Study On High Temperature Hot Deformation Behavior Of TC18 Titanium Alloy

TC18 titanium alloy has become a popular study material in the field of aviation due to its great comprehensive mechanical properties,and is often used in a large number of aircraft loadbearing components.Due to the high deformation resistance of TC18 titanium alloy in the process of high temperature thermal processing,especially sensitive to thermal deformation parameters,which makes the alloy difficult to deform during processing and thus leads to product defects,such as distortion,cracking and so on.Therefore,this paper investigates the flow behavior,constitutive model and thermal processing map of TC18 titanium alloy,which has certain theoretical significance and high degree of practical applicability for mastering the thermal deformation behavior of this alloy and optimizing the thermal processing process.The details of the study are as follows.First,the true stress-true strain curves of the alloy are plotted based on the data obtained from high-temperature hot compression experiments under each deformation condition.The deformation law of flow stress during thermal deformation is analyzed according to the curve and the deformation mechanism is described.When the strain rate is fixed,the flow stress in the true stress-true strain curve decreases noteblely due to the increase in temperature;when the deformation temperature is fixed,the flow stress in the curve rises with the increase in strain rate,indicating that the flow stress of TC18 titanium alloy is extremely sensitive to both temperature and strain rate.The deformation mechanism shifts from dynamic reversion to dynamic recrystallization when the deformation temperature is increased or the strain rate is decreased.Secondly,based on the true stress-true strain curve,two methods are used to construct the thermal deformation constitutive model of TC18 titanium alloy.The Arrhenius equation is used to describe the constitutive relationship between the flow stress and deformation parameters of TC18 titanium alloy,and the heat deformation activation energy Q is calculated.In addition,the thermal deformation model of TC18 titanium alloy is constructed again using BP neural network,and the weights and thresholds of the neural network model are optimized using genetic algorithm.The flow stresses of TC18 titanium alloy were predicted by both models,and the predicted and experimental values were compared and analyzed to verify the accuracy of both models using the correlation coefficient(R)and the average relative error(AARE),the results show that the BP neural network model optimized using genetic algorithm has better prediction performance,while the Arrhenius principal equation is more practical.Finally,the thermal processing map of TC18 titanium alloy at each strain were developed using the Dynamic Material Model(DMM)and Rheological instability criterion.The results of the thermal processing map show that the instability zone of TC18 titanium alloy during thermal processing usually occurs at low deformation temperature and high strain rate,and the instability zone increases with increasing strain.The power dissipation factor of the alloy increases as the deformation temperature increases and the strain rate decreases.