真空浇注条件下镁合金石膏型压力凝固的研究

研究真空浇注条件下镁合金石膏型压力凝固过程中浇注温度、真空度对镁合金流动性的影响及凝固压力、操作时间与镁合金拉伸性能的关系.结果表明,高真空度和高浇注温度条件下,可使镁合金液在石膏型中获得很好的充型.铸件凝固过程中,凝固压力越高,操作时间愈短,得到的铸件组织越致密,拉伸性能愈高.     

水玻璃的固化机理及其提高耐水性途径分析

伴随环保要求的提升和人们环境意识的提高,水玻璃作为一种环境友好型粘结剂受到广大研究者以及消费者的垂青。但水玻璃因其自身的结构而导致的固化后耐水性差的问题尚未得到彻底的解决,不同的固化途径而产生的固化机理亦是琳琅满目,从而使水玻璃在其应用原理以及解决其相应的缺陷上受到阻碍。笔者对近几年来所研究的水玻璃固化机理以及提高其耐水性途径进行了详细的分析,以促进和拓宽水玻璃的改性研究及应用领域。     

一种基于Ag-Sn等温凝固的圆片键合技术

研究了用Ag-Sn作为键合中间层的圆片健合。相对于成熟的Au-Sn键合系统(典型键合温度是280℃),该系统可以提供更低成本、更高键合后分离(De-Bonding)温度的圆片级键合方案。使用直径为100mm硅片,盖板硅片上溅射多层金属Ti/Ni/Sn/Au,利用Lift-off工艺来形成图形。基板硅片上溅射Ti/Ni/Au/Ag。硅片制备好后,将盖板和基板叠放在一起送入键合机进行键合。键合过程在N2气氛中进行,键合过程中不需要使用助焊剂。研究了不同键合参数,如键合压力、温度等对键合结果的影响。剪切强度测试表明样品的剪切强度平均在55.17MPa。TMA测试表明键合后分离温度可以控制在500℃左右。He泄漏测试证明封接的气密性极好。     

COOLING AND SOLIDIFICATION IN THE PRESENCE OF RECIPROCATING NON-NEWTONIAN SHEAR FLOW: THE LIVE FEED INJECTION MOULDING PROCESS

The cooling and solidification of a non-Newtonian material in the presence of reciprocating flow is analysed. Finite difference solutions are obtained for a transient I-space-dimensional model of a shear thinning material with temperature dependent properties, contained between plane parallel surfaces to which it looses heat by conduction in the presence of periodically reversing flow. This situation is relevant to live feed injection moulding - an advanced technology for thermoplastics processing - in which alternating direction shear is imposed on melt in the mould cavity during the holding and packing stages. Interest centres on the effects of flow on cooling and solidification and the development of frozen-in strains, which have important implications for the control of material alignment and mechanical properties of mouldings. The governing dimensionless parameters are identified and their influence investigated in a series of numerical simulations, where the parameters range over the following values: Pearson number 0.5-5.0; Brinkman number 0.001-1.0; Stefan number 0.1 - 1.0; Power Law Index 0.2-1.0; dimensionless flow reversal time 0.01-1.0. Two operating modes are considered: (1) fixed applied pressure gradient, with a falling flow rate; (2) flow rate initially fixed, followed by a falling flow rate period at a fixed limiting pressure gradient. In mode 1 operation viscous heating does not significantly influence cooling rates, but in mode 2, for high Brinkman numbers, a dynamic equilibrium temperature field can be attained. Complex profiles of frozen-in material strain through the thickness of the moulding are predicted. In mode 1 operation the magnitude of the principle stretch rises to a peak close to the mould walls, and falls in a series of oscillations to unity (zero deformation) on the centre plane. High values of the Pearson number or low values of the Power Law Index damp these oscillations. In mode 2 operation sharply alternating regions of high and low strain are obtained indicating a structure with alternating layers of high and low material orientation.     

Prediction of solidification behaviour and microstructure of Ni based alloys obtained by casting and direct additive laser growth

Numerical tools are now used widely in the prediction of material properties necessary in order to gain a better understanding of the relationship between material properties and performance, to improve the reliability of processes and the quality of the final product, and to reduce costs, waste and energy use. In this paper, the solidification properties and the microstructure of some commercial Ni based alloys were analysed and predicted numerically using the ProCAST software. The microstructure of a sample obtained by the direct additive laser growth, a new additive manufacturing technique based on the selective laser melting, is presented and discussed. Numerical approaches and software packages that can be used to model additive manufacturing processes are discussed and critically analysed.     

A solidification model for prediction of castability in the precipitation-strengthened nickel-based superalloys

The Scheil equation was used to model the solidification path, microsegregation of alloying elements in the interdendritic regions, solidification temperature ranges, and to predict the formation of secondary structures and the castability behavior of as-cast superalloys. 4 experimental alloys with pre-specified γ-Ti,Nb,Al,Mo composition containing different Nb, Ti and Al contents were designed using vacuum induction melting furnace. The produced as-cast superalloys were characterized using optical and scanning electron microscopy equipped with energy dispersive X-ray spectrometer and TG–DSC analysis. The experiments showed logic conformity to the modeling results. The model and experiment confirmed the highest segregation behavior for Ti and Nb. All the experimental superalloys indicated the remarkable tendency to form secondary eutectic structures at the last stages of solidification. Superalloy with chemical composition of γ-3.5%Mo,1.8%Al,4%Ti,2.9%Nb showed the shorter solidification temperature range and the best castability.     

DESCRIPTION OF A THERMO-INELASTIC BODY WITH A MOVING SOLID/LIQUID INTERFACE AND SIMULATION OF THE WELDING PROCESS

Conservation laws of mass, momentum, and energy for the thermo-inelastic body in the region involving the moving interface between the solid and liquid are introduced to derive jump conditions of velocity, stress, and energy on the interface as the sequence of local form for the generalized Reynolds transport theorem. The jump condition of energy is revealed to be the generalized Stefan condition for moving interface problems. The finite element formulation is used to analyze the modes of flow, deformation, and stresses in a melting or solidifying process by employing a viscoplastic constitutive equation that describes the mechanical behavior of both the solid and liquid phases. The mathematical formalism is applied to simulate bead-on plate welding.     

架空原油管道停输期间温降及原油凝固界面推进

由于架空原油管道没有土壤的蓄热来减缓管内原油的热散失,架空原油管道的温降过程往往成为决定整条管道允许停输时间的关键。根据原油温度划分管内原油为纯液油区、凝油区和纯固油区,并假设凝油区以已凝固原油、固体骨架和液态原油为填充相的多孔介质区域,该区域随着温降过程向管心推移。考虑了凝固潜热和空气横掠管道对流换热对原油温降过程的影响,建立了空气、管道与原油相互耦合的传热模型,并进行了数值模拟,数值结果表明停输前期管内原油的温度整体下降较快;在停输中后期,由于凝固潜热的释放,凝油厚度增加使得热阻增大,大大减缓了原油温度的降低;对流换热系数沿管道周向分布不均,导致管内原油温度周向分布不均和凝固界面中心偏离管道中心。     

等轴晶移动条件下铸锭凝固宏观偏析的数值计算方法

本文建立并推导了等轴晶移动条件下合金铸锭凝固的溶质传输守恒方程,并进一步推导了固相移动条件下铸锭凝固宏观偏析的数值计算方法。