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Study of cutting deformation in machining nickel-based alloy Inconel 718
International Journal of Machine Tools & Manufacture 51 (2011) 520–527
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Study of cutting deformation in machining nickel-based alloy Inconel 718 Gao Dong 1, Hao Zhaopeng n, Han Rongdi, Chang Yanli, J.N. Muguthu School of Mechanics Engineering, Harbin Institute of Technology, Harbin 150001, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 November 2010 Received in revised form 22 February 2011 Accepted 28 February 2011 Available online 10 March 2011
During the process of high-speed machining nickel-based alloy the material presents serrated chips. An experiment involving quick-stop device was conducted. The chip root obtained in the experiment was presented in a metallographic graph. Through the analysis of metallographic graph, the physical features showed that shear angle is reduced and shear plane is converted into shear body when serrated chips formed were analyzed. Conditions under which a crack appeared and adiabatic shear that occurred were also analyzed. Based on the research, shear strain, shear strain rate and shear stress model in the adiabatic shear band were established. The effects of cutting parameters on character of the serrated chip were studied through observing chip metallographic graph. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Inconel 718 Cutting deformation Serrated chip Shear body
1. Introduction In the process of cutting nickel-based alloy (Inconel 718), increase in cutting speed results in serrated chips formation. Serrated chips cause the vibration of cutting force. Cutting force signals are high sensitive carriers of information about the machining processes, such as tool state, cutting vibration. Furthermore, the saw tooth continue to impact the tool rake face and cutting edge. The generation frequency of serrated chip is very high. The stress and temperature shocks are generated in tool rake face because of this high-frequency load. These shocks make the tool generate micro-cracks so as to speed up the tool wear. It is therefore necessary to carry out a research on serrated chips at high cutting speeds. Many scholars have carried out researches on the formation process of serrated chips. Through various theoretical and experimental studies, they have made numerous achievements. Experimental materials used in research include titanium, stainless steel and hardened steel among other hard materials. Some results obtained coincided with adiabatic shear theoretical model, which indicates that the serrated chips form as a result of adiabatic shear caused by the localized shear deformation in the first deformation zone. Recht [1] proposed the theory of catastrophic thermoplastic shear. He claimed that catastrophic shear occurs when the local rate of change in temperature has a negative effect on strength, n
Corresponding author. Tel.: þ86 159 4599 4371. E-mail addresses: [email protected] (G. Dong), [email protected] (H. Zhaopeng). 1 Tel.: þ86 0451 86413810. 0890-6955/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2011.02.011
which is equal to or greater than the positive effect of strain hardening. He then applied the theory to interpret the formation of serrated chips when machining titanium. Komanduri and Borwn [2] and Komanduri et al. [3] observed fully developed catastrophic shear bands in the chips at cutting speed above 275 m/min when machining Ti-6A1-4V alloy and AISI 4340 steel. Komanduri presented a two-phase model for the process of chip formation after a careful study on chip formation of machining titanium and high strength steel. One stage involves gradual flattening (upsetting) of the wedge-shaped work material ahead of the tool. The other stage involves plastic instability leading to catastrophic shear. They thought catastrophic shear in a narrow band will be imminent only when: (a) the thermal energy is forced to concentrate in a narrow band due to conditions that permit very little heat dissipation into the chip and/or work material; (b) the geometric constraint effect and the stress state in the primary shear zone under the conditions of cutting are favorable.
Davies et al. [4,5] did some researches on the formation mechanism of serrated chip under the state of cutting AISI 52100. They proposed the model of the serrated chip formation to support Komanduri’s theory. They claimed that serrated chip began to form when heat conduction, heat convection and heat production ratio come to thermal equilibrium. When cutting speed increased to a certain critical value, the inner local stress of the chip changed suddenly. It destroyed the dynamic equilibrium state, and then the serrated chip is formed. Furthermore,
G. Dong et al. / International Journal of Machine Tools & Manufacture 51 (2011) 520–527
Nomenclature
bD
d vc f
g0 a kr L h H fz hD
cutting speed feed rate rake angle flank angle cutting edge angle tooth spacing saw tooth height chip thickness generation frequency of serrated chip cutting thickness
they concluded that when the serrated chip formed, the shear angle got smaller, and they gave the cutting equation. Wang [6] and Duan [7] in Dalian University of Technology also carried out a research on the theory of adiabatic shear of the serrated chip and microscopic structure in the adiabatic band and have made great achievements. There also exist other viewpoints on the theory of formation of serrated chips, which point out that periodical crack generated on free surface of the workpiece leads to serrated chip formation. The periodical crack theory got the strong support of the numerical simulation result. Elbestawi et al. [8] studied the mechanism of serrated chip formation when hard cutting AlSl1550. He considered that the reason why serrated chip generated was not just shear deformation. The theory of surface energy and strain energy density could give a good explanation on how the crack generated and grew. Chip formation during hard turning may be assumed to start with initiation of a crack near the free surface, which further propagates and ceases in the plastically deformed region close to the tip of the cutting edge. Poulachon and Moisan [9] studied chip formation mechanism when hard cutting 100Cr6. He concluded that the chip formation is governed by a more global physical quantity such as the generated energy, and by the consequent temperature. The brittle fracture then serves as the mechanism of serrated chip formation. The effects of the cutting speed and the hardness of workpiece on chip formation are interdependent. Obikawa and Usui [10] analyzed the serrated chip formation mechanism through finite element analysis. They applied a ductile fracture criterion on the basis of strain, strain rate, hydrostatic pressure and temperature to the crack growth during the chip formation. Hua and Shivpuri [11] proposed a new interpretation of chip segmentation in the cutting of Ti-6A1-4V. It is based on a finite element simulation of orthogonal machining of Ti6Al4V in which a dynamic flow stress model based on high strain rate and high temperature, and ductile fracture criterion based on the strain energy, were applied to the crack initiation during the chip segmentation. The simulation results showed that the changes in the stress state near the tool tip led to the crack propagation shifting from the tool tip to the free surface of the deformed chip in the shear zone, as shown in Fig. 1. They claimed that this
Fig. 1. Chip segmentation at medium cutting speed of 120 m/min.
