Microstructure and mechanical properties of rapidly solidified Fe-25%Cr-5%Al ribbons produced by planar flow casting

Catalyst substrate foils of the highly oxidation resistant Fe-25Cr-5Al alloy (mass contents in %) with a thickness ranging from 40 to 180 μm have been produced by planar flow casting. The rapidly solidified ribbons showed a monophase ferritic microstructure of columnar grains. The grain size determined over a section parallel to the ribbon wheel side ranged from 5 μm for the thin ribbons (40 μm) to 18 μm for the thicker ones {180 μm). This anisotropic columnar solidification microstructure exhibits a strong <100> fiber texture, with the fiber axis nearly perpendicular to the plane of the ribbon. The position of the maximum in the pole figure was tilted from the center point toward the casting direction. Results of uniaxial tensile tests showed that only the yield stress data of the ribbons in the as-cast condition are reproducible, whereas the ultimate tensile strength and the elongation to failure data show a wide scatter band. The fracture mode exhibits ductile features such as glide bands and dimples.     

Estimation of droplet solidification temperature in rapid solidification using in-situ measurements

Copper and D2 tool steel powders were produced using a drop tube-impulse atomization technique. In order to measure the radiant energy and droplet size of atomised D2 steel droplets, DPV-2000 (Tecnar Automation Ltée, St. Hubert Quebec, Canada) was utilised. In-situ velocity and droplet size of the atomised droplets were also measured using shadowgraphy technique (Sizing Master Shadow from LaVision GmbH in Gottingen, Germany). A 3D translation stage was designed, constructed and installed inside the drop tube system. DPV-2000 and shadowgraph were then mounted on the translation stage. The Cu droplets were primarily used to calibrate to particle size and velocity measurements between both instruments. Using this stage, online measurements were conducted at 4?cm, 18?cm and 28?cm distances for D2 droplets below the crucible. Using liquid (fully undercooled) and semi-solid behaviour of droplets, it was possible to estimate the droplet size and temperature at which recalescence ends. These values were then confirmed by the thermal model using experimentally estimated primary phase undercooling values.     

Sintering behaviour of Al-6061 powder produced by rapid solidification process

Abstract

Complex aluminium alloy components fabricated by powder metallurgy (P/M) offer the promise of a low cost and high strength-to-weight ratio, which meets the demands of the automotive sector. This paper describes the die compaction and sintering response of an atomised Al-6061 alloy powder containing Mg and Si produced by rapid solidification. A design of experiments is used involving three levels for each of the die compaction pressure, sintering temperature, peak temperature hold time and heating rate. Three trials were used to obtain the optimum press sinter processing conditions. Besides the mechanical properties, phase transformation and microstructure are investigated. Supplemental insight is gained through thermogravimetric analysis, differential scanning calorimetry and SEM with energy dispersive spectroscopy. Analysis of variation is used to quantify the contribution of each design variable to the mechanical properties.     

Modelling of crystal growth during the ribbon formation in planar flow casting

The crystalline solidification during rapid substrate quenching in planar-flow casting was simulated by using a numerical model based on a rapid solidification algorithm and the infinite viscosity approximation. The calculation shows that the existence of a real melt puddle shape suppresses undercooling and recalescence on the melt surface as well as on the solidifying ribbon. The melt puddle length is mainly determined by the heat-transfer coefficient. With increasing heat transfer across the melt – substrate interface the melt puddle length decreases. If the formation rate of critical nuclei on the substrate surface is low compared to the present cooling rate a large undercooling may occur. The performed calculations reveal that an undercooling of up to 600 K does neither affect the temperature distribution on the surface of the melt nor the melt puddle length, perceptibly. Therefore, investigations on microstructural features of rapidly quenched metals might give detailed information on the amount of undercooling present at the beginning of solidification.     

Morphological stability of a planar interface under rapid solidification conditions

The linear perturbation theory of Mullins and Sekerka for the stability of a planar interface is extended to the case of large thermal peclet numbers. It is shown that an absolute stability criterion for a planar interface exists for undercooled melts also. In light of these results, the conventional constitutional supercooling criterion is reexamined and a more general planar interface stability criterion is proposed which is valid for low as well as high growth rate conditions. The results of this stability analysis are applied to dendritic growth from pure undercooled melt.     

Heterogeneous nucleation during rapid solidification by laser surface melting

Crystal nucleation during rapid solidification generally occurs rather slowly except at active heterogeneities or unless very large undercoolings are achieved. Many typical microstructures are then essentially of columnar morphology, for example from the bottom surface of a melt-spun ribbon or from a heterigeneity on the surface of a powder particle. In such cases the microstructure may be considerably refined by the presence of many active nucleants for heterogeneous nucleation distributed throughout the melt. Such microstructural refinement is analysed here during rapid solidification of a laser-melted surface containing fine TiB₂ particles. Simulation of the cooling and solidification conditions confirms these particles to be highly effective nucleating substrates, capable of greatly increasing nucleating rates. As a result it is possible to obtain materials possessing greatly refined grain sizes.     

