Influence of thermomechanical processing on low cycle fatigue of prealloyed Ti-6AI-4V powder compacts
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I.
INTRODUCTION
RECENT developments in titanium powder metallurgy (PM) have been aimed at producing lower cost components, while maintaining mechanical properties at least equivalent to those exhibited by wrought material. 1-4 The static properties of PM Ti-6A1-4V, such as tensile behavior and fracture toughness, have been found to be equal to wrought levels. However, dynamic properties of PM material, such as low cycle fatigue (LCF) 5 and high cycle fatigue (HCF), 6 ~0 in which initiation plays a major role, can be degraded as a result of high contaminant levels. These contaminants result from insufficient attention being given to cleanliness during powder production and subsequent handling. During the past decade, only one type of prealloyed powder has been available in commercial quantities: the Rotating Electrode Process (REP) powder." This powder can contain various types of contaminants including: (a) tungsten particles from the tungsten cathode, (b) cross contaminant particles such as iron and superalloy powders, and (c) nonmetallic inclusions introduced during powder handling. Previous work on the effect of thermomechanical processing (TMP) on the microstructure of REP compacts indicated that mechanical behavior, LCF in particular, was degraded compared to wrought material.5 In contrast to the majority of other results reported, ~ that work 5 also showed fatigue crack initiation predominantly at tungsten particles rather than at crosscontaminant metal particles or nonmetallic inclusions. In the past few years, greatly improved REP powder has become available, and more recently a tungsten-free powder has been produced using a plasma source rather than the previously used tungsten arc. This new Plasma REP powder
is termed PREP. l~ However, even in extremely clean powder, a few, albeit small, contaminant particles exist. Complete removal of these by cleaning would make the powder prohibitively expensive.~'3 Thus, to make use of powder compacts in fracture critical applications, it is necessary either to accept some degradation in defect sensitive mechanical property behavior or to render innocuous the foreign particles present. One way to achieve this latter objective is to reduce the content of the contaminants below some critical levels. The levels required have been defined for superalloys, ~2 and a parallel study to determine these levels in the titanium system ~3is currently in progress. The critical parameters are likely to vary with changes in alloy strength level, ductility, and microstructure. It was the primary aim of the present work to vary the Ti-6A1-4V microstructure by TMP, and to investigate whether this would make consolidated powder less sensitive to premature crack initiation at tungsten or other contaminant particles in LCF tests. The REP powder used in this work was the cleanest available at the time the program was initiated and was produced using practices designed to give low tungsten and low cross-contaminant/nonmetallic inclusion contents. This powder was therefore used for the pro
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