j j1 j2 F
o g g_ t
521
cutting width average width of adiabatic shear zone shear angle wedge angle of cutting layer intersection angle between chip and the free surface of cutting layer cutting force angle between cutting force and shear band shear strain shear strain rate shear stress
change in crack initiation and propagation is the primary reason for the chip changing from discontinuous to a segregated continuous morphology. The cutting deformation in machining Inconel 718 presented specificity as its unique physical properties. There exist limitations when using either the adiabatic shear theory or the periodical crack theory to explain the cutting deformation in machining Inconel 718. The present study aims to increase the understanding of the cutting deformation mechanism in machining Inconel 718. 2. Materials and experimental procedures Workpiece used in this study is Inconel 718 bar with a diameter of 100 mm. The mechanical properties of the workpiece are presented in Table 1. Turning was conducted on a Metal lathe (Model CA6140) by dry machining with a CNMG160608-YGB202-type tool supplied by ZHUZHOU CEMENTED CARBIDE CUTTING TOOLS CO., LTD. The rake angle g0 was 01, flank angle a is 61 and the cutting edge angel kg was 901. Chip root was obtained by quick-stop tests. The quick-stop devices can make the cutting tool separate from the workpiece quickly during the process of cutting, and ‘‘froze’’ the cutting state when retracting. Cutting speeds vc are 20, 30, 40, 50, 60, 70, 80, 90 and 100 m/min. The feed rate f is 0.2 mm/r and the cutting depth ap is 2 mm. Chips obtained after turning were made into a specimen on mounting with mosaic materials. The metallographic graph of chips were obtained using metallurgical microscope after polishing and etching the chip root specimen, as shown in Fig. 2. 3. Results and discussion 3.1. Serrated chip formation processes According to Fig. 2, when the cutting speeds vc are 20 and 30 m/min, the cutting chip presented ribbon shape and it did not have concentration zone of shear slipping. When the cutting speed vc is higher than 40 m/min, the chip presented is serrated. The shear zone and crack can be seen clearly. Through the observation of metallographic specimens of chip root, a comparison was made, which summarized the process of formation of serrated chip. It contained elementary stage, transition stage and the final stage. With the cutting process progressing on, the shear angle is reduced, shear plane is converted into shear body and the strain in the shear zone got larger. The crack appeared when the material could not bear this large strain, and adiabatic shear happened along the crack direction. (1) Elementary stage: In the elementary stage, the cutting layer in front of tool presented wedge shape. The wedge angle is j1, and the shear angle j is larger than j1. The intersection angle between the chip and the free surface of cutting layer is j2, which is equal
522
G. Dong et al. / International Journal of Machine Tools & Manufacture 51 (2011) 520–527
Table 1 Mechanical properties of Inconel 718. Workpiece
Density r (kg/m3)
Yield strength s0.2 (Mpa)
Tensile strength sb (Mpa)
Elongation d5 (%)
Shrinkage c (%)
Toughness ak (J/cm2)
Inconel 718
8280
1260
1430
24
40
40
Fig. 2. Pictures of metallographic specimens of chip root under different cutting speed. (a) vc ¼ 20 m/min, (b) vc ¼30 m/min, (c) vc ¼ 40 m/min, (d) vc ¼ 50 m/min, (e) vc ¼80 m/min, and (f) vc ¼ 100 m/min.
Fig. 3. Elementary stage. (a) Chip root and (b) diagram of elementary stage.
to j1 in the elementary stage, as shown in Fig. 3. A new serrated chip formation began. (2) Transition stage: In the transition stage, as cutting went on, the wedge-shaped cutting layer keep piling up. The angle j2 decreased, as shown in Fig. 4.
As the angle j2 decreased, the compressive force of chip received by cutting layer increased, which led to shear slipping towards the free surface of cutting layer from the shear zone. The shear angle j kept decreasing. The cutting force increased as the shear angle reduced to balance with the compressive force. The shear body is generated gradually but
G. Dong et al. / International Journal of Machine Tools & Manufacture 51 (2011) 520–527
chip does form, as shown in Fig. 5. At this moment, a lot of elastic energy, deformation energy and deformation heat have been accumulated in the shaded zone. In the transition stage, these angles (j, j1, j2) must satisfy the following relationship for the serrated chip to form:
j 4 j1 4 j2 :
ð1Þ
(3) Final stage: In the final stage, the compressive force between chip and the cutting layer keep increasing as a result of cutting tool movement. At the same time, the shear body increased, along with it, the strain in the shear body increased until the shear
angle j reduced to be equal with j1. Stress concentration caused by dislocation is built up in the free surface or the hump caused by dislocation collapse around the deposition body. Both could result in crack in the free surface and around the deposition body. This was not only the effective structure bringing about brittle rupture but also the common origin of ductile rupture [12]. When stress concentration in the free surface of cutting layer caused by dislocation reach a certain limit, due to the workpiece material in the shear zone not being able to bear the high strain energy, crack appeared in the free surface of the shear body. For the time being, the shear slipping concentration did not appear in the shear body, as shown in Fig. 6. From this observation it can be concluded that elastic energy and deformation energy, which is built in the shear body is released fast along the crack. High amount of deformation heat is generated instantaneously and the adiabatic shear zone is formed. Recht [1] formulated a simple criterion for the catastrophic slip in the primary shear zone based on the thermo-physical response of the phenomenon involving strain hardening and thermal softening. He expressed this as 0r
Fig. 4. Relationship between j, j1 and j2 in transition stage.