双辊薄带连铸啮合点前流动与凝固特征分析

应用连铸专用软件Calcosoft模拟了双辊薄带连铸过程的流场和温度场,着重分析了啮合点钢液流动的特点.结合凝固组织研究,研究了啮合点前的钢液流动对凝固组织的影响.并且结合了啮合点处钢液流动的利用,认为将啮合点钢液控制在糊状区是最佳工艺条件.     

Modeling of crystal growth during rapid solidification

Models for computer simulation of solidification of alloys in two different geometries have been developed. These models allow us to study the roles of crystal nucleation and heat transfer and the effects of different modes of growth on the evolution of the microstructure during solidification. In particular, by applying the powerful and versatile tool comprised of the solute-drag model and the thermodynamic modeling of the alloy system, we are able to predict the formation of a microstructure consisting of alternating layers parallel to the growth front. This type of banded microstructure, experimentally observed in Al-Cu, Al-Fe, and Ag-Cu alloys, has not been satisfactorily explained before.     

Theory of microstructural development during rapid solidification

A theoretical model is presented for describing columnar (directional) growth of dendrites including growth rates in the range of the limit of absolute stability. A maximum growth rate for dendrites is predicted which is slightly below the limit of absolute stability of a planar interface. The changes in microstructures which may occur at high growth rates are discussed. Both the effect of the temperature dependant diffusion coefficient and the velocity and temperature dependant partition coefficient on microstructure characteristics are considered. The model is applied to the Ag-Cu system where detailed microstructural analysis has been published. The theoretical results are found to be in reasonable agreement with experimental data obtained from the literature.     

Simulation of the temperature distribution on the mold surface and inside casting during high-gradient directional solidification

In order to determine the temperature gradients in ingots fabricated by the directional solidification (DS) using an UVNS-6 installation produced by VIAM (Moscow), single-crystalline ingots made of VZhM3 nickel superalloy have been prepared. Herewith, DS technologies with liquid-metal cooling (LMC) and without it are used (the Bridgman–Stockbarger method). A tin melt is used as the LMC. Readings of thermocouples installed on the surface of a ceramic mold are recorded during ingot DS. The DS of ingots made of VZhM3 nickel superalloy using the UVNS-6 installation is simulated in the ProCast program using thermal properties of the alloy, ceramic mold, and parts of the DS installation found in articles, as well as boundary conditions between them (interface heat-transfer coefficients). The good coincidence of the calculated and experimental temperature distributions in the mold during solidification using the Bridgman–Stockbarger and the LMC technique is shown, which makes it possible to use simulation of the ingot fabrication in the ProCast program to predict the temperature gradient at the solidification front, the solidification front profile, and the size of the mushy zone (where the dendritic alloy structure is formed). The temperature gradient attained in the ingot in the case of using the Bridgman–Stockbarger method by the results of simulation was 36°C/cm. The temperature gradient in the case of applying the LMC technique is 204°C/cm; i.e., it turned out sixfold higher than that attained when using the Bridgman–Stockbarger technique. Thermal properties and boundary conditions can be demanded when performing computer simulating of nickel superalloys blade casting process.     

连铸流动与凝固耦合模拟中糊状区系数的表征及影响

分析提出了连铸流动与凝固耦合数值模拟中, 钢液在两相区流动时的糊状区系数(A_mush)与渗透率的关系; 通过建立大方坯连铸结晶器三维耦合数值模型, 揭示了不同糊状区系数对钢液流动、传热与凝固进程的影响, 以及早期相关研究结果差异的源头.结果表明: 糊状区系数越大, 钢液在糊状区内的流动阻力越强, 凝固时钢液流动速度降低越快.采用较大的糊状区系数时, 糊状区呈较窄的"带状"分布在固液相之间; 当糊状区系数较小时, 糊状区范围变大, 钢液在结晶器内温降过快, 自由液面处出现过冷现象, 凝固坯壳局部发生重熔.结合实验数据验证与模型分析, 认为糊状区系数取值1×108~5×108 kg·m-3·s-1可以较可靠地揭示连铸结晶器内的实际凝固现象.      

Fracture of precipitation-hardened copper alloy produced by semi-continuous casting at elevated temperature

In semi-continuously cast slabs of precipitation-hardened copper alloy, internal cracks are often observed after homogenization, although those defects can not be found out after casting. It is considered that, before stress relief takes place during homogenization, thermal stresses generated in the slab during casting will exceed the fracture stress. It is of interest to understand how internal stresses of slabs will affect the fracture during homogenization. Fracture temperatures of the material under static tensile stresses are investigated in an attempt to demonstrate the fracture evolved during homogenization.     

