Cryogenic Milling of Aluminium-lithium Alloys: Thermo-mechanical ...

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25.97 · Huazhong University of Science and Technology ... Technology for Automotive Components, Wuhan University of Technology, Wuhan 430070, China.

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ScienceDirect Procedia CIRP 31 (2015) 160 – 165

15th CIRP Conference on Modelling of Machining Operations

Cryogenic milling of Aluminium-lithium alloys: thermo-mechanical modelling towards fine-tuning of part surface residual stress Xiaoming Zhanga*; Haikuo Mub; Xinda Huanga; Zhongtao Fua; Dahu Zhuc ; Han Dinga a

State key Laboratory of Digital Manufacturing Equipment and Technology,Huazhong University of Science and Technology, Wuhan, 430074, China FAW-Volkswagen Automotive Co. Ltd, Changchun 130011, China c Hubei Key Laboratory of Advanced Technology for Automotive Components, Wuhan University of Technology, Wuhan 430070, China * Corresponding author. Tel.:+86-27-87559842; fax: +86-27-87559842. E-mail address: [email protected] b

Abstract

Cryogenic cooling is emerging nowadays as an effective method for high performance machining, improving the machining efficiency as well as the surface integrity of the machined parts. Aluminium–lithium (Al-Li) alloys will likely become the material of choice over composites as the fuselages of the aircraft due to its higher strength-to-weight ratio and excellent corrosion resistance. We study the influence of liquid nitrogen (LN2) cryogenic cooling in milling process on surface residual stress of the generated Al-Li alloy parts. The model of the intrinsic thermo-mechanical coupling characters during cryogenic milling process is developed, which determines the cutting temperatures and forces related to the process parameters. Models and experimental results indicate that the cutting forces change little, while cutting temperatures do much under the LN2 cryogenic cooling and conventional dry milling operations respectively. Then the residual stresses of part surfaces after milling under the two different conditions are measured. Measured results show that the residual compression forces(negative values) under the LN2 condition is much less than those under the dry milling operations. This states clearly that the cutting temperatures contribute much more than the cutting forces to the part surface residual stresses. .

© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2015 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of The International Scientific Committee of the “15th Conference on Modelling of Machining Operations”. Peer-review under responsibility of the International Scientific Committee of the “15th Conference on Modelling of Machining Operations Keywords: cryogenic cooling; milling; thermo-mechanical modelling; surface integrity

1. Introduction Al-Li alloy plays great role in the aircraft structures because of the higher strength-to-weight ratio and the excellent corrosion resistance. In the past, some Al-Li structures are made using the chemical-corrosion method, which in results leads to the environmental problem. Nowadays, mechanical milling replaces the chemical method in making these structures, improving the machining efficiency and avoiding the possibility of environmental pollutions. However, in a milling process the structure surfaces are subjected to the strongest loads as well as to environmental influences. In addition to the mechanical and thermal stressed, environmental influences and influences of other components, subjected to the cooling conditions, should be taken into account.

Coolant is of great importance during metal cutting process. Not only the tool wear but also the surface integrity of the machined surface is influenced dramatically by the cooling conditions due to the temperature rising in the second and tertiary cutting zone. However, conventional cutting fluid poses a threat to the worker and environment. What’s more, both the recycling and disposal of waste coolant will cost a lot. As a result, several new cooling methods are proposed in recent years, including minimum quantity lubrication (MQL), high-pressure coolant (HPC), and cryogenic cooling etc., as summarized in detail by Sharma et al.[1]. In these methods, cryogenic cooling with LN2 is emerging as a promising approach due to its non-toxic and excellent cooling effect. Wang and Rajurkar[2] reported the significant improvement of tool life and surface roughness in cryogenic machining of hard-to-cut materials. Hong et al.[3] found that cryogenic machining tends to increase the cutting force for

2212-8271 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the International Scientific Committee of the “15th Conference on Modelling of Machining Operations doi:10.1016/j.procir.2015.03.055

