Didier Bouvard

Study of particle rearrangement during powder compaction by the Discrete Element Method

This paper presents simulations of cold isostatic and closed die compaction of powders based on the Discrete Element Method. Due to the particulate nature of powders, densification of the compact proceeds both through the plastic deformation at the particle contact and the mutual rearrangement of particles. The relative weight of each mechanism on the macroscopic deformation process depends on the contact law, the relative density, and the type of stress exerted on the particles (shear or pressure). 3D computer simulations have been carried out to investigate the role of these parameters on the deformation mechanisms of powder compacts. The effect of rearrangement is studied by comparing simulations that use a homogeneous strain field solution for which local rearrangement is omitted and simulations that include local rearrangement. It is shown that local rearrangement has some effect on average quantities such as the average coordination number, the average contact area and the macroscopic stress. The effect on averaged quantities is much stronger for closed die compaction than for isostatic compaction. However the main effect of local rearrangement is to widen the distribution of the parameters that define the contact (contact area in particular). The results of these simulations are compared to available experimental data and to statistical models that use a homogeneous strain field assumption.

Study of the cold compaction of composite powders by the discrete element method

We present numerical simulations of the cold isostatic and close die compaction of mixtures of soft and hard powders. The simulations use the discrete element method with periodic boundary conditions on packing of 4000 spherical monosize particles. The two mechanisms that are generally put forward to explain the retarding effect of hard particles on the compaction have been analysed. First, we have studied in which condition the hard particles form a cluster that hinders the homogeneous distribution of the load. Friction between particles is shown to affect significantly the formation of such a cluster. Second, the additional deformation that soft particles must undergo in the presence of hard non-deformable particles has been evaluated. Related to this last issue, the importance of including hardening effects in the constitutive equation of the soft phase is demonstrated.

Viscosity of WC–Co compacts during sintering

Abstract This paper describes experiments performed on WC–Co compacts in order to measure the viscosities of a Newtonian constitutive law commonly used to simulate sintering. An intermittent loading method is used during two series of experiments. The first series are dedicated to determining the axial viscosity and takes place in a loading-dilatometer. The second one takes advantage of a video-extensometry device and provides results about the viscous Poissons ratio. The axial viscosity is obtained as a function of relative density and temperature. Viscosity shows strong exponential increase with increasing density during isothermal conditions but decreases from 10 to 1 GPa·s between 1100°C and 1325°C during a conventional sintering cycle. Viscous Poissons ratio shows low values at low densities and increases to 0.5 at full density.

Densification behaviour of mixtures of hard and soft powders under pressure

The densification behaviour of mixtures of soft and hard powders during pressing is investigated. Several experimental studies using materials with miscellaneous mechanical and morphological features are summarised. The influence of mixture characteristics on the densification kinetics is discussed and several mechanisms explaining the influence of hard particles on the densification behaviour are proposed. Two main cases are distinguished. When the applied pressure is low, with regard to the yield stress of soft particles, the densification is mainly due to particle rearrangement and is favoured by a high fraction of hard particles. When soft particle deformation is the main densification mechanism, hard particles hinder the densification, with more or less significance depending on whether they are mostly isolated, are grouped in aggregates or form a percolating network.

Near net shape processing of a sintered alumina component: adjustment of pressing parameters through finite element simulation

Near net shape forming of alumina powder by cold die pressing and pressureless sintering was investigated. From experimental data of triaxial compression test of alumina powder, a hyperbolic cap model with a critical state line was proposed for densification of alumina powder at room temperature. For pressureless sintering, the phenomenological model for densification and viscous behavior of alumina powder proposed by Kim and co-workers was used. The constitutive models were implemented into a finite element program (ABAQUS) to simulate densification of alumina powder under cold die pressing and pressureless sintering. Finite element results were compared with experimental data for density distribution and deformation of an alumina powder compact under cold die pressing and pressureless sintering. New conditions of compaction were then proposed to reduce the distortion of the sintered part.

