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Measurement and modeling of interface heat transfer coefficients

Description: The results of preliminary work on the modeling and measurement of the heat transfer coefficients of metal/mold interfaces is reported. The system investigated is the casting of uranium in graphite molds. The motivation for the work is primarily to improve the accuracy of process modeling of prototype mold designs at the Los Alamos Foundry. The evolution in design of a suitable mold for unidirectional solidification is described, illustrating the value of simulating mold designs prior to use. Experiment indicated a heat transfer coefficient of 2 kW/mS/K both with and without superheat. It was possible to distinguish between solidification due to the mold and that due to radiative heat loss. This permitted an experimental estimate of the emissivity, epsilon = 0.2, of the solidified metal.
Date: January 1, 1985
Creator: Rollett, A.D.; Lewis, H.D. & Dunn, P.S.
Partner: UNT Libraries Government Documents Department

Chemistry modification of high oxygen-carbon powder by plasma melting

Description: State-of-the-art melting of tantalum and tantalum alloys has relied on electron beam (EB) or vacuum-arc remelting (VAR) for commercial ingot production. The limited number of melting techniques for these materials are the result of high melting temperatures and reactivity with conventional mold/crucible materials. In addition, the required vacuum levels used in the EB and VAR processes limit these techniques to relatively low interstitial content material due toe extensive outgassing during melting. Plasma arc melting (PAM) provides and alternative for melting tantalum and offers the advantage or processing under inert or other gases rather than vacuum, The plasma process is not sensitive to materials outgassing and allows for the direct recycling of material that would otherwise be reprocessed by chemical extraction. The current work examines melting of high interstitial content tantalum powder by the plasma arc process. Various cover gases of argon-hydrogen and helium-hydrogen were investigated to determine best melt quality. Melted ingots were characterized by chemical and metallographic methods to determine overall interstitial content, compound and morphology.
Date: January 1, 1996
Creator: Dunn, P.S.; Korzekwa, D.R. & Garcia, F.G.
Partner: UNT Libraries Government Documents Department

Plasma arc melting of zirconium

Description: Zirconium, like some other refractory metals, has an undesirable sensitivity to interstitials such as oxygen. Traditionally, zirconium is processed by electron beam melting to maintain minimum interstitial contamination. Electron beam melted zirconium, however, does not respond positively to mechanical processing due to its large grain size. The authors undertook a study to determine if plasma arc melting (PAM) technology could be utilized to maintain low interstitial concentrations and improve the response of zirconium to subsequent mechanical processing. The PAM process enabled them to control and maintain low interstitial levels of oxygen and carbon, produce a more favorable grain structure, and with supplementary off-gassing, improve the response to mechanical forming.
Date: December 31, 1997
Creator: Tubesing, P.K.; Korzekwa, D.R. & Dunn, P.S.
Partner: UNT Libraries Government Documents Department

Chemistry modification of high oxygen-carbon powder by plasma melting: Follow up to complete the story

Description: State of the art melting of tantalum and tantalum alloys has relied on electron beam (EB) or vacuum arc remelting (VAR) for commercial ingot production. Plasma arc melting (PAM) provides an alternative for melting tantalum that contains very high levels of interstitials where other melting techniques can not be applied. Previous work in this area centered on plasma arc melt quality and final interstitial content of tantalum feedstock containing excessive levels of interstitial impurities as a function of melt rate and plasma gas. This report is an expansion of this prior study and provides the findings from the analysis of second phase components observed in the microstructure of the PAM tantalum. In addition, results from subsequent EB melting trials of PAM tantalum are included.
Date: March 1, 1998
Creator: Dunn, P.S.; Korzekwa, D.R.; Garcia, F.G. & Michaluk, C.A.
Partner: UNT Libraries Government Documents Department

