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

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 608; pp. 265 - 272
• Main Authors: Zhang, Tuoyang, Liu, Yong, Sanders, Daniel G., Liu, Bin, Zhang, Weidong, Zhou, Canxu
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier B.V 01.07.2014; Elsevier
• Subjects: 6Al–4V; Applied sciences; Corrosion; Corrosion mechanisms; Elasticity. Plasticity; Exact sciences and technology; Fine grain size; Heat treatment; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy; Microstructure; Sheet material; Superplastic forming; Superplasticity; Tensile tests; Titanium; Titanium base alloys; Fine grain size; Titanium; Superplasticity; Superplastic forming; 6Al–4V; Beta phase; Heat treatment; Material processing; Volume fraction; 6Al-4V; High temperature; Aspect ratio; Grain size distribution; Hydrofluoric acid; Test method; Oxidation; Rate constant; Elongation (mechanics); Strain rate; Grain size; Titanium alloy; Mechanical properties; Formability; Etching reagent; Nitric acid; Contamination; Tension test; Hot pressing; Tensile stress; Hot rolling; Fine grain structure; Microstructure
• DOI: 10.1016/j.msea.2014.04.098
• ISSN: 0921-5093

Fine-grained titanium 6Al–4V alloy, which typically has a grain size of about 1–2μm, can be made to superplastic form at around 800°C with special processing. The normal temperature for superplastic forming (SPF) with conventional titanium 6Al–4V sheet material is 900°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°C subsequent to hot rolling above the beta transus temperature, Tβ-(150–250°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 β phase volume fraction can contribute to higher superplastic elongations.