Determination of interfacial heat transfer coefficient and its application in high pressure die casting process

In this paper,the research progress of the interfacial heat transfer in high pressure die casting(HPDC)is reviewed.Results including determination of the interfacial heat transfer coefficient(IHTC),influence of casting thickness,process parameters and casting alloys on the IHTC are summarized and discussed.A thermal boundary condition model was developed based on the two correlations:(a)IHTC and casting solid fraction and(b)IHTC peak value and initial die surface temperature.The boundary model was then applied during the determination of the temperature field in HPDC and excellent agreement was found.

从工具理性到价值理性:儿童学习的理念转变

教育受制于社会政治和经济,童年被视为不成熟的人生准备阶段,以及传统读书功用论的影响,是导致儿童学习背离教育精神的重要原因。儿童学习应受价值理性的引领,儿童天性热爱并擅长学习,其学习动机应源自内驱力,其学习需要自主探索。在教育教学实践中,教育者要慎用外部的强化机制,创造有利于儿童学习的情境,让儿童探索自主学习的方式,以此来践行基于价值理性的学习理念。

Feature study of body-fitted Cartesian grids used in casting numerical simulation

A type of mesh called a body-fi tted Cartesian mesh,very different from the traditional structured body-fi tted mesh,is established.At f irst,the right parallelepiped mesh is generated,then,a feature analysis is done on the cross sections.These cross sections are the intersections of the casting shape with the right parallelepiped grids(under the Cartesian coordinate system).On the basis of the feature analysis,two sorts of body-f itted boundary grids,shape-keeping grids and shape-distortion grids,are def ined.Shape-distortion grids can be removed or weaken by increasing the number of grids or moving the coordinates of the mesh generation region,so actually the body-fitted Cartesian mesh generation is to get shape-keeping grids.A shape-keeping grid mainly consists of two sorts of surfaces(I type face and II type face),and each of them is joined by two types of points(I type point and II type point).If only these two types of points were given,the shape-keeping mesh would be constructed.In this paper,the cases of the above two boundary grids being generated were discussed.An algorithm was put forward to get the shape-keeping grids.Several body-fi tted Cartesian meshes generated on castings show the validity of the algorithm.The mesh generation examples show that the body-fi tted Cartesian mesh is more excellent than the right parallelepiped mesh in aspects of decreasing grids number and being closer to the shape of the casting solid.

压铸过程数值模拟技术研究进展

压铸作为一种有色金属近净成形制备工艺,具有生产效率高、产品尺寸精确、表面质量好等特点,被广泛应用于汽车、电子及通讯等领域。然而,由于合金熔体高速高压充填型腔,压铸充型及凝固过程极其复杂。综述了国内外压铸过程数值模拟技术的研究进展,阐述了当前压铸充型及卷气缺陷预测采用的模型与求解算法,按照在压室及压铸型腔两个阶段分析了熔体温度场模拟仿真的影响因素,重点论述了界面传热系数对压铸熔体温度场模拟仿真的影响,对比了不同的缩孔缩松缺陷预测判据,分析了压铸模具热平衡及寿命预测模型与方法。针对大型结构件一体化压铸带来的模拟仿真时网格数量及计算规模庞大问题,阐述了网格划分的方法,并总结了仿真效率提升方法。最后指出了当前压铸过程数值模拟研究存在的不足及发展方向。

压铸镁合金缺陷带的组织特征及形成机理

研究了压铸镁合金缺陷带的组织形貌及分布特征,建立缺陷带与压室预结晶(ESCs)、气缩孔及压铸工艺参数之间的对应关系,在此基础上探讨了缺陷带的形成及演化机理。结果表明,压铸镁合金截面凝固组织以缺陷带为界可划分为3个有明显组织特征差异的区域。随着压铸低速速度提高,镁合金凝固组织中ESCs含量逐渐降低,而缺陷带组织愈加明显并向压铸件中心靠拢,缺陷带宽度减小,其内部孔洞更加聚集;高速速度越大,压铸镁合金凝固组织中ESCs越分散,缺陷带向压铸件中心靠拢,压铸在无高速速度情况下,镁合金凝固组织横截面出现双缺陷带现象。压铸充型过程中金属液流的形态决定了缺陷带的分布位置和发展趋势,在高速金属液的剧烈冲刷及增压压力的作用下,靠近金属液流外轮廓的晶粒发生破碎或转动,在晶粒间形成大于剩余金属液体积的间隙,随着凝固的进行,形成沿液流轮廓分布孔洞聚集的缺陷带组织。