铸件自动剖分过程中点与三角形关系分类方法的确定

在对比了多种判断点与多边形关系的方法之后，提出了在铸件凝固数值模拟的自动剖分中，用面积法对点与三角形的关系进行分类是较为理想的，这种方法与其它方法相比有速度快和编程简单的特点．     

Numerical investigation on the heat transfer characteristics of a liquid-metal pool subjected to a partial solidification process

The main purpose of this paper is to investigate numerically the effects of solidification on the heat transfer characteristics of the liquid metal layer, for use in accident analyses. The situation is very similar to an overlying liquid melt pool that could be fooned in the reactor lower head during the late phase of a severe nuclear accident. Based on a computational model, MPCOOL, the numerical predictions were then assessed through a comparison with the experimental data that was obtained with various boundary temperature conditions and geometrical aspect ratios, especially for the Ra-Nu relationship. For the cases with solidification, the results of the comparison show that(a) the computational model does show a good agreement with heat transter rates inferred from the experimental data, with a few exceptions at the Ra numbers which suggest a turbulent transport; and also (b) the computational model underpredicts the heat transfer rates by about 6% than that inferred from the experimental data when it is integrally evaluated with the Ra-Nu correlation. The foregoing results are mainly due to the currently limited applicability of the computational model up to the laminar-to-turbulence transition flows and its application to the turbulence flows because it is always subjected to a model uncertainty between the laminar and turbulence. Next, an additional comparison for the cases with and without solidification was made to examine the effects of the solidification on the energy partition within the liquid metal layer and its effects on the directional heat transfer rates. The results of the comparison show that the computational model for the case without solidification predicts higher heat transfer rates by about 15% than when solidification is included, but there isn't any experimental data that directly supports this trend.     

Corrosion of Al—Fe in hydrochloric acid

Abstract

Nanoprocessing can be considered a distinct form of grain boundary engineering by which property enhancements are achieved by deliberately increasing the volume fractions of grain boundaries and triple junctions in a material. Electrodeposition has been shown to be a technologically viable production method to synthesize such materials both in bulk form and as thin films. The mechanical, magnetic, electrical and corrosion properties exhibited by nanocrystalline metals produced by this method make them strong contenders for a number of advanced materials applications.

Résumé

Nous pouvons considérer le nanotraitement comme une forme distincte de génie des joints de grains par lequel nous obtenons des améliorations de propriétés en augmentant délibérément le volume des fractions des joints de grains et des jonctions triples dans les matériaux. On a montré que l'électrodéposition est une méthode technologiquement viable pour synthétiser de tels matériaux, aussi bien en films minces que sous forme massive. Les propriétés mécaniques, magnétiques, électriques et corrosives des métaux nanocristallins obtenus par cette méthode, font d'eux des concurrents sérieux pour nombreuses applications de matériaux avancés.     

Investigation of crystallisation kinetics of Bi0·5Se99·5-xZnx glasses by differential scanning calorimetry

Abstract

The kinetics of crystallisation in Bi_0·5Se_99·5-xZn_x (x=0, 0·1, 0·2, 0·5, 1·0) glasses are studied by non-isothermal method using differential scanning calorimetry. Differential thermal analysis was performed at different heating rates of 5, 10, 15, and 20 K min^-1. The values of glass transition and crystallisation temperatures are found to be composition and heating rate dependent. From the heating rate dependence of the glass transition and crystallisation temperatures, the activation energy of crystallisation ΔE _c, the order parameter n and the enthalpy released ΔH _c are calculated. The thermal stability (T _c–T _g) was found to be maximum for Bi_0·5Se_99·5 glass, which suggest that this glass can be considered as a critical composition at which the system becomes chemically ordered. The crystallisation enthalpy ΔH _c is maximum for Bi_0·5Se₉₉Zn_0·5 glass, hence it is the least stable glass in the present system.     

Solution of the inverse problem in solidification of binary alloy by applying the ACO algorithm

The paper presents a solution of the inverse problem consisting in reconstruction of the heat flux and the distribution of temperature in the process of binary alloy solidification when the temperature measurements in the selected points of the alloy are known. The considered task is mathematically modelled by means of the heat conduction equation with the substitute thermal capacity and with the liquidus and solidus temperatures varying in dependence on the concentration of the alloy component, whereas for describing the concentration the lever arm model is applied. An important part of the procedure consists in minimization of some functional executed with the aid of ACO algorithm.     

Effect of Solidifcation Rate on Grain Structure Evolution During Directional Solidifcation of a Ni-based Superalloy

The effect of solidification rate on grain structure evolution during directional solidification(DS) of a Ni-based superalloy was explored.It was found that a high solidification rate led to sharper <001> texture and smaller grain size in the DS samples.One of the most important findings in this work was that such result was not in accordance with the general concept,and the sharper <001> texture was accompanied by the larger grain size.To explain the contradiction,the modeling samples with five grains were produced and the effect of solidification rate on the evolution of grain texture was illustrated based on the modeling samples.     

Computation of feed-paths for casting solidification using level-set-method

The aim of the present work is to compute feed-paths and hot-spots by combining level-set-method based sharp interface and feed-path model. The model is based on the solution of energy and level-set equations in solid and liquid, with Stefan condition on the interface. The energy and level-set equation are discretized using finite-volume and finite-difference method, respectively. Feed-path is computed by tracking mass-less particles along the liquid-solid interface during solidification using combined Eulerian-Lagrangian framework. The proposed model is benchmarked on six test cases, where temperature contours and solidification time are compared with a finite-element-method based commercial software. The capability to predict the temporal evolution of interface and to identify multiple hot-spots is validated with an industrial aluminum-alloy lug casting. The numerical as well as experimental validations demonstrate the effectiveness of level-set-method for feed-path calculation.     

Solidification crack initiation and propagation in pulsed laser welding of wrought heat treatable aluminium alloy

The effect of preheating on strain rate and liquid flowrate is analysed to develop a model for solidification cracking in successive laser weld spots in 2024 aluminium alloy. It is shown that the lower tendency for solidification cracking with preheating can be attributed to the lower strain rate but not the feeding rate. It is also shown there is a strong tendency for solidification crack propagation through the fusion lines between the consecutive weld spots where there is already a crack present at the fusion line. This observation can be attributed to the higher local strain rate and lower feeding rate.     

Modeling and characterization of grain structures and defects in solidification

The paper by Karma and Tourret (this volume) in this special issue focuses on multiscale modeling approaches ranging from atoms to microstructure. In the present one, the most recent and significant modeling contributions dealing with the scale of solidification from microstructure to grain structure are briefly reviewed. The paper also covers modeling of defect formation during the last stage of solidification, namely porosity and hot tearing. As will be shown, the field of solidification has taken advantage of several simulation and experimental tools which have become increasingly powerful and accessible over the past decade. The emphasis will be put on complex 2D and 3D models for which correlations with in situ observations using synchrotron radiation and/or combined orientation and metallography imaging have been made.