Energy Relationship

When a metal is plastically deformed by cold working, most of the mechanical energy of the deformation process is converted into heat. However, a small amount (approximately 5%) is stored in the metal, thereby raising its internal energy. The energy associated with permanent lattice strain or cold work is minimal for hard, brittle particles, but can be large for ductile materials. The energy expended to overcome the friction between particles is translated to heat and performs no useful work in milling.

If the temperature of the powder rises above a certain point, the cold worked metal particles may undergo recovery and recrystallization. Heat is generated by particle deformation, and by elastic deformation of metal grinding balls and grinding chamber walls. Figure 11 illustrates typical temperature versus milling time curves for dry vibratory milled Fe-27Ni-16Cr alloy powder (Ref 7). Generally, the temperature rises during severe cold working, abruptly falls just before cold work attains saturation levels, and then slowly decreases after extended milling times. Water-jacketed milling chambers usually are required for large, high-energy vibratory and attrition mills that reach temperatures above 200 °C (390 °F).

Fig. 11 Heating curve for 280 cm3 (17 in.3) stainless steel milling chamber during vibration milling of Fe-27Ni-16Cr
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