钛铝基合金扁形冷坩埚定向凝固数值计算与组织研究
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摘要
本文运用有限元数值计算与实验相结合的方法,研究了钛铝合金在宽厚比2.5:1电磁冷坩埚条件下的定向凝固。首先建立了空载坩埚三维简化与局部逼真模型,进行电磁特性分析。在此基础上,建立坩埚内钛铝熔体模型,对熔池内电磁场和流动场做了细致的研究。进而,在不同工艺条件下成功制备了钛铝合金扁锭试样,成分为Ti46Al0.5W0.5Si。结合有限元计算结果,借助高清数码相机、金相显微镜和扫描电镜等分析工具,对定向凝固组织进行了宏观和微观的分析研究。
数值计算结果表明:空载坩埚内BBz从底至上变化规律为先增大再减小,并在线圈中部靠上位置出现峰值,靠近开缝处的磁场明显较高且径向磁场很大。电流频率从100kHz变化到10kHz时,坩埚内磁场增加至原来2~3倍,但开缝处变化不明显。钛铝合金熔池内的流动呈交错式的回漩循环状,处于拐角附近的熔体向前上方流动,经过复杂的循环路径后又集中回流,凝固界面存在水平局部强流区,电流140A、50kHz时达到0.15mm/s以上。驼峰顶端出现向上高速流动,这可能会使驼峰失稳。随着电流增大,熔池流动形态基本不变但其流速增大。随着频率的降低,熔池内回漩强流区向熔池内部扩展且范围不断变大。
实验结果显示,试样凝固界面多呈“W”形,两侧的柱状晶组织较为平直,而中部组织呈“V”字形生长。综合计算结果分析表明:熔池内部热场分布不均匀,两边过热度较高而中部不足;凝固界面存在局部热流冲刷,大大降低了柱状晶生长前沿的形核率,使试样两边柱状晶可以持续生长;游离晶随热流漂移,集中分布在熔池中部,成为试样中部组织“V”字生长的原因之一。此外,试样侧面的表面质量明显优于正宽面,这也会对内部组织造成影响。对柱状晶的析出物分析发现,其富钨相和富硅相分别在晶内和晶界以不同的形式析出,富钨相沿晶界网状析出,而富硅相呈团簇状析出。
通过本课题的研究可知,在此坩埚结构确定的情况下,结合数值计算获得的电磁场与流场分布规律,通过实验分析钛铝合金定向凝固过程中的各种因素影响规律,揭示影响机理,合理调整相关工艺参数,可以获得电磁冷坩埚条件下优异的钛铝合金定向凝固组织。
In this paper, based on the combination of finite element numerical calculation and experimental method, studied the directional solidification of the TiAl alloy under the condition of the width and thickness ratio 2.5:1 near-rectangular electromagnetic cold crucible. The electromagnetic characteristics of the empty crucible model was firstly analyzed. On this basis, TiAl melt model was established, the electromagnetic field and flow field distribution of the pool were studied in detail. Furthermore, we prepare TiAl alloy flat ingot successfully under the conditions of different processes with the components of Ti46Al0.5W0.5Si. Combined the results of finite element calculation, macro- and micro-organization of directional solidification have been analyzed with high-definition digital cameras, optical microscopy, scanning electron microscopy and other analytical machines.
Numerical model results show that: from the bottom to the top of empty crucible, the internal axial magnetic field (BBz) increases to a maximum value at the position higher than the middle height of the coil, and then decreases with the increasing of the height, BzB which near slits is significantly higher and radial B is much more. When Current frequency changes from 100kHz to 10kHz, the inside magnetic field increases 2 to 3 times, but department does not change significantly in the slotted. The flow of TiAl alloy in the molten pool appears the swirling circular shape, the melt near the corner of the crucible mobilizes upwardly, then focus on the return flow through complex cycle path. There exists horizontal local strong current area. With the current increased, the mobility patterns will basically constant but its flow rate increases. Local velocity reached 0.15mm/s and above under 140A Current with 50kHz. Strong swirling flowing in pool extended to internal part and the scope enlarged gradually with the reduction of frequency.
