Effects of Initial Powder Compact Thickness, Lubrication, and Particle Morphology on the Cold Compaction Behavior of Ti
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TRODUCTION
THERE is a lot of interest in improving the performance of PM titanium and titanium alloys produced by consolidation of low cost CP titanium and titanium alloy powders using advanced consolidation processes.[1–9] Although cold compaction studies on titanium powder have attracted a lot of attention, there are still gaps in our understanding of the cold compaction behavior of titanium powder. Ma reported that the compaction behavior of titanium powder is similar to the compaction behavior of annealed copper powder.[10] Only 70 to 80 pct green density is achieved for the commonly used pressure range of 300 to 800 MPa and compact thicknesses of 5 to 100 mm.[2] Takamiya et al.[11] have reported that a higher density is achieved using ultrahigh pressure (>1400 MPa) or the application of warm or hot compaction, but such an approach requires the use of expensive equipment which increases the processing cost. Another important factor is the green density variation within a pressed powder compact.[12] Lou et al.[1] reported that the variation in green density of a Ti powder compact can be from 65 to 90 pct, but the density variation is not uniform from the top surface to the bottom surface.[13] Such a large and irregular variation of the green density within a powder compact JIA LOU and FEI YANG, Researchers, and BRIAN GABBITAS, Professor, are with the Waikato Centre for Advanced Materials, School of Engineering, The University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand. Contact e-mail: [email protected] DELIANG ZHANG, Professor, is with the School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, P.R. China. Manuscript submitted October 6, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
can cause severe distortion and non-uniform mechanical properties in sintered parts.[14] Lou et al.[15] demonstrated the benefits of an improved green density distribution on the mechanical properties of sintered Ti compacts. This work investigates the compaction behavior of HDH Ti powder which was produced using a hydrogenation–dehydrogenation (HDH) process. The study covers the two stages of powder compaction: particle rearrangement and plastic deformation of particles. It also involves theoretical calculations and an experimental investigation. The effects of initial powder compact thickness, the use of internal lubrication, and addition of PREP (Plasma Rotating Electrode Process) Ti powder, with a spherical particle shape, were studied and a comparison was made with the compaction behavior of aluminum and iron powders. The aim of this paper is to establish, for titanium, a relationship between the green density and the compaction conditions, such as powder morphology, powder mass, and lubrication condition. The effects of any change in compaction conditions on each stage are discussed. To date the compaction behavior of titanium has received little attention, but since there is significant published information on the compaction of iron and aluminum powders, a comparison between the c
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