压铸过程压室及铸型界面传热的研究

发布时间：2019-06-15 01:03

【摘要】：压铸作为一种先进的金属成形方法,具有生产效率高、铸件尺寸精度好、力学性能优良、易于成形薄壁复杂零件等优点,广泛地应用于汽车、航空航天、通信电子等领域。压铸过程中液态金属与压室及铸型的界面换热决定了铸件凝固的初始状态和凝固方式,是影响铸件质量的重要因素之一。因此,研究整个压铸过程中的传热条件,确定液态金属与压室及铸型之间的界面换热系数,建立精确有效的界面传热条件,对于优化压铸工艺、预测和控制铸件质量、避免铸造缺陷的产生以及模拟仿真技术在压铸行业的发展具有非常重要的意义。论文采用实验和数值模拟相结合的方法,对压铸过程液态金属与压室及铸型之间的界面传热问题进行了系统的研究。研究了采用热传导反算法确定界面换热系数这类反问题的测温难点,设计了用于研究压铸界面换热的测温方案,包括通用测温单元、专用测温压室以及实际压铸模具,系统进行了压铸测温实验,精确地获得了不同工艺条件下压室及铸型内部的温度数据。深入研究了热传导反问题的求解技术和凝固过程的界面换热机制,耦合液态金属在压室及铸型中的温度场求解,建立了考虑液态金属在压室流动的界面传热二维反算模型,优化了未来时间步长,分析了反算模型的稳定性条件,确定了压铸过程反算参数选择的可接受域,开发了压铸全过程界面传热反求程序。基于界面传热反算模型及程序,系统求解了液态金属在压室中的温度场及其不同位置的界面换热系数,结果表明静态无压射条件与常规压铸条件下液态金属与压室界面换热情况存在较大差异。同时,压室中部界面换热系数均随液态金属流动方向依次降低,压室壁温度也存在两端高中间低的分布。在常规压铸条件下,由于冲头运动的影响,压室末端换热系数存在双峰现象。分析了填充率、合金、低速、高速和增压等工艺参数的影响,预测了压室预结晶组织(ESCs)的形核及在铸件中分布。研究了液态金属与铸型界面换热系数的变化规律分析了充型过程及工艺参数的影响,建立了金属与铸型界面换热边界模型,并用于实际压铸件温度场的求解和热平衡分析,验证了反算模型的合理性。
[Abstract]:Die casting, as an advanced metal forming method, has many advantages, such as high production efficiency, good dimensional accuracy, excellent mechanical properties, easy to form thin-wall complex parts and so on. It is widely used in automobile, aerospace, communication electronics and other fields. The interface heat transfer between liquid metal and die chamber and mold determines the initial state and solidification mode of casting solidification, which is one of the important factors affecting the quality of castings. Therefore, it is of great significance to study the heat transfer conditions in the whole die casting process, to determine the interfacial heat transfer coefficient between liquid metal and die chamber and mold, and to establish accurate and effective interface heat transfer conditions for optimizing die casting process, predicting and controlling casting quality, avoiding casting defects and the development of simulation technology in die casting industry. In this paper, the interfacial heat transfer between liquid metal and die chamber and mold in die casting process is studied systematically by means of experiment and numerical simulation. The temperature measurement difficulty of determining interface heat transfer coefficient by heat conduction inverse algorithm is studied. The temperature measurement scheme used to study interface heat transfer in die casting is designed, including general temperature measurement unit, special temperature measuring pressure chamber and actual die casting die. The temperature measurement experiment of die casting is carried out, and the temperature data of die chamber and mold under different technological conditions are obtained accurately. The solving technology of inverse heat conduction problem and the interface heat transfer mechanism of solidification process are deeply studied, and the temperature field of liquid metal in pressure chamber and mold is solved. A two-dimensional inverse calculation model of interface heat transfer considering liquid metal flow in pressure chamber is established, the future time step is optimized, the stability condition of reverse calculation model is analyzed, and the receiving region of reverse calculation parameter selection in die casting process is determined. The reverse heat transfer program at the interface of die casting process is developed. Based on the inverse calculation model and program of interface heat transfer, the temperature field of liquid metal in pressure chamber and its interfacial heat transfer coefficient at different positions are solved systematically. the results show that the interface heat transfer between liquid metal and pressure chamber under static non-injection condition is quite different from that under conventional die casting condition. At the same time, the heat transfer coefficient of the middle interface of the pressure chamber decreases with the flow direction of liquid metal, and the temperature of the pressure chamber wall also has the distribution of high and low temperature at both ends. Under the condition of conventional die casting, due to the influence of punch movement, there is a bimodal phenomenon in the heat transfer coefficient at the end of the pressure chamber. The effects of filling rate, alloy, low speed, high speed and supercharging on the process parameters were analyzed, and the nucleation and distribution of (ESCs) in castings were predicted. The variation of heat transfer coefficient between liquid metal and mold was studied. The influence of filling process and process parameters was analyzed. The boundary model of heat transfer between metal and mold was established and applied to the solution of temperature field and thermal balance analysis of actual die castings, and the rationality of the reverse calculation model was verified.
【学位授予单位】：清华大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG249.2

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