A review of multi-physical fields induced phenomena and effects in spark plasma sintering: Fundamentals and applications

It has been known for decades that the application of pulsed direct current can significantly enhance the formability of metals. However, the detailed mechanisms of this effect have been difficult to separate from simple Joule heating. Here, we study the electroplastic deformation of Ti–Al (7 at.% Al), an alloy that is uniquely suited for uncoupling this behaviour because, contrary to most metals, it has inherently lower ductility at higher temperature. We find that during mechanical deformation, electropulsing enhances cross-slip, producing a wavy dislocation morphology, and enhances twinning, which is similar to what occurs during cryogenic deformation. As a consequence, dislocations are prevented from localizing into planar slip bands that would lead to the early failure of the alloy under tension. Our results demonstrate that this macroscopic electroplastic behaviour originates from defect-level microstructural reconfiguration that cannot be rationalized by simple Joule heating. Transmission electron microscopy reveals the electroplastic effects in a Ti–Al alloy, which can be uncoupled from Joule heating effects. Electropulsing during deformation enhances wavy slip of dislocations, reconfiguring the dislocation pattern, and hence increases the ductility.

Defect reconfiguration in a Ti-Al alloy via electroplasticity.

In this study, the effect of high current density electropulsing (103 A/mm2 for 150 μs) on the residual stress of quenched steel was investigated with the objective of determining the electroplastic effect on plastic strain and dislocation evolution. The residual stress and dislocation density, which were closely related to the plastic strain, were significantly reduced through electropulsing. The direct effect of drift electrons on the dislocation motion, instead of side effects such as skin, pinch, instantaneous thermal expansion stress, and Joule heating, primarily contributes to the plastic strain. Employing the thermally activated plastic flow concept, the drift electrons were also found to have an effect on the thermally activated dislocation motion in addition to the force exerted on the dislocations by an electron wind. Under the action of drift electrons, the movement of vacancies and dislocations were enhanced, which accelerated the dislocation annihilation. In contrast, affected by the electron wind force, Frank-Read source failed to produce a large amount of dislocations, thereby simultaneously decreasing the dislocation multiplication rate. Finally, the dislocation density decreased and the dislocation structure was rearranged parallel to the direction of motion of the drift electrons.

Dislocation structure evolution under electroplastic effect

The mechanical properties of metals change temporarily or permanently under application of electric currents under deformation. This phenomenon is often referred to as electroplasticity. The electroplasticity of metals and their alloys has been investigated under various loading conditions including tensile, compressive, bending, and hardness tests. Electrically-assisted manufacturing (EAM) utilizes this electroplasticity in manufacturing processes. Due to its technical advantages such as enhanced formability and reduced springback, EAM is energy efficient with reduced process time. Recent developments in EAM include various bulk deformation and sheet metal forming processes. This paper summarizes previously reported electroplastic behaviors of various metals or metal alloys and recent EAM processes. In addition, contemporary EAM patents are briefly reviewed.

/pdf/a-review-of-electrically-assisted-manufacturing-3zvbz29y4m.pdf

A review of electrically-assisted manufacturing

Crystal Lattice Defects

Investigations of the effects of an electric current on dislocation mobility and mechanical properties at low homologous temperatures (T < 0.5Tm) reveal a polarity effect and yield an electron wind force in some agreement with theory. An external directcurrent electric field has been reported to influence the creep rate of unalloyed metals at high homologous temperatures. During superplastic deformation of the 7475 Al alloy, such a field has been found to decrease the flow stress, reduce strain hardening, increase strain-rate hardening, reduce grain boundary cavitation and reduce grain growth. The effects of the field were polarity dependent and extended to the center of 1–2 mm thick specimens. No significant effect of the field on the flow stress occurred at low homologous temperatures. This suggests that the field influences atomic mobility through vacancy generation and/or migration. The occurrence of an uneven electron density at the interfaces between phases and at grain boundaries has been proposed as a factor, but this idea needs further consideration.

Electroplasticity—the effect of electricity on the mechanical properties of metals

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Electroplasticity—the effect of electricity on the mechanical properties of metals