Quantitative interpretation of parabolic oxidation behaviour of nitride powders

Abstract

The oxidation behaviour of nitride powder under the condition of diffusion controlled has been investigated from both experimental and theoretical aspects. The effects of factors, such as oxidation time t, temperature T, oxygen partial pressure Po₂, temperature increasing rate η and particle size R ₀, on the reacted fraction of oxidation have been studied, in which many factors have never been studied quantitatively before. The applications of the authors' model to both experimental data of AlN, BN and β-SiAlON powder provided by the authors' lab as well as the data of SiAlON and MgAlON powders offered by literature show that this model works well. Finally, after the calculation error comparison, it is clear to show that the authors' new model would lead to a more accurate calculation result against the model used in literatures currently.     

Effect of solidification velocity on weld solidification process of alloy tool steel

Abstract

In order to clarify the effect of solidification velocity on the weld solidification process of alloy tool steel during the welding, the information about microstructure evolution was obtained by the concurrent experiments of liquid tin quenching and time resolved X-ray diffraction technique using intense synchrotron radiation. It was found from the experiments that the solidification mode was transferred from an FA to an A mode at the high solidification velocity. The effect of solidification velocity on the phase selection during solidification between the primary δ-ferrite and γ-austenite was theoretically proved by the Kurz, Giovanola and Trivedi (KGT) model. It is thus explained that the solidification cracking susceptibility of the weld metal of alloy tool steel was enhanced due to the δ to γ transition of the primary phase.     

The effect of residual strains on the progressive damage modelling of environmentally degraded adhesive joints

This work is concerned with developing numerical modelling techniques for predicting the environmental degradation of adhesively-bonded joints. Associated experimental data are also reported. The moisture-dependent mechanical properties of the adhesive were obtained by testing bulk specimens also exposed to various moisture contents. The diffusion parameters for moisture in the adhesive were determined by carrying out gravimetric experiments on bulk adhesive samples. The moisture-dependent interfacial bond strength of the adhesive system investigated has been determined by testing a mixed mode flexure (MMF) specimen, exposed to obtain various levels of moisture content at the interface. Progressive damage in the joints was modelled with a two-parameter cohesive zone model (CZM). The CZM parameters were determined by correlating the experimental data obtained from the MMF test with results from the numerical simulation. The parameters were then used to predict the response of another configuration, the notched coating adhesion (NCA) specimen. When the residual stresses were neglected in the modelling, the predicted NCA response was seen to be in good agreement with the experimental data. However, initial simulations that included the residual stresses resulted in poor predictions of the NCA response. Creep tests on the saturated adhesive, at the ageing temperature, showed large viscoplastic deformations at low loads. Coupled diffusion-stress modelling, including viscoplastic material properties for the adhesive continuum, showed that the residual stresses for the aged specimens decreased significantly and thus did not contribute strongly to the environmental weakening. Good predictions were then obtained for the NCA tests.     

Application of diffusion bonding model to superplastic γ-TiAl alloys

Abstract

In the present work, the superplastic characteristics of some γ-TiAl alloys were analysed on the bases of experimental data reported in previous work, and their suitability for diffusion bonding was examined. The constitutive equation ε = A DGb/kT (b/d)^p (σ?σ ₀ /G)ⁿ can be used to describe superplasticity in γ-TiAl alloys, and the equation predicts n=2, p=2, and activity energy Q=220 kJ mol^?1. It is suggested that the diffusion bonding model for γ-TiAl alloys should be considered two stages, i.e. a plastic deformation stage and a void shrinkage stage including both a diffusion controlled process and a plastic controlled process. Theoretical predictions for obtaining good bonding conditions were in good agreement with experimental results. Using a theoretical bonding model, a prediction map showing the relationship among temperature, applied stress and time for high quality diffusion bonding of superplastic γ-TiAl alloys was obtained.     

Understanding the low temperature end of the hot ductility trough in steels

Abstract

The low temperature end of the hot ductility trough has been examined for steels which have been solution treated at ～1300°C before tensile testing in the temperature range of 1000–600°C. Failure in the trough in this region is intergranular ductile and occurs by strain intensification in the thin film of ferrite surrounding the prior austenite grain. The strain causes voiding to occur at the inclusions situated at the boundaries, the cavities gradually linking up to give failure. In steels which are solution treated before tensile testing, the depth of the trough is shown to be controlled by the volume fraction of the second phase particles, their size and the separation between the particles. Recovery in ductility on the low temperature side of the trough is solely dependent on being able to produce a sufficiently large quantity of ferrite to prevent strain concentration (～40%). Often this has to await the test temperature falling below the AR ₃ in which case wide troughs are formed. However, if conditions are right, very narrow troughs can be produced in which the ferrite that is formed is deformation induced. The width of the trough at the low temperature end of the trough is shown to decrease with increase in strain rate, refinement of the austenite grain size, increase in cooling rate from the solution treatment temperature, decrease in the volume fraction of sulphides situated at the austenite grain boundaries and reduction in the Mn and C contents. The depth of the trough decreases in a similar manner with all these variables except for C and Mn, where for the former there is no effect and for the latter, increasing the Mn level reduces the depth. Narrow troughs on this side of the trough are dependent on being able to form deformation induced ferrite in sufficiently large amounts so as to improve the ductility at temperatures above the AR ₃. A model is proposed to account for most of these observations.     

