發(fā)布時間：2018-05-14 02:04

  本文選題：COMSOL + Multiphysics　； 參考：《哈爾濱工業(yè)大學》2017年碩士論文

【摘要】：TiAl合金以其低密度、高模量和優(yōu)異的抗氧化、高溫強度、抗蠕變和阻燃等性能,被公認是最具發(fā)展?jié)摿Φ妮p質高溫結構材料,擁有廣闊的應用前景。粉末冶金法在消除宏觀偏析、疏松和近凈成形方面具有明顯優(yōu)勢,制備的TiAl合金的組織更加均勻細小,是TiAl合金一種重要的成形方法。但TiAl合金在高溫狀態(tài)下很不穩(wěn)定,化學活性高,會與坩堝及空氣發(fā)生嚴重的化學反應,使合金受到污染,因此通常采用真空感應熔煉和惰性氣體霧化法制備TiAl合金粉末。TiAl合金的熔煉與導流是霧化制粉的重要過程,但是熔煉、導流都是在高溫密閉的環(huán)境中進行,因此通過直接的實驗觀測難以對熔煉和導流規(guī)律進行細致深入的研究。采用數(shù)值模擬方法,可以獲得熔煉和導流過程溫度與流場的動態(tài)變化規(guī)律。本文使用COMSOL Multiphysics數(shù)值模擬軟件對電磁場、溫度場和流場多物理場耦合作用條件下的TiAl合金冷坩堝感應熔煉和導流行為的進行模擬分析獲得了熔煉及導流過程中溫度與流場的變化規(guī)律,這對于高性能TiAl合金粉末的制備具有重要的指導意義。對熔煉過程的模擬結果表明集膚現(xiàn)象很明顯,磁感應強度、感應電流、電磁力都只分布在TiAl合金和銅坩堝表面,且向內(nèi)快速衰減。另外,銅坩堝分瓣內(nèi)感應電流遠大于TiAl合金內(nèi)的感應電流。電磁力的方向指向合金內(nèi)部。坩堝與熔體之間存在接觸熱阻,接觸熱阻由微接觸熱阻和微氣隙熱阻組成。接觸壓力越小,微硬度越大,表面粗糙度越大,表面粗糙平均斜率越小,接觸熱阻越大。當參數(shù)設置為:表面粗糙度4um、表面粗糙平均斜率0.3、接觸壓力25KPa和微硬度300MPa時,微接觸熱阻R_s=1.7257K/W,微氣隙熱阻R_g=0.2703K/W,接觸熱阻R_c=0.2352K/W。TiAl合金熔煉時,坯料頂端外側的溫度先達到熔點,坯料開始熔化并在重力作用下流入到原本坯料和坩堝之間的縫隙中,繼而熔體和坩堝壁接觸被強制冷卻為固體,同時由于電磁力是徑向向內(nèi),電磁力把表層熔體向內(nèi)擠壓到坯料的頂部,隨著金屬熔化的進行,最終形成駝峰,同時熔融區(qū)內(nèi)部存在強烈的湍流。導流過程的模擬結果表明,隨著感應加熱進行,受入口溫度影響,導流管內(nèi)先后出現(xiàn)上下兩個熔融區(qū),堵塞導流管內(nèi)液相線向出口移動,在融合另一個液相區(qū)后到達出口,霧化繼續(xù)進行。導流管內(nèi)熔體穩(wěn)定流動時,整體上電磁場對導流管和熔體的溫度影響不明顯,3000Hz時,出口的最高溫度比無磁場時提高了2.5℃的,熔體內(nèi)外的溫度差從13℃減小到了7.5℃。熔體在導流管內(nèi)的速度從入口到出口之間出現(xiàn)了較大幅度的波動,但是出口的軸向速度大小無明顯影響,并且頻率越大,出口徑向向內(nèi)速度越大,熔體流出時的匯聚效果越好。
[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，

本文編號：1885820