In situ microtomography investigation of metal powder compacts during sintering

Abstract The mechanisms involved in shape changes arising during sintering of complex materials like iron-based systems are still poorly understood. New information can be obtained by use of advanced techniques such as microtomography. In this study, the microstructural evolution of a Distaloy AE powder compact and of loose copper powder is investigated during a thermal cycle at the European Synchrotron in Grenoble (France). Both materials are sintered in a furnace set in front of a high-energy X-ray source in 30–45 keV range. At various steps of sintering, hundreds of radiographs are taken with different orientations of the specimen. From these images the 3D microstructure is reconstructed. This non-destructive method provides the 3D microstructural evolution of the material during sintering. Local and statistical information can be obtained and will be used in the future for modelling the sintering process. Special attention is given to the anisotropy induced by prior compaction and to its evolution through sintering.

A phenomenological constitutive model for the sintering of alumina powder

Abstract This paper develops a phenomenological model for the densification and viscous behavior of alumina powder during sintering. The free sintering strain rate part and the viscoplastic strain rate part of the constitutive model were determined from experimental data by using the experimental method proposed for WC–Co mixture by Bouvard and co-workers. From densification data under free sintering by using reference and stairway heating cycles with samples of three different green densities, a simple densification equation was identified. This equation expresses the densification rate as a function of temperature, relative density, and green density. The predictive capability of the identified densification equation was discussed by investigating the densification of alumina powder during various sintering cycles and with different green densities. For the viscoplastic strain rate part of the constitutive model, the axial viscosity and the viscous Poissons ratio of an alumina powder compact were determined by using an intermittent loading method.

Analysis of die compaction of tungsten carbide and cobalt powder mixtures

AbstractThis paper investigates die compression of tungsten carbide–cobalt powder mixtures, which is an important step in the processing route of cemented carbides. Simple action compression of miscellaneous mixtures has been investigated with an industrial type press allowing measurement of the force applied on both upper and lower punches. In addition, an original apparatus was used to measure the average radial stress applied upon the die, and the frictional shearing force at the powder/die interface was determined through a powder/wall friction apparatus. A mechanical analysis of the data obtained for various tungsten carbide–cobalt powder mixtures provided the intrinsic stress v. density compressibility relation as well as the radial stress transmission coefficient and the powder/die friction coefficient. In this framework, the effect of several material and process parameters on the densification is discussed.

Constitutive behaviour of metal powder during hot forming. Part I: Experimental investigation with lead powder as a simulation material

Lead powder has been chosen as a simulation material for investigating the constitutive behaviour of metal powders during hot pressing. Uniaxial compression tests and die pressing tests with radial stress measurement have been performed at various temperatures in a wide range of strain rate. At low strain rate a classical viscoplastic behaviour, as already observed with industrial powders, has been obtained. At high strain rate, new phenomena due to the saturation of the viscosity of the material have been displayed. Experimental data show that the behaviour of the porous material is mainly related to the rheology of the constituent material of powder particles. Classical and more original rheological functions, that have been identified from these data, can be used, in a first approach, for any industrial powder with similar morphological and rheological features. All these results are used in the second part of this paper (Part II: Unified viscoplastic modelling) to discuss the capability of classical constitutive equations to describe such behaviour.

Constitutive behaviour of metal powder during hot forming.: Part II: Unified viscoplastic modelling

Constitutive equations used for the finite element simulation of hot isostatic pressing should describe both plastic and viscous strains. For this purpose the unified viscoplastic theory, which has originally been developed for dense metals, has been extended to porous materials. Classical Abouafs model, which is based on a symmetric, elliptic viscous potential, fits in with this theory if is associated with a non-power creep law. However experimental data obtained on a lead powder and presented in the first part of this paper (Geindreau et al.) cannot be described with such model. To get a better description, a new asymmetric viscoplastic potential with linear strain hardening is thus proposed and identified.