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Ti6Al4V caused by the hardening of workpiece at lower temperature. In contrast, the lubrication effect of LN2 is verified by experiments in [4], which reduce the friction force. Recently, the influences of cryogenic cooling on the enhancement of surface integrity attract extensive attention. Biermann et al. [5] conducted cryogenic milling of aluminium alloys using Carbon-dioxide snow and found that the surface quality was improved due to the suppression of burr formation. Pušavec et al. [6] investigated the influence of cryogenic machining on the surface integrity of Inconel718. Their experimental results show that in cryogenic machining the compressive zone was extended from 40ȝm to 70ȝm and the hardness on the surface increased from 500HV to 800HV. Pu et al. [7] reported that cryogenic machining of AZ31B Mg alloy can get 10 times larger compressive areas in residual stress profiles and introduced significant grain refinement, which remarkably improved the corrosion/wear resistance of the machined surface. Rotella et al.[8] investigated the effects of different cooling conditions on the surface integrity of Ti6Al4V and the result indicated that cryogenic cooling contributed to make products having better surface integrity with smaller grains in contrast to MQL machining. However, the relationships between the process parameters and the surface integrity are addressed little. Especially, the control factors directly affecting the part surface residual stress are not revealed adequately. The remainder of this paper offers the followings: - We report the experiments of great compression stress improvement under LN2 conditions. End milling tests under the dry cutting and LN2 conditions with the same process parameters are carried out and the residual stress on the Al-Li surfaces after milling operations are measured and compared. - To explain the experiment results, the machining thermalmechanical model is presented to understand the mappings from the process parameters to the cutting forces and temperatures, respectively. The model results are consistent with the measurement results of cutting forces and temperatures in milling process. The model and measurement results show that with the same process parameters the maximal cutting forces remain unchanged under the LN2 and dry cutting conditions, however the part temperatures vary greatly under the two different coolant conditions, respectively. Based on the model the relationship among the process parameters--cutting forces and temperatures--the residual stresses are revealed. 2. Experiments setup and results The cutting experiments are carried out on a milling center VMC-50. The used cutter is a four-flute end mill with center tooth supplied by SECO Company. The cutter parameters are given in Table 1. The part for cutting test is the Al-Li alloy 100mm*100mm*6mm rectangular block.

(b)

(a)

(c)

Fig. 1. The experimental setup for cutting tests and measurements: (a) cutting tests, (b) test specimen mounted on the dynamometer, (c) Thermocouple implanted in the test specimen

Table 1. Cutter parameters of the cutter SECO JABRO-HPM-JHP770 Rake angle Helix angle Diameter Tooth Number E (deg) D r (deg) D(mm) 10

40

0

4

To obtain the cutting forces and temperatures, the dynamometer Kistler9257B and T-style thermocouple are used, respectively. As shown in Fig. 1(c), a 1mm*88mm slot is made on the Al-Li block, then the thermocouple (coppery line) is implanted into the slot. The block with the thermocouple is clamped with another Al-Li block by screws, see Fig. 1(b). The specimen for milling tests, including the two Al-Li blocks with implanted thermocouple, is mounted on the dynamometer, which is fixed on the rotating platform of milling center, see Fig.1(c). We conduct series of cutting experiments with different process parameters, listed in Table 2. With these process parameters the cutting forces along three orthogonal directions and the cutting temperatures under the LN2 and dry conditions are measured and shown in Table 2. From Table 2, we can see that the tendency of the changes of cutting forces with the same process parameters under the LN2 and dry conditions respectively, are very small. In contrast to the small changes of cutting forces, the obvious changes of cutting temperatures can be observed, under the LN2 and dry conditions, respectively. To tell if the experimental results are general for milling of Al-Li, we will give the model explanations in the next section. After measuring the cutting forces and temperatures in milling of Al-Li, we measure the residual stresses of the part surface after milling operations with the same process parameters under the LN2 and dry conditions, respectively.

