Development of fine-grain size titanium 6Al-4V alloy sheet material for low temperature superplastic forming

Fine-grained titanium 6Al-4V alloy, which typically has a grain size of about 1-2 mu m, can be made to superplastic form at around 800 degrees C with special processing. The normal temperature for superplastic forming (SPF) with conventional titanium 6Al-4V sheet material is 900 degrees C. The lower temperature performance is of interest to the Boeing Company because it can be exploited to achieve significant cost savings in processing by reducing the high-temperature oxidation of the SPF dies, improving the heater rod life for the hot presses, increasing operator safety and replacing the chemical milling operation to remove alpha case contamination with a less intensive nitric hydrofluoric acid etchant (pickle). In this report, room temperature tensile tests and elevated temperature constant strain rate tensile tests of fine-grained Ti-6Al-4V sheets provided by the Baoti Company of Xi'an, China, were conducted according to the test method standards of ASTM-E8 and ASTM-E2448. The relationships among the processing parameters, microstructure and superplastic behavior have been analyzed. The results show that two of the samples produced met the Boeing minimum requirements for low-temperature superplasticity. The successful material was heat-treated at 800 degrees C subsequent to hot rolling above the beta transus temperature, T-beta-(150-250 degrees C). It was found that the sheet metal microstructure has a significant influence on superplastic formability of the Ti-6Al-4V alloy. Specifically, fine grains, a narrow grain size distribution, low grain aspect ratio and moderate beta phase volume fraction can contribute to higher superplastic elongations. (C) 2014 Elsevier B.V. All rights reserved.