Advanced manufacturing by spray forming: Aluminum strip and microelectromechanical systems Page: 2 of 8
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ALUMINUM ALLOY STRIP
Currently, nearly all commercial aluminum sheet is produced by conventional ingot metallurgy (I/M) hot
mill processing. Rapid solidification/near-net-shape processing by spray forming offers technical and economic
advantages. Cost savings will largely result from process simplification and the need for fewer unit operations.
In a recent technoeconomic analysis , industry savings of $15MM per year (10% overspray) or $27MM per
year (no overspray) were estimated for an annual production rate of 2.1 billion pounds of spray-formed
aluminum sheet. Energy savings for high volume strip/sheet alloys are substantial and result from eliminating
intermediate, energy intensive, hot rolling steps necessary with conventional I/M processing. For example, each
ton of spray-formed aluminum sheet saves 4.2 X 106 Btu compared to I/M processing. This amounts to a
savings of 4.4 X 1012 Btu/year for the U.S. aluminum industry if 25% of current U.S. aluminum sheet production
is converted to spray forming. Technical benefits include improved metallurgical homogeneity, elimination of
macrosegregation, refined equiaxed grain structures, and improved impurity tolerance.
Strip Preparation and Properties
The approach used to produce aluminum strip on a continuous basis, depicted schematically in Figure 1,
has also been used to spray-form steel strip and metal-matrix-composite strip. The alloy to be sprayed is melted
under an inert atmosphere, superheated about 150*C, and pressure-fed into a de Laval (converging/diverging)
spray nozzle of our own design. A high-temperature, high-energy inert gas flow rapidly disintegrates the molten
aluminum into fine droplets that are entrained by the jet in a directed flow, and deposited onto a grit-blasted,
water-cooled, mild steel drum. The resultant strip is detached and hot rolled to full density. A purged nitrogen
atmosphere within the spray apparatus minimizes oxidation of the melt, surface oxidation of the strip, and in-
flight oxidation of the atomized droplets.
Nitrogen is used as the atomizing gas because it does not react with aluminum under spray conditions. A
gas-to-metal mass flow ratio (G/M) of about 0.3 is typically used, and metal mass flow rates are in the range 500
to 3000 lb/h per inch of nozzle width transverse to the flow direction. This nozzle dimension is scaled for a
desired sheet width or mass throughput. For example, a 24 in. wide nozzle produces strip at a rate of
approximately 12,000 to 72,000 lb/h. Overspray losses, defined as unconsolidated particulate and thin edge
trimmings, are about 9% for a bench-scale (-3/4 in. wide) nozzle, and decrease as the nozzle is scaled.
The cooling rate of droplets in 6061 aluminum spray jets was estimated by measuring the dendrite cell size
in polished/etched powders. Powder was partitioned into size bands using sieves of 300, 212, 177, 149, 125, 75,
63, 45, 38, 25, 20, 15, 10, and 5 pm. In general, dendrite cell size increased with increasing powder size,
consistent with previously published results on gas atomized aluminum alloys [9-13]. For example, the cell size
increased from about 1.8 pm for a 20 m particle, to about 9 pm for a 200 pm particle. Cell size was found to
follow a power law relationship with powder size. Measured cooling rates (Figure 2) ranged from about 102 to
- . -. -Substrate
Figure 1. Schematic of Approach for Producing Aluminum Alloy Strip.
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McHugh, K.M. Advanced manufacturing by spray forming: Aluminum strip and microelectromechanical systems, article, December 31, 1994; Idaho Falls, Idaho. (digital.library.unt.edu/ark:/67531/metadc681986/m1/2/: accessed March 24, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.