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Table 2. Cutting conditions and results under different cooling type Cooling type LN2

Dry

Case number

Spindle speed (rpm)

Radial depth of cut (mm)

Axial depth of cut (mm)

Feed per tooth (mm)

Fx _ max (N)

Fy _ max (N)

Fz _ max (N)

T ( 0C )

1

3000

1

3

0.1

117.7

94.12

50.36

29.8

2

3000

2

3

0.1

248.9

130.4

96.87

30.3

3

3000

3

3

0.1

314.7

122.2

115.7

37.8

4

3000

4

3

0.1

360.5

127.3

125.4

42.7

5

3000

1

3

0.1

116.2

98.68

50.76

35.5

6

3000

2

3

0.1

222.4

129.6

89.34

61.8

7

3000

3

3

0.1

319.5

132.3

118.9

76.9

8

3000

4

3

0.1

375.3

132.7

129

86.5

The tests results indicate that the Al-Li part surfaces stresses are compression ones, negative values after milling operations under dry and LN2 conditions. With the same process parameters the compression stresses under LN2 condition is about 100MPa less than that under dry milling condition, see Fig.2. From Fig.3, we can see that the residual compression stresses are approximately linear with the changes of part surface temperatures under dry milling operations. However, the linear relationship between the part surface temperatures and residual stresses do not applicable for that under LN2 milling operations, see Fig.4. Fig. 2. Effect of cutting depth on the changes of residual stresses under dry and LN2 conditions

3. Experiment results explanation with modelling of thermal-mechanical characters in Al-Li end milling process In this section, we give the milling force and temperature models to reveal the effect of process parameters on the milling forces and temperatures under the dry and LN2 coolant conditions, respectively. 3.1 Milling force calculation in end milling based on the predictive model

Fig.3. Effect of part surface temperatures on the changes of residual stresses under dry condition

Fig. 4. Effect of part surface temperatures on the changes of residual stresses under LN2 condition

In this section, an analytical mode for cutting forces is developed based on the predictive machining theory proposed by Li et al. [9, 10, 11]. The model predicts the milling forces from the input data of workpiece material properties, tool geometry and cutting conditions. This is done by discretizing the cutting edge into a number of slices along its axis direction. The cutting action of each slice is modeled as oblique cutting process with the inclination angle which is equal to the helix angle of the cutter. The cutting forces in oblique cutting are predicted using the predictive machining theory [9]. The total forces in end milling are then calculated from the sum of the forces acting on at all slices.

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specific heat of this alloy are taken as 2500kg/m3, 168W/mK and 1203J/kgK, respectively. These known parameters including workpiece material properties, tool geometry and cutting conditions are input to the predictive oblique model [10] to predict cutting forces ( dFt , dFr , dFa ) for each slice. The predicted cutting forces in local coordinates for each slice are transformed in global coordinates in the following matrix form: ­ dFx ½ ª  cos I j  sin I 0 º ­ dFt ½ ° ° « ° °  cos I 0 »» ® dFr ¾ ®dFy ¾ « sin I ° dF ° « 0 0 1»¼ °¯dFa °¿ ¯ z¿ ¬ (4) The total cutting forces in the global x, y and z directions are calculated by summing the differential cutting forces for each slice ( dFx , dFy , dFz ).

z

i0

dz

ap

z

y dFa

ae

nr

I

o

dFr dFt

x

Fig. 5. Model for end milling process

The milling process and the geometry of an end milling cutter is shown in Fig. 5. In order to predict the cutting forces, for each tool rotation ( M ), the end mill is discretized into r number of slices along its axis in the z direction. For each slice of j-th cutting edge at the axial location z , the lag angle in global coordinates {o - xyz} , I j ( z ) , measured from the + y

We give the simulation results of cutting forces under the LN2 coolant and dry conditions, shown in Figs. 6 and 7, respectively. It can be seen that the predicted results are agreement with those in Table 2.

