Atomization and deposition process for spray metal forming. 

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... Department of Defense (DoD) and industry have placed emphasis on the production of materials that are lighter and cheaper, but also have higher performance. The Applied Research Laboratory at Penn State is currently developing advanced materials processes that addresses the manufacture of low-cost, lightweight, high-performance materials. Spray metal forming has emerged as a viable process for producing unique materials. To meet stated DoD and industry objectives, ARL’s Material Science Division has established a spray metal forming facility that specializes in aluminum alloys. Spray-formed billets are now being produced on a routine basis. These billets are combined, in the downstream manufacturing process, with extrusion or forging, and then coupled with high-speed machining to produce components in final form. Spray metal forming is a rapid solidification technology for producing semi-finished tubes, billets, plates, and simple forms in a single integrated operation. The most widely used spray metal process is patented by Osprey Metals Ltd., Neath, Wales. 1 Spray forming involves converting a molten metal stream into a spray of droplets by high-pressure gas atomization ( Figure 1). The droplets cool rapidly in flight and ideally arrive at a collector plate with just enough liquid content to spread and completely wet the surface. The metal then solidifies into an almost fully dense preform with a very fine, uniform microstructure. Steel, copper, nickel-based superalloys, and aluminum alloys have been successfully spray formed. A schematic of the spray forming plant at ARL Penn State is shown in Figure 2. Metal is placed in a crucible and heated to the desired temperature under an inert atmosphere, usually argon or nitrogen. The spray chamber is purged with nitrogen until there is a very low level of oxygen present. A stopper rod is lifted and the metal stream flows through the nozzle in the bottom of the crucible. Pressure in the crucible is increased to maintain a constant metal flowrate. High-pressure nitrogen jets at near-sonic velocity atomize the molten stream and carry it toward the collector plate. The collector plate is placed on a ram that rotates and withdraws so that the buildup of the billet is uniform and the spray height or distance the metal droplets travel is the same during the entire spray forming operation. This ensures the microstructure and the precipitates are uniform in size and distribution throughout the entire billet. It is also possible to inject particulate reinforcement such as silicon-carbide (SiC) into the atomized metal spray. These particles are deposited with the molten metal on the surface of the billet to produce a metal matrix composite (MMC) with a uniform distribution of the particulate. See Figure 3. Process variables are closely controlled to produce the desired cooling rate and microstructure. These include the gas-to-metal (G/M) ratio, commonly defined as cubic meters of gas per kilogram of metal, the pour temperature, spray height or metal droplet flight distance, and metal flowrate. These variables are adjusted to optimize the desired properties and to reduce production costs. Rapid solidification processes such as spray metal forming offer some distinct advantages over conventional ingot metallurgy processing. Superior properties due to fine grain sizes; a fine, homogeneous distribution of second phase precipitates; and the absence of macrosegregation result from cooling rates on the order of 10 3 to 10 5 K/s (although not as high as other rapid solidification processes; e.g. gas atomization processes approach 10 6 K/s). 2 The higher range in cooling rate in spray forming is obtained by higher G/M ratios. In certain alloy systems, a high volume fraction of fine (0.05 to 0.2 μ m) intermetallic dispersoids may be obtained with high G/M ratios. Most other rapid solidification processes produce metal powders as the first step in processing. The aluminum alloy powders are atomized in air to form a thin layer of oxide to make the powders safer to handle. Subsequently, these metal powders must be sieved, classified, and consolidated in an inert atmosphere. Spray metal forming offers the distinct advantage of skipping these intermediate steps by atomizing and collecting the spray in the form of a billet in a single operation. Also, the elimination of powder handling reduces oxide content and enhances ductility. The Penn State spray metal forming facility is a research scale plant that is capable of producing billets, plates, and tubes. The melt capacity is 145 lbs. of aluminum. Billets with diameters from 6 to 10 inches can be produced with lengths up to 16 inches. Plates can be produced that are 6 inches wide by 12 inches long with thickness up to 0.8 inches. Tube sizes range from 3 to 9 inches inside diameter with wall thickness up to 1 inch. A powder feeder may be used to inject particulate reinforcement into the gas stream near the point of atomization to form a metal matrix composite with uniform distribution of the injected particulate. The Metals and Ceramics Processing Department has facilities to extrude rod stock from spray-formed billets, characterize microstructure by preparing and examining metallography samples, and perform hardness tests. The department’s extrusion press can extrude materials up to 2 inches in diameter at temperatures up to 970 ° F and loads approaching 80 tons. Metallography facilities include an automatic polisher and an inverted metallograph equipped with image analysis software. Heat treating furnaces are also available within the department. Using other Penn State facilities, department personnel perform tensile, dynamic modulus, and dilatometer (coefficient of thermal expansion) tests; scanning, backscatter, and transmission electron microscopy; electron microprobe; and x-ray diffraction. In addition, a wealth of facilities and expertise are within easy access at Penn State’s Materials Research Institute. When necessary, these tests can be preformed within a week to give an initial characterization of an alloy. Many different aluminum alloys and SiC-particulate reinforced aluminum MMCs have been ...