Prediction of solidification behaviour via microstructure models based on granular structures

Abstract

Two important factors affecting hot tearing – semi-solid constitutive behaviour and grain percolation – have been simulated through the use of microstructure models based on granular structures. The semi-solid model geometry is based on a modified Voronoi tessellation, and includes rounded corners to approximate an equiaxed-globular grain structure with liquid surrounding the grains. The percolation model combines solidification and thermodynamic aspects to predict the gradual transition within the mushy zone from a continuous liquid to a coherent solid network, while the constitutive behaviour model uses experimentally-derived data to describe the behaviour of the solid grains. By performing a series of model runs over ranges of grain size and fraction solid, the simulations have revealed an important link between grain size, semi-solid yield stress, strain localisation, and grain coalescence. Furthermore, the models provide insight on the relative importance of each mechanism on hot tear formation, and show promise for improving quantitative hot tearing predictions.     

双超声场对A356半固态浆料凝固组织的影响

试验研究了双超声场中,施振温度和处理区域距离,对流经处理区的A356铝合金半固态浆料凝固组织的影响规律。结果表明,双超声场对凝固组织的细化、球化效果较好。浇注温度为630℃并施振所获得的金相组织晶粒最细小,球化最均匀。而随着施振温度升高,晶粒反而变得粗大。同时,由于超声波的衰减,当处理区域距离为5mm时,获得的组织最好。     

连铸工艺对方坯凝固组织影响的数值模拟

铸坯凝固组织特征对于连铸坯粗坯质量具有重要影响。通过建立重轨钢U71Mn方坯连铸过程中传热和凝固组织模拟数学模型,研究了CA模型特征参数和连铸操作工艺参数对于凝固组织特征的影响规律。研究结果表明：平均形核过冷度控制柱状晶区域,随着该值的增大,柱状晶区域也随之增大;最大形核密度控制晶粒尺寸,该值越大晶粒的尺寸则越小。连铸拉坯速度波动在±0.5m/min时,最终铸态组织特征并没有明显变化,而浇注温度对于最终铸坯组织结构具有较大的影响,晶粒平均半径由15℃的1.555mm增加到45℃的1.721mm,并且中心等轴晶比例逐渐减小。因此,连铸生产过程中,在满足钢液顺利浇注的条件下,降低浇注钢液的过热度会很大程度上改善重轨钢内部组织结构,提高铸坯内部质量。     

Progress on modeling and simulation of directional solidification of superalloy turbine blade casting

Directional solidified turbine blades of Ni-based superalloy are widely used as key parts of the gas turbine engines.The mechanical properties of the blade are greatly influenced by the final microstructure and the grain orientation determined directly by the grain selector geometry of the casting.In this paper,mathematical models were proposed for three dimensional simulation of the grain growth and microstructure evolution in directional solidification of turbine blade casting.Ray-tracing method was applied to calculate the temperature variation of the blade.Based on the thermo model of heat transfer,the competitive grain growth within the starter block and the spiral of the grain selector,the grain growth in the blade and the microstructure evolution were simulated via a modified Cellular Automaton method.Validation experiments were carried out,and the measured results were compared quantitatively with the predicted results.The simulated cooling curves and microstructures corresponded well with the experimental results.The proposed models could be used to predict the grain morphology and the competitive grain evolution during directional solidification.     

A three-phase simulation of the effect of microstructural features on semi-solid tensile deformation

A direct finite-element microstructure model for prediction of the deformation behavior of semi-solid metallic alloys is presented. The 2D model geometry is based on a modified Voronoi tessellation, and includes rounded corners to approximate an equiaxed-globular grain structure, liquid surrounding the grains, and micro-porosity. An elasto-plastic empirical constitutive equation is derived for the solid grains, while the liquid is approximated as a perfectly plastic material with a very low yield stress. The resulting three-phase model was used to investigate the effects of fraction solid, porosity, and grain size on the constitutive behavior of a semi-solid aluminum alloy, AA5182. The model predictions were validated against experimental data at high fraction solid. These simulations reveal a strong correlation between semi-solid grain size and yield stress, and between porosity and strain localization. The application of direct finite-element simulations is shown to be an effective technique for examining the effects of microstructure phenomena on the macro constitutive behavior of semi-solid materials.     