After the specimen cut along the axial line of width surface, grinding, polishing and corrosion, it is found that solidification interface mostly present "W" shape, and the grains are is much straight on both sides, the central organization growth with "V" shape. With the combination of computation, the results show: the heat distribution is varying in melt pool, overheating is higher on both sides and lower in midst; local heat flow scouring exist on solidification interface which greatly reduces the nucleation rate in front of columnar crystal, so that columnar crystal is sustainable of growth on both sides of the sample; free crystal drifts with the flux and concentrated in the central pool, becoming one of the reasons of the "V" growth in central organization. In addition, the quality of sample surface in side is better than the front wide side, which will has impact on the internal structures.
It has been found from the precipitation of columnar crystals that the tungsten-rich phases and silicon-rich phases precipitate in grains and grain boundaries in different forms, W-rich phases precipitate reticular along the boundaries while the silicon-rich phases are cluster-like precipitates.
It can be seen from this subject that once the structure of this crucible was defined, through combining the distribution rules of the electromagnetic and flow field which obtained by numerical calculation, analyzing various influent factors through the experimental process of TiAl alloy directional solidification, revealing the influencing mechanism, adjusting process parameters reasonably, ideal TiAl-based alloy directionally solidified structures can be obtained under the conditions of electromagnetic cold crucible.
数值计算结果表明:空载坩埚内BBz从底至上变化规律为先增大再减小,并在线圈中部靠上位置出现峰值,靠近开缝处的磁场明显较高且径向磁场很大。电流频率从100kHz变化到10kHz时,坩埚内磁场增加至原来2~3倍,但开缝处变化不明显。钛铝合金熔池内的流动呈交错式的回漩循环状,处于拐角附近的熔体向前上方流动,经过复杂的循环路径后又集中回流,凝固界面存在水平局部强流区,电流140A、50kHz时达到0.15mm/s以上。驼峰顶端出现向上高速流动,这可能会使驼峰失稳。随着电流增大,熔池流动形态基本不变但其流速增大。随着频率的降低,熔池内回漩强流区向熔池内部扩展且范围不断变大。
实验结果显示,试样凝固界面多呈“W”形,两侧的柱状晶组织较为平直,而中部组织呈“V”字形生长。综合计算结果分析表明:熔池内部热场分布不均匀,两边过热度较高而中部不足;凝固界面存在局部热流冲刷,大大降低了柱状晶生长前沿的形核率,使试样两边柱状晶可以持续生长;游离晶随热流漂移,集中分布在熔池中部,成为试样中部组织“V”字生长的原因之一。此外,试样侧面的表面质量明显优于正宽面,这也会对内部组织造成影响。对柱状晶的析出物分析发现,其富钨相和富硅相分别在晶内和晶界以不同的形式析出,富钨相沿晶界网状析出,而富硅相呈团簇状析出。
通过本课题的研究可知,在此坩埚结构确定的情况下,结合数值计算获得的电磁场与流场分布规律,通过实验分析钛铝合金定向凝固过程中的各种因素影响规律,揭示影响机理,合理调整相关工艺参数,可以获得电磁冷坩埚条件下优异的钛铝合金定向凝固组织。
In this paper, based on the combination of finite element numerical calculation and experimental method, studied the directional solidification of the TiAl alloy under the condition of the width and thickness ratio 2.5:1 near-rectangular electromagnetic cold crucible. The electromagnetic characteristics of the empty crucible model was firstly analyzed. On this basis, TiAl melt model was established, the electromagnetic field and flow field distribution of the pool were studied in detail. Furthermore, we prepare TiAl alloy flat ingot successfully under the conditions of different processes with the components of Ti46Al0.5W0.5Si. Combined the results of finite element calculation, macro- and micro-organization of directional solidification have been analyzed with high-definition digital cameras, optical microscopy, scanning electron microscopy and other analytical machines.
