基于有限元法的定向凝固过程温度场数值模拟的研究

发布时间：2019-03-29 06:32

【摘要】：在航空航天领域中,主要采用定向凝固工艺生产涡轮叶片,而高性能的涡轮叶片一直是阻碍我国实现“大飞机”梦的最大障碍。实际定向凝固过程是十分复杂的材料成形过程,而且需要特别考虑辐射换热的影响。采用定向凝固温度场模拟技术,能够对铸件在凝固过程中的温度场演变过程进行有效的分析,预测可能出现的缺陷,为优化定向凝固工艺提供理论指导。本文对有限元定向凝固温度场数值模拟所涉及的各个方面进行了深入研究,包括定向凝固温度场数学建模、有限元数值求解、液态金属冷却液换热处理和定向凝固温度场分析判据等,开发了有限元定向凝固温度场数值模拟系统。首先,由于辐射换热在定向凝固温度场数值模拟中需要重点考虑,采用射线追踪法对其进行处理,并结合计算机图形学得到辐射换热边界条件的控制方程。通过对定向凝固过程进行一系列相对合理的假设和简化,建立了定向凝固过程的温度场模型,根据有限元数值计算理论,结合定向凝固过程温度场的控制方程和边界条件,详细地推导出定向凝固过程温度场数值模拟的有限元离散过程和求解方法。其次,采用等效比热法和温度校正方法共同处理铸件潜热,使其满足能量守恒原则。针对定向凝固工艺的随型型壳边界,采用智能化查找型壳内外表面的算法,自动区分各材质的内外表面,避免用户手动选择的繁琐操作,并采用盒子树法处理各个接触表面的对流换热边界条件,能够在不过多要求网格质量的基础上,较为合理地处理各材质间的对流换热边界条件。由于LMC(Liquid Metal Cooling)工艺中型壳会逐渐浸入液态金属冷却液,为了避免直接求解所带来的网格重新划分难题,采用随时间和温度变化的等效换热系数来处理型壳与冷却液间的换热。实际定向凝固过程中需要避免等轴晶即杂晶的出现,为了对HRS(High Rate Solidification)和LMC定向凝固温度场模拟结果进行分析,采用G/L判据来预测铸件可能出现杂晶的部位。同时,为了保证有限元模拟系统的计算效率,提出局部矩阵的概念,在有限元程序处理过程中分开组装各材质的计算矩阵。通过实现上述各关键技术,开发了有限元HRS和LMC定向凝固温度场数值模拟系统。最后,分别采用温度场数值模拟系统和通用化有限元平台ANSYS计算典型工字件的空冷过程温度场,对比发现两者的计算结果基本一致,验证了本文温度场数值模拟系统中有限元算法的准确性。采用温度场数值模拟系统计算一组熔模铸造工艺的温度场,其中初始方案由于阀盖件中部散热条件差,模拟结果预测其中部会出现缩孔缩松缺陷,通过实际生产得以验证。改进工艺之后,加快了阀盖件中部的降温速率,消除了孔松缺陷,实际也生产出合格的阀盖件,验证了温度场数值模拟系统的实用性。采用定向凝固温度场数值模拟系统对带冠涡轮叶片分别进行HRS和LMC工艺模拟,并通过设置不同的抽拉速度进行多方案分析,模拟结果与实际过程相吻合,证明了本文的有限元定向凝固温度场数值模拟系统的可靠性,能够为实际定向凝固生产提供科学指导。
[Abstract]:In the field of aeronautics and astronautics, a directional solidification process is used to produce turbine blades, while the high-performance turbine blades have been the biggest obstacle to the realization of the "a large plane" 's dream in our country. The actual directional solidification process is a very complicated material forming process, and it is necessary to take special consideration of the influence of radiation heat transfer. By adopting the directional solidification temperature field simulation technology, the temperature field evolution process in the solidification process of the casting can be effectively analyzed, the possible defects can be predicted, and the theoretical guidance is provided for optimizing the directional solidification process. In this paper, the numerical simulation of the finite element directional solidification temperature field is deeply studied, including the mathematical modeling of the directional solidification temperature field, the finite element numerical solution, the liquid metal cooling liquid heat treatment and the directional solidification temperature field analysis criterion, etc. The numerical simulation system of the finite element directional solidification temperature field is developed. First, because the radiation heat transfer needs to be taken into consideration in the numerical simulation of the directional solidification temperature field, the ray tracing method is adopted to deal with it, and the control equation of the radiation heat transfer boundary condition is obtained by computer graphics. The temperature field model of the directional solidification process is established by a series of relative reasonable assumptions and simplification to the directional solidification process, and the control equation and the boundary condition of the temperature field of the directional solidification process are combined according to the finite element numerical calculation theory. The finite element discrete process and the method for solving the numerical simulation of the temperature field in the directional solidification process are derived in detail. Secondly, the latent heat of the casting is treated by using the equivalent specific method and the temperature correction method so as to satisfy the energy conservation principle. aiming at the along-the-type shell boundary of the directional solidification process, an intelligent searching type shell inner and outer surface algorithm is adopted, the inner and outer surfaces of each material are automatically distinguished, the tedious operation of the manual selection of the user is avoided, and the convection heat exchange boundary conditions of each contact surface are treated by adopting a box tree method, And the convection heat exchange boundary conditions between the materials can be more reasonably processed on the basis of more demanding the grid quality. As the medium shell of the LMC (Liquid Metal Cooling) process is gradually immersed in the liquid metal cooling liquid, the heat exchange between the shell and the cooling liquid is treated by the equivalent heat exchange coefficient which is changed with time and temperature in order to avoid the problem of re-partitioning the grid brought by the direct solution. In order to analyze the simulation results of HRS (High Rate Solid) and LMC directional solidification temperature field, the G/ L criterion is used to predict the potential of the casting. At the same time, in order to ensure the calculation efficiency of the finite element simulation system, the concept of the local matrix is put forward, and the calculation matrix of each material is assembled separately during the process of the finite element program. The numerical simulation system of the directional solidification temperature field of the finite element HRS and the LMC was developed by the key techniques. In the end, the temperature field of the air-cooling process of the typical I-piece is calculated by using the temperature field numerical simulation system and the general-purpose finite element platform ANSYS, and the results of the comparison are basically the same, and the accuracy of the finite element algorithm in the numerical simulation system of the temperature field is verified. The temperature field of a set of investment casting process is calculated by the temperature field numerical simulation system, and the initial scheme is due to the poor heat dissipation condition of the middle part of the valve cover. After the improvement process, the cooling rate of the middle part of the valve cover is accelerated, the defect of the hole is eliminated, the qualified valve cover is actually produced, and the practicability of the temperature field numerical simulation system is verified. By adopting the directional solidification temperature field numerical simulation system, the HRS and the LMC process simulation are respectively carried out on the crown turbine blades, and the multi-scheme analysis is carried out by setting different drawing speeds, and the simulation results are consistent with the actual process, The reliability of the numerical simulation system of the finite element directional solidification temperature field is proved, and the scientific guidance can be provided for the actual directional solidification production.
【学位授予单位】：华中科技大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：V261.31

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