Processing and Properties of FeAl Sheets Obtained by Roll Compaction and Sintering of Water Atomized Powder
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tion of powder by roll compaction; de-bindering of green sheets followed by partial sintering; cold rolling, simultaneous annealing, and sintering of FeAl sheets. In step 1, pre-alloyed powders of FeA1 were obtained by water atomization of a molten FeAl alloy obtained by inductive melting of electrolytic grade iron, 99.99% aluminum, and Ferro-alloys of zirconium, molybdenum, and boron in air. In step 2, iron aluminide powders were consolidated into green sheets by roll compaction technique to a green density of 52%. In step 3, green sheets were de-bindered in an inert atmosphere for over 8 hr in the temperature range of 350 to 600 °C. De-bindered sheets were partially sintered in a vacuum furnace to provide a reasonable tensile strength for cold rolling. In step 4, cold rolling was carried out in multiple passes using a tungsten carbide rolling mill backed with hardened steel rolls until 60% reduction was achieved. In step 5, sheets were annealed in a vacuum furnace to remove the cold work prior to repeating cold rolling and annealing steps to obtain a sheet of 0.2mm thickness. A detailed description of the processing steps can be found in Deevi et all', and in Hajaligol et al 12 FeAI
Pure
Manufacturing
Green
Green Sheet
Powder
Elements (Fe,Al, etc.)
Manufacturing
Powder
Sintering Annealing
C
Porous I R-llig 1hee
Desirable Sheet Properties Reached?
Binder Burn out and Initial Sintering
NO YES
Fig. 1. A flowchart of the process steps employed to obtain thin sheets.
CONSOLIDATION OF POWDER INTO SHEET Among the powder processing techniques, roll compaction is extensively used by the industry to manufacture specialty alloys in various thicknesses by compacting irregular shape powders with the help of polymeric binders such as methocel, polyvinyl alcohol etc. Green sheets of FeAl of approximately 0.66 mm thickness were obtained using water atomized powder of -125 / + 47 gtm size distribution mixed with an organic binder such as methocel or polyvinyl alcohol. A schematic illustration of roll compaction is shown in Figure 2. The roll compaction works on the same principles as roll pressing and involves compaction of solid particles between two counter rotating rolls. A blended mixture of water atomized powder and binder were fed through the hopper into the feed zone of the rolls to obtain green sheets of FeAl of 0.66mm. The densities of the roll comapcted sheets can be varied by adjusting the polymeric binder, powder size, and size distribution of FeAI alloy powder. Hopper
Powder
Fig. 2. A schematic of roll com-
Compaction Roll
pacting machine for manufacturing green FeAI sheets. Compacted Sheet Conveyer
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THERMOMECHANICAL PROCESSING OF FeAl SHEETS The green sheets were first de-bindered in an oven in the temperature range of 350 to 600'C with flowing nitrogen to facilitate complete decomposition of the binders. The sheets were then transferred to a vacuum furnace to partially sinter at 1260 TC in a vacuum of 10 4 torr. Vacuum heating to 1260 'C also decomposes the remaining binder, an
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