Thermal Processes in the Heating of Powder Compacts of Metals and Their Compositions I. Recrystallization Thermokinetics of Copper Compacts

Abstract

The thermal processes of recrystallization in porous copper compacts heated after cold pressing in a steel die were experimentally studied. The recrystallization stages, with temperatures corresponding to data determined with metallographic and other research methods, were identified. Only the first tempering stage was found to occur uniformly throughout the entire volume at a heating temperature of 190°C. Upon further heating, a linear stage ensued, where the temperature remained consistent throughout the entire volume of the material, persisting until the next stage. This was accompanied with energy release, as confirmed by a zero temperature gradient across all regions within the sample. With an increase in temperature to 210–215°C, a thermal surge was observed and visible temperature asynchronization was noted in individual volumes of the sample. However, this was followed by a temperature gradient between individual volumes. At this stage, asynchronized thermal behavior within the sample was observed for the first time, as evidenced by the emergence of thermal waves. Subsequent stages demonstrated nonlinear thermal behavior, evidenced by several competing processes leading to the wave-like transfer of energy accumulated as the copper powders deformed during cold pressing. Like recrystallization processes with first-order reactions in the high-temperature synthesis of intermetallic and other compounds, the emergence of thermal waves was due to several competing processes. Thus, if microplastic deformation processes occurred during recrystallization, then traveling waves could arise in the system, which was actually revealed. This could lead to thermal interference and subsequently to local buildup of thermal energy, potentially causing a sharp increase in temperature in individual areas of the wires deformed during switching and, as a result, their ignition. The temperature surges observed were likely to cause the combustion of insulating materials. Therefore, this can explain the causes of accidents that occur in the operation of complex mechanisms with numerous electrical circuits.