3.1 Experimental conditions
The material used in this experiment are 2024 Al alloy and CFRP, and the fiber type of CFRP is T700. Al alloy with a size of 150 mm×100 mm×2 mm and CFRP with a size of 150 mm×95 mm×5 mm are selected as shown in Fig. 6. The material of the fouredge milling tool is cemented carbide (tungsten steel) and the diameter is 5 mm. The diameter of the hole is 6 mm. The basic parameters of the milling tool are shown in Table 1.
Table 1
Overall length
(mm)

Blade length
(mm)

HRC

Helical angle
(°)

20

15

58

35

3.2 Experimental design
According to literature [33,34], it can be seen that the most important factors of influence the quality of holemaking are spindle speed and pitch. Secondly, some studies have found that holemaking of applying preload has significant inhibitory effect on the burr height and damage of the interlayer gap. This is because the formation of burrs between laminations is inhibited by inhibiting the extra gap under the action of flow force when holemaking of applying preload [35]. Therefore, this paper chooses the preload as the third influence factors for the helical milling of laminated materials. Through experiments, it is found that when the spindle speed is between 2000 r/min and 3000 r/min, the pitch is between 1.5 mm and 2.2 mm, which has greater impact on the quality of the hole.Therefore, the selected factor levels are shown in Table 2.
Table 2
Level

Spindle speedA
(rpm)

PitchB
(mm)

PreloadC
(N)

1

2000

0.1

240

2

3000

0.2

360

3

4000

0.3

480

3.3 Analysis of Al alloy/CFRP
Taking spindle speed, pitch and preload as design variables and taking exit burrs of Al alloy and damage values of CFRP as response values. The 17 groups of response surface experiments were obtained by BBD experimental design as shown in Table 3.
Table 3 Experimental results
No.

Spindle speedA
(rpm)

PitchB
(mm)

PreloadC
(N)

Results
(mm)

1

3000

0.3

240

44.2 0.59 0.545 40.6

2

3000

0.2

360

16.2 0.41 0.19 11.7

3

3000

0.1

480

27.2 0.16 0.11 24.2

4

2000

0.2

240

44.8 0.855 0.415 36.9

5

3000

0.1

240

32.5 0.23 0.15 28.9

6

3000

0.2

360

16.2 0.42 0.19 12

7

4000

0.2

480

38.6 0.13 0.13 32.5

8

3000

0.2

360

17.8 0.415 0.195 11.5

9

2000

0.2

480

38.2 0.79 0.455 35.4

10

2000

0.3

360

48.5 1.25 0.875 45.2

11

3000

0.2

360

15.6 0.415 0.195 12.2

12

3000

0.3

480

34.3 0.52 0.39 38.4

13

2000

0.1

360

39.8 0.71 0.21 31

14

4000

0.3

360

43.5 0.44 0.29 38.9

15

3000

0.2

360

15.8 0.4 0.185 12.5

16
17

3000
4000

0.2
0.1

240
360

41.5 0.31 0.16 36.1
42.0 0.12 0.065 32.4

The maximum burr for the Al alloy at the entrance and exit are listed in Table 3 respectively. Through the nonlinear fitting method, the variables and the size of the burr at the entrance and exit of the Al alloy are subjected to multiple regression fitting and the following mathematical model of the multiple regression equation is obtained. Where Aspindle; Bpitch; Cpreload.
$$\begin{gathered} H=273  0.09995A  293.35B  0.4185C  0.018AB \hfill \\ {\text{+}}7.708 \times {10^{  6}}AC  0.0958BC+1.6677 \times {10^{  5}}{A^2} \hfill \\ +1045.25{B^2}+5.401 \times {10^{  4}}{C^2} \hfill \\ \end{gathered}$$
7
After the mathematical model of the regression equation is obtained, it is necessary to further test the fit and reliability of the model itself. The regression equation analysis of variance table is obtained as shown in Table 4.
It can be seen from Table 4that the F value is 108.95 and the Prob > F value is 0.0001. It is less than 0.05 that the model is significant, which means that the regression equation is valid and the significance is very high. The Prob > F value of lack of fit of the model is 0.0597 greater than 0.05. This indicates that it is not significant. It can be seen from the table that the Prob > F values of B and C are far less than 0.05, which indicates that their influence on holemaking is relatively large.
Table 4
Source of variance

