Powder Metallurgical Processing of Rhenium

The unique attributes of rhenium make it the material of choice for propulsion system engineers. Its tensile strength at 1900 deg C (3452 deg F) is around 193 MPa (28 ksi), and rhenium is immune to thermal shock: no ductile-brittle transition temperature has been observed. It is compatible with most propellents, and in the environs of space it can successfully endure several thermal cycles with ease. However, making components for space applications is another matter. Rhenium presents a formidable challenge to the process engineer. It has a melting point of 3180 deg C (5756 deg F), second only to tungsten. Its elastic modulus is about 420 GPa (61 Msi), more than twice that of steel, and its work-hardening coefficient is about 0.5, comparable to that of copper and brass. These characteristics limit room-temperature working to a few percent strain, and render single-point machining processes impractical. In addition, rhenium surface oxides readily sublime between 600 and 1200 deg C (1112 and 2192 deg F). Therefore, working of rhenium at elevated temperatures without a protective environment is not possible. Lastly, rhenium is an expensive material, currently costing about $1800 per kilogram. These qualities all make rhenium an ideal candidate for net-shape processing. To eliminate the need for machining, powder injection molding and cold isostatic pressing have been developed with improved net shape capability. This article describes these two processes, as well as a compaction modeling program developed to optimize tool design.