Constitutive analysis of hot deformation behavior of a Ti6Al4V alloy using physical based model

Abstract

The effect of deformation parameters on the flow behavior of a Ti6Al4V alloy has been studied to understand the deformation mechanisms during hot compression. Cylindrical samples with partially equiaxed grains were deformed in the α+β phase region at different thermo-mechanical conditions. To develop components with tailored properties, the physically based Estrin and Mecking (EM) model for the work hardening/dynamic recovery combined with the Avrami equation for dynamic recrystallization was used to predict the flow stress at varying process conditions. The EM model revealed good predictability up to the peak strain, however, at strain rates below 0.01 s⁻¹, a higher B value was observed due to the reduced density of dislocation tangles. In contrast, the flow softening model revealed higher value of constants a and b at high strain rates due to the reduction in the volume fraction of dynamic recrystallization and larger peak strain. The predicted flow stress using the combined EM+Avrami model revealed good agreement with the measured flow stress resulted in very low average absolute relative error value. The microstructural analysis of the samples suggests the formation of coarse equiaxed grains together with the increased β phase fraction at low strain rate leads to a higher flow softening.

Introduction

The hot deformation of Ti6Al4V alloy is considered as one of the most difficult processes due to its narrow processing window of time and temperature, to produce components with controlled microstructure and improved mechanical properties [1], [2]. Extensive research has already been carried out to understand Ti6Al4V alloy deformation behavior for different initial microstructures and processing conditions [3], [4], [5], [6], [7], [8], [9]. Studies have shown that the initial microstructure, significantly affects the flow behavior, and deformation temperature and deformation rate control the flow behavior and the final properties of the alloy. Further, the high diffusion coefficient of bcc β phase at high temperature single phase region (i.e. β) leads to better ductility and ease of deformation than the two phase region (i.e. α+β), where the combination of hcp α and bcc β makes the processing more difficult [10]. Though the processing is relatively difficult in the α+β phase region, a good combination of strength and toughness is obtained during the α+β phase deformation than the β phase. Therefore, it is essential to elucidate the deformation behavior of Ti6Al4V alloy in the α+β phase to improve the mechanical properties of components.

The hot deformation behavior usually consisted of the work hardening during the initial stage up to the peak strain and the flow characteristics beyond the peak strain. Multiple approaches are available to predict the flow behavior under multiple deformation conditions. The evolution of constitutive equations, where the instantaneous stress can be calculated as a function of deformation parameter and material property, are widely used [11], [12], [13], [14], [15]. This can be classified into two groups, namely i) empirical and ii) physical models. The empirical models are straight forward, where only some material constants are involved without physical meaning. On the other hand, the physical models constitute the micro-mechanisms involved during deformation [16]. A few studies employed semi empirical and artificial neural network (ANN) models to predict the flow stress of Ti6Al4V [13], [17], [18], [19], [20]. Limited attempts have been made, to the authors' knowledge, to predict the flow behavior of Ti6Al4V alloy through a physical approach because of the material's heterogeneous deformation behavior and complex microstructural evolution. The physical based internal state variable (ISV) approach, which relates the material properties to its microstructural and deformation mechanism, is used to predict the flow behavior for each individual phase of Ti6Al4V [21], [22], [23], [24], [25]. However, a combined approach to predict the work hardening/dynamic recovery up to the peak strain and the flow softening behavior beyond the peak strain has not yet been developed for Ti6Al4V alloy.

In this work, a physically based EM model [26], which predicts the work hardening/dynamic recovery behavior, and the Avrami equation to predict the flow softening due to dynamic recrystallization [27] were considered. Thus, the combined EM+Avrami model was used to predict the flow stress in the initial stages of deformation and beyond the peak strain during hot deformation of Ti6Al4V alloy in the α+β phase region. The flow behavior analysis and the microstructural evolution were carried out to understand the thermo-mechanical behavior. The model constants were identified for varying hot deformation conditions in the α+β phase region. Finally, the predicted flow stresses were compared with the measured flow stress to analyze the accuracy of the developed model.