523
@t=@g r1: ð@t=@yÞðdy=dgÞ
ð2Þ
On the basis of the analysis on the formation process of the serrated chip, the processes can be concluded in the following steps: the compressive force of chip received by free surface of cutting layer (shear zone) increases, shear angle decreases, shear
A
A2
Fig. 5. Transition stage. (a) Chip root and (b) diagram of transition stage.
A
A1
Fig. 6. Final stage. (a) Chip root and (b) diagram of final stage.
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G. Dong et al. / International Journal of Machine Tools & Manufacture 51 (2011) 520–527
plane converts into shear body and builds up, heat accumulates, the crack appears in the free surface, energy is released and adiabatic shear happens.
6.0 5.5 5.0
3.2. Modeling of shear strain, shear strain rate and shear stress in the adiabatic shear band
(1) Shear strain in adiabatic shear band: A DBN g¼ 1 ,
d
2.5 2.0 1.5 1.0 0.5 40
ð5Þ
t
¼
g
80
ð6Þ
7 shear strain
h hD h A1 D ¼ , H H sin j1
AA1 =v
70
8
hD h : Hd sin j1
g
60 vc(m/min)
9
6 4
ð7Þ
¼
gv , AA1
DF A1 F ¼ , CD AA1
shear strain
5 3 2
The values of shear strain, with different cutting speed and feed rate, have been got by these formulas according to the Eq. (7). The variation in shear strain with different cutting speeds and feed rates is shown in Fig. 8. (2) Shear strain rate g_ in adiabatic shear band:
g_ ¼
50
10
A1 DBN h ¼ , A1 D H
g¼
3.0
ð3Þ ð4Þ
A1 DBN ¼
3.5
0.0
hD , sin j1
A1 D ¼
shear strain
4.0 shear strain
According to the process of the serrated chip formation, the following geometry relationship was found as shown in Fig. 7. AC was parallel to A1D in the Fig. 7. All of these parameters can be obtained through experiments. The model of shear strain, shear strain rate and shear stress in the adiabatic shear band can be established using these parameters. Then, the values of shear strain, shear strain rate and shear stress with different cutting speed and feed rate have been obtained by these formulas.
4.5
1 0 0.06 0.08 0.10 0.12 0.14 0.16 0.18 f (mm/rev)
ð8Þ
DF ¼
hD , cos g0
ð13Þ
HL , hD
ð14Þ
AA1 ¼
A1 FCD , DF
ð10Þ
AA1 ¼
A1 F ¼
H , cos g0
ð11Þ
g_ ¼
ð12Þ
0.26
Fig. 8. Variation of shear strain g with vc and f. (a) vc–g and (b) f g.
ð9Þ
CD ¼ L,
0.20 0.22 0.24
gvhD HL
:
ð15Þ
The values of shear strain rate, with different cutting speed and feed rate, have been got by these formulas according to the Eq. (15). The variation in shear strain rate with different cutting speeds and feed rates was shown in Fig. 9. (3) Shear stress t in adiabatic shear band: With the formation of adiabatic shear band, Fig. 10 showed every cutting geometry parameter.
The shear stress can be written as [13]
t¼
Fig. 7. Geometry diagram of serrated chip.
F cos o : bD hD =sin j1
ð16Þ
The values of shear stress, with different cutting speed and feed rate, have been got by these formulas according to the Eq. (16). The variation in shear stress with different cutting speeds and feed rates was shown in Fig. 11.
G. Dong et al. / International Journal of Machine Tools & Manufacture 51 (2011) 520–527
525
1.4
shear stress
shear strain rate 1.2 shear stress (MPa)
shear strain rate (s-1)
3
x10 650 600 550 500 450 400 350 300 250 200 150 100 50 0
1.0 0.8 0.6 0.4 30
30
40
50
60 v c (m/min)
70
40
50
60
80 1.4
shear strain rate
1000 900
shear stress (MPa)
1.2
1100 shear strain rate (s-1)
80
shear stress
3
x10 1300 1200
70
vc (m/min)
800
1.0 0.8 0.6
700 0.4
600
0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 f (mm/rev)
500 400
Fig. 11. Variation of shear stress t with vc and f. (a) vc t and (b) f t.
0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 f (mm/rev) Fig. 9. Variation of shear strain rate g_ with vc and f. (a) vc g_ and (b) f g_ .
H
h
L
Fig. 12. Geometry character of serrated chip.
The generation frequency of serrated chip can be expressed as fz ¼
vc f sin kr =H : L
ð17Þ
Fig. 10. Geometric diagram of shear stress.
3.3. Effects of cutting parameters on geometry characteristic of serrated chip The effects of cutting speed and feed rate on the geometry characteristics of serrated chip were also studied. The parameters of collected are shown in Fig. 12.
(1) Effects of cutting speed on geometry characteristic of serrated chip: The experiment has been carried out using different cutting speeds as: 40, 50, 60, 70 and 80 m/min. The feed rate f and cutting depth ap are constants. They are 0.16 mm/r and 1 mm, respectively. The tool edge angle kg is 451 and rake angle g0 is 61.
526
G. Dong et al. / International Journal of Machine Tools & Manufacture 51 (2011) 520–527
0.55
0.14
0.10
0.50 f =0.1mm/rev ap=1mm
f =0.1mm/rev ap=1mm
0.45
0.08
h/H
tooth spacing L(mm)
0.12
0.06
0.40
0.04 0.35
0.02 0.00 40
50
60 vc(m/min)
70
80
0.30 40
50
60 vc(m/min)
70
80
generation frequency fz (Hz)
x103 400 350
f =0.1mm/rev ap=1mm
300 250 200 150 40
50
60 vc(m/min)
70
80
Fig. 13. Effects of cutting speed on geometry characteristic of serrated chip. (a) Variation of tooth spacing L with cutting speed vc, (b) variation of h/H with cutting speed vc, and (c) variation of generation frequency of serrated chip with cutting speed vc.
