Experiments and modeling of rapid solidification of plasma-sprayed yttria-stabilized zirconia

Thermal barrier coatings (TBCs) are widely used to increase the working temperature and improve the high-temperature corrosion resistance of base materials. Thermal spraying methods such as air plasma spraying (APS) are convenient techniques to deposit TBCs. This work examines the rapid solidification of APS-deposited yttria-stabilized zirconia (YSZ), by modeling the non-equilibrium solidification of a single molten particle. The model solves the so-called hyperbolic equations for heat and mass transfer to predict interface undercooling and velocity as a function of time, and also predicts the morphology of the solidification front (and thus the microstructural characteristics of rapidly solidified YSZ) as a function of interface velocity. Results are presented of a single particle solidifying onto a steel substrate, and onto a previously deposited particle. The numerical results are also compared to experimental data of the microstructure of a YSZ splat deposited by APS, to validate the model.     

The manufacturing significance of solidification modeling

Modeling of solidifcation is becoming increasingly feasible, even in manufacturing-scale processes. Certain critical features of solidification can be modeled very accurately and used to predict casting results, including defects. The advent of faster computers and continuing software development bode well for the future of modeling in foundry operations.     

The current status of titanium rapid solidification

Rapid solidification can have a variety of effects on the microstructure, constitution and mechanical properties of titanium-base alloys. Numerous processing methods can be used to form a wide range of rapidly solidified compositions. Among the alloy classes studied— conventional alloys, rare-earth systems, metalloid systems, eutectoid systems and intermetallics— rare-earth systems and intermetallics hold the greatest promise for developing improved alloys. Such alloys are intended for elevated-temperature aerospace applications.     

凝固微观组织的多层次模拟

通过引入溶质再分配、溶质扩散、界面能各向异性和界面曲率,构建了描述合金凝固微观组织形态演变的元胞自动机模型.在介观和微观尺度上的模拟结果表明,该模型可有效地解决微观组织形成的多尺度问题.模拟结果清晰地再现了与实测结果相一致的枝晶形态和生长现象.根据分形理论采用分维定量比较了模拟结果和实验结果,两者的计盒维数分别为1.703和1.694,阐述了分维定量表征枝晶形貌的物理含义.模拟结果表明熔体过冷度和树枝晶的计盒维数呈近似抛物线关系.     

Trapping of vacancies by rapid solidification

A model has been developed for the process of trapping of vacancies in rapid solidification of pure metals, which includes the effect of solute drag where vacancies play the role of solute. Within a reasonable range of parameter values it predicts that substantial trapping cannot occur unless the solidification velocity is 1 m/s or higher. It is demonstrated that the intrinsic mobility of the liquid/solid interface should not be evaluated without considering the effects of vacancy trapping and solute drag caused by vacancies. The model is applied to copper and unknown parameters are evaluated from information on the solidification velocity as a function of undercooling.     

The formation of randomly textured magnesium alloy sheet through rapid solidification

The rapid solidification of three magnesium-based alloys has been carried out on copper substrates with four tailored surface morphologies. Increasing surface roughness improved the wetting characteristics of the substrates, as did coating with carbon. Of the alloys tested, AZ31 showed inferior wetting behaviour due to the formation of alumina oxides at the interface between the melt and the copper substrate. This behaviour was obviated by the addition of calcium to the melt. The pure magnesium castings all produced a strongly columnar region and showed a weak crystallographic texture. The alloy castings produced weakly columnar regions with an essentially random texture. The alloys cast onto the carbon-coated substrates produced a fully equiaxed microstructure due to the inhibition of heterogeneous nucleation on the substrate surface. It is demonstrated that a randomly textured microstructure with equiaxed grains can be produced through rapid solidification and the implications of this microstructure for the ductility of magnesium sheet is discussed.     

Modelling dendrite evolution under rapid solidification conditions

Abstract

The evolution of a single dendrite is simulated by the cellular automaton (CA) method, in which the influences of interface velocity on the solute partition coefficient ahead of the interface and liquidus slope are investigated under rapid solidification conditions. Furthermore, the kinetic undercooling ahead of the interface is also taken into account, which is neglected under normal solidification conditions. The simulated dendrite morphology is compared with that of the equilibrium conditions and it is shown that the interface easily loses stability around the dendritic tip causing more and more secondary dendrite arms to appear. In addition, the solute partition coefficient increases when the interface velocity increases, which leads to a decrease in solute microsegregation.     