Titanium-tantalum alloy development

Description: Research has been underway at Los Alamos National Laboratory for several years to develop an alloy capable of containing toxic materials in the event of a fire involving a nuclear weapon. Due to their high melting point, good oxidation resistance, and low solubility in molten plutonium, alloys based on the Ti-Ta binary system have been developed for this purpose. The course of the alloy development to-date, along with processing and property data, are presented in this overview.
Date: April 1, 1996
Creator: Cotton, J.D.; Bingert, J.F.; Dunn, P.S.; Butt, D.P. & Margevicius, R.W.
Partner: UNT Libraries Government Documents Department

The influence of melting process and parameters on the structure and homogeneity of titanium-tantalum alloys

Description: Alloys of titanium with refractory metals are attractive materials for applications requiring high temperature strength and corrosion resistance. However, the widely different characteristics of the component elements have made it difficult to produce sound, compositionally homogeneous ingots using traditional melting techniques. This is particularly critical because the compositional ranges spanned by the micro- and macrosegregation in theses systems can easily encompass a number of microconstituents which are detrimental to mechanical properties. This paper presents results of a study of plasma (PAM) and vacuum-arc (VAR) melting of a 60 wt% tantalum, 40 wt% titanium binary alloy. The structural and compositional homogeneity of both PAM consolidated + PAM remelted, and PAM consolidated + VAR remelted ingots were characterized and compared using optical and electron microscopy and x-ray fluorescence microanalysis. Additionally, the effect of melting parameter, including melt rate and magnetic stirring, was studied. Results indicate that PAM remelting achieves more complete dissolution of lie starting electrode, due to greater local superheat, than does VAR remelting. PAM remelting also produces a finer as-solidified grain structure, due to the smaller molten pool and lower local solidification times. Conversely, VAR remelting produces an ingot with a more uniform macrostructure, due to the more stable movement of the solidification interface and more uniform material feed rate. Based on these results, a three-step process of PAM consolidation, followed by a PAM intermediate melt and a VAR final melt, has been selected for further development of the alloy and processing sequence.
Date: December 31, 1995
Creator: Dunn, P.S.; Korzewka, D.; Garcia, F.; Damkroger, B.K.; Van Den Avyle, J.A. & Tissot, R.G.
Partner: UNT Libraries Government Documents Department

Plastic Flow Characteristics of Uranium-Niobium as a Function of Strain Rate and Temperature

Description: The stress-strain response of uranium-niobium alloys as a function of temperature, strain-rate and stress-state was investigated. The yield and flow stresses of the U-Nb alloys were found to exhibit a pronounced strain rate sensitivity, while the hardening rates were found to be insensitive to strain rate and temperature. The overall stress-strain response of the U-6Nb exhibits a sinusoidal hardening response, which is consistent with multiple deformation modes and is thought to be related to shape-memory behavior.
Date: January 4, 1999
Creator: Cady, C.M.; Gray, G.T. III; Hecker, S.S; Thoma, D.J.; Korzekwa, D.R.; Patterson, R.A. et al.
Partner: UNT Libraries Government Documents Department

Strengthening mechanisms of tungsten powder reinforced uranium

Description: Tungsten powder reinforced uranium exhibits a three-fold increase in yield strength due to precipitation hardening. The tungsten-rich interphase precipitates form at moving phase boundaries during slow cooling. Further increases in yield strength, attained with increasing tungsten content, are due to composite strengthening; this is verified by increasing elastic modulus with increasing tungsten content. Age hardening behavior is observed, with strengthening occurring at aging temperatures low in the alpha phase. Aging higher in alpha gives initial strengthening followed by rapid overaging. Beta phase aging results in a very soft structure with precipitates visible optically. Wrought material exhibits significant strain hardening as well as composite strengthening due to elongation of the tungsten particles. 7 refs., 15 figs., 4 tabs.
Date: January 1, 1989
Creator: Lewis, M.A.K.; Hill, M.A.; Rollett, A.D.; Dunn, P.S.; Mortensen, A. (Los Alamos National Lab., NM (USA) & Massachusetts Inst. of Tech., Cambridge, MA (USA))
Partner: UNT Libraries Government Documents Department