Comparison of SPS Processing Behavior between As Atomized and Cryomilled Aluminum Alloy 5083 Powder
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RODUCTION
THE low density and high strength of aluminum alloys make them attractive lightweight replacements for steel alloys. The properties of the aluminum alloys can be further improved through various manufacturing techniques. For parts produced via powder metallurgy, cryogenic attrition, here on referred to as cryomilling, can be used to improve part performance by nanostructuring the powder to be consolidated into the final part. For this research, cryomilling is the low energy ball milling of powder in liquid nitrogen.[1] This process can produce easy to handle micron-sized powder agglomerates that are made up of nano-grained material.[1,2] Processing under liquid nitrogen can also protect the new particle surfaces that are being formed as the existing particles are broken up.[1,2] It can also lead to nitrogen inclusions within the material that can lead to higher strengths and improved grain stability during consolidation.[3,4] After cryomilling, a processing technique needs to be chosen that can consolidate the material to a fully dense part without significant grain
FRANK KELLOGG is with the SURVICE Engineering, Belcamp, MD 21017. Contact e-mail: [email protected] BRANDON McWILLIAMS, JENNIFER SIETINS, ANIT GIRI, and KYU CHO are with the Weapons & Materials Research Directorate, U.S. Army Research Laboratory, Aberdeen Proving Ground, MD 21005. Manuscript submitted April 26, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
growth. For this study, spark plasma sintering (SPS) was chosen. The SPS process, due to its inherently high heating and cooling rates, is especially attractive for the consolidation of nanostructured materials.[5–9] SPS can also be used to sinter materials in a few minutes that would, under other situations, take days.[10–14] However, most of the SPS research has concentrated on the structure and properties of the processed material, and not what is actually happening to the material. Much of the research into the mechanics of the SPS process have focused on diffusion and electromigration.[15–20] SPS has previously shown success consolidating cryomilled aluminum 5083 and that cryomilled 5083 demonstrated enhanced sintering kinetics compared to the as atomized powder.[21,22] The enhanced kinetics observed in the cryomilled powder were attributed to the increased surface energy of the cryomilled powder and neglected the effect of powder condition on the current pathway and heating rate effects during processing. Altering the current pathway during SPS will change the heating rates experienced by the sample powder and will change how the sample densifies.[23] In this study, aluminum 5083 powder was processed via SPS in both the as atomized and cryomilled forms using either a conductive graphite die or an electrically insulating boron nitride (BN) die with the goal to see how (1) nanostructuring of the material changes processability and (2) how altering current pathways can alter powder densification. In the case of the graphite die, the effective circuit of the system is dependent
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