The experimental results showed that saw tooth spacing (L), the ratio of saw tooth height to chip thickness (h/H) and generation frequency of the serrated chip (fz) increased with the increasing of cutting speed, as shown in Fig. 13. (2) Effects of feed rate on geometry characteristic of serrated chip: The experiment has been carried out using different feed rates as: 0.08, 0.12, 0.16, 0.2 and 0.24 mm/r. The cutting speed vc and cutting depth ap are constants. They are 60 m/min and 1 mm, respectively. The tool edge angle kg is 451 and rake angle g0 is 61. The experimental results showed that when feed rate changed from 0.08 to 0.16 mm/r, saw tooth spacing (L) and the ratio of saw tooth height to chip thickness (h/H) increased with the increase in feed rate. When feed rate changed from 0.16 to 0.24 mm/r, saw tooth spacing (L) and the ratio of saw tooth height to chip thickness (h/H) decreased with the increasing of feed rate. However, it is opposite for the changes of the generation frequency of serrated chip (fz) with feed rate, as shown in Fig. 14.
4. Conclusions The chip root was obtained through the experiment involving quick-stop device and then was made into metallographic specimens. Some correlated theories were applied to study the
deformation process when machining Inconel 718 after the observation of metallographic specimens of chip root and chip micro-morphology. Based on this study, the following conclusions can be drawn: (1) Through the observation of metallographic specimens of chip root, the results showed that when vc is less than 40 m/min, the chip presented ribbon shape. When vc is higher than 40 m/min, the chip presented serrated. (2) The formation mechanism of serrated chip can be summarized as: with the compressive force in the shear zone increased, the shear angle gradually decreased and the shear plane transformed into shear body. At last, the serrated chip is generated as the interaction of the crack in the free surface and the adiabatic shear. (3) By analyzing the formation process and the characteristic of serrated chip, it is proposed that the calculation model of strain and strain rate in the adiabatic shear zone has been constructed. (4) Cutting speed and feed rate are sensitive factors that affect shear stress, shear strain, shear strain rate and so on. Through the observation of chip morphology and careful calculation, the effects of cutting speed and feed rate on geometry characteristic of serrated chip were studied in this paper. With increasing cutting speed, saw tooth spacing (L), the ratio of saw tooth height to chip thickness (h/H) and generation
0.24 0.22 0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00
527
0.50
vc=80m/min ap=1mm
vc=80m/min ap=1mm
0.45
h/H
tooth spacing L(mm)
G. Dong et al. / International Journal of Machine Tools & Manufacture 51 (2011) 520–527
0.40 0.35 0.30 0.25
0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28 0.30
0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26
f (mm/rev)
f (mm/rev)
generation frequency fz (Hz)
x103 360 340
vc=80m/min ap=1mm
320 300 280 260 240 220 200 180 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26
f (mm/rev) Fig. 14. Effects of feed rate on geometry characteristic of serrated chip. (a) Variation of tooth spacing L with feed rate f, (b) variation of h/H with feed rate f and (c) variation of generation frequency of serrated chip with feed rate f.
frequency of the serrated chip (fz) increased. When feed rate changed from 0.08 to 0.16 mm/r, saw tooth spacing (L) and the ratio of saw tooth height to chip thickness (h/H) increased with the increase in feed rate. However, when feed rate changed from 0.16 to 0.24 mm/r, the values of L and h/H decreased. It is opposite for the changes in the generation frequency of serrated chip (fz) with feed rate. Acknowledgements This work was supported by ‘‘863 key projects of China— highefficiency cutting technology and applications of large components of nickel-based alloy (2009AA044301)’’. References [1] R.F. Recht, Catastrophic thermoplastic shear, Journal of Applied Mechanics— Transactions of the ASME 86 (1964) 189–193. [2] R. Komanduri, R.H. Borwn, On the mechanism of chip segmentation in machining, Journal of Engineering for Industry 103 (1981) 33–51. [3] R. Komanduri, T. Schroeder, J. Hazra, et al., On the catastrophic shear instability in high-speed machining of an AISI 4340 Steel., Transactions of the ASME: Journal of Engineering for Industry 104 (1982) 121–131.
[4] M.A. Davies, Y. Chou, C.J. Evans, On chip morphology, tool wear and cutting mechanics in finish hard turning, Annals of the CIRP (1996) 77–82. [5] M.A. Davies, T.J. Burns, C.J. Evnas, On the dynamics of chip of formation in machining hard metals, Annals of the CIRP 46 (1997) 25–30. [6] Wang Minjie, Dynamic mechanical properties of metal and thermoplastic shear instability in the orthogonal cutting experiments. Degree of Doctor of Philosophy in Engineering, Dalian University of Technology, 1989. [7] Duan Chunzheng, Study on microcosmic mechanism of adiabatic shear behavior in orthogonal cutting of high strength steel. Degree of Doctor of Philosophy in Engineering, Dalian University of Technology, 2004. [8] M.A. Elbestawi, A.K. Srivastava, T.I. El-Wardany., A model for chip formation during machining of hardened steel, ClRP Annals—Manufacturing Technology 45 (1996) 71–76. [9] G. Poulachon, A.L. Moisan., Hard turning: chip formation mechanisms and metallurgical aspects, Journal of Manufacturing Science and Engineering— Transactions of the ASME 122 (2000) 406–412. [10] T. Obikawa, E. Usui, Computational machining of titanium alloy-finite element modeling and a few results, Journal of Manufacturing Science and Engineering—Transactions of the ASME 118 (1996) 208–215. [11] Hua Jiang, Shivpuri Rajiv, Prediction of chip morphology and segmentation during the machining of titanium alloys, Journal of Materials Processing Technology 150 (2004) 124–133. [12] J. Friedel, writings. Translated by Wang Yu. Dislocations. Beijing: Science and Technology Press.1984. [13] Zhongshan, Kazuo, Translated by Li Yunfang. Metal Cutting Theory. Machinery Industry Press (1985) 93–96.
