Low-Cost Fabrication of Tungsten-Rhenium Alloys for Friction Stir Welding Applications
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DUCTION
ADVANCING the solid-state joining technique of friction stir welding (FSW) to join high strength, high-temperature alloys is limited by available tooling materials. Since the FSW process joins materials at roughly 0.7 to 0.9 their homologous temperature, tooling for steels must use materials that retain their strength at temperatures in excess of 1000 °C.[1,2] Among the limited alloys that meet this high-temperature requirement are polycrystalline cubic boron nitride (PCBN)[3,4] and the family of tungsten (W) alloys.[5,6] While PBCN is a brittle ceramic, the family of tungsten (W) single-phase alloys retains ductility across the temperature range of interest[7,8] especially with refined microstructures.
JUDY SCHNEIDER, JORDAN TERRELL, and LAURA FARRIS are with the Department of Mechanical and Aerospace Engineering, University of Alabama in Huntsville, 310 Sparkman Drive, Huntsville, AL 35899. Contact e-mail: [email protected] DENNIS TUCKER is with the National Aeronautics and Space Administration, Marshall Space Flight Center, Marshall Space Flight Center, AL 35812. TODD LEONHARDT is with Rhenium Alloys, 38683 Taylor Parkway, North Ridgeville, OH 44035. HENNIG GOLDBECK is with the Bundesanstalt fu¨ r Materialforschung und -pru¨ fung, Unter den Eichen 87, 12205 Berlin, Germany. Manuscript submitted June 14, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Typically, single phase, solid solution-strengthened alloys are desirable for their high temperature stability; however, the high strength is accompanied by a corresponding decrease in ductility. Rhenium (Re) is a crucial alloying element in the W family of alloys due to its special strengthening effects, often referred to as the ‘‘Re effect’’.[9–13] Since maintaining a fine grained microstructure further improves the ductility of the strengthened W-Re alloy the use of carbide additives, such as hafnium carbide (HfC), is used to pin grain boundaries thereby further increasing the grain size stability at elevated temperatures.[14–21] The high temperature stability and ductility across a range of temperatures, enables the W-Re alloys to be machined to form the desired tool shape. Re is soluble in W up to 27 pct Re, with 25 pct Re obtaining the highest strength.[22,23] Above the solubility limit, the intermetallic r phase forms reducing the mechanical properties.[24] In high-melting temperature materials with low diffusion rates, such as the W alloys, powder metallurgy has traditionally been used to sinter the powders at temperatures in excess of 2000 °C for times as long as 24 hours.[25] Reducing the powder size using ball milling can reduce the diffusion distance to shorten the sintering time. However, the long ball milling times, in excess of 20 hours, required to produce nanocrystalline powders typically results in contamination from the milling medium.[5,26–30] Densification can be further inhibited due the increased surface area which increases the amount of native surface oxides. Thus, most of the
sintering studies found that to obtain densities above 90 pc
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