Abstract
When a current is involved, as in spark plasma sintering, metallic powders are heated by the Joule effect through both tool and specimen. Other mechanisms might occur, but it is difficult to separate the role of the temperature from the role of the current inside the sample as, in most cases, the two parameters are not controlled independently. In this paper, the consolidation and the densification of a pure copper powder were studied in three configurations for obtaining different electric current paths: (i) current flowing through both the powder and the die, (ii) current forced into the powder and (iii) no current allowed in the powder. Electrical conductivity measurements showed that even low-density samples displayed higher conductivities than graphite by several orders of magnitude. FEM simulations confirmed that these copper specimens were mainly heated by the graphite punches. No modification of the microstructure by the flow of current could be observed. However, the absence of current in the specimen led to a decrease in densification. No significant temperature difference was modeled between the configurations, suggesting that differences are not linked to a thermal cause but rather to a current effect.
Highlights
The correlation between electrical conductivity and density was investigated via electrical conductivity measurements at room temperature on samples sintered at different temperatures in the Spark plasma sintering (SPS) device (Figure 8), in order to estimate the conductivity of samples during the sintering process after the very first step, where grain-to-grain contacts are formed
The densification curves of a pure copper powder were analyzed for three different configurations and with different thermocouple positions, from the relative electrical conductivities of the sample and the tool, from FEM simulations and from scanning electron microscope (SEM) observations of fracture surfaces
This could be explained by different actual thermal cycles applied to the samples, due to thermal gradients inside the system
Summary
Tokita first suggested that the formation of sparks and plasma enhanced consolidation and densification mechanisms [1], but the lack of evidence for spark discharge and plasma resulted in further research on the mechanisms occurring in SPS devices
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