The stochastic modeling of solidification structures in alloy 718 remelt ingots

A stochastic numerical approach was developed to model the formation of grain structure and secondary phases during the solidification of nickel-based alloy 718 remelt ingots. The significance of the present stochastic approach is that the simulated phases can be directly compared with actual phases from experiments at two different scales: grain characteristics can be visualized at the macroscale, while the amount, size, and distribution of secondary phases can be viewed at the microscale. The computer becomes a “dynamic metallographic microscope.” Stochastic modeling was applied to simulate the formation of solidification phases (γprimary phase and NbC and eutectic γ-Laves secondary phases) during the solidification of vacuum-arc-remelted and electroslag-remelted alloy 718 ingots. Modeling results, such as pool profile, grain-growth pattern, grain structure (both columnar and equiaxed grains), columnar-to-equiaxed transition, grain size, and secondary dendrite arm spacing, as well as amount, size, and location of both eutectic γ-Laves and NbC phases compared well with experimental data for cast alloy 718. This research demonstrates that the stochastic approaches are relatively fast, comprehensive, and more accurate than the deterministic approaches in predicting the solidification characteristics of remelt ingots and are mature enough to be used effectively by the metal industry for process development and optimization. Laurentiu Nastac earned his Ph.D. in metallurgical and materials engineering at the University of Alabama at Tuscaloosa in 1995. He is currently a senior staff engineer at Concurrent Technologies Corporation. Dr. Nastac is a member of TMS. Suresh Sundarraj earned his Ph.D. in mineral engineering at the University of Minnesota in 1994. He is currently a process modeling engineer for Concurrent Technologies Corporation. Dr. Sundarraj is also a member of TMS. Kuang-O Yu earned his Ph.D. in metallurgical engineering at the University of Kentucky in 1978. He is currently director of research and development at RMI Titanium Company. Dr. Yu is also a member of TMS. Yuan Pang earned his M.S. in mechanical engineering at the University of Akron in 1977. He is currently a principal engineer at Concurrent Technologies Corporation.     

液态金属快速凝固过程的PFM模型

近年来发展起来的PFM模型根据"溶质捕捉"和"溶质牵引"两种机制对金属及合金快速凝固过程进行数学描述及模拟, 较好地反映了金属凝固的物理本质. 介绍了PFM模型发展过程、研究现状及理论模型, 并对发展前景进行了展望.     

Solidification characteristics of near rapid and supercooling directional solidification

1INTRODUCTIONTraditionalD.S.,suchasHRS,LMCandzonemeltingliquidmetalcoolingD.S.tech-niquesareconductedwithatemperaturegradientGL>0attheheadofS-Linterface,andcanob-tainafineorsuperfinedendriticmicrostructure.Intherecentyears,withthedevel0pmentofdeepsupercoolingandsupercoolings0lidificationtechniquethesolidificationbehaviorofmeltun-dersupercooledstatewithGL>0attheS-Lin-terface,hascausedgreatattentions.Intheearly1980s,L..[l]investigatedthedynamicsuper-coolingsolidification0fsuperalloys…     

液态金属Ga的快速凝固模拟

采用分子动力学方法对液态Ga 的凝固过程进行了模拟研究. 结果表明, 与通常液态及非晶态金属相反, 随着温度降低, 与二十面体相关的1551键型数越来越少, 而与菱面体相关的1311, 1301和1201键型数目显著增加, 最后形成以菱面体结构单元为主的非晶态结构. 以1.69×1012K/s的速度冷却时, 得到非晶态结构; 以1.01×1011K/s的速度冷却时, 发生晶化, 结晶转变温度约为144K.     

焊接熔池快速凝固过程的微观组织演化数值模拟

基于晶粒形成原理和枝晶生长动力学特点,建立了焊接熔池凝固过程中的形核、枝晶生长、溶质再分配及扩散的二维数学物理模型,对焊接熔池快速凝固过程中柱状晶向等轴晶的转变以及不同的冷却速度对这一转变过程的影响进行了模拟.结果表明,焊接熔池在快速冷却凝固过程中,溶质再分配与扩散明显;柱状晶向等轴晶转变时,熔池中心等轴晶凝固排出的溶质使柱状晶尖端浓度急剧升高,抑制了柱状晶的生长;冷却速度越大,柱状晶越容易向等轴晶转变,且转变所需时间越短.     

过共晶Al-Si合金快速凝固组织演变

以群体动力学模型为基础,在充分考虑合金的热物性参数、过饱和度以及第二相形核率变化的基础上,提出描述该合金快速凝固组织演变的数学模型,并结合A390合金进行计算。结果表明:随着雾化合金液滴尺寸的减小,平均冷却速度增加,而初生Si相长大时间减少,所析出初生Si相的尺寸及体积分数也不断减小;当粉末尺寸减小到某一临界值时,初生Si相消失,从而使不同尺寸粉末的微观组织发生很大的变化。计算结果与实验结果吻合较好,说明此提出的模型可以较好地预测过共晶Al-Si合金快速凝固组织的形成,从而为进一步研究高硅合金材料的组织演变奠定基础。     

Hydrogenation reaction of Mg-Based alloys fabricated by rapid solidification

Mg-23.5wt%Ni-xwt%Cu (x=2.5, 5 and 7.5) alloys for hydrogen storage were prepared by melt spinning and crystallization heat treatment. The alloys were ground by a planetary ball mill for 2 h in order to obtain a fine powder. The Mg-23.5Ni-5Cu alloy had crystalline Mg and Mg₂Ni phases. Mg-23.5Ni-5Cu had an effective hydrogen capacity of near 5 wt%. The activated Mg-23.5Ni-5Cu alloy absorbed 4.50 and 4.84 wt%H at 573K under 12 bar H₂ for 10 and 60 min, respectively, and desorbed 3. 21 and 4.81 wt%H at 573K under 1.0 bar H₂ for 10 and 30 min, respectively. The activated Mg-23.5Ni-5Cu alloy showed a quite high hydriding rate like Mg-10Fe₂O₃, and higher dehydriding rates than the activated Mg-xFe₂O_3?yNi. This likely resulted because the melting before melt spinning process has led to the homogeneous distribution of Ni and Cu in the melted Mg, and the Mg-23.5Ni-5Cu alloy has a larger amount of the Mg₂Ni phase than the Mg-xFe₂O_3?yNi alloy.     

