Convection during thermally unstable solidification of Pb-Sn in a magnetic field

Convection and macrosegregation in directionally solidified hypoeutectic Pb-38 wt pct Sn and hypereutectic Pb-64.5 wt pct Sn have been examined during upward and downward growth. Temperature fluctuations are observed along the length of the melt column during downward growth. With increasing Rayleigh number, these fluctuations change from none, to cyclic, to time periodic having multiple harmonics, and finally to random. At the higher convective driving force of 350 K temperature inversion, the transverse magnetic field decreased convective levels, strong random temperature fluctuations (flows) becoming smaller and periodic. The maximum field of 0.45 T was unable to completely eliminate convection. For the lower convective driving force of 150 K temperature inversion, the 0.05 T magnetic field decreased flows, and at 0.15 T, the field caused a dramatic decrease in the characteristic frequency of the temperature fluctuations, indicating a change in the nature of the flow, the waveform of the temperature fluctuations changing from sinusoidal to a pulsed wave. Temperature fluctuations and time delays between thermocouples were used to estimate flow velocities. Irrespective of the convection in the bulk melt (ahead of the mushy zone), longitudinal macrosegregation occurs only if the interdendritic melt mixes with the bulk melt.     

The sublimation of an electrically conducting droplet through the use of an external alternating magnetic field

A nondimensionalized mathematical model is developed for a stationary, electrically conducting, spherical droplet undergoing equilibrium sublimation. Sublimation is caused by the presence of an external, alternating magnetic field generated by a current carrying solenoid. The model incorporates the influence of magnetic field parameters, phase change, and radiative heat transfer. Numerical results from the model are compared with sublimation rates obtained from experimentation.     

Effects of forced electromagnetic vibrations during the solidification of aluminum alloys: Part I. solidification in the presence of crossed alternating electric fields and stationary magnetic fields

A new magnetohydrodynamic method of transmitting forced vibrations to solidifying aluminum alloy melts has been developed. Contrary to the case of the conventional mechanoacoustic systems, this device lends itself very well to a fundamental investigation. The relatively accurate knowledge of both the electromagnetic pressure and the local velocity peaks has enabled us to reveal the specific effects of the oscillatory flow and of the cavitation phenomena on grain refinement. It has been shown that the cavitation threshold depends both on the surface state of the crucible internal walls and on the electromagnetic pressure peak. In the presence of well-developed cavitation situations, a very fine and homogeneous microstructure has been observed throughout the ingot. A laboratory prototype of a new magnetohydrodynamic cavity resonator, allowing for significant energy saving and likely to be used for industrial applications, including the elaboration of metal matrix composites by means of a preform infiltration process, was also the subject of experimentation.     

The mechanisms of formation and prevention of channel segregation during alloy solidification

Conditions for the formation of macroscopic segregation channels have been examined in the ammonium chloride-water and lead-tin systems, using base chilled molds. Such channels develop when the rejected solute is less dense than the solvent and are therefore a result of density inversion, but slow (≺5 rpm) rates of mold rotation, about axes inclined to the vertical by 20 deg to 30 deg, throughout the time of solidification, effectively prevent the formation or propagation of these channels. Artificially created channels or those momentarily blocked fail to continue and are overgrown, but channels can be initiated by drawing liquid upward from close to the growth front in fine capillaries. Examination of these effects leads to the conclusion that channels originate at the growth front, rather than within the dendritic array, and that their formation is necessarily preceded by a liquid perturbation from the less dense boundary layer into the supernatant, quiescent bulk liquid. Intermittent ‘solute fingers’ are then fed by dendritic entrainment to produce stable convective plumes and concomitant channels. It is considered that the effects of mold precession are primarily caused by translation of bulk liquid across the dendritic growth front, shearing off convective perturbations from the boundary layer before they have time to develop. The nature of the liquid movements is discussed and shown to be a function of the mold dimensions. The inclination of the gravitational vector within the solid-liquid, dendritic array is considered to be of secondary importance to the formation or prevention of channels. This paper is based on a presentation made at the symposium “Fluid Flow at Solid-Liquid Interfaces” held at the fall meeting of the TMS-AIME in Philadelphia, PA on October 5, 1983 under the TMS-AIME Solidification Committee.     

