Flow stress correction for hot compression of titanium alloys considering temperature gradient induced heterogeneous deformation

Uniaxial compression is widely employed to investigate the constitutive behavior and microstructural developments of metallic materials at elevated temperatures. However, the heat exchange between work-piece and anvils leads to temperature gradient and inhomogeneous deformation within the work-piece, which weakens the reliability of the testing method. The problem is more serious for titanium alloys due to the poor thermal conductivity and high sensitivity of deformation resistance on temperature. To quantify the effect of temperature gradient, a coupled thermo-electric-mechanical FE model is developed for hot compression of TA15 titanium alloy with a Gleeble thermal simulator. The precise temperature control is achieved via a negative feedback control algorithm. It is found that the temperature gradient prior to deformation is sensitive to the electrical contact conductance between specimen and anvils as well as the heat transfer between the anvil and anvil base. Bulging is sensitive to temperature gradient rather than interfacial friction. The measured flow stress is smaller than the true value at large strain for the titanium alloy, which is contrary to conventional perception. Thus, the traditional flow stress correction method degrades the accuracy of flow stress measurement. A correction procedure is developed based on inverse analysis of the FE simulation. The results can reduce the systematic error of flow stress measurement of titanium alloys with Gleeble system.