Influence of β phase fraction on deformation of grains in and around shear bands in machining of titanium alloys
Introduction
Titanium alloys have α and β phases at room temperature. It is possible to obtain a wide range of mechanical properties in titanium alloys by changing the proportion of phase compositions. The phase composition can be altered by either adding alloying elements or changing cooling rates during their fabrication. α phase has hexagonal crystal structure and has a limited number of slip systems. Therefore, titanium alloys with higher proportion of α phase are difficult to process [1]. However, these alloys are more temperature resistant due to lower contents of β phase fraction. It may be noted that the lower the β phase fraction, the higher the temperature stability. The β phase has body centre cubic structure and has more number of slip systems. Therefore, a higher proportion of β phase in titanium alloys improves their processability. However, it reduces the high temperature stability of β rich titanium alloy. Thus, the proportion of β phase in titanium alloys significantly changes mechanical properties of titanium alloys and their consequent applications. The β phase fraction in α alloy is low, it is relatively higher in α+β alloy, and is the highest in β rich alloy. Table 1 illustrates the changes that occur in the mechanical properties and consequent applications of the titanium alloys with increasing β phase fraction.
The machinability of titanium alloys mainly depends upon the temperature stability of the alloys, which as mentioned earlier is a function of β phase fraction in titanium alloys. The key phenomenon which plays an important role during machining of titanium alloys is the concentration of heat in the shear zone that leads to thermal softening and formation of localized shear bands [2]. The shear localization causes fluctuations in cutting forces, coupled with lower elastic modulus of work material, introduces fluctuations in cutting forces and vibrations in the tool-work-machine system [3]. The proportion of β phase also influences the shear band formation phenomenon as it changes the temperature stability of the material.
Very few studies have been carried out to understand the role of β phase fraction in the machinability of titanium alloys. Most of the studies have been carried out on a few aspects of composition but by and large these studies do not correlate the results to the composition i.e. β phase fraction. In a comparative analysis of two titanium alloys, Ti6Al4V and Ti54M, it was found that cutting forces and tool wear were lower during machining of Ti54M. Formation of adiabatic shear bands was observed in chips of both the alloys [4]. In another study, milling of β 21S alloy was done to investigate optimum machining parameters [5]. The influence of processing parameters on milling of alpha titanium alloy was studied [6] and a relation between magnitude of tool wear and dynamics of cutting force was arrived at.
Beside these studies, there are other notable studies address correlations between microstructural parameters of a material such as grain size and shape, alloying elements, precipitates hard particles on the formation of shear bands. It is known that the mechanism of shear band formation involves dynamic recrystallization which more prominently occurs when the grains size is finer [7]. Also, presence of hard particles or secondary precipitates in the microstructure of α alloy promotes occurrence of adiabatic shear bands during machining [8]. An increase in the shear band thickness was observed in Ti–3Al–5Mo–5V alloys with larger size of α plates [9]. Lamellar microstructure of Ti6Al4V has more tendency to form shear bands than bi-modal microstructure because it prevents slip transfer across and adjacent to α and β interface [7]. In addition, critical strain for the shear localization is less in lamellar microstructure, which increases its susceptibility to shear band formation [10]. Refining of the inter-lamellar spacing and colony size of α+β lamellar microstructure is observed to promote dynamic recrystallization [11]. A comparative study of high strain rate deformation of commercially pure (CP) titanium and Ti6Al4V show occurrence of extensive twinning in the CP titanium, which prevents dynamic re-crystallization. On the other hand, in Ti6Al4V alloy, absence of twinning leads to re-crystallization of grains. This shows that twinning retards the dynamic re-crystallization process as it does not store or supply the energy required for dynamic recrystallization [12]. Another microstructural parameter that influences the formation of shear bands is stacking fault energy. A high stacking fault energy material shows more dynamic recovery and thus suppresses dynamic re-crystallization and subsequently causes a delay in the shear band formation [13].
The above research shows that a strong relationship exists between microstructural parameters and the shear band formation during a machining process. However, very few studies have focussed on the influence of β-phase fraction on the material deformation during a machining process. Various micro-mechanisms like dynamic recovery, dynamic recrystallization, grain boundary sliding operate during a machining process. The proportion of β-phase fraction changes the intensity of one micro-mechanism over the other, and accordingly, the characteristics of shear bands and deformation of grains in the surrounding region change. Micro-mechanism in deformation is also a function of β phase fraction in the titanium alloys. It influences microstructural and geometrical changes in the shear bands that occur during machining. Concurrently, when the chip is being generated, a newly generated surface also undergoes severe deformation during machining. Thus, it is important to study the kind of deformation that occurs on the machined surfaces and the microstructural changes that occur within them. This work therefore undertakes two dimensional wedge (orthogonal) cutting experiments to investigate the microstructural changes that occur in a chip and on the resulting machined surfaces.
Section snippets
Experimental details
This section discusses the details of the theme of experiments, specifications of work materials and experimental procedure.
Results and discussions
In this section, optical images of the chips of three titanium alloys were analysed to understand the grains deformation during machining process. The deformation of grains in the chip roots and chips was correlated with the original microstructure of the respective titanium alloys. Also, shear band configuration which includes spacing between shear bands and their thickness have been analysed for the three titanium alloys. Further, analysis on micro-mechanisms involved in the shear band
Prediction of shear band configuration
This section describes an analytical evaluation of shear band configuration. Research by Wright and Ockendon [27] predicted spacing between shear bands using perturbation method. In this method, segment spacing was related to the characteristics perturbation wavelength. Molinari et al. [25] considered the influence of cutting speed to modify the perturbation analysis of Wright and Ockendon [27]. However, this analysis predicts the spacing between the shear bands to a limited range of cutting
Conclusions
From the above discussion, the following conclusions are drawn:
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An increase in β phase fraction limits the spread of material deformation in the shear band and reduces the spacing between them. An increase in the cutting speed reduces the shear band thickness but increases the inter band spacing.
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With an increase in β phase fraction, a part of non-indexable region on EBSD scan increases. It is the smallest in α alloy and the largest in β rich α+β alloy. This shows that the extent of recovery in
Acknowledgements
The authors gratefully acknowledge the partial support provided for this work by National Centre for Aerospace Innovation and Research, IIT-Bombay, a Dept. of Science and Technology-Government of India, The Boeing Company and IIT Bombay Collaboration.
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