Impact extrusion is a process in which metal is forced
to flow under the influence of high compressive stresses. The mechanism
depends on whether working occurs above or below the temperature of
recrystallization. Wagner1 has differentiated between these two types of
activity by calling them the thermoplastic range and the crystoplastic
range.
The thermoplastic range lies above the temperature of recrystallization and
the structure of the crystals is not stable in this range. Thus, interatomic
stresses, resulting from flow in this range, can be partially relieved by
atomic rearrangement at in the operating temperature. This rearrangement
however requires a finite time interval. Lead and tin are in the
thermoplastic at room temperature, whereas aluminum, zinc and magnesium are
in the range at slightly elevated temperatures. Disturbance of the slip
planes causes work – hardening, but is relived to some extent by
recrystallization – the actual relief depending on time and
temperature.
The crysto plastic range is below the temperature of recrystallization. In
this range, plastic flow causes permanent deformation of the structure with
areas of relatively high energy at the slip planes. According to Wagner,
there are three distinct regions in the crysto plastic range.
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