Analysis of the Cold Compaction Behavior of Titanium Powders: A Comprehensive Inter-model Comparison Study of Compaction
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I.
INTRODUCTION
A. Background
IN recent years, the powder metallurgy (PM) industry has witnessed a great deal of progression in as far as improvements in powder compaction technologies are concerned; Froes[1] has documented the renewed interest in titanium PM in particular. Cold pressing is by far technically the simplest and most accessible processing technique available in PM portfolio of technologies.[2] It remains one of the most important and primarily used technologies in the fabrication of near net-shape parts.[3] The microstructure and properties of the final sintered powder compacts are strongly influenced by the quality of the green compact. The quality of the green compact, in turn, is determined by the densification mechanisms and the behavior of the powder during compaction.[4,5] Several studies in literature have established relationships between the compaction pressure, the powder characteristics (such as impurity levels and particle size shape), different compaction route (cold, hot, or dynamic compaction), and the obtained properties of the compacts (density, sinterability and strength) with varying degrees of success.[1,2,6–18] Studies into the cold compaction of titanium and titanium-based powder materials are necessitated by fact that titanium PM also offers improved chemical homogeneity and refined microstructures[2] in addition to a cost and energy-consumption reduction benefit—given the high cost of titanium powder material. In addition, challenges associated with titanium powder compaction RONALD MACHAKA, Senior Researcher, and HILDA K. CHIKWANDA, Research Group Leader, are with the Titanium Centre of Competence, Council for Scientific and Industrial Research, P.O. Box 395, Pretoria 0001, South Africa, and also with the Light Metals, Materials Science and Manufacturing, Council for Scientific and Industrial Research, P.O. Box 395, Pretoria 0001, South Africa. Contact e-mail: [email protected] Manuscript submitted November 4, 2014. Article published online July 9, 2015 4286—VOLUME 46A, SEPTEMBER 2015
such as (i) the obvious high reactivity of titanium powder material in air, (ii) its inherent difficulty to press into green bodies due to its high hardness and inductile properties,[13] (iii) problems associated with compact cold welding to the die wall[9,11,13,14,19] as well as (iv) the high ejection force required.[9,11,13] These challenges, for example, further justify the continued interests in the cold compaction behavior of titanium powders. The significant influence of the cold compaction characteristics on the densification of titanium powders has been a subject of several studies. Some noteworthy findings from literature are summarized in Table I. According to Table I, many research articles have reported on the compressibility behavior of titanium powders[1,2,6–14,17,20,22,25]; evidently, only a very few have paid particular attention to the analysis of the cold compaction behavior of titanium powders using existing or new compaction equations, see References 10, 12, 14, 22, and 25 for example. T
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