Manuscript received 21 November 1979. S. T. S. Al-Hassani, BSc, MSc, PhD, is in the Mechanical Engineering Department, and T. J. Davies, PhD, MIM, is in. the Metallurgy Department, Joint UMIST/University of Manchester Bu:ilding, Manchester. Metal powders can be compacted using a high-voltage electrical discharge. The experimental set-up consists of a relatively simple electricqJ circuit using a column of metal powder as a short circuit resistance across a capacitor bank. This isshown in Fig. 1. The capacitor bank used in the present work'consisted of fifteen 5 . 3 ,uF capacitors which could be charged to 20 kV and discharged through an ignition switch, allowing the instantaneous discharge as well as the instantaneous voltages at both ends of the powder column to be measured. More detailed descriptions and analyses of the electrical circuit and the discharge conditions have been given previously. 1-3 Some typical shapes which have been compacted are shown in Fig. 2. These range from simple cylindrical shapes to flats and tubes. The materials successfully compacted included powders of pure iron, plain -carbon steel, stainless steel, pure nickel and nickel alloys, cobalt and cobalt alloys, aluminium and aluminium alloys, tin, chromium, and molybdenum. The mesh sizes of the powders ranged from -150 to -400 mesh and component sizes ranged from 50 to 500mm length and 4 to.8 mm diameter. The effect of varying the discharge voltage on the' structure of the preforms is shown in Fig. 3. This illustrates the fact that at low discharge voltages the discharge jumps from one longitudinal 'fibre' to another, forming a loose network of fibres with some transverse bridging. With increasing voltage, there is an increase in number and shortening in length of the transverse bridging units; the similarity between this lateral contraction and the 'pinch effect' of magnetohydrodynam,.. ics has been noted previously. 2 Whereas the inductance of the powder column did not change appreciably 'with discharge voltage, the ohmic resistance underwent drastic reduction, as shown in Fig. 4. This drop in resistance is coincidental with good metaIl metal contact and the disruption of oxide films on particle surfaces; an example of these contacts forming liquid phase necks between. particles is shown in Fig. '5. Hardness measurements showed that the powders were fully annealed during the discharge compaction. Preform density and bending strength increased with increased energy input up to a limiting charged voltage. Densities 9f up to 96% theoretical density were obtained. by coldswaging the preforms. It was noted experimentally that particular powders had a range of discharge voltages within which sound handleable compacts could be produced. This is shown schematically in Fig. 6, where the minimum voltage indicates a voltage below which preforms were not sufficiently strong, and the maximum a' voltage above which the powder column
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