Evolution of microstructure in semi-solid slurries of rheocast aluminum alloy

Semi-solid metal processing is being developed in die casting applications to give several cost benefits. To efficiently apply this emerging technology, it is important to understand the evolution of microstructure in semi-solid slurries for the control of the rheological behavior in semi-solid state. An experimental apparatus was developed which can capture the grain structure at different times at early stages to understand how the semi-solid structure evolves. In this technique, semi-solid slurry was produced by injecting fine gas bubbles into the melt through a graphite diffuser during solidification. Then, a copper quenching mold was used to draw some semi-solid slurry into a thin channel. The semi-solid slurry was then rapidly frozen in the channel giving the microstructure of the slurry at the desired time. Samples of semi-solid 356 aluminum alloy were taken at different gas injection times of 1, 5, 10, 15, 20, 30, 35, 40, and 45 s. Analysis of the microstructure suggests that the fragmentation by remelting mechanism should be responsible for the formation of globular structure in this rheocasting process.     

铝钛二元合金凝固过程数学模型的建立及应用

针对Ti〈0.15%的Al-Ti二元合金,基于三个假设：一个晶核生长过程中所产生的成分过冷为邻近晶粒的形核提供所需的形核过冷度;两个形核事件发生的距离相当于一个晶粒尺寸;忽略界面前沿温度梯度和凝固过程中结晶潜热的析出,在此基础上建立了Al-Ti二元合金凝固过程的数学模型并进行数值计算,计算了成分过冷度与相对晶粒尺寸。应用于电解低钛铝合金和熔配铝钛合金,发现电解低钛铝合金在试验室条件下的形核过冷度ΔTn=0.5~1.0K,低于传统熔配加钛的Al-Ti二元合金（ΔTn=0.8~1.5K）。对于研究电解低钛铝基合金的细化机理提供了数值基础。     

等温处理对挤压态AZ80A镁合金半固态组织的影响

采用应变诱发熔化激活法(SIMA)工艺制备了AZ80A镁合金半固态坯料,研究了保温温度和保温时间对半固态组织的影响。结果表明:随着保温温度的升高和保温时间的增加,AZ80A镁合金的平均晶粒尺寸与液相率都呈上升趋势,形状因子呈先增大后减小的趋势。半固态组织由α-Mg晶粒、Al、Zn元素富集形成的晶界处液相和晶内“小液池”组成,其组织演变分为初始晶粒合并长大,晶粒球化、彼此分离,最终合并粗化3个阶段。采用该种方法制备AZ80A镁合金半固态坯料时合适的保温温度为550 ℃、保温时间为45 min,此时半固态组织的平均晶粒尺寸、形状因子和液相率分别为89 μm、0.795和26.7%。     

铸造铝合金微观组织模拟

用凝固过程数值模拟技术预测铝合金铸件的晶粒组织 ,进而预测铸件力学性能已成为国内外铸造领域的研究热点 ,M.Rappaz和 J.A.Spittle,S.G.R.Brown等学者基于决定论、随机论方法提出了预测铸件晶粒组织形成的计算模型 ,模拟晶粒的二维生长与实验结果很符合。相场方法是一种三维微观组织的直接模拟方法。建立了相场模型 ,模拟晶粒的三维生长 ,反映了凝固因素对晶粒形状的影响 ,也反映了扰动对树枝晶枝晶结构的影响。进一步建立多晶粒的生长模型 ,预测二次枝晶臂的间距 ,建立力学性能与二次枝晶臂的间距的关系模型     

导流器对自孕育铸造法制备AM60镁合金半固态浆料的影响(Ⅲ)

采用自孕育铸造法制备AM60镁合金半固态坯料,研究导流器参数对半固态组织的影响规律及组织形成机理。结果表明:导流器能有效促进合金组织由树枝晶向等轴晶及颗粒状晶的转变;熔体处理温度和孕育剂加入量一定时,导流器的相关参数,如角度、长度、温度、混合段的凹槽宽度,均影响合金的传热及流动,进而影响其凝固组织;在导流器角度为30°～45°、长度为500 mm、混合段凹槽宽度为50 mm、且通水冷却时可以获得细小颗粒状或蔷薇状的半固态坯料,初生α-Mg的平均晶粒尺寸为37.5μm;减小导流器出口宽度有利于增强液体紊流,促进晶粒均匀分布;导流器促进了熔体非均质形核及激冷晶的形成,增强了晶粒增殖;熔体对流使合金的温度场和浓度场更均匀,晶粒生长以颗粒状生长、蔷薇晶根部颈缩熔断和磨圆熟化机制为主。     

