DEVELOPMENT AND APPLICATIONS OF TITANIUM-ALLOY SP-7OO WITH HIGH FORMABILITY

A new beta-rich alpha + beta titanium alloy, SP-700, was designed to i mprove hot workability and mechanical properties over Ti-6Al-4V alloy. The chemical composition of the alloy is Ti-4.5Al-3V-2Mo-2Fe, and par ticularly enhanced properties include superplasticity, hardenability, and fatigue strength. Owing to its extremely fine microstructure and l ow beta-transus temperature, SP-700 is superplastically formable at te mperatures below 1073 K without significant increase in flow stress. M ore advantageously, diffusion bonding is also accomplished around this temperature. The low temperature SPF/DB process not only saves die ma terial life and process costs, but also reduces alloy degradation from exposure al elevated temperatures, e.g., grain growth and oxidation. Isothermal and conventional hot forging are also performed better with SP-700 than ordinary titanium alloys like Ti-6Al-4V. The alloy is hea t treatable and one of its many mechanical properties is that high str ength over 1200 MPa is attained in solution treatment and aging condit ion. The advantage to this is that the moderately stable beta phase to lerates air fooling for solution treatment. Thermal distortion in work pieces, which would be caused by rapid cooling, is consequently avoid ed. In addition, the alloy is cold workable in such cases as rolling a nd blanking, when annealed around 1073 K. Furthermore, fatigue life an d resistance to hydrogen-assisted stress corrosion cracking are improv ed due to reduced aluminum content. The superior properties of SP-700 over conventional titanium alloys make new applications possible in no n-aerospace fields including sporting goods, hand tools, and wrist wat ches. Golf drivers, with club heads made of SP-700, are sold commercia lly Crampons, a new product of SP-700, withstand cold blanking and ben ding which reduces production costs and extends life. Due to its high forgeability, SP-700 is expected to extend its application to the prod uction of steam turbine blades. Details of practical applications have been demonstrated together with advantages of properties and fabricat ion for the blades.