Computational modeling of the cold compaction of ceramic powders

Abstract

A finite-element model of the cold compaction of ceramic powders by uniaxial pressing is developed and validated by comparison with experimental data. The mechanical behavior of processing powders is assumed according to the modified Drucker-Prager cap model. The frictional effects and the mechanical behavior of tools involved in the process are taken into account. The proposed model allows evaluation of the density distribution into the processed part, as well as stress and strain fields. Variations of the density distribution due to the unloading and the ejection of the part are evaluated Keywords: cold compaction, powder metallurgy, finite-element method, numerical modeling, DruckerPrager cap model Introduction.Manufacturing processes that allow obtaining near net-shape products using powders as raw material are referred to as powder metallurgy. The most relevant steps of powder metallurgy are the production of (pure or pre-alloyed) powders, the powder cold compaction process, and the sintering process. Sometimes, after these processes, the part is subjected to finishing processes, such as recompaction, resintering, heat treatment, and machining operations, to improve its geometrical and mechanical properties. The physical and chemical properties of processing powders are very important to achieve the desired mechanical properties of the final product.Metallic or ceramic powders can be obtained by several processes: mechanical processes, such as machining, sledging, grinding, granulating, shot blasting, and atomization; physical or physical-chemical processes, such as condensation and thermal decomposition; chemical processes, such as oxide reduction, solution precipitation, and corrosion; and electrolytic methods. The obtained powders are then subjected to mechanical and thermal treatments to improve density, to mix different kinds of powders, and to remove hardening effects and impurities.Cold compaction processes are used to shape powders into porous parts, called greens, whose mechanical resistance allows manipulating them during the sintering process. Typical processes of powder compaction are uniaxial pressing, isostatic pressing, high speed forming, extrusion forming, vibration forming, centrifugal forming, rolling forming, continuous forming, gravity forming, and mixture forming. The sintering process is heat treatment used to achieve stable cohesion of compacted powders. The sintering temperature is lower than the melting temperature of the main component of the powder mixture or of all different powders.The powder compaction process is very important to obtain a final product characterized by the desired mechanical and geometrical features. Indeed, plasticity phenomena, internal friction of a porous medium, and frictional effects between the die wall and the processing material may induce inhomogeneous density and residual stress distributions. As a consequence, during the sintering process, nonuniform shrinkage, local distortions, or cracks may occur. Nowadays, the powder metallurgy industry is base...

Cold compaction of ceramic powder: Computational analysis of the effect of pressing method and die shape

“…The modified Drucker-Prager/cap model [22] is a multisurface elastoplastic model that allows controlling the dilatancy by a hardening cap intersecting a fixed failure envelope in a nonsmooth fashion. The Drucker-Prager/cap model consists of a failure line with a nonassociated flow rule and an associated high triaxiality elliptical cap.…”

Application of tomographic reconstruction techniques for density analysis of green bodies

“…Not only do these variations usually decrease product performance, they also amplify uncertainty in material behavior which is unacceptable when producing highperformance ceramics. Currently, the ceramics industry heavily relies on the process of trial-and-error to determine optimal mold geometry and forming pressures for a given piece [6].…”