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Appendix K TONNAGE POWDER METALLURGY DIJ PONT TITANIUM TONNAGE POWDER METALLURGY The barrier to important lower cost titanium mill products, based on du Pont's investigations In the 1950s and 1960s and highlighted in Chapter 11, may be the economical production of titanium powder containing less than 50 parts per million of by-product chloride. Du Pont did not solve this problem and, consequently, stopped its titanium TPM development in 1962. Its technologic achievements on the remainder of the TPM processes were considerable, however, and are outlined in thi s appendix. this problem and, Du Po nt f ound i t that, when ground in the presence of salt, produced a high purity, acicular powder. This powder was ideally suited for processing it directly to mill products (USP 2984560 and USP 30723473. - - could produce a f riable sponge from sodium reduction Du Pont then experimented with compacting these powders directly in the nip of a rolling mill to produce sheet and with the extrusion of green compacted billets to bar, tubing, and shapes. These experiments were so promising that du Pont proceeded to scale up powder manufacture to tonnage size batches. It established a direct powder conversion process for mill product manufacture at a commercial-size facility in Baltimore, Maryland. Sheet was produced by compacting powder continuously in the nip between two rolls. Through the development of a feed hopper and an edge control system, sheet with high green strength was produced (U. S . Patent 3530210 and U. S . Patent 3478136~. Sheet formed in these nips was 75 percent of theoretical density at a minimum. When the powders used were optimum in particle size distribution, the as-compacted density was in excess of 90 percent of theoretical. In contrast to the free-flowing microspheres required for the precision molding process, the du Pont direct powder rolling process requires dendritically shaped particles that interlock. The green sheet was coiled and sintered of f line in a continuous, inert gas, Wintering furnace. The sheet then was finished cold with intermediate anneals as necessary . Excellent control of final gauge was achieved along with high ductility and superior surface finish. Since all mill processing was done at room temperature, the hardness of the sheet was dependent only on the purity of the starting powder, the quality of the sintering atmosphere, and the time at temperature. 207
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208 The process also proved to be successful for producing continuou s 6A1-4V alloy sheet. Elemental powders were blended with master alloy powders and fed to the roll ni p through a hopper that prevented segregation of the master alloy powder. Most of the deformation necessary to reach f inal dimensions in the alloy sheet could be carried out through the ductile matrix of a partially homogenized alloy. Final homogenization through a 4-hour treatment at 1100°C, followed by finish rolling resulted in high-strength alloy foils with typical 6-4 properties. This sheet could readily be formed and spot welded into high-strength honeycomb structures (U. S. Patent 3084042) . Bars, tubing, and shapes were produced by ache extrusion of hydrostatically compacted billets. In some cases, extrusion was to finished dimensions; in others, to near finished dimensions followed by heat treatment and either rolling or drawing. Essential to the extrusion of these products was a lubrication process that served to permit streamlined flow of the billet through the die during extrusion (U.S. Patent 3481762~. Most titanium and blended titanium alloy billets were extruded below 400°C and contamination was not detectable. Billets up to 12 inches in diameter by 24 inches long were extruded. Reduction ratios of up to 40:1 were accomplished. Hydrostatic pressures of up to 8000 psi gave compacted densities as high as 97 percent. Theoretical density was readily achieved upon extrusion. Alloys extruded in the partially homogenized condition could be completely homogen] zed by annealing at temperatures of 1100°C for 4 hours either prior to or after redrawing. Du Pont tests could detect no differences in the mechanical properties between these materials and those from melted and wrought products of equivalent hardness. Tons of titanium powder were processed by du Pont. Du Pont never test marketed these metal products because there was one flaw that, in du Pont's judgment, would make their product noncompetitive with melted and wrought products. The titanium and titanium alloy mill products produced from powders contained very small quantities of chlorides trapped within the microstructure. These chlorides volatilized rapidly during welding and caused a buildup of salts on the tungsten welding electrode that resulted in an unstable arc. Chlorides in these products ranged f ram 0.05 percent to 0.01 percent. Experiments involving the dilution of these chloride levels with powder produced from hydrided and ground pure titanium that had been previously arc melted to remove the chlorides resulted in a judgment that satisfactory weldability would be reached at a chloride level of 0.005 percent or less. Achieving these chloride levels was judged at that time to be impractical f ram the manuf acturing standpoint . Theref ore, the entire program was discontinued based on the conviction that a marginally weldable product would not be acceptable commercially. Technology has advanced significantly since du Pont's work In the late 1950s. Twenty years of technical advances all over the world may well contain the clue to the manufacture of chloride-f ree powder. Indeed, D-H Titanium Company's report on its electrolytic titanium powder
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209 at the May 1980 Kyoto conference stated: "The low chloride content assures clean melting characteristics and makes the metal ideal for conversion into powder for use in powder metallurgy applications re quiring products f ree of internal porosity and having good weldability." New personnel, with new technical information and new viewpoints might well solve the trace chloride problem in titanium powder. Vacuum arc melting and large ingot technology have stood the test of time f or important parts of the broad titanium field (e.g. for large forgings and for thick plates), but for strip, particularly alloy strip, and for tubing, bars and shapes, direct powder conversion would appear to have signif icant advantages . As described in Chapter 10 sour-crude tubing could lead to a much larger titanium industry based on civilian demand and, thus, provide the capacity fly-wheel needed in case of a national emergency. Buchovecky, K. E., and Patton, REFERENCES Du Pant Patent s L. W., U.S. Patent 3478136, 11/11/69. Dombrowski, H. S., U.S. Patent 2984560, 5/16/61. Dombrowski, H. S., U.S. Patent 3072347, 1/8/63. Patton, W. L., U.S. Patent 3530210, 9/22/70. Wartel, W. S ., Wasilewski, R. J., and Pollock, W. I., U. S. Patent 3084042 4/2/63.
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9 - BIBLlOGRAPHIC DATA SHEET 4. ' ~1c Anti Subtitle ~1.~ Ti tani~nn: Pas t, Present, and Future 7 Auchot(s ) Panel on Assessment of Titanium Availability: Current and Future Needs _ . 9. Pcrforming, Organ~zar~on.Name and Address Nat tonal Materials Advisory Board National Research Council 2101 Constitution Avenue, N.W. Washington, D. C. 20418 _ 12. Sponsoring Organization .~;ame and Address Federal Emergency Management Agency SOO C Street, S.W. Washington, D. C. 20472 5. S~ppiomentary Notes 16. Abstracts _ 2. - 3. P
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