Hot pressing is a process in which pressing and sintering take place simultaneously. In addition to the commonly used resistance and induction heating techniques, there are new processes and equipments such as vacuum hot pressing and vibratory hot pressing.
●The process of putting the final additive powder into the graphite press and simultaneously sintering and pressing is called hot pressing.
●Although hot pressing is carried out simultaneously with the two processes of pressing and sintering, there is no difference in principle with ordinary cold pressing and sintering. In terms of effectiveness, hot pressing considerably reduces the time for phase changes and the formation of alloys. Therefore, it can be said that the hot pressing process is an activated sintering process. The densification processes of the alloys produced by the two production methods are compared. The alloy density-sintering time curve shows that the sintering process takes 1-2 hours to complete, whereas hot pressing takes only 3-10 minutes.
●The sintering process during hot pressing is a highly unbalanced process. When the mixed powder is heated continuously under pressure, the oxide film on the powder surface is broken under pressure and can be reduced by carbon. However, unlike the sintering process, hot pressing does not allow sufficient time to equilibrate the solubility of WC in Co and no recrystallisation process through the liquid phase takes place during the hot pressing process. As a result, the shrinkage of hot pressed products differs from that of the normal sintering method.
●In addition, some studies have shown that the shrinkage of hot pressed products is caused by the flow process. Prior to the appearance of the liquid phase, it relies mainly on plastic flow. When the liquid phase appears, the rearrangement of the carbide particles under pressure is also known as liquid flow, which causes shrinkage of the pressed billet. Diffusion and recrystallisation are difficult due to the short hot pressing time.
●In addition to temperature and pressure promoting shrinkage during sintering, the holding time also has an effect on the density of the hot pressed product. The higher the temperature and the greater the eye force, the stronger the contraction of the pressed billet with time at the beginning of the hot pressing and the shorter the time required for the density to stop changing almost completely. Once a certain holding time has been reached, the density of the briquettes will no longer continue to increase and there is no point in extending the holding time.
●In addition, powders with crystal defects can achieve greater alloy density because the presence of defects can activate the hot pressing process.
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