Sum of square

F

Prob > F

Model

2285.37

108.95

< 0.0001

A

4.06

1.74

0.2284

B

105.13

45.10

0.0003

C

76.26

32.72

0.0007

AB

12.96

5.56

0.0505

AC

3.42

1.47

0.2649

BC

5.29

2.27

0.1757

A2

1171.11

502.45

< 0.0001

B2

460.02

197.37

< 0.0001

C2

254.69

109.27

< 0.0001

Residual

16.32



Lack of fit

13.31

5.90

0.0597

It can be seen from Fig. 7(a) that the residual arrangement is approximately a straight line. It can be seen from Fig. 7(b) that each experimental point is relative dispersed. All these indicate that the model has high concordance between the prediction value of the burr height of the Al alloy and the experimental value.
The RSM is used to analyze the results and while one experimental factor remains unchanged, the influence of the interaction between other experimental factors on the burr height of the Al alloy at the entrance and exit are explored.
It can be seen from Fig. 8(a) that when the preload remains constant, with the increase of the spindle speed, the Al alloy burr height first decreases and then increases. When the spindle speed is about 3000 r/min, the burr height increases. The value tends to be stable and the burr height decreases to the lowest value when the pitch is about 0.18 mm.
It can be seen from Fig. 8(b) that when the pitch remains constant, the burr height first decreases and then increases with the increase of the spindle speed and the overall change trend is obvious. When the spindle speed is around 3000 r/min, the burr height trend to be stable. And the burr height first decreases and then tends to be stable with the increase of preload, but the overall change trend is not obvious. When the spindle speed is around 3000 r/min and the preload is around 380 N, the burr height is reduced to the lowest level.
It can be seen from Fig. 8(c) that when the spindle speed remains constant, the burr height first decreases with the increase of the pitch, tends to be stable and then increases greatly. This shows that the burr height trend to be stable when the pitch is in the range of 0.1–0.2 mm. However, the efficiency of holemaking is considered. The pitch is 0.2 mm. And the burr height first decreases and then tends to be stable with the increase of the preload. When the preload reaches about 380 N, the burr reaches the lowest level and tend to be stable state.
The RSM is used to predict the experimental results of Al alloy, with the minimum burr height at the exit as the goal, the optimal parameters combination is obtained as shown in Fig. 9.
For CFRP, the maximum tear value at the entrance and exit is used as the criterion for analysis. Therefore, the tear value of CFRP are listed in Table 3.
The multiple regression fitting is performed on the variables and the exit tear value of CFRP by the nonlinear fitting method, and the following the multiple regression equation is obtained. Table 5 is the regression equation of variance analysis table.
$$\begin{gathered} H=1.81875  1.2708 \times {10^{  3}}A  2.34B+3.5552 \times {10^{  3}}C \hfill \\  5.5 \times {10^{  4}}AB  1.7708 \times {10^{  7}}AC+2.632 \times {10^{  17}}BC \hfill \\ +1.85875 \times {10^{  7}}{A^2}+3.2125{B^2}  4.80035 \times {10^{  6}}{C^2} \hfill \\ \end{gathered}$$
8
Table 5
Source of variance