Effects of latent heat of fusion on thermal processes in laser welding of aluminium alloys

Abstract

Based on the Green's function method, a mathematical model allowing for the latent heat of fusion and solidification is developed to describe the steady state, two-dimensional heat flow during welding of thin plates. It is demonstrated that the latent heat has a pronounced effect on shape and size of the weld pool and mushy zone. The thermal efficiency of base metal fusion by a line heat source η _t can exceed 0·4839 considerably if the latent heat is taken into account. It is shown that the known simplified approaches for considering the latent heat can introduce large errors into the estimation of η _t. The calculated and experimental weld pool shapes are compared.     

Numerical simulation and experimental validation of slag entrainment during tundish draining

Abstract

During continuous steel casting slag entrainment from the tundish into the mould has to be avoided. Since slag entrainment most likely occurs at low tundish levels, this paper focuses on the behaviour of the steel phase and the covering liquid slag during tundish draining. The process of slag entrainment is modelled both numerically by a volume of fluid (VOF) approach and physically by corresponding water-oil experiments. Thereby, the influence of a centric stopper and additional inserts on the critical suction level is studied. With help of the numerical model these results are subsequently extrapolated to real plant applications with steel and slag as liquid phases. The numerical simulations have been proven to reasonably depict the onset of slag entrainment during tundish draining and their results agree with corresponding experiments.     

Mathematical model for instant interface equilibrium temperature at time τ=O+ of solid additive–melt bath system

Abstract

The instant interface equilibrium temperature at time τ=0⁺ in close form is obtained from a mathematical model which has been developed for a solid additive–melt bath system. It is a function of the Stefan number S _t and the property ratio B as well as the initial temperature of the solid additive θ _i. For B→∞ 0 ≤ S _t ≤ ∞, or S _t→0, 0 ≤ B ≤ ∞, the instant interface equilibrium temperature becomes the freezing temperature of the bath material, whereas it takes the temperature θ _i of the additive before its immersion in the bath once B→0 for 0 ≤ S _t ≤ ∞. In the case of S _t→∞, 0 ≤ B ≤ ∞, it becomes θ _e = [(24B)^1/2 + 3θ _i]/[(24B)^1/2 + 3].     

Methodology to improve applicability of welding simulation

Abstract

The objective of this paper is to demonstrate a new simulation technique which allows fast and automatic generation of temperature fields as input for subsequent thermomechanical welding simulation. The basic idea is to decompose the process model into an empirical part based on neural networks and a phenomenological part that describes the physical phenomena. The strength of this composite modelling approach is the automatic calibration of mathematical models against experimental data without the need for manual interference by an experienced user. As an example for typical applications in laser beam and GMA–laser hybrid welding, it is shown that even 3D heat conduction models of a low complexity can approximate measured temperature fields with a sufficient accuracy. In general, any derivation of model fitting parameters from the real process adds uncertainties to the simulation independent of the complexity of the underlying phenomenological model. The modelling technique presented hybridises empirical and phenomenological models. It reduces the model uncertainties by exploiting additional information which keeps normally hidden in the data measured when the model calibration is performed against few experimental data sets. In contrast, here the optimal model parameter set corresponding to a given process parameter is computed by means of an empirical submodel based on relatively large set of experimental data. The approach allows making a contribution to an efficient compensation of modelling inaccuracies and lack of knowledge about thermophysical material properties or boundary conditions. Two illustrating examples are provided.     

TINA Network Resource Information Model

The TINA (Telecommunications InformationNetworking Architecture) Network Resource InformationModel (NRIM) is a technology-independent network levelresource model that provides the foundation for the management and control of networks envisionedin the TINA architecture. Such networks are capable ofsupporting multimedia and multipoint communicationsessions and may consist of network elements based on different technologies. Different portionsof such networks will be under the control of differentnetwork administrations. NRIM describes such a networkabstractly in terms of network elements, aggregations of network elements, the topologicalrelationship between the elements, transport entities,and endpoints of transport entities. In addition, NRIMdefines management support objects for alarmsurveillance, resource configuration, and accounting. NRIM isindependent of the architecture of the individualmanagement functions. Further, the model is describedabstractly in terms of objects and relationships and it is independent of the protocols used forcommunication between management functions. Informationmodels for the management of specific networktechnologies can be derived from the generic modeldefined by the NRIM. This paper presents an overview ofthe NRIM emphasizing the recent advances made in theNRIM specification.