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Proceedings Paper

Processing method and process modeling of large aperture transparent magnesium aluminate spinel domes
Author(s): Jian Yu; Brandon McWilliams; Steven Kilczewski; Gary Gilde; Ashley Lidie; James Sands
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Paper Abstract

Polycrystalline spinel serves as an alternative to materials such as sapphire and magnesium fluoride that are currently being used in electromagnetic window applications such as missile domes, where high strength, high hardness and high transmittance in the visible and infrared spectra are required. The cubic crystal lattice of spinel imparts an isotropy to the bulk optical property, which eliminates optical distortion due to birefringence that occurs in sapphire and other non-cubic materials. The current study is to find a reliable manufacturing process to produce large magnesium aluminate spinel domes from powder consolidation efficiently. A binder-less dry ball milling process was used to deflocculate the spinel powder to increase its fluidity in an effort to ease the shape-forming. Dry ball milling time trials were conducted at several intervals to determine the appropriate level of time required to break up both the hard and soft agglomerates associated with the virgin spinel powder. The common problems encountered in dry powder shape-forming are crack growth and delamination of the green body during cold isostatic pressing (CIPing). The cracking and the delamination are due to the buildup of stress gradients on the green body that are created by the frictional force between the powder and the die wall or mold wall. To understand the stresses during the CIPing process, a finite element analysis of stresses on the green body was conducted. The simulation was used to evaluate the effect of die tooling and process characteristics on the development of stress gradients in the green body dome. Additionally, the effect of friction between the die wall and powder was examined by the simulation. It was found that by mitigating the frictional forces, cracking and delamination on the green body could be eliminated. A stepped-pressure CIPing technique was developed to reduce stress gradient build-up during CIPing. Also, oleic acid lubricant was applied to the die wall to reduce the wall friction between the powder and the die itself. As a result of these two above-mentioned methods, it was demonstrated that it is possible to consolidate a binder-free powder into large defect-free domes.

Paper Details

Date Published: 27 April 2009
PDF: 8 pages
Proc. SPIE 7302, Window and Dome Technologies and Materials XI, 730209 (27 April 2009); doi: 10.1117/12.817909
Show Author Affiliations
Jian Yu, U.S. Army Research Lab. (United States)
Brandon McWilliams, U.S. Army Research Lab. (United States)
Steven Kilczewski, U.S. Army Research Lab. (United States)
Gary Gilde, U.S. Army Research Lab. (United States)
Ashley Lidie, U.S. Army Research Lab. (United States)
James Sands, U.S. Army Research Lab. (United States)

Published in SPIE Proceedings Vol. 7302:
Window and Dome Technologies and Materials XI
Randal W. Tustison, Editor(s)

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