Microstructure evolution of adiabatic shear bands and mechanisms of saw-tooth chip formation in machining Ti6Al4V

doi:10.1016/j.msea.2011.10.050

Materials Science and Engineering: A

Volume 531, 1 January 2012, Pages 155-163

https://doi.org/10.1016/j.msea.2011.10.050 Get rights and content

Abstract

The formation of saw-tooth chip is one of the primary characteristics in high-speed cutting and in the machining of difficult-to-cut materials, such as titanium and its alloys, hardened steels. The saw-tooth chips obtained from Ti6Al4V turning were examined geometrically and metallurgically. The segment spacing, adiabatic shear band (ASB) width and the degree of segmentation were determined by micrographic observations. The microstructure evolution of ASB in terms of cutting speed was analyzed. The mechanism of saw-tooth chip formation was discussed depending on the adiabatic shear sensitivity of workpiece materials when processed at high strain and strain rates. Experimental results show that the evolution of microstructure inside the ASBs is: deformed band → deformed band + transformed band → transformed band with cutting speed increasing. As for those workpiece materials possessed high adiabatic shear sensitivity, the catastrophic instability resulting in the formation of saw-tooth chip is thermoplastic instability; as for those workpiece materials that is insensitive to shear localization, the instability in primary shear zone is periodic cracks originated at free surface ahead of tool; as for those workpiece materials in which adiabatic shear sensitivity is situated between the above two kind of materials, the interaction of thermoplastic instability and periodic cracks results in saw-tooth chip formation.

Highlights

► Disclose microstructure evolution of ASB with cutting speed. ► Microhardness distribution across ASB under different cutting speeds. ► Analyze the forming process of ASB during saw-tooth chip forming. ► Discuss mechanism of saw-tooth chip formation based on adiabatic shear sensitivity.

Introduction

The chip generated in high-speed cutting prefers to present saw-tooth chip. Saw-tooth chip is also commonly observed in machining difficult-to-cut materials, such as titanium and its alloys, hardened steels, and nickel–iron based super-alloys. There has been a significant amount of research reported on saw-tooth chip formation since the chip morphology and chip formation mechanism are important as many parameters in metal cutting, such as cutting force and temperature, tool wear, machined surface finish and integrity, are related to the process of chip formation.

Poulachon and Moisan [1] thought that the two main parameters influencing the saw-tooth chip morphology are the hardness of materials and cutting speed. Yang and Li [2] thought that the chip saw-tooth degree decreases with cutting speed increasing, while it increases with feed speed increasing. Dolinšek et al. [3] carried out the experimental investigations of chip shape, chip microhardness and segmentation frequency. Duan and Wang's results show that the width and spacing of ASBs decrease with the increase of cutting speed [4]. Davies et al. [5] thought that segment spacing increases monotonically with cutting speed but appears to be asymptotic to some limiting value.

Two different mechanisms have been adopted to describe saw-tooth chip formation under different machining conditions and materials. Nakayama et al. [6], Nakayama [7], Shaw and Vyas [8], Vyas and Shaw [9], Elbestawi et al. [10], König et al. [11], Poulachon and Moisan [1], [12] have suggested that a crack initiates periodically at the free surface of the workpiece ahead of the tool and propagates towards the tool tip. Thus the periodic cracks originated at free surface are the root of saw-tooth chip formation. Komanduri et al. [13], Davies et al. [5], [14], Semiatin and Rao [15], Hou and Komanduri [16] have proposed thermoplastic instability as the root cause of saw-tooth chip formation. This mechanism is often referred to the formation of adiabatic shear band in which the rate of thermal softening exceeds the rate of strain hardening. Recent publications tend to support the thermoplastic shear instability theory. Gente and Hoffmeister [17] introduced a new quick stop method to obtain partially formed chip at extremely high cutting speed and found the fact that concentrated strain can be observed in the area next to the tool tip first, which indicates the starting point of adiabatic shear. Barry and Byrne [18] studied the mechanism of chip formation in machining hardened steels and concluded that the primary instability resulting in the formation of saw-tooth chip is initiation of adiabatic shear at the tool tip and propagation partway towards the free surface. Puerta Velásquez et al. [19] performed metallurgical analysis on chips obtained from high speed machining of Ti6Al4V alloy and thought that microscopical observations are in agreement with the catastrophic thermoplastic shear model for saw-tooth chip formation, instead of the periodic crack initiation. Cotterell and Byrne [20] recorded saw-tooth chip formation cycle of Ti6Al4V by high-speed imaging system and found that the thermoplastic shear instability resulted in saw-tooth chip formation during machining of titanium alloy Ti6Al4V.

A considerable amount of effort has been devoted to the microstructure and metallurgy of ASB [4], [18], [21], [22]. Bayoumi and Xie [21] studied the effect of the cutting conditions on the formation of shear bands in Ti6Al4V. The analyses of segmentation frequency and of phase change within shear bands were carried out. Molinari et al. [22] investigated the influence of the cutting speed and of the cutting depth on the level of cutting forces and on the shear band morphology and spacing. They concluded that the band width varies as the inverse of the cutting speed. The microstructures of the ASBs during cutting 30CrNi3MoV were also characterized by Duan and Wang [4]. The micrographic observations show that the microstructure between the matrix and the center of the ASB gradually changes, and that the martensitic phase transformation and recrystallization may occur in the ASB.

The above overview shows that the influence of cutting conditions on saw-tooth chip morphology is inconsistent and the mechanism of saw-tooth chip formation has not been well understood. In addition, little effort has been placed on the microstructure evolution of ASB in terms of cutting speed.

