Study of the High‐Efficiency Superplastic Deformation of Ti–6Al–2.5Mo–1.5Cr–0.5Fe–0.3Si Alloy Based on the Strain Rate Sensitivity Index m

Abstract

Ti–6Al–2.5Mo–1.5Cr–0.5Fe–0.3Si is an (aþb) Ti-alloy and widely used in aerospace industries for fabrication of some critical engine components, including blade and turbine disk, for its stable mechanical properties at working temperature. [1–2] This Ti-alloy is also used for in other structural components in aircrafts. Similar to other high strength Ti-alloys, the difficulty in fabrication of complex components by using this Ti-alloy has become the bottleneck for its wide applications. It is thus crucial to develop efficient manufacturing technologies such as superplastic deformation (SPD) process to make complex components via fully employing its plastic deformation capacity. In tandem with this, the SPD or the SPD-based isothermally forging process is a promising forming process for fabrication of the complicated Ti-alloy aerospace components. For the SPD process to fabricate large and complex structural components, especially for net-shape or near netshape parts in aerospace industries, the SPD-based isothermal forging process is the key process. [3] In addition, the current SPD processes requires pre-processing such as severe plastic deformation, protium treatment, and thermo mechanical processing, [4–12] to ensurethe unique phase and microstructure condition of the sample materials can be obtained. The aim of the pre-processing is to seek the good superplasticity for the SPD of materials To develop efficient SPD processes, lot of efforts have been provided to conduct researches on the in-depth understanding of SPD mechanism, microstructure evolution, and the modeling and simulation of the SPD processes. The deformation behavior of SPD was first investigated via tensile test. [13–22] The deformation mechanism, microstructure evolution, phase transformation, and recrystallization were also explored and studied. [23–31] These prior arts provide a good basis and fundamental for the understanding of SPD processes. In this research, the so-called maximum strain rate sensitivity index (m) superplastic tension (MaxmSPD) [32–34] is first used to study the SPD of Ti–6Al–2.5Mo–1.5Cr–0.5Fe– 0.3Si. To overcome the inefficiency of the MaxmSPD process, which is a crucial barrier to the wide engineering applications of the process, a new tensile test with the high-efficiency superplastic deformation (SPD) based on the strain rate sensitivity index m (m-HESPD) is developed. Using this tensile test, the SPD of the Ti-alloy is conducted under the strain rate between the upper strain rate, at which the maximum m is obtained, and the lower strain rate, under which the minimum m occurs. The minimum m must meet the requirement of SPD, viz., m� 0.3. The strain rate for the m-HESPD is designed via considering the deformation efficiency. In this research, Tialloy Ti–6Al–2.5Mo–1.5Cr–0.5Fe–0.3Si is used as the testing material for realization of the above-mentioned SPD processes.