Numerical model results show that: from the bottom to the top of empty crucible, the internal axial magnetic field (BBz) increases to a maximum value at the position higher than the middle height of the coil, and then decreases with the increasing of the height, BzB which near slits is significantly higher and radial B is much more. When Current frequency changes from 100kHz to 10kHz, the inside magnetic field increases 2 to 3 times, but department does not change significantly in the slotted. The flow of TiAl alloy in the molten pool appears the swirling circular shape, the melt near the corner of the crucible mobilizes upwardly, then focus on the return flow through complex cycle path. There exists horizontal local strong current area. With the current increased, the mobility patterns will basically constant but its flow rate increases. Local velocity reached 0.15mm/s and above under 140A Current with 50kHz. Strong swirling flowing in pool extended to internal part and the scope enlarged gradually with the reduction of frequency.
After the specimen cut along the axial line of width surface, grinding, polishing and corrosion, it is found that solidification interface mostly present "W" shape, and the grains are is much straight on both sides, the central organization growth with "V" shape. With the combination of computation, the results show: the heat distribution is varying in melt pool, overheating is higher on both sides and lower in midst; local heat flow scouring exist on solidification interface which greatly reduces the nucleation rate in front of columnar crystal, so that columnar crystal is sustainable of growth on both sides of the sample; free crystal drifts with the flux and concentrated in the central pool, becoming one of the reasons of the "V" growth in central organization. In addition, the quality of sample surface in side is better than the front wide side, which will has impact on the internal structures.
It has been found from the precipitation of columnar crystals that the tungsten-rich phases and silicon-rich phases precipitate in grains and grain boundaries in different forms, W-rich phases precipitate reticular along the boundaries while the silicon-rich phases are cluster-like precipitates.
It can be seen from this subject that once the structure of this crucible was defined, through combining the distribution rules of the electromagnetic and flow field which obtained by numerical calculation, analyzing various influent factors through the experimental process of TiAl alloy directional solidification, revealing the influencing mechanism, adjusting process parameters reasonably, ideal TiAl-based alloy directionally solidified structures can be obtained under the conditions of electromagnetic cold crucible.
引文
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2李成功,傅恒志,于翘.航空航天材料.北京:国防工业出版社,2002,16
3 Loria E A. Gamma titanium aluminides as prospective structural materials[J ]. Intermetallics, 2000 , 8: 1339-1345
4黄伯云.钛铝基金属件化合物.长沙:中南工业大学出版社,1999:288~314
5 Choudhury A, Blum M. Economical production of titanium-aluminide automotive valves using cold wall induction melting and centrifugal casting in a permanent mold. Vacuum, 1996, 47:829~831
6 Chen RR, Ding HS, Bi WS, et al. Electromagnetic cold crucible technology and its application. Rare Metal Materials and Engineering. 2005, 34(4):511~514
7 Makino H, Kuwabara M, Asai S. Process analysis of non-contact continuous casting of materials using cold crucible. ISIJ International. 1996, 36(4):380~387
8 Durand F. The electromagnetic cold crucible as a tool for melt preparation and continuous casting. International Journal Of Cast Metals Research. 2005, 18(2):93~107
9王艳丽.钛基合金近距形冷坩埚连续熔铸与定向凝固工艺基础研究.哈尔滨工业大学博士学位论文. 2007
10 (俄)库兹明诺夫,洛曼诺娃,奥西科.冷坩埚法制取难熔材料.北京:冶金工业出版社, 2006
11 Umbrashko A, Baake E, Nacke B, et al. Experimental investigations and numerical Modeling of the melting process in the cold crucible. Compel-The International Journal For Computation And Mathematics In Electrical And Electronic Engineering. 2005, 24(1):314~323
12 Westphal E, Muhlbauer A, Muiznieks A. Calculation of electromagnetic fields in cylindrical induction systems with slitted metallic walls. Electrical Engineering. 1996, 79(4):251~263
13 Gombert D, Richardson JR. Cold-crucible induction melter design and development. Nuclear Technology. 2003, 141(3):301~308
14 Sugilal G. Experimental study of natural convection in a glass pool inside a coldcrucible induction melter. International Journal Of Thermal Sciences. 2008, 47(7):918~925
15 Shi G, Atkinson HV, Sellars CM, et al. Application of the Generalized Pareto Distribution to the estimation of the size of the maximum inclusion in clean steels. Acta Materialia. 1999, 47(5): 1455~1468
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