Sum of square

F

Prob > F

Model

1.38

128.56

< 0.0001

A

0.87

726.60

< 0.0001

B

0.31

261.25

< 0.0001

C

0.022

18.02

0.0038

AB

0.012

10.13

0.0154

AC

1.806E003

1.51

0.2585

BC

0.000

0.000

< 0.0001

A2

0.15

121.79

< 0.0001

B2

4.345E003

3.64

0.0981

C2

0.020

16.84

0.0045

Residual

1.194E003



Lack of fit

2.710E003

27.14

0.054

The F is used to judge the significant degree of each item. A large F value and small Prob > F value represent higher significance of the relevant items [36]. The F value is 128.56 and the Prob > F value is less than 0.0001. Pitch and the interaction of pitch and preload have very significant influence on the tear. Therefore, within the parameter range selected in the experiment, the sequence of influence degree on the response value are pitch > spindle speed > preload.
The distribution of experimental data in Fig. 10(a) is approximately a straight line, and the experimental points in Fig. 10(b) are also relative dispersed. In general, this model is reliable for predicting the tear value of CFRP holes.
The RSM is used to analyze the results, and while one experimental factor remains unchanged, the influence of the interaction between other experimental factors on the burr height of the Al alloy at the entrance and exit is explored.
It can be seen from Fig. 11(a) that when the preload remains constant, with the increase of the spindle speed, the tear value of the CFRP at the entrance first decreases and then remains stable. At about 3000 r/min of spindle speed, the tear value of CFRP tends to be stable. When the spindle speed is about 3500 r/min and the pitch is about 0.11 mm, the burr height is reduced to the lowest.
It can be seen from Fig. 11(b) that the interaction between the spindle speed and the preload is not significant. When the pitch remains constant, the tear value of the CFRP at the entrance and exit decreases first and then tends to be stable with the increase of the spindle speed, and the overall change trend is not obvious. When the spindle speed is around 3600 r/min, the tear value began to stabilize. And the tear value decreases first and then tends to be stable with the increase of the preload, but the overall change trend is obvious. When the spindle speed is about 3600 r/min and the preload is about 430 N, the tear value of the hole edge is reduced to the lowest.
It can be seen from Fig. 11(c) that when the spindle speed remains constant, the burr height first increases slightly and then greatly increases with the increase of the pitch, which shows that when the pitch is in the range of 0.1–0.13 mm, the tear value along the hole tends to be stable. When the spindle speed remains constant, the burr height decreases first and then tends to be stable with the increase of the preload. When the preload reaches about 450 N, the tear value along the hole reaches the minimum and tends to be stable state.
The RSM is used to predict the experimental results of CFRP, with the minimum tear value at the exit as the goal, the optimal process parameters combination is obtained as shown in Fig. 12.
3.4 Analysis of CFRP/ Al Alloy
Similar to the analysis of the measurement results of the laminated materials, the tear value of the CFRP material and the maximum burr at the entrance and exit of the Al alloy are also used as the response value. And the 17 groups of response surface experiments are obtained by the BBD experimental design. The tear value for the CFRP at the entrance and exit are listed in Table 3, and the following mathematical model of multiple regression equation is obtained. Table 6 is the regression equation of variance analysis.
$$\begin{gathered} H=0.61687  3.72 \times {10^{  4}}N  2.56375{a_p}  2.1979 \times {10^{  4}}F \hfill \\  1.1 \times {10^{  3}}N{a_p}  1.45833 \times {10^{  5}}NF  2.39588 \times {10^{  3}}{a_p}F \hfill \\ +8.0123 \times {10^{  8}}{N^2}+8.8875a_{p}^{2}+1.31076 \times {10^{  6}}{F^2} \hfill \\ \end{gathered}$$
9
It can be seen from Table 6 that the F value is 36.05, and the Prob > F value is less 0.0001. It less than 0.05 indicates that the model term is significant. At the same time, the interaction between the pitch and the spindle speed also has very significant influence on the tear value. Within the range of parameters selected in the experiment, the significant degree of their influence on the response value are pitch > spindle speed > preload.
Table 6
Source of variance

Sum of square

F

Prob > F

Model

0.64

36.05

< 0.0001

A

0.21

107.2

< 0.0001

B

0.31

154.03

< 0.0001

C

4.28E003

2.15

0.1858

AB

0.048

24.35

0.0017

AC

1.225E003

0.62

0.4582

BC

3.306E003

1.66

0.2381

A2

0.027

13.60

0.0078

B2

0.033

16.73

0.0046

C2

1.500E003

0.75

0.4138

Residual

1.988E003



Lack of fit

0.060

7.08

0.0445

The distribution of experimental data in Fig. 13 (a) approximates a straight line. Each experimental point in Fig. 13(b) is also relative dispersed. In general, this model is reliable for the predicting the tear value of CFRP holes.
It can be seen from Fig. 14(a) that when the preload remains constant, with the increase of the spindle speed, the tear value at the entrance and exit has decreasing trend. When the spindle speed is about 3600 r/min, the tear value reaches the minimum and when the pitch is about 0.1 mm, the tear value is reduced to the minimum.
It can be seen from Fig. 14(b) that when the pitch remains constant, the tear value at the entrance and exit decreases with the increase of the spindle speed. When the spindle speed is around 3600 r/min, the tear value tends to be stable. When the pitch remains constant, the tear value along the hole slowly decreases with the increase of the preload and then tends to be stable, but the decreasing trend is slow. When the spindle speed is around 3700 r/min and the preload is around 460 N, the tear value is reduced to the minimum.
It can be seen from Fig. 14(c) that when the spindle speed remains constant, the tear value at the entrance and exit decreases with the decrease of the pitch, but the influence of preload on the tear value is not obvious, and the trend of change is also slow.
The RSM is used to predict the experimental results of CFRP, with the minimum tear value at the exit as the goal, the optimal process parameters combination is obtained as shown in Fig. 15.
Similarly, the measured burr height for the Al alloy at the entrance and exit are listed in Table 3, and the following multiple regression equation is obtained. Table 7 is the regression equation of variance analysis table.
$$\begin{gathered} H=250.125  0.076935A  356.65B  0.49467C \hfill \\  0.01925AB  4.375 \times {10^{  6}}AC+0.052083BC \hfill \\ +{1.3547510^{  5}}{A^2}+1134.75{B^2}+6.73437 \times {10^{  4}}{C^2} \hfill \\ \end{gathered}$$
9
Table 7
Source of variance