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Study of cutting deformation in machining nickel-based alloy Inconel 718 Gao Dong 1, Hao Zhaopeng n, Han Rongdi, Chang Yanli, J.N. Muguthu School of Mechanics Engineering, Harbin Institute of Technology, Harbin 150001, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 November 2010 Received in revised form 22 February 2011 Accepted 28 February 2011 Available online 10 March 2011
During the process of high-speed machining nickel-based alloy the material presents serrated chips. An experiment involving quick-stop device was conducted. The chip root obtained in the experiment was presented in a metallographic graph. Through the analysis of metallographic graph, the physical features showed that shear angle is reduced and shear plane is converted into shear body when serrated chips formed were analyzed. Conditions under which a crack appeared and adiabatic shear that occurred were also analyzed. Based on the research, shear strain, shear strain rate and shear stress model in the adiabatic shear band were established. The effects of cutting parameters on character of the serrated chip were studied through observing chip metallographic graph. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Inconel 718 Cutting deformation Serrated chip Shear body
1. Introduction In the process of cutting nickel-based alloy (Inconel 718), increase in cutting speed results in serrated chips formation. Serrated chips cause the vibration of cutting force. Cutting force signals are high sensitive carriers of information about the machining processes, such as tool state, cutting vibration. Furthermore, the saw tooth continue to impact the tool rake face and cutting edge. The generation frequency of serrated chip is very high. The stress and temperature shocks are generated in tool rake face because of this high-frequency load. These shocks make the tool generate micro-cracks so as to speed up the tool wear. It is therefore necessary to carry out a research on serrated chips at high cutting speeds. Many scholars have carried out researches on the formation process of serrated chips. Through various theoretical and experimental studies, they have made numerous achievements. Experimental materials used in research include titanium, stainless steel and hardened steel among other hard materials. Some results obtained coincided with adiabatic shear theoretical model, which indicates that the serrated chips form as a result of adiabatic shear caused by the localized shear deformation in the first deformation zone. Recht [1] proposed the theory of catastrophic thermoplastic shear. He claimed that catastrophic shear occurs when the local rate of change in temperature has a negative effect on strength, n
Corresponding author. Tel.: þ86 159 4599 4371. E-mail addresses: [email protected] (G. Dong), [email protected] (H. Zhaopeng). 1 Tel.: þ86 0451 86413810. 0890-6955/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2011.02.011
which is equal to or greater than the positive effect of strain hardening. He then applied the theory to interpret the formation of serrated chips when machining titanium. Komanduri and Borwn [2] and Komanduri et al. [3] observed fully developed catastrophic shear bands in the chips at cutting speed above 275 m/min when machining Ti-6A1-4V alloy and AISI 4340 steel. Komanduri presented a two-phase model for the process of chip formation after a careful study on chip formation of machining titanium and high strength steel. One stage involves gradual flattening (upsetting) of the wedge-shaped work material ahead of the tool. The other stage involves plastic instability leading to catastrophic shear. They thought catastrophic shear in a narrow band will be imminent only when: (a) the thermal energy is forced to concentrate in a narrow band due to conditions that permit very little heat dissipation into the chip and/or work material; (b) the geometric constraint effect and the stress state in the primary shear zone under the conditions of cutting are favorable.
Davies et al. [4,5] did some researches on the formation mechanism of serrated chip under the state of cutting AISI 52100. They proposed the model of the serrated chip formation to support Komanduri’s theory. They claimed that serrated chip began to form when heat conduction, heat convection and heat production ratio come to thermal equilibrium. When cutting speed increased to a certain critical value, the inner local stress of the chip changed suddenly. It destroyed the dynamic equilibrium state, and then the serrated chip is formed. Furthermore,
G. Dong et al. / International Journal of Machine Tools & Manufacture 51 (2011) 520–527
Nomenclature
bD
d vc f
g0 a kr L h H fz hD
cutting speed feed rate rake angle flank angle cutting edge angle tooth spacing saw tooth height chip thickness generation frequency of serrated chip cutting thickness
they concluded that when the serrated chip formed, the shear angle got smaller, and they gave the cutting equation. Wang [6] and Duan [7] in Dalian University of Technology also carried out a research on the theory of adiabatic shear of the serrated chip and microscopic structure in the adiabatic band and have made great achievements. There also exist other viewpoints on the theory of formation of serrated chips, which point out that periodical crack generated on free surface of the workpiece leads to serrated chip formation. The periodical crack theory got the strong support of the numerical simulation result. Elbestawi et al. [8] studied the mechanism of serrated chip formation when hard cutting AlSl1550. He considered that the reason why serrated chip generated was not just shear deformation. The theory of surface energy and strain energy density could give a good explanation on how the crack generated and grew. Chip formation during hard turning may be assumed to start with initiation of a crack near the free surface, which further propagates and ceases in the plastically deformed region close to the tip of the cutting edge. Poulachon and Moisan [9] studied chip formation mechanism when hard cutting 100Cr6. He concluded that the chip formation is governed by a more global physical quantity such as the generated energy, and by the consequent temperature. The brittle fracture then serves as the mechanism of serrated chip formation. The effects of the cutting speed and the hardness of workpiece on chip formation are interdependent. Obikawa and Usui [10] analyzed the serrated chip formation mechanism through finite element analysis. They applied a ductile fracture criterion on the basis of strain, strain rate, hydrostatic pressure and temperature to the crack growth during the chip formation. Hua and Shivpuri [11] proposed a new interpretation of chip segmentation in the cutting of Ti-6A1-4V. It is based on a finite element simulation of orthogonal machining of Ti6Al4V in which a dynamic flow stress model based on high strain rate and high temperature, and ductile fracture criterion based on the strain energy, were applied to the crack initiation during the chip segmentation. The simulation results showed that the changes in the stress state near the tool tip led to the crack propagation shifting from the tool tip to the free surface of the deformed chip in the shear zone, as shown in Fig. 1. They claimed that this
Fig. 1. Chip segmentation at medium cutting speed of 120 m/min.
