The effects of ball size distribution on attritor efficiency
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INTRODUCTION
MECHANICAL alloying (MA) has proven to be a versatile alternative to traditional powder processing techniques for producing a wide range of materials and material microstructures. Originally developed by Benjamin and coworkers in the mid-to-late 1960s for production of oxide dispersion superalloys,V.:.31MA has since been found viable for synthesizing nonequilibrium phases and structures and amorphous, nanocrystalline, and intermetallic materials (e.g., References 4 through 18). Considerable effort has gone into developing a better understanding of the local dynamics of MA; [19-25]i.e., the deformation undergone by the powder particles entrapped between colliding media, the temperatures the powder experience during such a collision, the proclivities for powder particle fracture and coalescence, etc. In addition, studies relating to defining the milling conditions--and thermodynamic and kinetic factors--leading to amorphization, to compound formation, etc. have also been conducted.12~331 A wide range of mills can be used for MA. The mills vary between large diameter ball mills, which have significant charge capacities and low energy densities, and small mills (e.g., a SPEX* mill), which have low charge *SPEX is a trademark of SPEX Industries, Edison, NJ.
capacities and high energy densities. A lesser number of studies (although some have been noteworthy) have been directed at delineating the geometrical features and mechanical characteristics of these, and other, devices commonly used for MA. The work described here relates to an attritor, a device having an intermediate charge capacity and energy density, to a SPEX mill, and to a commercial ball mill. We have investigated the effect of using differently sized balls on attritor media dynamics, with the long-range purpose of im-
T.M. COOK, Graduate Research Assistant, and T.H. COURTNEY, Professor and Chair, are with the Department of Metallurgical and Materials Engineering, Michigan Technological University, Houghton, MI 49931. Manuscript submitted November 21, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
proving attritor efficiency. The study involves high speed filming of a transparent operating attritor from which the media dynamics pertaining to attrition milling with differently sized grinding balls can be contrasted and compared to analogous dynamics pertinent to milling with a single sized ball. The cinematographic observations are complemented by empirical studies of MA kinetics relative to the two situations. The work of Rydin et al.t34j provided the impetus for our study. So it is worthwhile to review the salient conclusions from this previous work. Using high speed cinematographic observations, Rydin et al. divided the attritor into several regions characterized by different media dynamics and powder contents. To a first approximation, the attritor is considered composed of a core region (where the impeller arms are located) and the region outside this core. The core is characterized by a high media velocity and relatively high ener
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