【作者】 姜俊侠；

【导师】 陈立亮； 吴志超；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2008， 博士

【摘要】 干砂负压消失模铸造技术(简称LFC)是新一代的精确铸造成型技术,被誉为“代表21世纪的铸造新技术”。泡沫模样是消失模铸造成败关键,而泡沫珠粒的充填是获得优质泡沫模样的关键工序之一。泡沫珠粒在模具中充填不均或不密实会使模样出现残缺不全或融合不充分等缺陷,影响产品的表面质量。利用计算机对泡沫珠粒射料充填过程的流动场进行数值模拟,定量地模拟射料过程的影响因素,有利于优化射料工艺、提高效率和减少废品,具有重要的理论及应用价值。本文基于分子动力学模拟,建立模拟离散颗粒流动的离散单元模型(DEM)。采用硬球模型处理高浓度颗粒相中离散颗粒间的碰撞过程,并提出搜索颗粒碰撞事件的优化技术,以减少计算机计算颗粒间碰撞事件的工作量,为数值模拟奠定了坚实的理论基础。开发了基于DEM硬球模型的消失模射料充填过程数值模拟通用程序,预报模具型腔内泡沫珠粒的运动规律及特性。采用可视化实验研究三维模具型腔内泡沫珠粒充填规律,将可视化实验的结果与DEM模拟结果对比,验证模型物理概念的合理性和计算方法的正确性,考察泡沫珠粒的受力情况,进而讨论充填运动机理。模拟结果中的泡沫珠粒的流动趋势与实验结果基本上是一致的,证明使用该数学模型对射料充填过程进行数值模拟是一种行之有效的方法。利用现有商业化软件Arena-flow模拟的结果与自行开发的仿真系统模拟的结果进行对比验证,发现在两把料枪射料充填情况下,两种模拟结果非常接近,更进一步证实本研究具有较好的可信度。泡沫模样的质量除与发泡模具、发泡成型工艺有关外,射料工艺也是影响模样质量的关键因素。射料是成型的基础,不同结构的模具必须选用合适的射料工艺。射料工艺不合理会导致模具型腔内珠粒不紧实,使泡沫模样上产生收缩和变形的缺陷。因此用数值模拟方法代替传统试错法的实验手段优化射料工艺可大大提高效率,节约成本。影响射料充填的主要因素有:射料口的位置、排气塞数量和位置、射料压力、原始珠粒的选择等。本文根据DEM模型对不同工艺条件下的射料充填过程进行数值模拟,为实际生产中工艺参数的确定提供可靠依据。发动机缸体作为发动机中最重要的部件之一,其尺寸较大、结构复杂、壁厚较薄又很不均匀(最薄处仅为3～5mm),同时其工艺范围狭窄,影响因素众多,是典型难以成型的复杂铸件。消失模铸造的特点决定了它比较适合于缸体铸件生产,其工艺为分片制取泡沫模样,模样粘合成箱体零件再进行铸造。合理的分片是获得优质铸件的关键。本研究采用水平分型与竖直分型相结合工艺,既保证了曲轴箱的原始结构设计不变又减轻了零部件的重量。分片模具制取泡沫模样过程中,泡沫珠粒在模具薄壁区中易出现充填不满、不均或不紧实会使模样出现残缺不全或融合不充分等缺陷,进而影响产品的表面质量。本文运用影响射料充填的因素及工艺参数优化的模拟分析结果,对实际生产中的工艺方案进行改进,优化后的工艺方案在复杂模具薄壁区和模具的边角处,特别是有肋板,或加强筋的地方,充填效果良好,在实际生产中得到了合格的泡沫模样,大大减少了传统工艺的试模次数。对消失模铸造中模样的生产具有重要的指导意义。更多还原

【Abstract】 Lost foam casting, the new generation of precision casting process, is called to be the representation of advanced casting technology of 21th century. In lost foam casting (LFC) ,the foam pattern is the key qualification, and the beads filling process is quite crucial to ensure the high quality of the foam pattern. Filling without uniformity and denseness will cause various defects and affect the quality of the surface of the casting. The influential factors of beads filling process are realized in this research. And optimizing beads filling process, enhancing efficiency, decreasing waster and so on are obtained by the numerical simulation of beads filling process using computer. All of these are profoundly valuable for the theory and practical application.Based on molecular dynamics simulation, the Discrete Element Method (DEM) is established to simulate the flow of discrete particles. In the high dense particle phase, the collision processes of discrete particles are simulated by rigid sphere model which uses the optimized technology to reduce computer workload on the occurrences of searching collision. An universal program of simulating LFC beads filling process based on rigid sphere model is developed which can forecast movement law and character of foam beads in die cavity.By comparing the result of visual experiments with simulation results, the rationality of such physics concept and correctness of computational method can be tested, and further more, the force distribution on beads and movement mechanism of filling process will be analyzed. Since the movement flow of simulations is quite in accord with that of experiments, such numerical simulation then can be taken as a kind of effective method to analyze the filling process. What’s more, using the existing software Arena-flow to simulate the process, and on the condition of two inlet to fill cavity, we can find that the simulation result of Arena-flow is also close to that of the method in this paper, which further ensures the correctness of DEM.In fact, the factors which affect the quality of foam patterns include molds, process control of expanding beads. Besides, bead filling process is also a crucial factor for the quality of patterns, for that the filling process is the basis of molding, and different molds should match respective filling process. Therefore, adopting numerical simulation method rather than conventional repeated experiment to optimize the filling process can improve efficiency and reduce cost greatly. Factors which may influence the filling process include the position of beads inlets, the amount and position of outlets, the filling pressure, choosing beads etc. In this paper, different filling processes are tested by numerical simulation adopting DEM, the results and analysis provide evidences of deciding the technological parameters in practical production.Cylinder with the complicated structure, large dimension, and uneven thickness of wall (the thinest thickness can be 3～5mm) is one of the most important part of an engine, and then the most difficult casting product. Thus, the best casting process to product engine cylinder should be LFC which uses several foam patterns glued to form a cylinder pattern for casting. How to rationally parting the pattern is then to be the key of gaining high quality products.In this paper, vertical parting and horizontal parting are combined to make sure that the structure of cylinder will not be deformed and the weight of the casting can also be reduced. When producing the foam pattern, some defeats such as inadequate filling, insufficient comapction of beads are apt to occur in the thin-wall part, which will surely affect the surface quality of casting products. By analyzing the factors of bead filling and optimizing technological parameters, the practical production is improved, and good filling process is acquired, especially in the thin-wall part and some corners of the mold. The improved process can gain qualified patterns and greatly reduce times of test, thus has a profound meaning on foam pattern production in LFC.更多还原