j j1 j2 F
o g g_ t
521
cutting width average width of adiabatic shear zone shear angle wedge angle of cutting layer intersection angle between chip and the free surface of cutting layer cutting force angle between cutting force and shear band shear strain shear strain rate shear stress
change in crack initiation and propagation is the primary reason for the chip changing from discontinuous to a segregated continuous morphology. The cutting deformation in machining Inconel 718 presented specificity as its unique physical properties. There exist limitations when using either the adiabatic shear theory or the periodical crack theory to explain the cutting deformation in machining Inconel 718. The present study aims to increase the understanding of the cutting deformation mechanism in machining Inconel 718. 2. Materials and experimental procedures Workpiece used in this study is Inconel 718 bar with a diameter of 100 mm. The mechanical properties of the workpiece are presented in Table 1. Turning was conducted on a Metal lathe (Model CA6140) by dry machining with a CNMG160608-YGB202-type tool supplied by ZHUZHOU CEMENTED CARBIDE CUTTING TOOLS CO., LTD. The rake angle g0 was 01, flank angle a is 61 and the cutting edge angel kg was 901. Chip root was obtained by quick-stop tests. The quick-stop devices can make the cutting tool separate from the workpiece quickly during the process of cutting, and ‘‘froze’’ the cutting state when retracting. Cutting speeds vc are 20, 30, 40, 50, 60, 70, 80, 90 and 100 m/min. The feed rate f is 0.2 mm/r and the cutting depth ap is 2 mm. Chips obtained after turning were made into a specimen on mounting with mosaic materials. The metallographic graph of chips were obtained using metallurgical microscope after polishing and etching the chip root specimen, as shown in Fig. 2. 3. Results and discussion 3.1. Serrated chip formation processes According to Fig. 2, when the cutting speeds vc are 20 and 30 m/min, the cutting chip presented ribbon shape and it did not have concentration zone of shear slipping. When the cutting speed vc is higher than 40 m/min, the chip presented is serrated. The shear zone and crack can be seen clearly. Through the observation of metallographic specimens of chip root, a comparison was made, which summarized the process of formation of serrated chip. It contained elementary stage, transition stage and the final stage. With the cutting process progressing on, the shear angle is reduced, shear plane is converted into shear body and the strain in the shear zone got larger. The crack appeared when the material could not bear this large strain, and adiabatic shear happened along the crack direction. (1) Elementary stage: In the elementary stage, the cutting layer in front of tool presented wedge shape. The wedge angle is j1, and the shear angle j is larger than j1. The intersection angle between the chip and the free surface of cutting layer is j2, which is equal
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Table 1 Mechanical properties of Inconel 718. Workpiece
Density r (kg/m3)
Yield strength s0.2 (Mpa)
Tensile strength sb (Mpa)
Elongation d5 (%)
Shrinkage c (%)
Toughness ak (J/cm2)
Inconel 718
8280
1260
1430
24
40
40
Fig. 2. Pictures of metallographic specimens of chip root under different cutting speed. (a) vc ¼ 20 m/min, (b) vc ¼30 m/min, (c) vc ¼ 40 m/min, (d) vc ¼ 50 m/min, (e) vc ¼80 m/min, and (f) vc ¼ 100 m/min.
Fig. 3. Elementary stage. (a) Chip root and (b) diagram of elementary stage.
to j1 in the elementary stage, as shown in Fig. 3. A new serrated chip formation began. (2) Transition stage: In the transition stage, as cutting went on, the wedge-shaped cutting layer keep piling up. The angle j2 decreased, as shown in Fig. 4.
As the angle j2 decreased, the compressive force of chip received by cutting layer increased, which led to shear slipping towards the free surface of cutting layer from the shear zone. The shear angle j kept decreasing. The cutting force increased as the shear angle reduced to balance with the compressive force. The shear body is generated gradually but
G. Dong et al. / International Journal of Machine Tools & Manufacture 51 (2011) 520–527
chip does form, as shown in Fig. 5. At this moment, a lot of elastic energy, deformation energy and deformation heat have been accumulated in the shaded zone. In the transition stage, these angles (j, j1, j2) must satisfy the following relationship for the serrated chip to form:
j 4 j1 4 j2 :
ð1Þ
(3) Final stage: In the final stage, the compressive force between chip and the cutting layer keep increasing as a result of cutting tool movement. At the same time, the shear body increased, along with it, the strain in the shear body increased until the shear
angle j reduced to be equal with j1. Stress concentration caused by dislocation is built up in the free surface or the hump caused by dislocation collapse around the deposition body. Both could result in crack in the free surface and around the deposition body. This was not only the effective structure bringing about brittle rupture but also the common origin of ductile rupture [12]. When stress concentration in the free surface of cutting layer caused by dislocation reach a certain limit, due to the workpiece material in the shear zone not being able to bear the high strain energy, crack appeared in the free surface of the shear body. For the time being, the shear slipping concentration did not appear in the shear body, as shown in Fig. 6. From this observation it can be concluded that elastic energy and deformation energy, which is built in the shear body is released fast along the crack. High amount of deformation heat is generated instantaneously and the adiabatic shear zone is formed. Recht [1] formulated a simple criterion for the catastrophic slip in the primary shear zone based on the thermo-physical response of the phenomenon involving strain hardening and thermal softening. He expressed this as 0r
Fig. 4. Relationship between j, j1 and j2 in transition stage.
