Enhanced Sintering Kinetics in Aluminum Alloy Powder Consolidated Using DC Electric Fields
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ANCES in the production of aluminum alloy powders has led to dramatic increases in specific strength of bulk aluminum alloys manufactured through powder metallurgy routes.[1–3] Traditional powder metallurgy operations such as press and sinter and hot pressing require exposures to high temperatures for extended periods of time which generally results in significant grain growth and a loss of the benefit of the initial microstructure of the feedstock material.[4] Alternative sintering techniques using electric fields, such as spark plasma sintering (SPS), have been successfully utilized to consolidate nanocrystalline aluminum alloy powders which retain grain size of the feedstock.[5–8] These electric field-assisted techniques have several benefits including high heating rates greater than 1000 K/min, rapid sintering, and reduced sintering temperatures.[9–11] These features are desirable to reduce manufacturing costs, increase throughput, and minimize grain growth to produce bulk nanocrystalline solids.[11,12] Despite these benefits SPS generally results in complex stress, thermal, and electric gradients within the specimen which can lead to heterogeneous microstructures and properties in the sintered BRANDON MCWILLIAMS and JIAN YU, Materials Engineers, are with the U.S. Army Research Laboratory, Weapons & Materials Research Directorate, Aberdeen Proving Ground, MD 21005; Contact e-mail: [email protected] FRANK KELLOGG, Research Engineer, is with Bowhead Science and Technology, Belcamp, MD 21017. STEVEN KILCZEWSKI, Research Engineer, is with TKC Global, Herndon, VA 20171. Manuscript submitted May 26 2016. Article published online November 16, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
component.[13–17] The complex coupling of thermal–electric phenomena with stress[17] makes it difficult to study and quantify the contribution of individual mechanisms[11] to the enhanced sintering kinetics often observed when using this process in comparison to traditional techniques (i.e., no electric field), such as hot pressing.[18] Thermal effects include resistive Joule heating of the tooling (punches and die)[13,19] as well as localized Joule heating at particle contacts.[20] Stress effects include plastic deformation and creep.[21,22] Electric field effects on the sintering of metallic powders include dielectric breakdown of surface oxides,[23] enhanced mass transport through electromigration,[24] and electric field contributions to the plastic response of the particles.[25–27] A relatively new electric field-assisted sintering technique, which has come to be known as flash sintering, has been widely used to rapidly (order of seconds) consolidate ceramic powders using direct current (DC) fields.[28,29] The flash sintering process uses larger electric fields (typically 100 to 1000 V/cm) and lower current densities than SPS-type processes. The advantages of the flash sintering process include reduced sintering temperatures, times, and power requirements compared to other techniques such as SPS and traditional powder processing opera
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