-axis clockwise, is calculated as follows: 2 z tan i0 (1) I j ( z ) M  ( j  1)I p  D where I p 2S N f is the pitch angle and D is the diameter of the cutter, N f the number of teeth. In addition, I j ( z ) is used to determine whether this part of the cutting edge is in cut or out of cut. If the pitch angle I j ( z ) is greater than the entrance angle and less than the exit angle of the cutter, this part is in cut, otherwise, it is out of cut. The entrance and exit angles are determined based on the milling type. The undeformed chip thickness h(I j ) varies

Case 1

periodically with the immersion angle I j ( z ) and is written as:

h(I j )

ft sin I j ( z )

(2)

where f t is the feed per tooth. The part material used for the helical end milling is the Aluminium alloy, and the flow stress is calculated using the Johnson-Cook constitutive model: Ws =



1 ª A B J 3 «¬



n 3 º ª¬1  C ln J * º¼ ª¬1  T *m º¼ »¼

Case 2

(3)

where W s is the shear flow stress of the part material and J is the shear strain. J* J J0 is the ratio of the shear strain rate to a reference strain rate J0 (10-3s-1 is taken in this analysis).

T * (T  Tr ) (Tm  Tr ) is the homologous temperature, T is the workpiece temperature, Tr is the room temperature ( Tr 298 K for dry milling and Tr 77 K for LN2 coolant) and Tm 6500 C . A, B, n, C , m are material constants with the values 527MPa, 575Mpa, 0.72, 0.017,1.61, respectively [12]. The values of the density, thermal conductivity, and

Case 3

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Xiaoming Zhang et al. / Procedia CIRP 31 (2015) 160 – 165

Case 8

Case 4 Fig. 6 Cutting forces under LN2 condition with process parameters given in Table 2 (Cases 1-4)

Fig. 7 Cutting forces prediction results under dry condition with process parameters given in Table 2 (Cases 5-8)

3.2 Part surface temperature modeling in end milling A mathematical model, fully considering the tool path and chip formation mode, for the prediction of workpiece temperature during the milling operation is proposed by our previously work [13]: 2D Rw qm I  I  I SO v(l cos Os  a p sin Os ) 1 2 3

T



I1

³D

I2

³

I3

³

Case 5

( x  l cos Os ) v 2D

xv 2

(x 

( x  a p sin Os ) v 2D ( x  l cos Os ) v 2D

2D 1 v  tan Os )e  u k0 ( u )( tan Os 2D v

l u v e k0 ( sin Os 2D

( x  l cos Os  a p sin Os ) v 2D ( x  l cos Os ) v 2D

4D 2 2 u  zi 2 ) du v2

4D 2 2 u  zi 2 )du v2

2D § · ¨ a p sin Os  l cos Os  v u  x ¸  u v ¨ ¸e k0 ( 2D sin Os cos Os ¨¨ ¸¸ ¹ ©

4D 2 2 u  zi 2 ) du v2

(5) where D is the thermal diffusivity of the workpiece, Rw the energy partition to the workpiece, qm the heat flow out from the shear surface, O the thermal conductivity of the workpiece, v the heat source speed, here it equals to the work speed, Os the edge inclination angle of the cutting tool, l the Case 6

contact length between tool and workpiece, a p the axial depth of cut. The temperature drop in the workpiece due to the cryogenic cooling is given by the equation˖ li w / 2 § 1 q 1 · (6) T _ cool  cool ³ ³ ¨  ¸dzdx 2S kt 0  w / 2 © Ri Ric ¹ where, li is the distance from the tool tip to which the cryogenic cooling is acting, w is the axial depth of cut. , and Ri ( X  xi )2  (Y  yi )2  Z 2

Ric Case 7

( X  (2L2  xi ))2  (Y  yi )2  Z 2

.

The

heat

loss

intensity qcool h (T  T0 ) , where h is the overall heat transfer coefficient, T0 is the room temperature. In terms of the above cutting temperature model, the cutting temperatures under dry and LN2 conditions are given in Figs. 8. It can be observed that the cutting temperatures obtained from the predictive model, are consistence with the experimental data shown in Table 2.