功率超声双振动头处理A356半固态熔体的研究

利用两套自制的功率超声双振动头试验装置,对A356铝合金流动熔体进行处理获得半固态浆料,对比分析了1.0kW和1.5kW超声换能器交错式排布流道及对向式排布流道装置处理A356合金的凝固组织,研究了合金熔体在运动状态中加上功率超声场影响下的凝固机理,分析了功率超声的机械效应、热效应及声流效应对半固态金属结晶的影响。结果表明,浆料经处理后的试样金相组织的细化、球化效果与振动头的摆放位置有关,交错式比对向式处理效果好;流道上的凝料因激冷和整个凝固过程都在流道上受到振动影响而更细小,流出流道的晶粒易长大,受到超声和斜流道影响而球化更好。     

Al-5Ti-B和稀土Y复合添加对A356合金半固态凝固组织的影响

采用稀土Y及Al-5Ti-B中间合金对A356铝合金进行细化处理,并用低温浇注技术制备半固态坯料.研究了单独添加Al-5Ti-B和稀土Y以及两者复合添加时对A356铝合金半固态凝固组织的影响.研究结果表明,Al-5Ti-B和稀土Y对A356铝合金半固态初生相的形貌及尺寸均有不同程度的改善.将两者复合添加时,可显著提高合金中初生相的形貌及尺寸,得到细小、圆整且分布均匀的半固态组织,其添加量的最佳组合为0.02％的Ti与0.5％的Y.探讨了两者复合添加对合金凝固组织的细化机理.     

Metastable structure of austenite base obtained by rapid solidification in a semi-solid state

Material processing in a semi-solid state with rapid solidification is an innovative technology, which enables us to produce complex-shaped semi products in one operation. Unconventional properties and microstructures can be obtained in this way. Material processing in a semi-solid state has been used for materials with lower melting temperatures, particularly for Al alloys.This paper concentrates on the development of new technologies for production of miniature thin-walled steel components with complicated shapes. Ledeburitic steel with 1.8% of carbon and 11% of chromium was chosen for this experimental study. This material was used to produce very small thin-walled semi products. From the initial structure consisting of primary and secondary carbides distributed in a ferrite matrix was obtained a microstructure with over 90% of metastable austenite after cooling from the semi-solid state. The main aim of this experimental program was to describe the effect of two different methods of heating to the semi-solid state. The first method used unique heating equipment, combining high frequency and resistance heating. The second method consisted of conventional heating in a furnace. The influence of the cooling rate on the development of the microstructure was investigated. If was found that both heating and cooling rates influence grain size and the size and the morphology of carbide network placed between the globular austenite grains.Structure analysis was performed with the help of light microscopy, laser scanning confocal microscopy and scanning electron microscopy. EDX analysis was applied to determine chemical profiles of phases and X-ray diffraction phase analysis was used to establish volume fraction of austenite in the final microstructure. Characterisation of individual structural components was further completed by hardness measurements.     

冶金硅定向凝固法制备太阳能级多晶硅及其微观组织与位错(英文)

以冶金硅为原料,探索采用具有高温度梯度的真空定向凝固技术制备低成本太阳能级多晶硅,并研究其在不同生长条件下的微观组织特征、晶界与晶粒大小、固液界面形貌以及位错结构。结果表明,当凝固速率低于60μm/s时,能获得具有高密度和良好取向的定向凝固多晶硅棒状试样,硅晶粒大小随凝固速率的增大而减小;在控制凝固过程,获得平的固液界面形貌是获得沿凝固方向排列柱状晶的关键;由于硅的小平面生长特性,微观组织中出现了位错生长台阶和孪晶结构;在晶粒中,位错分布呈现不均匀性,并且位错密度随凝固速率的增加而增加;在此基础上,讨论了多晶硅的生长行为以及位错形成机制。     

