Milling dynamics: Part III. integration of local and global modeling of mechanical alloying devices
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I.
INTRODUCTION
MECHANICALLY alloyed powders must be consolidated to render them useful engineering solids. Consolidation is usually accomplished through hot deformation (e.g., extrusion and hot isostatic pressing). Densification mechanisms, and the resulting densification rates, depend on powder characteristics such as particle size and hardness. Powder microstructure is also altered during consolidation. Two-phase dispersions generally coarsen and lamellar structures (frequently generated through mechanical alloying (MA)) spheroidize, tl,2J The rate at which these structural changes occur during consolidation depends upon the microstructural scale of the as-milled powder. Thus, it is useful to compare how different MA devices yield differently characterized powders with respect to powder size, hardness, and microstructural scale. Differences in these powders resulting from employing different milling devices are related to different dynamics of the mills. In previous work, t3,4]we have separately considered attritor and SPEX* mill dynamics, two commonly used MA *SPEX is a trademark of SPEX Industries, Edison, NJ.
devices. In this article, we compare the global mechanics of these mills and assess how disparities between them affect processing. While such a comparison between mills provides a basic test of the modeling described in our previous work, a model is of greater value if it has predictive, as opposed to merely descriptive, capabilities. While our model should be considered no more than a framework for future refinement, an attempt to extrapolate serves to illustrate some of its potential, as well as possibly offering insight into the MA process. Thus, in this article, we follow two converging paths. In the first, we examine how global processing parameters may effect the processed powder product. Global parameters can often be varied by materials processors, and they can be considered in the design of a new mill. In the second, we examine the ways in which
D. MAURICE, NRC Research Associate, is with the Albany Research Center, Albany, OR 97321. T.H. COURTNEY, Professor and Chair, is with the Department of Metallurgical and Materials Engineering, Michigan Technological University, Houghton, MI 49931. Manuscript submitted August 15, 1995. METALLURGICALAND MATERIALSTRANSACTIONS A
starting properties of the powders may influence processing kinetics. While less control can be exercised here, an understanding of these effects may lead to some improvements in milling efficiency.
II.
COMPARISON OF MILL DYNAMICS
In this section, we review the global mechanics of attritors and assess how the disparities in impact velocity and frequency between an attritor and a shaker mill affect processing. In the first article of this series,t3j we described a technique that allowed us to approximate the frequency and velocity of ball impacts in different locations within an attritor. Lead balls (having the same size as the steel balls being used) were placed in strategic locations within the attritor. The balls were suff
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