Pierre Dorémus

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.

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.

Numerical Simulation of Cold Compaction of 3D Granular Packings

During cold compaction processes loose powder is pressed under tooling action in order to produce complex shaped engineering components. Here, the analysis of the plastic deformation of granular packings is of fundamental importance to the development of computer simulation models for industrial forming processes. Powders can be idealized by packing discrete particles, where each particle is a sphere meshed with finite elements. During pressing, particles are deformed by elastic-plastic hardening where friction is present at each contact. The pressing of an isolated particle followed by a body centered cubic packing was compared with numerical prediction and experimental data. The analysis was focused on the interaction between particles and the global response expressed in force-displacement curve during compaction. The accuracy of the numerical models was also analyzed for high relative densities up to 0.95.

High-velocity compaction and conventional compaction of metallic powders; comparison of process parameters and green compact properties

Abstract High-velocity compaction (HVC) has been used for some years now with commercially available high-energy hydraulic presses. Compact densification is achieved in less than 0.01 s, shorter than with a conventional process (about 1 s) but longer than with explosive compaction (0.001 s). Powders are compacted by means of an adjustable energy instead of force or displacement. This work presents a comparison between conventional compaction (CC) and HVC using an industrial hydraulic press (18 kJ) and a laboratory press (380 J). The velocity of the hammer producing the impact is therefore restricted to 10 m/s. Six powders were compacted using the HVC process and the conventional die compaction process. Differences and similarities are examined considering process characteristics (punch forces, ejection force, and tooling strength) and the characteristics of green compacts (densities, dimensions, and cohesion).