A Combined Electromagnetic Levitation Melting,Counter‐Gravity Casting,and Mold Preheating Furnace for Producing TiAl Alloy

In this work, the authors describe the development of TiAl castings over a wide range of approaches. To overcome casting defects and cracks that appear in TiAl castings, a novel furnace is designed and constructed. The design combines induction skull melting, counter‐gravity casting, and mold heating, which facilitates both filling and microstructure formation via a controllable process. This aim is to improve shaping capabilities and microstructural control for TiAl castings. Melting and casting experiments on TiAl alloys with a nominal composition corresponding to Ti–48Al–2Cr–2Nb (at%) are carried out and discussed. X‐ray examinations indicate that the shaping of the TiAl components dose not contain macro casting defects, validating the advantages of this technique. The results are of interest to researchers devoted to technical innovations and modifications for TiAl casting at the industrial scale.

     

Measurements of thermophysical properties of molten silicon by a high-temperature electrostatic levitator

Several thermophysical properties of molten silicon measured by the high-temperature electrostatic levitator at JPL are presented. They are density, constant-pressure specific heat capacity, hemispherical total emissivity, and surface tension. Over the temperature range investigated (1350<T _m<1825 K), the measured liquid density (in g·cm⁻³) can be expressed by a quadratic function,p(T)=p _m−1.69×10⁻⁴(T−T _m)−1.75×10⁻⁷(T−T _m)² withT _m andp _m being 1687 K and 2.56 g·cm⁻³, respectively. The hemispherical total emissivity of molten silicon at the melting temperature was determined to be 0.18, and the constant-pressure specific heat was evaluated as a function of temperature. The surface tension (in 10⁻³ N·m⁻¹) of molten silicon over a similar temperature range can be expressed by σ(T)=875–0.22(T−T _m). Invited paper presented at the Fourth Asian Thermophysical Properties Conference, September 5–8, 1995, Tokyo, Japan.     

Effects of Sample Size on Surface-Tension Measurements of Nickel in Reduced-Gravity Parabolic Flights

Surface tensions of molten metals have been reported in the literature by application of many standard techniques: sessile-drop, maximum bubble pressure, pendant-drop, and capillary-rise methods. Great experimental care must be exercised to ensure the absence of contamination, and containerless techniques based upon the classical theory of oscillations of a liquid drop are being developed for high-precision measurements on reactive alloys. Droplet positioning and heating can be efficiently accomplished by electromagnetic levitation, although additional modes of oscillation can be excited and the fundamental oscillation mode can be shifted to higher frequencies due to asymmetries in droplet shape when experiments are performed in earth-based laboratories. These additional factors associated with 1 g experiments significantly complicate data analysis. An electromagnetic levitator has been developed at Auburn University to test containerless processing methods for characterizing the surface tension of high temperature, reactive melts. Recent oscillating drop experiments with nickel samples utilizing electromagnetic levitation in the low-g environment of NASA's KC-135 research aircraft have shown droplet oscillations in the primary mode and at the fundamental frequency. A series of experiments was performed with droplets covering a range of sizes (i.e., mass), and the largest samples exhibited the largest deviations from Rayleigh's simple theory. The smallest samples exhibited oscillatory behavior consistent with Rayleigh's simple theory. An uncertainty analysis showed that the oscillating-drop technique should provide uncertainties in surface tension of ±0.1 to 2.0percnt; depending upon the uncertainty in the mass of the sample.     

随机载荷作用下输送机永磁支撑装置的自适应特性研究

为了保证低阻力永磁带式输送机的正常稳定运行,对随机载荷作用下输送机悬浮支撑装置的自适应特性进行仿真研究。在合理假设的基础上,建立了悬浮支撑装置的纵向随机振动模型;经过三维实体造型,自适应控制系统建模,外部随机载荷的定义及边界条件的施加等环节,完成跨平台多软件联合仿真环境下的悬浮支撑装置虚拟样机,并进行仿真模拟。仿真结果表明,在随机载荷的作用下,输送带运行状态失稳;随着输送带悬浮气隙的变化,磁悬浮力进一步调整,从而能再次达到系统平衡,与实验结果相符。永磁悬浮带式输送机悬浮支撑装置具有自适应动态调整功能。