523
@t=@g r1: ð@t=@yÞðdy=dgÞ
ð2Þ
On the basis of the analysis on the formation process of the serrated chip, the processes can be concluded in the following steps: the compressive force of chip received by free surface of cutting layer (shear zone) increases, shear angle decreases, shear
A
A2
Fig. 5. Transition stage. (a) Chip root and (b) diagram of transition stage.
A
A1
Fig. 6. Final stage. (a) Chip root and (b) diagram of final stage.
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plane converts into shear body and builds up, heat accumulates, the crack appears in the free surface, energy is released and adiabatic shear happens.
6.0 5.5 5.0
3.2. Modeling of shear strain, shear strain rate and shear stress in the adiabatic shear band
(1) Shear strain in adiabatic shear band: A DBN g¼ 1 ,
d
2.5 2.0 1.5 1.0 0.5 40
ð5Þ
t
¼
g
80
ð6Þ
7 shear strain
h hD h A1 D ¼ , H H sin j1
AA1 =v
70
8
hD h : Hd sin j1
g
60 vc(m/min)
9
6 4
ð7Þ
¼
gv , AA1
DF A1 F ¼ , CD AA1
shear strain
5 3 2
The values of shear strain, with different cutting speed and feed rate, have been got by these formulas according to the Eq. (7). The variation in shear strain with different cutting speeds and feed rates is shown in Fig. 8. (2) Shear strain rate g_ in adiabatic shear band:
g_ ¼
50
10
A1 DBN h ¼ , A1 D H
g¼
3.0
ð3Þ ð4Þ
A1 DBN ¼
3.5
0.0
hD , sin j1
A1 D ¼
shear strain
4.0 shear strain
According to the process of the serrated chip formation, the following geometry relationship was found as shown in Fig. 7. AC was parallel to A1D in the Fig. 7. All of these parameters can be obtained through experiments. The model of shear strain, shear strain rate and shear stress in the adiabatic shear band can be established using these parameters. Then, the values of shear strain, shear strain rate and shear stress with different cutting speed and feed rate have been obtained by these formulas.
4.5
1 0 0.06 0.08 0.10 0.12 0.14 0.16 0.18 f (mm/rev)
ð8Þ
DF ¼
hD , cos g0
ð13Þ
HL , hD
ð14Þ
AA1 ¼
A1 FCD , DF
ð10Þ
AA1 ¼
A1 F ¼
H , cos g0
ð11Þ
g_ ¼
ð12Þ
0.26
Fig. 8. Variation of shear strain g with vc and f. (a) vc–g and (b) f g.
ð9Þ
CD ¼ L,
0.20 0.22 0.24
gvhD HL
:
ð15Þ
The values of shear strain rate, with different cutting speed and feed rate, have been got by these formulas according to the Eq. (15). The variation in shear strain rate with different cutting speeds and feed rates was shown in Fig. 9. (3) Shear stress t in adiabatic shear band: With the formation of adiabatic shear band, Fig. 10 showed every cutting geometry parameter.
The shear stress can be written as [13]
t¼
Fig. 7. Geometry diagram of serrated chip.
F cos o : bD hD =sin j1
ð16Þ
The values of shear stress, with different cutting speed and feed rate, have been got by these formulas according to the Eq. (16). The variation in shear stress with different cutting speeds and feed rates was shown in Fig. 11.
G. Dong et al. / International Journal of Machine Tools & Manufacture 51 (2011) 520–527
525
1.4
shear stress
shear strain rate 1.2 shear stress (MPa)
shear strain rate (s-1)
3
x10 650 600 550 500 450 400 350 300 250 200 150 100 50 0
1.0 0.8 0.6 0.4 30
30
40
50
60 v c (m/min)
70
40
50
60
80 1.4
shear strain rate
1000 900
shear stress (MPa)
1.2
1100 shear strain rate (s-1)
80
shear stress
3
x10 1300 1200
70
vc (m/min)
800
1.0 0.8 0.6
700 0.4
600
0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 f (mm/rev)
500 400
Fig. 11. Variation of shear stress t with vc and f. (a) vc t and (b) f t.
0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 f (mm/rev) Fig. 9. Variation of shear strain rate g_ with vc and f. (a) vc g_ and (b) f g_ .
H
h
L
Fig. 12. Geometry character of serrated chip.
The generation frequency of serrated chip can be expressed as fz ¼
vc f sin kr =H : L
ð17Þ
Fig. 10. Geometric diagram of shear stress.
3.3. Effects of cutting parameters on geometry characteristic of serrated chip The effects of cutting speed and feed rate on the geometry characteristics of serrated chip were also studied. The parameters of collected are shown in Fig. 12.
(1) Effects of cutting speed on geometry characteristic of serrated chip: The experiment has been carried out using different cutting speeds as: 40, 50, 60, 70 and 80 m/min. The feed rate f and cutting depth ap are constants. They are 0.16 mm/r and 1 mm, respectively. The tool edge angle kg is 451 and rake angle g0 is 61.
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0.55
0.14
0.10
0.50 f =0.1mm/rev ap=1mm
f =0.1mm/rev ap=1mm
0.45
0.08
h/H
tooth spacing L(mm)
0.12
0.06
0.40
0.04 0.35
0.02 0.00 40
50
60 vc(m/min)
70
80
0.30 40
50
60 vc(m/min)
70
80
generation frequency fz (Hz)
x103 400 350
f =0.1mm/rev ap=1mm
300 250 200 150 40
50
60 vc(m/min)
70
80
Fig. 13. Effects of cutting speed on geometry characteristic of serrated chip. (a) Variation of tooth spacing L with cutting speed vc, (b) variation of h/H with cutting speed vc, and (c) variation of generation frequency of serrated chip with cutting speed vc.
