Date of Award

5-1973

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Supervisor

Professor M.C. de Malherbe

Abstract

The purpose of this work was to examine the processes of isostatic compaction and hydrostatic extrusion and to evaluate their potential for the cold consolidation of metal powders to bar stock.

Equipment for isostatic compaction and hydrostatic extrusion at pressures up to 1600 MN/m² is described. A numerical technique is presented for calculating the internal pressure required to produce a given bore strain in a tapered pressure vessel set of the type used. The method is applicable to open or closed ended vessels of elastic-plastic work hardening material and assumes the Mises criterion of yielding.

From a review of the literature of powder compaction it is concluded that there is some confusion as to the role and extent of plastic deformation in powder compaction and that the means by which compacts consolidate and achieve green strength is uncertain. It is suggested that the Shapiro-Konopicky pressure-density equation has most capability for further development.

Results of mechanical testing, metallographic examination and x-ray diffraction analyses of some atomized iron powder compacts are presented, together with a metallographic examination of compacted spherical superalloy powders. From these it is concluded that extensive plastic deformation occurs even during the first stage of compaction, but is not solely responsible for consolidation. A sequence of compacting mechanisms is described for the iron powder and it is suggested that the transition from stage 1 to stage 2 compaction corresponds to the change from local to homogeneous plastic flow.

Torre's model of a hollow sphere subjected to external pressure, that was developed to represent the compaction behavior of a porous body, has been modified to cover strain hardening of the material. Theoretical predictions of density are compared with experimental results for Atomet 28 iron powder and Alcoa grade 1202 aluminum powder. There is good agreement in the second stage of compaction; outside of this stage the theoretical values are higher than the experimental results. Some possible reasons for the discrepancy are discussed.

Results are presented showing the extrusion pressure required for iron compacted at different pressures; from these it is concluded that the extrusion characteristics of compacts can be influence by their porosity and an expression is derived relating the extrusion pressure to the relative density of the compact.

Results of mechanical testing and metallographic and fractographic examinations of extruded aluminum compacts are presented, together with their extrusion characteristics. These show that good bonding can be developed in these compacts by hydrostatic extrusion at reduction ratios of 6.25 and that their strengths can be higher than wrought material of similar composition. This strength improvement is attributed to the strain hardening undergone by the material during compaction. An interpretation of the mechanism of bonding is also given.

Although it is shown that isostatic compaction and hydrostatic extrusion can be used to produce well bonded bar material from metal powders, it is suggested that the potential of the method is limited by the very high pressures that would be required to produce materials of commercial interest.

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