Introduction

MILLING OF MATERIALS, whether hard and brittle or soft and ductile, is of prime interest and of economic importance to the P/M industry. Mechanical comminution is the most widely used method of powder production for hard metals and oxide powders. Secondary milling of spongy cakes of oxide-reduced, atomized, or electrolytic powders is the most common milling process; hammer and rod mills are used for this type of milling. Depending on the degree to which the material is sintered, either primary particle size distribution is reestablished during milling or larger agglomerates are produced.

Mechanical comminution is restricted to relatively hard, brittle metals (electrolytic iron or bismuth, for example), some reactive metals such as beryllium and metal hydrides, ductile metals used for producing metal flakes, and chemically embrittled materials such as sensitized stainless steel.

Increasing interest in metal powder with particle sizes that are finer than the particle sizes of powders produced by atomization has reactivated interest in milling, particularly in solid-state alloying or high-energy milling. However, milling of metal powders has received minimal attention to date. Research conducted on ball milling of metals is primarily proprietary and empirical, and thus restricted to specialized P/M applications. Objectives of milling include:

• Particle size reduction (comminution or grinding)

• Particle size growth

• Agglomeration

• Solid-state alloying (mechanical alloying)

• Solid-state blending (incomplete alloying)

• Modifying, changing, or altering properties of a material (density, flowability, or work hardening)

• Mixing or blending of two or more materials or mixed phases

• Nonequilibrium processing of metastable phases such as amorphous alloys, extended solid solutions, and nanocrystalline structures

The milling operation fractures, deforms (cold works), or cold welds the impacted particles. Milling also may produce polymorphic transformations (Ref 1), as is the case with lead oxide and stainless steel (Ref 2). The specific effect that milling has on a powder depends on the physical and chemical properties of the powder, the vacuum, gaseous, or liquid environment in which the operation is conducted, and milling conditions. Selection of the milling process is based on the desired result of the milling operation, the behavior of the powder under milling conditions (if known), the characteristics of the powder, and the physical and mechanical properties of the material.

During milling, four types of forces act on particulate material: impact, attrition, shear, and compression. Impact is the instantaneous striking of one object by another. Both objects may be moving or one may be stationary. Attrition is the production of wear debris or particles created by the rubbing action between two bodies. This type of milling force is preferred when the material is friable and exhibits minimal abrasiveness. Shear consists of cutting or cleaving of particles and usually is combined with other types of force. Shear contributes to fracturing by breaking particles into individual pieces with a minimum of fines. Compression is the slow application of compressive forces to a body (crushing or squeezing of particulate material). This type of milling action usually is associated with jaw crushers and the breaking of large agglomerates of hard, nonductile material.

Design and process improvement has been based primarily on empirical and semi-empirical data. Advancement has been made in understanding the nature of brittle fracture of single particles of relatively homogeneous materials, such as glass. Fundamental research on milling ductile metals is lacking, although much is known phenomenologically about the process. Comminution is not adequately understood as it applies to the detailed mechanism involved when particles are impacted by colliding mediums.

Fracture occurs in hard, brittle materials, with minimal particle deformation and agglomeration by welding. Particle deformation results from attempts to comminute or blend particulate materials. Deformation, cold welding, and fracturing occur in varying degrees with both hard and soft ductile materials. Although powders of <100 mesh (<150 /'m) are most frequently milled, particles up to 6 mm (0.2 in.) and larger can be processed.

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