定向凝固过程中的流动效应

在类合金(NH₄Cl-H₂O溶液)定向凝固晶体生长实验装置上,利用ϕ30μm煤粉作示踪粒子,再现糊状区内微通道流以及通道出口处的流体流动,并测算了各处流体的瞬时速率.分析认为:凝固初期,糊状区内固相体积分数较大,内部流体流动受阻;随着固相体积分数减少,糊状区孔隙率增大,流体充分发展;当平均固相体积分数降至0.42,接近最小值0.38时,当量雷诺数达到临界值(247),糊状区内形成微通道;随着通道宽度逐渐扩大,液相区内热流体进入微通道.微通道内稀冷液体向上流,浓热液体向下流,促使通道内溶液再结晶.     

高温合金定向凝固过程中通道偏析的物理模拟研究

利用NH4Cl-H2O溶液凝固实验,对高温合金定向凝固中的通道偏析进行物理模拟,研究通道偏析的形成过程以及溶液中NH4Cl的质量分数w(NH4Cl)和初始基板温度对通道偏析数目和形貌的影响。结果表明,除了w(NH4Cl)为15%外,w(NH4Cl)为24%、27%和30%的NH4Cl-H2O溶液凝固时均形成通道偏析。随凝固时间延长,通道偏析数目先逐步增加然后减少,形貌辨识程度增加。随w(NH4Cl)增加,通道偏析数目减少,形貌辨识程度增加。随初始基板温度下降,通道偏析数目减少,形貌辨识程度增加。影响通道偏析的主要因素可归结为影响偏析形成和通道形成的因素,包括合金成分、温度梯度和凝固速率。     

交变磁场作用下特种合金的凝固特性

以高温合金(K403)为实验材料,对中、小尺寸特种合金在交变磁场作用下成形过程的凝固特性进行研究,以期加深对凝固组织细化机理的认识,进而实现对凝固组织的优化控制。在研究过程中首先试验、研究了高温合金无接触和软接触电磁成形的凝固特性,探讨了电磁致流、磁化套以及抽拉速度对凝固组织的影响,进而分析了该特种合金电磁成形凝固组织的细化机理。结果表明:使用磁化套可以获得平直、细化的定向枝晶组织;在该文实验条件下,当抽拉速度为9.8mm/min时,一次枝晶间距能减小到80μm;电磁致流导致二次枝晶臂脱落是无接触电磁成形凝固组织细化的主要机理,而枝晶尖端开裂是软接触电磁成形凝固组织细化的主要机理。     

Effect of the specific features of solidification of an ML10 magnesium alloy on the zirconium segregation during melting

The causes of the formation of shrinkage defects that are often observed in castings of the ML10 alloy have been elucidated. The solidification of the magnesium ML10 alloy is considered using vertical sections of phase diagrams calculated with the Thermo-Calc program. The cause of the zirconium segregation in the alloy is found to be the formation and subsequent sedimentation of not only a zirconium-containing phase (Zr) at the beginning of solidification but also a zirconium-rich liquid L ₂. During melting, this liquid also deposits on the crucible bottom, and the liquid formation is favored by the existence of iron and silicon impurities in the alloy in concentrations permissible by State Standard GOST. Overheating of the metal, which is often used to prevent immiscibility during melting in steel crucibles, enhances immiscibility because of iron dissolution and the formation of the zirconium-rich liquid.     

Mediated short-term influence of alternating magnetic field on the epileptic brain

The changes of bioelectric activity of the brain hemispheres were studied under the influence of an alternating magnetic field (AMF), directed on the hand of 20 epileptic patients and 18 healthy normals. Under AMF influence there was intensification of epileptic activity--elevation of the focal bioelectric characteristics and of the processes of generalization. It permitted to apply AMF for diagnosis of epileptic focus. Under AMF influence more stable changes in posterio-parietal areas of the right hemisphere occur. The authors came to the conclusion that AMF effects were mainly realized with participation of right hemisphere.     

