TiAl合金熔炼及导流过程的多场耦合作用规律研究

发布时间：2018-05-14 02:04

  本文选题：COMSOL + Multiphysics　； 参考：《哈尔滨工业大学》2017年硕士论文

【摘要】：TiAl合金以其低密度、高模量和优异的抗氧化、高温强度、抗蠕变和阻燃等性能,被公认是最具发展潜力的轻质高温结构材料,拥有广阔的应用前景。粉末冶金法在消除宏观偏析、疏松和近净成形方面具有明显优势,制备的TiAl合金的组织更加均匀细小,是TiAl合金一种重要的成形方法。但TiAl合金在高温状态下很不稳定,化学活性高,会与坩埚及空气发生严重的化学反应,使合金受到污染,因此通常采用真空感应熔炼和惰性气体雾化法制备TiAl合金粉末。TiAl合金的熔炼与导流是雾化制粉的重要过程,但是熔炼、导流都是在高温密闭的环境中进行,因此通过直接的实验观测难以对熔炼和导流规律进行细致深入的研究。采用数值模拟方法,可以获得熔炼和导流过程温度与流场的动态变化规律。本文使用COMSOL Multiphysics数值模拟软件对电磁场、温度场和流场多物理场耦合作用条件下的TiAl合金冷坩埚感应熔炼和导流行为的进行模拟分析获得了熔炼及导流过程中温度与流场的变化规律,这对于高性能TiAl合金粉末的制备具有重要的指导意义。对熔炼过程的模拟结果表明集肤现象很明显,磁感应强度、感应电流、电磁力都只分布在TiAl合金和铜坩埚表面,且向内快速衰减。另外,铜坩埚分瓣内感应电流远大于TiAl合金内的感应电流。电磁力的方向指向合金内部。坩埚与熔体之间存在接触热阻,接触热阻由微接触热阻和微气隙热阻组成。接触压力越小,微硬度越大,表面粗糙度越大,表面粗糙平均斜率越小,接触热阻越大。当参数设置为:表面粗糙度4um、表面粗糙平均斜率0.3、接触压力25KPa和微硬度300MPa时,微接触热阻R_s=1.7257K/W,微气隙热阻R_g=0.2703K/W,接触热阻R_c=0.2352K/W。TiAl合金熔炼时,坯料顶端外侧的温度先达到熔点,坯料开始熔化并在重力作用下流入到原本坯料和坩埚之间的缝隙中,继而熔体和坩埚壁接触被强制冷却为固体,同时由于电磁力是径向向内,电磁力把表层熔体向内挤压到坯料的顶部,随着金属熔化的进行,最终形成驼峰,同时熔融区内部存在强烈的湍流。导流过程的模拟结果表明,随着感应加热进行,受入口温度影响,导流管内先后出现上下两个熔融区,堵塞导流管内液相线向出口移动,在融合另一个液相区后到达出口,雾化继续进行。导流管内熔体稳定流动时,整体上电磁场对导流管和熔体的温度影响不明显,3000Hz时,出口的最高温度比无磁场时提高了2.5℃的,熔体内外的温度差从13℃减小到了7.5℃。熔体在导流管内的速度从入口到出口之间出现了较大幅度的波动,但是出口的轴向速度大小无明显影响,并且频率越大,出口径向向内速度越大,熔体流出时的汇聚效果越好。
[Abstract]:TiAl alloys have been recognized as the most promising lightweight high-temperature structural materials for their low density, high modulus and excellent properties of oxidation resistance, high temperature strength, creep resistance and flame retardancy. Powder metallurgy has obvious advantages in eliminating macro segregation, porosity and near net forming, and the microstructure of the prepared TiAl alloy is more uniform and finer, which is an important forming method for TiAl alloy. However, the TiAl alloy is unstable at high temperature and has high chemical activity, which will result in serious chemical reaction with crucible and air, resulting in contamination of the alloy. Therefore, the melting and flow conduction of TiAl alloy powder. Tial alloy prepared by vacuum induction melting and inert gas atomization is an important process of powder atomization, but the melting and diversion are carried out in a high temperature airtight environment. Therefore, it is difficult to study the melting and conducting laws through direct experimental observation. The dynamic variation of temperature and flow field in smelting and conducting processes can be obtained by numerical simulation. In this paper, COMSOL Multiphysics numerical simulation software is used to simulate the electromagnetic field. Under the coupling of temperature field and flow field, the variation of temperature and flow field in the process of melting and conducting in cold crucible of TiAl alloy is obtained by simulating the induction melting and conducting behavior of TiAl alloy. This is of great significance for the preparation of high performance TiAl alloy powder. The simulation results of melting process show that the skin collecting phenomenon is very obvious. The magnetic induction intensity, inductive current and electromagnetic force are distributed only on the surface of TiAl alloy and copper crucible, and decay rapidly inward. In addition, the inductive current in copper crucible is much larger than that in TiAl alloy. The direction of the electromagnetic force is directed to the inner of the alloy. There is contact thermal resistance between crucible and melt, which consists of micro contact thermal resistance and micro air gap thermal resistance. The smaller the contact pressure, the greater the microhardness, the smaller the average slope of surface roughness, and the greater the contact thermal resistance. When the parameters are as follows: surface roughness 4um, average slope of surface roughness 0.3, contact pressure 25KPa and microhardness 300MPa, micro-contact thermal resistance RSP 1.7257K / W, micro-air gap thermal resistance RGG 0.2703K/ W, contact thermal resistance R_c=0.2352K/W.TiAl alloy melting, the temperature outside the top of the billet reaches the melting point first. The billet begins to melt and flow under gravity into the gap between the billet and the crucible, and then the contact between the melt and the crucible wall is forced to cool to a solid, and because the electromagnetic force is radial inward, The surface melt is squeezed inwards to the top of the billet by electromagnetic force. With the melting of metal the hump is formed and there is strong turbulence in the melting zone. The simulation results of the diversion process show that, with the induction heating and the influence of the inlet temperature, the upper and lower melting zones appear successively in the diversion pipe, and the liquid phase line in the diversion pipe moves towards the outlet, and the liquid phase line moves to the outlet after merging the other liquid phase region. The atomization continued. When the melt flows steadily in the diversion tube, the temperature difference inside and outside the melt decreases from 13 鈩，

本文编号：1885820