Densification of titanium powder during hot isostatic pressing
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INTRODUCTION
H O T isostatic pressing (HIP) of powders, rapidly solidified ribbons or flakes, or castings is becoming increasingly important as a process to eliminate porosity. In the absence of fundamental studies of HIP, unnecessarily high temperatures and pressures plus long times are commonly used to obtain full density. This may be particularly undesirable when HIP is used to consolidate rapidly solidified ribbon or powder particles, and low temperature/short times are desired to retard microstructural coarsening. The dynamic response of metal powders to the simultaneous application of temperature and pressure during hot isostatic pressing is a complex process during which particle rearrangement, plastic deformation, and creep occur. I1-9] Most of the experimental data available [~'3'7'81 concern the temperature-pressure-time conditions required to achieve a fully dense compact in order to optimize the mechanical properties, especially fracture resistance. Recently, several theoretical models 13'7-10] - have been proposed to predict densification during HIP of powders on the basis of creep and plastic deformation mechanisms during the compaction of monosized spherical powder particles. The development of these models relies extensively on experiments by Swinkels et al.,[7] who examined the hot isostatic pressing behavior on lead, tin, and polymethylmethacralate powders. In particular, the analysis of Arzt et al. 13j incorporated plastic yielding, power-law creep, and diffusional densification in order to predict densification of equisized, spherical powders as a function of HIP parameters. Subsequently, Helle et al.fS] have extended this model such that densification rates are predicted under a wider range of densification mechanisms. Both analyses[3'8jprovide the basis for constructing HIP maps. More recently, Carroll has analyzed the problem of rapid density increases associated with an initial rapid change in pressure (30 seconds rise time).[g] In addition, Nair and Tien have proposed an analysis of HIP for the compaction of powders of unequal size. tl~ This study determines the densification behavior during the hot isostatic pressing of commercially pure (CP) titanium
B. K. LOGRASSO, formerly with the Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931, Is Research Engineer, Department of Materials Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180-3590. D. A. KOSS is Professor and Chairman, Metals Science and Engineering Program, The Pennsylvania State University, University Park, PA 16802. Manuscript submitted March 31, 1987. METALLURGICALTRANSACTIONS A
in the form of both spherical and angular shaped powders which are subject to HIP resulting in densities ranging from packing density to full density. The densification behavior is examined in terms of the microstructural characteristics of the compact, and the experimental results are compared with predictions of existing theories. In particular it will be shown that the analysis of Arzt et al.
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