压铸充型过程卷气及压室预结晶组织的数值模拟研究

发布时间：2018-03-11 00:29

本文选题：压力铸造　切入点：表面张力　出处：《清华大学》2016年博士论文　论文类型：学位论文

【摘要】：卷气气孔和压室预结晶粗大组织是压铸工艺中最常见的两种缺陷,而这些缺陷的形成及后续变化又都与压铸的充型过程相关。因此借助计算机数值模拟技术研究压铸充型过程,预测卷气及压室预结晶组织分布,对优化压铸工艺设计、提高压铸件的质量具有重要的意义。基于Youngs界面假设开发了三维空间自由表面重构算法;基于算子不分裂原则开发了流体运输算法;基于切网格剖分系统开发了对应的控制方程求解算法和后处理程序;分别验证了上述算法的可靠性和优越性。通过求解各类表面的法向量,考虑表面张力,建立了压铸充型过程的数值模拟模型,并利用有限差分法离散求解控制方程;搭建了水力学模拟实验平台,进行两套模具的水模拟实验;将实验结果与模拟结果进行定性与定量的对比,验证了模型的准确性以及考虑张力后的模拟精度变化。基于卷入气泡在充型过程中的演变过程,开发了气泡搜索、气泡破碎、气泡移动的算法,建立了压铸卷气缺陷预测模型;分别比较宏观气泡和微观气泡的模拟结果与实验结果,验证了模型的预测能力。基于压铸充型模型和颗粒轨道模型建立了压室预结晶组织流动分布预测模型;对压室预结晶颗粒进行了受力分析,并利用实验参数设置了模型的初始条件;对比了压室、铸件整体、铸件剖面上预结晶组织分布的模拟结果和金相结果,验证了模型的预测能力。对拉伸棒展开多参数模拟实验,观察了拉伸棒各方向截面上预结晶组织的移动趋势,统计了预结晶颗粒的无量纲平均距离及其周期性波动,总结了预结晶组织的主要分布规律,进而提出了分布规律对缺陷带形成影响的假说,并用不同参数条件下的实验结果进行了验证。综合运用各类模型,对实际的压铸阶梯件、支架件、碰撞盒件进行了充型、卷气分布、压室预结晶分布的模拟,将模拟结果与X射线检测结果、工业CT断层扫描结果、金相照片统计结果对比,验证了模型程序能应用于实际压铸工艺,并通过改进压铸件设计来体现模拟对压铸工艺优化的指导作用。
[Abstract]:The two most common defects in the die casting process are the air volume porosity and the coarse pre-crystallization structure of the pressure chamber, and the formation and subsequent changes of these defects are related to the filling process of the die casting, so the mold filling process of the die casting is studied by computer numerical simulation technology. It is important to predict the distribution of precrystalline structure in the volume gas and pressure chamber for optimizing the design of die casting process and improving the quality of die casting. Based on the assumption of Youngs interface, a 3D free surface reconstruction algorithm is developed. Based on the principle of operator non-splitting, the fluid transportation algorithm is developed, and the corresponding control equation solving algorithm and post-processing program are developed based on the tangent mesh generation system. The reliability and superiority of the above algorithms are verified respectively. By solving the normal vectors of various surfaces and considering the surface tension, the numerical simulation model of the die casting filling process is established, and the governing equations are discretized by the finite difference method. The hydraulic simulation experiment platform was built to carry out the water simulation experiment of two sets of molds, and the qualitative and quantitative comparison between the experimental results and the simulation results was carried out. The accuracy of the model and the change of simulation accuracy after considering the tension are verified. Based on the evolution of the involved bubble in the filling process, the algorithms of bubble searching, bubble breaking and bubble moving are developed, and the prediction model of the gas defect in the die casting coil is established. The prediction ability of the model is verified by comparing the simulation results of macro and micro bubbles with the experimental results. Based on the die casting mold model and the particle orbit model, the prediction model of the flow distribution of precrystalline microstructure in the pressure chamber is established. The stress of precrystalline particles in the pressure chamber is analyzed, and the initial conditions of the model are set up by using the experimental parameters, and the simulation results and metallographic results of the distribution of precrystalline microstructure in the die chamber, the whole casting and the section of the casting are compared. The prediction ability of the model was verified. The multi-parameter simulation experiment was carried out on the tensile rod. The moving trend of the precrystalline structure on each cross section of the tensile rod was observed, and the dimensionless average distance and its periodic fluctuation of the precrystalline particles were calculated. The main distribution laws of precrystalline structure are summarized, and the hypothesis of the influence of distribution law on the formation of defect band is put forward, and the experimental results under different parameters are verified. The supporting parts, impact box parts were simulated with filling, roll air distribution and pressure-chamber pre-crystallization distribution. The simulation results were compared with the results of X-ray examination, the results of industrial CT tomography, and the statistical results of metallographic photographs. It is verified that the model program can be applied to the actual die casting process, and the simulation can be used to guide the die casting process optimization by improving the die casting design.
【学位授予单位】：清华大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG249.2

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