Initial Stage Of Densification Research Articles

Hot pressing of tungsten and its pseudoalloys. II

1. It has been established that densification in the hot pressing of tungsten and its pseudoalloys is accomplished through several processes, each of which predominates in a particular density range. At a relative density of 0.45-0.6, regrouping and dense packing of particles play the main part. In the density range 0.6–0.8, the process is controlled chiefly by plastic deformation in the particle contact zones. At higher densities, diffusional creep becomes the dominant factor. 2. Mathematically, the densification process in the hot pressing of tungsten and two-component tungsten-containing mixtures in which one component becomes transformed in the course of pressing into a liquid phase having no chemical affinity for the solid-phase component, can be satisfactorily described on the basis of the mechanism of volume-viscous flow of a compressible liquid within the framework of the Koval'chenko-Samsonov equation at relative densities of the tungsten skeleton of 0.6–0.95 provided that the viscosity coefficient is proportional to the square of isothermal holding time. At an initial relative density of the tungsten skeleton of < 0.6, it is necessary to introduce an additional term describing the initial stage of densification.

Electron metallography of powder beryllium — a hot-pressing mechanism and the role of oxygen impurities

The application of electron microscopy to the study of hot-pressing beryllium powder is described. Microstructural examination revealed that the principal impurity in consolidated metal was beryllium oxide which was introduced into the material as a surface layer on the original powder particles. The oxide in the compact, although distributed as a network of fine precipitates surrounding relatively precipitate-free regions, was not concentrated at the final grain boundaries. As the oxide was found to be stable in beryllium, behaving as a form of reference marker, it was deduced that this network was the remains of the oxide present on the surface of the powder particles. Clusters of very small grains were observed in material pressed at low temperatures, indicating that nucleation takes place in the initial stages of compaction, the formation of nuclei being related to the highly deformed contact regions between projections on the particle surfaces. As consolidation proceeds, the grains have grown and the final compact contains a grain structure superimposed on, but not directly related to, the original oxide network. It is therefoie concluded that the hot-pressing process is analogous to hot-working, with definable stages of nucleation and recrystallisation taking place, and that consolidation has been facilitated by promotion of the bulk flow characteristics during the working. The improved strength of beryllium of powder origin, as compared with metal fabricated from cast ingot, is ascribed mainly to a finer grain size, the oxide having a beneficial effect in minimising grain growth. It is shown that ~ 0.2 wt % oxygen is sufficient to maintain a thermally stable structure up to temperatures ~ 10 °C below the melting point of beryllium, oxide levels in excess of this being of little further benefit with respect to grain size control. It is also concluded that the distribution of oxide precipitates has by no means been optimised to achieve maximum lattice hardening and since the limited solubility found for oxygen in beryllium appears to preclude the possibility of modifying the precipitate dissemination by thermal treatment, it is suggested that recycling the powder may be advantageous.

On the Hausner Ratio and its relationship to some properties of metal powders

The ratio between the tap density and apparent density of a powder mass is termed the Hausner Ratio. Values of this ratio, flow times and angles of repose are compared for three types of dry powder and for two types of oiled copper powder. The results indicate that the Hausner Ratio measures the friction condition in a moving powder mass. It is concluded that the Hausner Ratio is a characteristic of a powder in addition to angle of repose and flow time. Knowledge of the Hausner Ratio is of use in gaining a basic understanding of the mechanism in sieving and of the initial stage of compaction of powders.

Vacuum Hot Pressing of Tungsten Powders

In order to study the densification behaviors, several tungsten powders were hot-pressed in vacuum at 1100-1300°C for 10 min under the pressure of 750-875kg/cm2. The density of the sintered specimens was 60-82% of the theoretical value, indicating that they are still in the initial stage of densification. Without additives, higher density is achieved with fine powders, but the density is not a simple function of the average particle size. In mixed powders, different densification behaviors were observed at lower temperatures and the densification is not affected significantly by the powder parameters. Summarizing, important factors in densification are powder, temperature and pressure in the order stated.

Densification of the Plastic Ball Compacts during the Hot-Pressing

For the purpose of getting a better insight into the initial stages of densification during the hot-pressing, the hot-pressing experiments using the compacts of amorphous polystyrene sphere were performed. The size of the amorphous polystyrene plastic sphere was 0.695mm in diameter.The obtained experimental data showed that the densification curves were divided into three parts. These parts were characterized by an initial rapid densification followed by an intermediate stage such as Murray's relation and finally a much slower process. On the other hand, it has been reported that the rapid densification in the initial stage was attributed to; (a) the particle rearrangement, (b) the influence of the porosity on the applied load, (c) the plastic flow within the solid particle.To ensure these effects, the creep experiments using the sintered polystyrene sphere compacts which had various porosities were performed. From these experiments, it was found that the visco-elastic deformation within polystyrene itself and the influence of the porosity on the applied load could be effective, the former in paticular. In spite of these facts, the rearrangment of the spheres within the sintered body could not be observed in these experimental conditions. These were the contrast to the reports which had been presented.In the intermediate stage of densification, Murry's relation was valid for the experimental data. In this stage, it was reasonable that the densification proceeded by the viscous flow characteristics of polystyrene sphere.The final stage of the densification which was a much slower process was attributed to the pore closure as some investigaters reported.Besides, the effects of porosity to the creep rate in the sintered polystyrene plastic sphere compacts were determined. As the results, the following experimental equations were obtained.J/J0=1+5PJ=J0 exp 3.7PJ0: creep compliance for the zero porosity specimenJ: creep compliance for the P % porosity specimenP: prorosity %

Hot‐Pressing of Alumina Powders at Low Temperatures

A better insight into the initial stages of densification during the hot‐pressing of ceramics can be obtained by performing the experiments at low temperatures. The parameters which have been examined are pressure, temperature, and the particle size of the alumina used. The results obtained preclude plastic flow as the predominant mechanism responsible for the large increases in density observed initially. Particle rearrangement by boundary sliding is suggested as the process responsible for the initial increase in compact density.