Effect of Current Pathways During Spark Plasma Sintering of an Aluminum Alloy Powder

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SPARK plasma sintering (SPS), also known as field-assisted sintering (FAST), is a relatively recent processing technique that has been shown to effectively sinter materials that are otherwise very hard, or very time consuming, to sinter.[1–5] SPS has also shown great promise in sintering nanostructured materials; its rapid heating and cooling rates lead to greatly reduced processing times that key to maintaining the nanostructuring of powders used to create bulk materials.[6–10] However, outside of exploiting the thermal processing rates of the SPS process, there is a lack of understanding and research into the fundamentals of what is occurring during the SPS process. Most of the research has postulated that current, and or field effects, influence dislocation or defect mobility.[8,11–16] More recently, the development of flash sintering has shown that large electrical fields can sinter materials at much lower temperatures than with traditional hot pressing techniques.[17–19] It is often assumed that during the processing of metallic powders via SPS that the current passes through the powder compact resulting in rapid direct heating of the sample and enhanced sintering kinetics.[1,3,12,13,20] However, the lower applied pressures typically used in SPS may not be sufficient to produce conducting pathways in these materials resulting in orders of magnitude lower effective conductivity of the

FRANK KELLOGG, Materials Engineer, is with Bowhead Science and Technology, Belcamp, MD 21017. BRANDON MCWILLIAMS and KYU CHO, Materials Engineers, are with the U.S. Army Research Laboratory, Weapons & Materials Research Directorate, Aberdeen Proving Ground, MD 21005. Contact e-mail: franklin.r. [email protected]. Manuscript submitted June 9, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A

compact.[21,22] Additionally, in many cases, the production of metallic powders may result in the formation of oxide shells around the particles resulting in dramatically reduced electrical conductivity and further exacerbating (or completely dominating) the flow of current in the effective circuit of the SPS system. In this work, the influence of thermal and electrical field effects were separated by altering the current path during sintering of aluminum alloy 5083 powder using either conductive graphite dies or resistive boron nitride (BN) dies. Due to the possible passage of current through both the powder and the die, the graphite system may have both coupled thermal and electric field effects, while the BN dies will force the current to pass only through the powder bed, thus, isolating the effect of electric current on the sintering kinetics.

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PROCEDURE

Five grams of aluminum alloy 5083 powder were packed into a thin-walled graphite or boron nitride (BN) die. The powder was sieved with a 325 mesh and had an average particle size of 11 lm, a particle size distribution ranging from 1 to 35 lm (measured via a Horiba LA 910 Laser Light Scattering Particle Size Analyzer and SEM analysis, Figure 1). Both die types had an inner diameter of 25.4 mm an

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