Milling Processes

Changes in powder particle morphology (Fig. 12) that occur during milling of metal powders are produced by the following processes:

Process

Symbolic

designation

Microforging

M

Fracture

F

Agglomeration

A

Welding (atomic bonding)

Aw

Mechanical interlocking

Am

Autohesion (van der Waals forces)

Aa

Deagglomeration

D

Fig. 12 Scanning electron micrographs of Haynes Stellite 21 cobalt-base alloy powder. Milled in ethyl alcohol with aluminum nitrate grinding aid. (a) As-received powder. (b) After 1 h. (c) After 2 h. (d) After 4 h. (e) After 8 h. (f) After 16 h. (g) After 32 h. (h) After 64 h

Microforging. The initial and predominant process during milling is compression shape forming of ductile metal particles by impact from the milling medium, as shown in Fig. 6(b). Individual particles, or a cluster of particles, are impacted repeatedly by milling medium so that they deform (flatten) with little or no net change in individual mass.

Fracture. After a period of milling, individual particles deform to the extent that cracks initiate, propagate, and ultimately fracture the particles. Fissures, cracks, defects, and inclusions in particles facilitate fracturing. Particles formed from irregular or spongy particles contain fissures and cracks that facilitate compression, fatigue failure, and fragmentation, compared to the smooth, relatively nonporous spheroidal particles formed by atomization. Some friable metals (antimony, bismuth, silicon, manganese, and chromium, for example) and brittle intermetallic alloys (such as copper-aluminum, aluminum-magnesium, copper-tin, nickel-iron, nickel-zirconium, and phosphorus-copper) can be milled easily into powders.

Agglomeration of particles may occur by welding, mechanical interlocking of spongy or rough surfaces, or autohesion. Autohesion is the molecular interaction of particles among themselves, characterized by van der Waals forces. Deagglomeration is a process that breaks up agglomerates formed by autohesion without further disintegration of the individual powder particles.

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