Ti-6Al-4V Production-Scale Sheet for Low Temperature SPF Applications

Part manufacturers have sought to reduce the temperatures used for the superplastic forming (SPF) of Ti-6Al-4V sheet through reduction in the grain size.  Some of the primary reasons include; poor life of the stainless / alloy steel dies, platens and heating elements at typical SPF forming temperatures of 1650-1700°F (899-927°C), and also formation of an alpha case layer on titanium during forming.  Dies are expensive to manufacture, typically have long lead times and require significant cleaning and rework when used at higher temperatures / multiple thermal cycles.  Furthermore, the alpha case layer formed on the titanium parts must be removed through conditioning and chemical milling processes that result in reduced yield and produce associated process wastes.  Therefore, part manufacturers comfortable with the Ti-6Al-4V alloy have requested fine grain Ti-6Al-4V sheet in order to take advantage of low forming stress at temperatures less than 1500°F (816°C) in order to minimize these deleterious effects.  Here, the mechanical properties, SPF characteristics (flow stress, m-value) as a function of strain rate and temperature, and microstructure of RTI Ti-6Al-4V production fine grain as compared to standard sheet will be discussed, along with results from RTI internal SPF trials.