Xiaoming Zhang et al. / Procedia CIRP 31 (2015) 160 – 165

Cases 1 and 5

165

LN2 coolant is helpful to improve the structural fatigue, comparing with the conventional coolant conditions. As we know the cutting forces and temperatures act as the main control parameters to adjust the part surface residual stresses, so the cutting forces and temperatures in milling process are measured. An important phenomenon is found that the changes of cutting forces under the dry and LN2 milling operations respectively are very small. In contrast the tendency of changes of cutting temperatures under the two different coolant conditions is very obvious. We use the cutting forces and temperatures models to explain why the cutting forces under the two different coolant conditions vary little, while the cutting temperatures do much. So we can draw the conclusion that the cutting temperatures make the main contributions to improve the residual compression stresses in milling of Al-Li. Acknowledgements This work was partially supported by the National Natural Science Foundation of China (51375005) and the National Basic Research Program of China (2014CB046704). References

Cases 2 and 6

Cases 3 and 7

Cases 4 and 8 Fig. 8 Cutting temperatures prediction results with process parameters given in Table 2

4. Conclusions We use LN2 as the coolant for milling of Al-Li and find the residual compression stress is less than that using the conventional dry milling operations. The results indicate that

[1] V.S. Sharma, M. Dogra, N.M. Suri, Cooling techniques for improved productivity in turning, International Journal of Machine Tools and Manufacture 49 (2009) 435-453. [2] Z.Y. Wang, K.P. Rajurkar, Cryogenic machining of hard-to-cut materials, Wear 239 (2000) 168 - 175. [3] S.Y. Hong, Y. Ding, W. Jeong, Friction and cutting forces in cryogenic machining of Ti̢6Al̢4V, International Journal of Machine Tools and Manufacture 41 (2001) 2271 - 2285. [4] S.Y. Hong, Y. Ding, J. Jeong, Experimental evaluation of friction coefficient and liquid nitrogen lubrication effect in cryogenic machining, Machining Science and Technology 6 (2002) 235 - 250. [5] D. Biermann, M. Heilmann, Improvement of workpiece quality in face milling of aluminum alloys, Journal of Materials Processing Technology 210 (2010) 1968-1975. [6] F. Pusavec, H. Hamdi, J. Kopac, I.S. Jawahir, Surface integrity in cryogenic machining of nickel based alloyüInconel 718, Journal of Materials Processing Technology 211 (2011) 773 - 783. [7] Z. Pu, J.C. Outeiro, A.C. Batista, O.W. Dillon, D.A. Puleo, I.S. Jawahir, Enhanced surface integrity of AZ31B Mg alloy by cryogenic machining towards improved functional performance of machined components, International Journal of Machine Tools and Manufacture 56 (2012) 17 27. [8] G. Rotella, O.W. Dillon Jr, D. Umbrello, L. Settineri, I.S. Jawahir, The effects of cooling conditions on surface integrity in machining of Ti6Al4V alloy, The International Journal of Advanced Manufacturing Technology 71 (2014) 47 - 55. [9] B. Li, X. Wang, Y. Hu, C. Li, Analytical prediction of cutting forces in orthogonal cutting using unequal division shear-zone model, The International Journal of Advanced Manufacturing Technology 54 (2011) 431-443. [10] B. Li, Y. Hu, X. Wang, C. Li, X. Li, An analytical model of oblique cutting with application to end milling, Machining Science and Technology 15 (2011) 453-484. [11] Z. Fu, X. Zhang, X. Wang, W. Yang, Analytical modeling of chatter vibration in orthogonal cutting using a predictive force model, International Journal of Mechanical Sciences 88 (2014) 145-153. [12] N.S. Brar, V.S. Joshi, B.W. Harris, M. Elert, M.D. Furnish, W.W. Anderson, W.G. Proud, W.T. Butler, Constitutive model constants for Al7075-T651 and Al7075-T6. 2009, pp. 945. [13] D. Zhu, Thermal modeling and tool wear prediction in cutting of nickelbased superalloys. In: Postdoctoral Thesis. Huazhong University of Science and Technology 2013. (in Chinese)

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