In this work, the influence of cutting conditions on saw-tooth chip morphology and microstructure evolution of ASB in terms of cutting speed are presented during turning Ti6Al4V. Findings are obtained on the basis of optical microscope, scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) analysis and measurement of chip microhardness. Also, the mechanism of saw-tooth chip formation is discussed by analyzing the initiation of ASB and formation of saw-tooth chip obtained from Ti6Al4V and hardened 42CrMo turning.

Section snippets

Experimental setup

The workpiece materials used in the experiments was titanium alloy Ti6Al4V with chemical composition specified in Table 1. These cylinders were annealed at 780 °C for 1.5 h and then cooled at ambient temperature. The oxidized surface layer was removed by turning before experiments. The microstructure of the annealed Ti6Al4V is shown in Fig. 1. The alloy Ti6Al4V is an important α + β titanium alloy. The microstructure presents a three dimension complexity with features at different scale levels

Influence of cutting parameters on characteristics of chip morphology

Generally, the parameters that depict chip morphologies are segment spacing p, ASB width δ and degree of segmentation defined as G_s = (h₁ − h₂)/h₁ [23]. The parameters are illustrated in Fig. 2. All measurements of the chip morphology characteristics were carried out for 10 neighboring segments.

Microstructure changes inside ASB with cutting speed

Fig. 6 shows the microstructure evolution of ASB with the increasing of cutting speed. In Fig. 6(a), the slipping deformation that is the precursors of ASB can be observed. However at the low cutting speed of 30.2 m/min, the instability is weak, and the shear localization is not as sharp as that of high speed. In this case, only the incipient separation of segments is presented as shown in Fig. 7. By increasing cutting speed to 59.6 m/min cutting speed leads to complete separation of segments and

Discussion on mechanism of saw-tooth chip formation

There are significant disagreements and controversies on the mechanism of saw-tooth chip formation. Essentially, the point of disagreement is the mechanism of catastrophic failure in the primary shear zone.

Conclusion

(1)
Segment spacing, ASB width and degree of segmentation increase with cutting speed, feed rate increasing, and with rake angle decreasing.
(2)
The microhardness of ASBs increases with the increase of cutting speed from 440.8 HV_0.025 to 507.3 HV_0.025 in contrast to the surrounding matrix microhardness of 400.5 HV_0.025 when cutting speed increases from 30.2 m/min to 281.3 m/min. The average microhardness outside ASB is close to the microhardness of the workpiece materials (390 HV_0.025).
(3)
The microstructure

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (Nos. 51075155, 50930005); Project No. 2009ZM0168, financially supported by the Fundamental Research Funds for the Central Universities, is also appreciated.

References (23)

S. Dolinšek et al.
J. Mater. Process. Technol.
(2004)
M.A. Davies et al.
CIRP Ann.
(1996)
K. Nakayama et al.
CIRP Ann.
(1988)
M.C. Shaw et al.
CIRP Ann.
(1993)
M.A. Elbestawi et al.
CIRP Ann.
(1996)
W. König et al.
CIRP Ann.
(1993)
G. Poulachon et al.
CIRP Ann.
(1998)
M.A. Davies et al.
CIRP Ann.
(1997)
S.L. Semiatin et al.
Mater. Sci. Eng.
(1983)
Z.B. Hou et al.
Int. J. Mech. Sci.
(1997)

A. Gente et al.

CIRP Ann.

(2001)

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Citation Excerpt :
The serrated chip, cyclic cutting force and cyclic temperature fluctuation are produced simultaneously with almost same frequency. The main reason might be the machining of Ti-6Al-4V created high strain rates which caused thermoplastic instabilities within the adiabatic shear banding, this induced the cutting force and temperature changed correspondingly [38,39]. In order to better understand the wear failure mechanism, a correlation was established between the resistance to thermal fatigue shock and cutting performance of the coated tools during machining Ti-6Al-4V alloy.
The multilayer nano-coating shows excellent thermal stability and oxidation resistance which could be attributed to the multielement composition and multilayer structure. It is widely used in the machining of hard-to-cut materials, such as titanium alloy. The formation of serrated chip during cutting process of Ti-6Al-4V would result in the ultra-high frequency thermal fatigue shock on the coating surface, and this could induce the thermal fatigue failure of coating. In this study, the ultra-high frequency (20 kHz) thermal fatigue shock testing was developed to investigate the thermal fatigue failure mechanisms of the bilayer TiSiN/TiAlN coating and multilayer TiSiN/TiAlN nano-coating. It was achieved by using the pulsed laser system to irradiate the coating surface under different shock cycles, which could produce alternating thermal stress on coating surface. This creates a close simulation of the actual working conditions in high-speed machining of Ti-6Al-4V with serrated chip produced. Moreover, the high temperature oxidation experiments were performed to study the oxidation resistance of coating. The failure mechanisms of coating/substrate system under the constant thermal loads and dynamic thermal loads were compared and analyzed. The cutting performance of related coated tools was evaluated after turning Ti-6Al-4V. The correlation between the thermal fatigue shock resistance of coatings at ultra-high frequency and the wear behavior of coated tools was built. It can be concluded that the multilayer TiSiN/TiAlN nano-coating showed better properties of the resistance to thermal fatigue shock in machining Ti-6Al-4V.

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Microstructure evolution of adiabatic shear bands and mechanisms of saw-tooth chip formation in machining Ti6Al4V

Abstract

Highlights

Introduction

Section snippets

Experimental setup

Influence of cutting parameters on characteristics of chip morphology

Microstructure changes inside ASB with cutting speed

Discussion on mechanism of saw-tooth chip formation

Conclusion

Acknowledgements

J. Mater. Process. Technol.

CIRP Ann.

CIRP Ann.

CIRP Ann.

CIRP Ann.

CIRP Ann.

CIRP Ann.

CIRP Ann.

Mater. Sci. Eng.

Int. J. Mech. Sci.

CIRP Ann.