Acta mater. 49 (2001) 1471–1477 www.elsevier.com/locate/actamat EFFECTS OF LUBRICATION AND ASPECT RATIO ON THE CONSOLIDATION OF METAL MATRIX COMPOSITES UNDER CYCLIC PRESSURE G. JIANG, G. S. DAEHN†, J. J. LANNUTTI, Y. FU and R. H. WAGONER Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210, USA ( Received 28 July 2000; received in revised form 28 December 2000: accepted 28 December 2000 ) Abstract—The effects of powder compaction under cyclic load were compared to traditional single-cycle compaction. The effect of interparticle friction as modified by lubrication and compact aspect ratio received particular attention. Mixtures of Al and Al 2 O 3 were consolidated at room temperature in contained uniaxial consolidation experiments. The experiments showed that in static compaction, lubrication aids densification at low pressures but can inhibit consolidation at high pressures. Enhanced densification was observed follow- ing pressure cycling. These improvements were more pronounced in compacts having smaller aspect ratios. The efficiency of pressure cycling was reduced by the lubricant. Lubrication also decreased the effects of aspect ratio in both the static and cyclic compaction cases. Although lubrication did increase density uniform- ity, the resulting compact green strength was much lower. Both single and double action compaction were studied and the best green strength and density distribution were obtained with double-action compaction under cyclic pressure without lubricant.  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Powder consolidation; Composites; Pressure cycling 1. INTRODUCTION In powder metallurgy, consolidation mechanisms of cold compaction include powder rearrangement, plas- tic deformation and fragmentation (for brittle solids). For randomly packed metal powders, particle rearrangement and localized plastic deformation occur concurrently. This regime is often referred to as stage I [1]. As density increases, plastic deformation flattens the surface of the particles and increasingly inhibits particle motion. Therefore rearrangement becomes quite difficult and plastic deformation of the ductile particles dominates [2]. At higher pressures, particle rearrangement disappears and plastic defor- mation is the only densification mechanism. This stage is referred to as stage II. Transition from stage I to stage II typically occurs when the pressure exceeds the bulk yield stress of the material and plas- tic flow becomes fairly homogeneous instead of local [3]. Friction plays an important role during compaction. Friction between the die wall and compact hinders pressure transmission and produces density gradients † To whom all correspondence should be addressed. Tel.: +1-614-688-6779; fax: +1-614-292-1537. E-mail address: daehn.1@osu.edu (G. S. Daehn) 1359-6454/01/$20.00  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII:S1359-6454(01)00026-X within compacts [4, 5]. One of the best-known predic- tive models of density gradients was proposed by Thompson which is based upon an analysis of the pressure distribution in both pressing and radial direc- tions [6]. Thompson’s model corresponds well with several experimental measurements [6–9]. It has been experimentally demonstrated that the use of a lubricant can decrease density variation by promoting more homogeneous pressure transmission [5, 8, 10]. There are two ways to use lubricant, either a homogeneous distribution of lubricant can be applied to the inside of the die wall or the powder mass itself can be lubricated. Using the “pin and disc” method, Mallender [11] examined the friction coef- ficients between pins of iron powder compacts and high-chromium die steel disc. The friction coef- ficients of dry pins of iron powder compacts were 0.35–0.50, and were reduced to 0.07–0.10 when lubri- cated by zinc stearate [11]. Rice and Tangzelius [12] found that the addition of zinc stearate increased the die-filling density by promoting the powder flowability. By studying the effects of admixed zinc stearate content in iron pow- der, Turenne and his co-workers [13] found that rela- tive density of total solids (iron powder and lubricant) increases with the admixed zinc stearate content; while the relative density considering iron only