冷速对IN718凝固过程中偏析和Rayleigh数的影响

利用SEM和EDS研究了在冷速分别为1,3和6K·min^-1时高温合金IN718的凝固组织、液体中Nb和Mo含量变化,并计算了液体密度差和相对Rayleigh数与温度的关系.结果表明:由于偏析,液体中Nb、Mo含量和液体密度差随着温度的降低而增加;冷速越低,Nb、Mo偏析越严重,同温度下液体密度差越大.相对Rayleigh数的计算表明,不同冷速下相对Rayleigh数都在1330℃存在最大值,随着冷速降低,其值急剧增加,黑斑形成趋势增大,大铸锭中冷速低于1K·min^-1时会形成黑斑.     

冷却速度对Waspaloy合金凝固过程中偏析和液体密度的影响

通过不同冷速下的重熔凝固实验,利用扫描电镜和能谱仪系统研究了冷速变化对Waspaloy合金凝固过程偏析行为和液体密度的影响.结果表明:随着冷速的增大,Waspaloy合金中元素的偏析会减小,凝固过程中偏析最严重的是富集于枝晶间的正偏析元素Ti;元素偏析导致剩余液体的密度发生变化,而且液体密度基本上呈现反转趋势,当凝固温度到1300℃时,出现最大的密度反转.     

帘线钢凝固过程中钛和氮偏析及TiN夹杂尺寸计算

 为了提高帘线钢盘条中TiN夹杂的检测效率,从热力学方面探讨了帘线钢盘条中TiN夹杂析出的条件以及尺寸的计算。热力学分析表明,由于溶质元素钛和氮偏析,造成TiN夹杂在固液两相区中析出,且氮元素是影响TiN夹杂长大的最主要因素,经过验证发现,TiN夹杂热力学长大公式的尺寸计算值与实际值基本吻合,平均相对误差为8.62%。提出了合适的数学模型对TiN夹杂尺寸与钛和氮质量分数的数据进行模拟,对TiN夹杂的尺寸进行预测并验证模型的合理性。结果表明,模型的平均相对误差仅为0.95%,可以运用到实际检测工作中,提高钛夹杂的检测效率。     

Effect of computational domain size on the mathematical modeling of transport processes and segregation during directional solidification

Using a finite-element simulator, a directionally solidified hypoeutectic Pb-Sn alloy was modeled in two dimensions to determine the effect of the height of the overlying liquid on convective transport and macrosegregation. It was determined that, while the strength of the convection in the overlying liquid depends on the square root of its height, one need not model the entire domain to predict freckling. Furthermore, the assumption of a constant thermal gradient in the liquid causes the predicted convection to be somewhat weaker than the convection in the temperature field used in directional solidification processing.     

纤维增强铝合金复合材料凝固偏析的数值模拟

根据凝固过程中的守恒原理,建立了铝合金复合材料凝固过程的数学模型.根据所建立的模型对Al2O3f/Al 4 5Cu复合材料进行数值模拟,模拟结果表明:Al2O3短纤维周围有低熔点物质及溶质富集;随冷却速度增大,复合材料基体偏析加剧;随复合材料中纤维体积分数增大,基体中偏析减小.     

电流对电渣重熔过程多场耦合行为和凝固参数影响的数值模拟

利用了FLUENT软件建立了电渣重熔(ESR)过程二维轴对称瞬态模型.使用有限体积法求解耦合的电磁场、动量和能量守恒方程,铸锭的凝固过程由焓-多孔介质模型处理,系统地研究了电流对电渣重熔GH984G凝固过程多场耦合行为和凝固参数的影响规律.模拟结果表明:电流由2.3kA增大为2.5kA,体系中电流密度、磁感应强度、洛伦...     

Dynamics of cardiac and skeletal muscle lactate dehydrogenase activity following a single exposure to an alternating magnetic field]

Analysis of records of microscopic slides from 33 patients with pure squamous carcinoma of the bladder who underwent radical cystectomy and lymphadenectomy disclosed a 5-year survival rate of 48 per cent. This survival rate correlated closely with pathologic stage and histologic grade of tumor and was significantly better than for each corresponding stage of transitional cell carcinoma of the bladder. The favorable survival rate and reasonably low operative mortality rate support aggressive therapy of this uncommon lesion.