Solid-state foaming of titanium by superplastic expansion of argon-filled pores
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Solid-state foaming of commercial purity titanium was achieved by hot-isostatic pressing of titanium powders in the presence of argon, followed by expansion of the resulting high-pressure argon bubbles at ambient pressure and elevated temperature. The foaming step was performed under isothermal conditions or during thermal cycling around the ␣/ allotropic temperature of titanium. Such thermal cycling is known to induce transformation superplasticity (TSP) in bulk titanium due to the complex superposition of internal transformation stresses and an external biasing stress; TSP was found to be active during foaming, where the deviatoric biasing stress was provided by the internal pore pressure. As compared to isothermal control experiments where foam expansion occurred by creep only, TSP foaming under thermal cycling conditions led to significantly higher terminal porosity (41% as compared to 27%). The foaming rates were also higher for the TSP case before pore growth ceased. Additionally, foaming experiments were conducted under an externally applied uniaxial tensile stress of 1 MPa. This procedure resulted in foaming kinetics and porosities similar to those achieved without an external stress and, for the TSP case, led to high aspect ratio pores elongated in the direction of the applied external stress.
I. INTRODUCTION
Titanium-based foams have many potential applications due to titanium’s outstanding mechanical properties, low density, and high chemical resistance. Potential uses include load-bearing applications as sandwich cores in the aerospace and ship-building industries1,2 and as porous implants in the biomedical industry, for which the excellent biocompatibility and fatigue properties of titanium are also essential.1–5 Several different methods exist for metallic foam fabrication, as reviewed in Refs. 1 and 2. Some metallic foam can be formed in the melt, e.g., by adding foaming agents (gas or gas-producing solids) during solidification, by lost-foam casting, by infiltration of a granular bed, or by gas-eutectic transformation.1,2 While these techniques have been commercially successful in fabricating, e.g., aluminum and zinc foam,1,2 they are unsuitable for foaming titanium and titanium alloys due to the high melting temperature and melt reactivity of titanium. Furthermore, sintering of loose powders gives only low porosities (except when a fugitive filler is used6) while sintering of short wires is uneconomical for titanium. Slurry foaming, electrolytic deposition, and metal a)
Present address: Medtronic AVE, 2345 Circadian Way, Santa Rosa, CA 95407.
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J. Mater. Res., Vol. 16, No. 5, May 2001 Downloaded: 13 Mar 2015
sputtering on a polymer substrate are also unsuitable for titanium foams due to contamination from the polymer substrate during its removal by burn-off. Solid-state foaming methods are thus more promising options for titanium foam production. A novel technique for solid-state processing of Ti– 6Al–4V foams was first described and patented by Kearns et al
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