The experimental results showed that saw tooth spacing (L), the ratio of saw tooth height to chip thickness (h/H) and generation frequency of the serrated chip (fz) increased with the increasing of cutting speed, as shown in Fig. 13. (2) Effects of feed rate on geometry characteristic of serrated chip: The experiment has been carried out using different feed rates as: 0.08, 0.12, 0.16, 0.2 and 0.24 mm/r. The cutting speed vc and cutting depth ap are constants. They are 60 m/min and 1 mm, respectively. The tool edge angle kg is 451 and rake angle g0 is 61. The experimental results showed that when feed rate changed from 0.08 to 0.16 mm/r, saw tooth spacing (L) and the ratio of saw tooth height to chip thickness (h/H) increased with the increase in feed rate. When feed rate changed from 0.16 to 0.24 mm/r, saw tooth spacing (L) and the ratio of saw tooth height to chip thickness (h/H) decreased with the increasing of feed rate. However, it is opposite for the changes of the generation frequency of serrated chip (fz) with feed rate, as shown in Fig. 14.
4. Conclusions The chip root was obtained through the experiment involving quick-stop device and then was made into metallographic specimens. Some correlated theories were applied to study the
deformation process when machining Inconel 718 after the observation of metallographic specimens of chip root and chip micro-morphology. Based on this study, the following conclusions can be drawn: (1) Through the observation of metallographic specimens of chip root, the results showed that when vc is less than 40 m/min, the chip presented ribbon shape. When vc is higher than 40 m/min, the chip presented serrated. (2) The formation mechanism of serrated chip can be summarized as: with the compressive force in the shear zone increased, the shear angle gradually decreased and the shear plane transformed into shear body. At last, the serrated chip is generated as the interaction of the crack in the free surface and the adiabatic shear. (3) By analyzing the formation process and the characteristic of serrated chip, it is proposed that the calculation model of strain and strain rate in the adiabatic shear zone has been constructed. (4) Cutting speed and feed rate are sensitive factors that affect shear stress, shear strain, shear strain rate and so on. Through the observation of chip morphology and careful calculation, the effects of cutting speed and feed rate on geometry characteristic of serrated chip were studied in this paper. With increasing cutting speed, saw tooth spacing (L), the ratio of saw tooth height to chip thickness (h/H) and generation
0.24 0.22 0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00
527
0.50
vc=80m/min ap=1mm
vc=80m/min ap=1mm
0.45
h/H
tooth spacing L(mm)
G. Dong et al. / International Journal of Machine Tools & Manufacture 51 (2011) 520–527
0.40 0.35 0.30 0.25
0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28 0.30
0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26
f (mm/rev)
f (mm/rev)
generation frequency fz (Hz)
x103 360 340
vc=80m/min ap=1mm
320 300 280 260 240 220 200 180 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26
f (mm/rev) Fig. 14. Effects of feed rate on geometry characteristic of serrated chip. (a) Variation of tooth spacing L with feed rate f, (b) variation of h/H with feed rate f and (c) variation of generation frequency of serrated chip with feed rate f.
frequency of the serrated chip (fz) increased. When feed rate changed from 0.08 to 0.16 mm/r, saw tooth spacing (L) and the ratio of saw tooth height to chip thickness (h/H) increased with the increase in feed rate. However, when feed rate changed from 0.16 to 0.24 mm/r, the values of L and h/H decreased. It is opposite for the changes in the generation frequency of serrated chip (fz) with feed rate. Acknowledgements This work was supported by ‘‘863 key projects of China— highefficiency cutting technology and applications of large components of nickel-based alloy (2009AA044301)’’. References [1] R.F. Recht, Catastrophic thermoplastic shear, Journal of Applied Mechanics— Transactions of the ASME 86 (1964) 189–193. [2] R. Komanduri, R.H. Borwn, On the mechanism of chip segmentation in machining, Journal of Engineering for Industry 103 (1981) 33–51. [3] R. Komanduri, T. Schroeder, J. Hazra, et al., On the catastrophic shear instability in high-speed machining of an AISI 4340 Steel., Transactions of the ASME: Journal of Engineering for Industry 104 (1982) 121–131.
[4] M.A. Davies, Y. Chou, C.J. Evans, On chip morphology, tool wear and cutting mechanics in finish hard turning, Annals of the CIRP (1996) 77–82. [5] M.A. Davies, T.J. Burns, C.J. Evnas, On the dynamics of chip of formation in machining hard metals, Annals of the CIRP 46 (1997) 25–30. [6] Wang Minjie, Dynamic mechanical properties of metal and thermoplastic shear instability in the orthogonal cutting experiments. Degree of Doctor of Philosophy in Engineering, Dalian University of Technology, 1989. [7] Duan Chunzheng, Study on microcosmic mechanism of adiabatic shear behavior in orthogonal cutting of high strength steel. Degree of Doctor of Philosophy in Engineering, Dalian University of Technology, 2004. [8] M.A. Elbestawi, A.K. Srivastava, T.I. El-Wardany., A model for chip formation during machining of hardened steel, ClRP Annals—Manufacturing Technology 45 (1996) 71–76. [9] G. Poulachon, A.L. Moisan., Hard turning: chip formation mechanisms and metallurgical aspects, Journal of Manufacturing Science and Engineering— Transactions of the ASME 122 (2000) 406–412. [10] T. Obikawa, E. Usui, Computational machining of titanium alloy-finite element modeling and a few results, Journal of Manufacturing Science and Engineering—Transactions of the ASME 118 (1996) 208–215. [11] Hua Jiang, Shivpuri Rajiv, Prediction of chip morphology and segmentation during the machining of titanium alloys, Journal of Materials Processing Technology 150 (2004) 124–133. [12] J. Friedel, writings. Translated by Wang Yu. Dislocations. Beijing: Science and Technology Press.1984. [13] Zhongshan, Kazuo, Translated by Li Yunfang. Metal Cutting Theory. Machinery Industry Press (1985) 93–96.