Phase field modeling of dendritic coarsening during isothermal solidification

Dendritic coarsening in Al-2mol%Si alloy during isothermal solidification at 880K was investigated by phase field modeling. Three coarsening mechanisms operate in the alloy: (a) melting of small dendrite arms; (b) coalescence of dendrites near the tips leading to the entrapment of liquid droplets; (c) smoothing of dendrites. Dendrite melting is found to be dominant in the stage of dendritic growth, whereas coalescence of dendrites and smoothing of dendrites are dominant during isothermal holding. The simulated results provide a better understanding of dendrite coarsening during isothermal solidification.     

Advances in multi-scale modeling of solidification and casting processes

The development of the aviation, energy and automobile industries requires an advanced integrated product/process R&D systems which could optimize the product and the process design as well. Integrated computational materials engineering (ICME) is a promising approach to fulfill this requirement and make the product and process development efficient, economic, and environmentally friendly. Advances in multi-scale modeling of solidification and casting processes, including mathematical models as well as engineering applications are presented in the paper. Dendrite morphology of magnesium and aluminum alloy of solidification process by using phase field and cellular automaton methods, mathematical models of segregation of large steel ingot, and microstructure models of unidirectionally solidified turbine blade casting are studied and discussed. In addition, some engineering case studies, including microstructure simulation of aluminum casting for automobile industry, segregation of large steel ingot for energy industry, and microstructure simulation of unidirectionally solidified turbine blade castings for aviation industry are discussed.     

双带快冷法制备1050纯铝带材

介绍了双带快冷法制备金属带材新技术的基本原理和特点. 针对1050纯铝带材的成形, 从理论和实验两个方面分析了采用孔径为6.mm的5孔喷嘴时, 熔体射流的稳定性和冷却带线速度对带材质量的影响; 考察了不同冷却带线速度条件下成形带材的组织和力学性能. 发现冷却带线速度越高, 所成形带材的组织越细小、力学性能越好; 在本实验条件下, 稳定制备1050纯铝带材的合理工艺参数是: 浇注温度710.℃, 气体压力0.007.5.MPa, 冷却带线速度2.m/s.     

Microstructure of a two-phase titanium alloy by rapid solidification technique

Ribbons of the two-phase titanium alloy were fabricated by single-roller rapid solidification technique, and aged at high temperature. The microstructure of ribbon samples were characterized with X-ray diffractometer （XRD） and environmental scanning electron microscope （ESEM）. The microstructures of the alloy are composed of a phase and supersaturated β phase, and X-ray diffraction results show that all peaks of the a and β phases shift slightly to smaller angles, which can be explained by the disordering growth pattern caused by the rapid solidification process. After aging at 960℃in vacuum, the ribbon is composed of homogeneous a phase and β phase.     

Complete experimental test of kinetic models for rapid alloy solidification

The interface response functions for rapid solidification of a non-dilute binary alloy were measured in the regime of partial solute trapping, where substantial discrepancies exist among predictions for the interfacial undercooling in various models. We used pulsed laser melting of Si-As on insulating substrates to enforce planar solidification spanning the velocity range 0.2–2 m/s. Nanosecond-resolution electrical measurements of the time-dependent melt depth and of the electrical resistivity of a buried Pt thin film thermometer permitted us to determine the solidification velocity and the temperature of the crystal/melt interface. With composition–depth profile measurements we also determined the nonequilibrium partition coefficient. The measured velocity-dependence of the interface temperature and partition coefficient are quantitatively consistent with the continuous growth model without solute drag of M. J. Aziz and T. Kaplan [Acta Metall. 36, 1335 (1988)] and are qualitatively and quantitatively inconsistent with all models exhibiting a significant solute drag effect. Elements of a potential explanation are proposed using the solute drag model of M. Hillert and B. Sundman [Acta Metall. 24, 731 (1976)] to investigate the origin of the solute drag effect in terms of irreversible processes occurring within a diffuse interface.     

A novel technique of rapid solidification net- form materials synthesis

A novel method of net-form materials synthesis that is related to spray forming has been conceived, and initial verification experiments have been successfully completed. The method uses streams of ultra-uni-form droplets of molten metal that are deposited onto a substrate in a controlled environment. The drop-lets are directed onto the substrate with measured angular dispersions as small as 2 × 10^-6 radians, and speed dispersions as small as 3.5 × 10^-7 times the average speed of the droplet, thereby facilitating the synthesis of small, detailed parts as well as large bulk parts with high resolution and a uniformly fine grain structure.