREM法抑制7075铝合金半连铸坯中溶质元素宏观偏析的机理进行了分析和说明。     

Simulation of Grain Growth during Solidification of Twin-Roll Continuously Cast Pure Aluminum with a Modified CA Model

基于枝晶生长的基本传输过程和元胞自动机(Cellular Automaton,简称CA)-有限元(Finite Element,简称FE)模型基本原理,建立了适应双辊连续铸轧纯铝薄带工艺特点的凝固过程形核和晶体生长的数学模型。模型耦合了宏观温度场和微观组织模拟计算,考虑了溶质扩散、曲率过冷和潜热释放等重要因素的影响,能够模拟凝固过程中枝晶生长的形态。应用本模型对双辊连续铸轧纯铝薄带凝固过程中等轴晶生长、等轴晶多晶粒生长及柱状晶生长、柱状晶向等轴晶演化进行模拟并与实验结果进行对比,模拟结果与实验结果吻合较好     

电阻点焊熔核微观组织模拟

基于有限差分思想建立了电阻点焊的三维宏观传热模型,并在此模型基础上通过进一步细化网格差分得到枝晶形核和生长所需的温度信息.基于元胞自动机方法建立枝晶形核和生长的微观模型,将已建立的宏观温度场模型和微观元胞自动机模型互相耦合起来,建立了电阻点焊熔核微观组织模拟的宏微观耦合模型.利用耦合模型模拟了5754铝合金电阻点焊一定参数下熔核的微观组织形成过程,所得的模拟结果为"柱状+等轴"晶组织,与试验结果吻合较好.