Research on the thermal forming limit diagram (TFLD) with the M-K model for Ti-6Al-4V alloy

The formability of Ti-6Al-4V titanium alloy at high temperatures is critical in assisting the manufacture of aircraft components. The predicted forming limit curves (FLCs) of Ti-6Al-4V under elevated temperatures are mainly determined by the constitutive equation introduced into the M-K model. In the article, the thermal tensile test (650–750 °C, strain rate 0.1–0.001 s⁻¹) was performed, and the relevant flow curve was fitted by the Backofen constitutive equation considering the variation of material parameters. The M-K model was established to predict the forming limit diagram (FLD) of Ti-6Al-4V alloy at various temperatures and strain rates based upon the Backofen model and Von Mises yield criterion, which was further verified by the hot Nakajima-type bulging experiment. The bulging experiment was also conducted by the finite element method (FEM) to obtain the limit dome height (LDH) at various temperatures under the strain rate of 0.1s⁻¹. Moreover, the internal forming mechanism of Ti-6Al-4V alloy with strain rate and temperature was analyzed from the micro-perspective. The contrast of experimental and simulation results indicated that the formability of the alloy increased with increasing temperature and decreasing strain rate. The M-K model utilizing the Backofen model conduces to the accurate thermal forming limit diagram (TFLD) prediction of Ti-6Al-4V.