Modeling characteristics for solidification in single-crystal,investment-cast superalloys

Macroscopic traits of solidification such as thermal gradient, solidification rate, and other criteria functions were extracted from computer modeling data of a single-crystal, investment-cast superalloy. The thermal histories were used to predict the presence or absence of grain defects in the shaped castings. Freckle defects were found on castings with various processing parameters, even though the traditional microstructural defect map predicted no freckles and a consistent microstructure. The findings suggest that the traditional defect map criteria are insufficient to accurately describe the formation of these and other single-crystal grain defects. Hence, additional criteria for porosity formation and other phenomena were investigated and found to be sensitive to changes in solidification conditions and grain defect formation. The additional functions were found to be necessary to fully capture the total energy of all solidification phenomena. When these functions are used in conjunction with the traits of the microstructural defect map, the prediction of defects in single-crystal castings can more accurately describe solidification under the stringent requirements of single-crystal turbine components.     

A1-4.5Cu合金半固态坯料制备研究

研究了稀土Ce对铝铜合金微观组织的影响以及A14.5Cu合金半固态坯料的制备工艺.实验表明:在Al-4.5Cu合金中加入稀土Ce,细化了合金的微观组织,固液两相区间变窄,改善了合金的凝固性能.浇注温度和浇注方式对合金坯料制备也有明显的影响.在700℃采用斜坡振动铜模水冷浇注可制备性能优良的稀土Al-4.5Cu半固态坯料,其组织为细小均匀的等轴球状晶,且制备工艺简单.     

Thermal Modeling of Spot Welds

Solidification structure and austenite decomposition in some spot welded 0.05% C-0.3% Mn steels have been studied with the aid of a numerical thermal model. The model is a one dimensional finite difference formulation, and accounts for melting, temperature dependent thermal properties, effective heat transfer in the liquid, and contact resistances. The model was used to predict solidification conditions during resolidification of the nugget. These solidification conditions were correlated with the primary dendrite spacing using available primary dendrite spacings modeling. The model was also used to predict cooling rates in the temperature regime for austenite decomposition. These cooling rates were used to interpret the underlying microstructure.     

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.     

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Ⅱ.模型的应用

利用等轴球晶三相凝固模型模拟了A356铝合金半固态浆料冷却斜槽法制备过程,研究了晶粒密度、尺寸及固相分数的分布与工艺参数的关系．结果表明,在斜槽浇注处晶粒形核密度最大,在斜槽末端晶粒尺寸、固相分数最大,在铸型中这三者的最终分布大致均匀．适当降低浇注温度有助于提高斜槽上形核密度和固相分数及降低晶粒尺寸．此外,还模拟了热溶质对流及补缩流在Al-4％CU（质量分数）合金等轴球晶凝固过程所起的作用,以及晶粒运动及补缩流对合金自由表面和宏观偏析形成的影响．结果表明,凝固初期热对流及补缩流为主导,凝固中期热溶质对流为主导,凝固后期补缩流为主导;晶粒运动受阻程度直接影响自由表面形状,补缩流考虑与否导致完全不同的宏观偏析图．实验测得晶粒尺寸与模拟结果分布较相似,但其绝对值存在较大差异．     

Simulation of solid deformation during solidification: Shearing and compression of polycrystalline structures

Deformation of the semi-solid mush during solidification is a common phenomenon in metal casting. At relatively high fractions of solid, grain boundaries play a key role in determining the mechanical behavior of solidifying structures, but little is known about the interplay between solidification and deformation. In the present study, a polycrystalline phase-field model is combined with a material point method stress analysis to numerically simulate the coupled solidification and elasto-viscoplastic deformation behavior of a pure substance in two dimensions. It is shown that shearing of a semi-solid structure occurs primarily in relatively narrow bands near or inside the grain boundaries or in the thin junctions between different dendrite arms. The deformations can cause the formation of low-angle tilt grain boundaries inside individual dendrite arms. In addition, grain boundaries form when different arms of a deformed single dendrite impinge. During compression of a high-solid fraction dendritic structure, the deformations are limited to a relatively thin layer along the compressing boundary. The compression causes consolidation of this layer into a fully solid structure that consists of numerous sub-grains. It is recommended that an improved model be developed for the variation of the mechanical properties inside grain boundaries.