Milling Parameters and Powder Characteristics

Grinding elements in ball mills travel at different velocities. Consequently, collision force, direction, and kinetic energy between two or more elements vary greatly within the ball charge. Frictional wear or rubbing forces act on the particles, as well as collision energy. These forces are derived from the rotational motion of the balls and movement of particles within the mill and contact zones of colliding balls.

Surface roughness of the balls is a significant factor in milling--the rougher the surface, the greater the frictional energy expended, thus causing increased abrasive action. Frictional forces exert a wearing action between particles in contact with one another and reduce particle size by attrition, producing wear debris. Impact forces effectively reduce particle size in grinding applications involving coarser particle fractions or hard and/or brittle materials.

Impact forces are desirable for deformation processing of metal powders, such as the production of flake powders. Figure 8 shows the change in flake width of iron powder with milling time in a vibratory ball mill. Attrition grinding is most effective in fine grinding both hard and ductile materials and in mechanical alloying. In most ball mills, particles are subjected to both impact and attrition forces. When forces are highly energetic, welding may occur between powder particles, between the powder and the balls, and between powder particles and the walls of the chamber.

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0 10 20 30 40 50 60 70 Milling time, h

Fig. 8 Relationship between flake size and milling time for electrolytic iron milled in a Megapact vibratory ball mill. Source: Ref 6

Force of impact is directly proportional to the mass of the milling medium. Consequently, processing rate and the forces acting on particles are a direct function of the effective diameter and mass of the milling bodies. Actual collision forces vary widely from theoretical values because of the complex motion of the medium and varying velocity, path lengths, and trajectories of individual milling bodies. Milling fluid viscosity also effects collision rate.

Generally, the size and density of the milling medium selected are determined by the deformation and fracture resistance for metals. For hard, brittle materials, fracture resistance is the only selection criterion. Large, dense grinding mediums are used to mill larger and stronger particulates, whereas smaller diameter mediums are used for finer grinding. For example, ceramic beads having a diameter of 1.6 mm (0.06 in.) and a density of 3.9 g/cm3 may be used for reducing coarse, thin metal flakes to very fine flakes. Milling elements of such a small size cannot be used effectively in conventional tumbler ball mills. For fine grinding of tough, hard materials, tungsten carbide mediums are recommended.

Often, a compromise must be made in the selection of ball size and material. Although tungsten carbide balls are economical for use in small-scale experimental mills or for milling expensive materials, costs may be prohibitive for some large-scale milling operations. Some mediums, such as stainless steel grinding balls, may not be readily available in large quantities in certain sizes; thus, optimum milling conditions may not be satisfied.

The surface area and particle size of nonmetallic materials change continuously. Total surface area of the medium increases as particle size decreases. With metals, however, two additional factors must be considered in addition to comminution. Metal particles may agglomerate by cold welding at impact, thus consuming surface area and causing a shape change because of ductility and the ability to cold weld.

Thus, depending on the dominant process during milling (fracturing, welding, or microforging), a particle may (a) become smaller in size through fracturing, (b) grow in size through agglomeration by welding, or (c) change from an equiaxed shape to a platelet or flake-like particle by microforging. For metals, changes in surface area and particle size measurements do not provide a meaningful criterion for comparing the effect of changes in process parameters, competing milling processes, and equipment.

A more useful criterion for assessing the milling process is one that reflects the spectrum of structural and physical changes occurring in the metal and that is sensitive to differences in processing parameters. Microhardness measurement of individual particles large enough to accommodate the smallest Knoop or diamond pyramid hardness indentation provides a suitable measure of the effect of milling on metal powders, because hardness is a measure of cold work and internal defects produced by milling. As shown in Fig. 9, hardness measurement is most meaningful up to milling times that produce maximum levels of cold works.

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in rt 400

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0 10 20 30 40 50 60 70 Milling time, h

0 10 20 30 40 50 60 70 Milling time, h

Fig. 9 Effect of milling time on microhardness of Nickel 123 powder

X-ray line broadening is sensitive to both the amount of cold work and the refinement in crystalline structure that occurs with continued kneading and working of the metal well after saturation cold work. Changes in the deformation rate of a metal powder produced by a given set of milling parameters are shown by the relationship of x-ray line broadening to milling time (Fig. 10).

Fig. 10 Relationship between x-ray line broadening and vibratory milling time for Nickel 123 powder. Numbers in parentheses refer to coordinates of atomic planes in face-centered cubic structure

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