Sum of square

F

Prob > F

Model

2223.11

376.46

< 0.0001

A

9.25

14.09

0.0071

B

271.45

413.70

< 0.0001

C

18.00

27.43

0.0021

AB

14.82

22.59

0.0021

AC

1.10

1.68

0.2360

BC

1.56

2.38

0.1667

A2

772.78

1177.76

< 0.0001

B2

542.17

826.30

< 0.0001

C2

395.96

603.47

< 0.0001

Residual

1.988E003



Lack of fit

0.060

7.08

0.0445

It can be seen from Table 7 that the F value is 376.46, and the model Prob > F value is less than 0.0001. If it is less than 0.05, the model is significant. The pitch and the interaction between the pitch and the spindle speed also have very significant influence on the tear.
The distribution of experimental data in Fig. 16(a) approximates a straight line. The experimental points in Fig. 16(b) are also relative dispersed. In general, this model is reliable for the predicting the burr height.
It can be seen from Fig. 17(a) that when the preload remains constant, with the increase of the spindle speed, the burr height of the Al alloy at the exit first decreases and then increases, and when the spindle speed is about 3000 r/min, the burr height tends to be stable. When the spindle speed is about 3500 r/min, the burr height begins to rise. When the pitch is between 0.15 mm and 0.22 mm, the burr height tends to be stable.
It can be seen from Fig. 17(b) that when the preload remains constant, the burr height first decreases and then increases with the increase of the spindle speed. The influence is very small. When the spindle speed is around 2900 r/min, the burr height begins to be stable. Under the condition that the pitch remains constant, the burr height only increases slightly with the increase of the preload, but the overall change is not obvious. When the spindle speed is around 2900 r/min and the preload is around 360 N, the burr height is reduced to the lowest level.
It can be seen from Fig. 17(c) that when the spindle speed remains constant, the burr height first decreases with the increase of the pitch, tends to be stable and then increases greatly, which indicates that the size of the pitch should be 0.16–0.21 mm range to choose.
Based on the analysis of these four groups, it can be seen that the factors of influence the quality of holemaking are pitch > spindle speed > preload. However, the preload has great influence on the quality of the interlayer gap of the holemaking materials.
The RSM is used to predict the experimental results of Al alloy, with the minimum burr height at the exit as the goal, the optimal parameters combination is obtained as shown in Fig. 18.
Taking into account the holemaking efficiency and holemaking quality, the optimal parameters combination for the Al alloy are obtained: the spindle speed is 3000 r/min, the pitch is 0.17 mm, and the preload is 380 N. The optimal parameters combination of CFRP are obtained: the spindle speed is 3700 r/min, the pitch is 0.11 mm, and the preload is 430 N.
3.5 Analysis of cutting force
In the machining process, important physical quantities such as the cutting state of the material, cutting quality and tool life can be reflected by cutting force and axial force. Therefore, it is of great significance to study the cutting force and axial force in the cutting process.
The radial force in the holemaking process is composed of the combined force of the FC3D 120 threedimensional force sensor in the X direction and the Y direction. The axial force is composed of the force in the Z direction of the sensor. The USB3200 signal collector is mainly used to process the collected signal information and convert it into a signal that can be recognized by the computer. The function of the computer monitor is to display the collected signal visually. The data collection for the force is shown in Fig. 19.
Figures 20(a) and 20(b) are the comparative analysis diagrams of the X and Y direction force and the axial force of the helical milling hole. The axial force during the holemaking process of the laminated materials can be roughly divided into five stages. The first stage is the entrance stage of milling CFRP. The second stage is the stable milling CFRP. The third stage is the transition stage of milling CFRP/Al alloy laminations. The fourth stage is stable milling Al alloy. The fifth stage is the milling Al alloy exit stage.
Figures 20(a) and 20(b) are machined with the optimal parameters combination of CFRP and Al alloy respectively. Compared with Fig. 20(a), the axial force of CFRP in Fig. 20(b) is about 12 N larger (The axial force are about 39 N and about 50 N respectively), the fluctuation is greater, and the quality of the obtained hole wall is poor. Therefore, the process parameters are not suitable for milling holes of CFRP. On the other hand, although the axial force of Al alloy is 11N in Fig. 20(b) greater than Fig. 20(a) (The axial force are about 41 N and about 52 N respectively), but the fluctuation is small and the efficiency is high. Therefore, this parameters are suitable for milling holes of Al alloy. Although the axial force using the process parameters of Fig. 20(a) is relatively small, it is suitable for CFRP and Al alloy. But because of the low efficiency of this parameters, and the axial force will increase sharply when the milling tool transitions from CFRP to Al alloy. Therefore, holemaking with variable parameters is considered to observe whether this sudden change in force can be eliminated.