高拉速下凝固进程的试验测量与模型预测

 根据京唐高拉速试验特点,结合板坯连铸机设备和洁净钢连铸工艺特点和要求,建立连铸板坯凝固传热模型并结合射钉法测量综合预测了1.9～2.4 m/min高拉速条件下铸坯的凝固坯壳厚度和凝固终点位置。综合研究表明,结合射钉试验和数值模拟能更精确跟踪铸坯的凝固进程,为高拉速试验提供准确的凝固信息,并能为评价连铸机综合冷却能力、优化二冷制度和轻压下工艺提供合理的参考信息。     

An evaluation of the creep properties of two Al-Si alloys produced by rapid solidification processing

Tensile creep tests were conducted on two Al-Si alloys produced by rapid solidification: an Al-Si-Ni-Cr alloy and an Al-Si-Cu-Fe alloy, designated alloys A and B, respectively. The creep curves of these two alloys in the temperature range from 493 to 573 K were markedly different, with alloy A exhibiting a normal creep curve with a very short tertiary region and alloy B exhibiting an extended tertiary stage associated with strain localization. The minimum creep rates varied, with the applied stress raised to exponents of ∼9.0 and ∼8.5 for the two alloys, respectively. The hardness of alloy B decreased with time during the creep testing, but there was little or no change in the hardness of alloy A. These differences in the creep and hardness characteristics are attributed to the evolution of precipitates within the two alloys during creep testing. A detailed analysis shows that, over the temperature range examined experimentally, alloy A exhibits a creep strength that is superior to conventional Al-based alloys and comparable to, or even higher than, some SiC-reinforced Al composites.     

Large undercooling,rapid solidification,and nucleation mechanism during multistage atomisation

Abstract

The large undercooling and rapid quenching that can occur during multistage atomisation are studied both theoretically and experimentally. Results show that these two effects are closely related to and promoted by each other. The level of undercooling for droplets is dependent on alloy composition, powder particle size, and atomisation condition. The cooling rates of droplets depend heavily on their particle size. Both experiments and theoretical analysis on Sn-Pb alloys reveal that a certain function of undercooling has a linear relationship with the logarithm of the mean powder particle particle size. Based on the present results, a mechanism of nucleation preferred on surface oxide is proposed to give a quantitative interpretation of the experimental observations.     

氩气雾化高温合金粉末的凝固组织特征

为深入了解高温合金粉末粒度细化对粉末涡轮盘性能的影响,对不同尺寸氩气雾化高温合金粉末的凝固组织特点进行了分析和研究.结果表明:AA粉表面和内部凝固组织主要为树枝晶和胞状晶组织,晶界和枝晶间分布少量富Ti、Nb的MC型碳化物.随着粉末尺寸的减小,冷却速率增大,组织从树枝晶向胞状晶转变,基体γ相晶面间距和点阵常数增加.小尺...     

Mathematical modelling of fluid flow,heat transfer and solidification phenomena in continuous casting of steel

A steady state, two-dimensional mathematical model for continuous billet casting operations has been developed. Towards this, governing equations of fluid flow and heat transfer together with their appropriate set of boundary conditions were derived and solved numerically via a control volume based implicit finite difference procedure (e.g., SIMPLE). The effect of various assumptions and procedures applied to modelling of turbulence phenomena, thermal buoyancy, flow through the mushy zone, free surface conditions etc., on the sensitivity of the computed results was investigated computationally. Of all these, modelling of heat and fluid flow phenomena in the mushy region was found to have relatively more effect on the predicted results. In addition to these, a set of three different billet casting operations reported in literature were simulated mathematically and direct comparisons were made between predicted and observed solidified shell profiles. Such comparisons demonstrated reasonable to excellent agreement between theory and experiments.     

Quantification of microsegregation during rapid solidification of Al-Cu powders

A new technique is introduced to quantify microsegregation during rapid solidification. The quantification involves calculation of the average solute solubility in the primary phase during solidification of an Al-Cu binary alloy. The calculation is based on using volume percent eutectic and weight percent of second phase (in the eutectic), which were obtained experimentally. Neutron diffraction experiments and stereology calculation on scanning electron microscope images were done on impulse atomized Al-Cu alloys of three compositions (nominal), 5 wt pct Cu, 10 wt pct Cu, and 17 wt pct Cu, atomized under N₂ and He gas. Neutron diffraction experiments yielded weight percent CuAl₂ data and stereology yielded volume percent eutectic data. These two data were first used to determine the weight percent eutectic. Using the weight percent eutectic and weight percent CuAl₂ in mass and volume balance equations, the average solute solubility in the primary phase could be calculated. The experimental results of the amount of eutectic, tomography results from previous work, and results from the calculations suggest that the atomized droplets are in metastable state during the nucleation undercooling of the primary phase, and the effect of metastability propagates through to the eutectic formation stage. The metastable effect is more pronounced in alloys with higher solute composition.     

连铸初始凝固区域传热研究进展

连铸初始凝固阶段的传热决定了初始凝固坯壳的形成,讨论了初始凝固坯壳的两大形成机理,探讨了初始凝固阶段的传热行为,分析了连铸中各种因素对初始凝固阶段传热的影响,进一步解释了电磁连铸技术对初始凝固传热的改善.