Hot Forming Characteristics of SP -700 Titanium Alloy

摘要

SP-700 is a relatively new titanium alloy consisting of a nominal chemical composition of Ti-4.5 Al-3V-2Fe-2Mo (mass%). This alloy was originally developed for applications using superplastic forming (SPF) technology at temperatures below 800℃, and therefore this alloy exhibits an inherent fine microstructure in the range of 3 - 5 μm in average primary a-grain size. The superplastic forming of titanium alloys at temperatures below 800℃ offers a number of technological and industrial advantages, namely a significant extension of the tool life and the SPF press maintenance cycle, as well as a significantly reduced level of the usage on the environmentally destructive chemical milling process for removing α-case. It is important to note that all those advantages reduce overall production costs and increase production efficiency in superplastic forming of titanium alloys. In this paper, superplastic forming characteristics of SP-700 are presented along with a discussion on fundamental SPF properties such as m-value and flow stress as a function of the temperature and the strain rate. This alloy exhibits excellent m-value characteristics at a wider range of strain rate. For example, the m-value reaches its highest at 775℃, and remains over 0.5 at strain rates slower than 10~(-3) s~(-1). Also discussed is SP-700's capability of creep forming and hot sizing. Residual stress of SP-700 reduces to less than 20% of initial level by relaxation at temperatures over 600℃ for 30min. Examples of industrial applications in such forming areas are also presented with emphasis on production advantages as compared with other titanium alloys.