Si3N4 graded ceramic tools at high speed machining

Chapter 2

Performance of Sialon/Si3N4 graded ceramic tools at high speed machining Guangming Zhenga and Jun Zhaob a

School of Mechanical Engineering, Shandong University of Technology, Zibo, China; School of Mechanical Engineering, Shandong University, Jinan, China

b

Chapter outline 1 Introduction 2 Design and fabrication of the Sialon/Si3N4 graded ceramic tool 2.1 Design model and raw materials 2.2 Fabrication process 2.3 Mechanical properties and microstructure 3 High speed turning of Inconel 718 3.1 Experimental method 3.2 Chip patterns 3.3 Cutting force and cutting temperature 3.4 Tool life

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3.5

Failure characteristics and failure mechanisms 3.6 Machined surface roughness 4 High speed milling of Inconel 718 4.1 Experimental method 4.2 Chip patterns 4.3 Cutting force and cutting power 4.4 Failure characteristics 4.5 Failure mechanisms and self-sharpening 4.6 Machined surface roughness 5. Summary

39 41 45 45 46 48 50 52 58 59

1 Introduction Ni-based alloy has been widespread employed in engine parts [1,2], because of the excellent mechanical properties on elevated temperature. During the high speed machining (HSM) process, the cutting features of sharply tool wear, high cutting force and cutting temperature, and poor machined surface integrity were demonstrated [3,4]. However, the cutting forces, cutting heat, and specific shearing energy can be reduced at higher cutting speeds [1,5,6]. For Ni-based alloy, some machining difficulties may be relieved by applied the cutting condition of high speeds or ultra high speeds. With the increasing demand for this kind of material in the key industries (such as aerospace, energy, automobile, shipbuilding, etc.), the development of innovative high performance tools and High Speed Machining. http://dx.doi.org/10.1016/B978-0-12-815020-7.00002-3 Copyright © 2020 Elsevier Inc. All rights reserved.

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the determination of reasonable cutting process have been facing unprecedented challenges. Initially, in order to design of heat-shielding structure materials, the concept of functionally graded materials (FGMs) was proposed [7,8]. After that, FGM was applied to the development of cutting tool, and the high thermal and mechanical properties were obtained [9,10]. Research on metal cutting indicated that the FGM tool expressed higher tool lives, compared with the homogeneous cutting tool [11]. Si3N4 ceramics had higher strength, toughness, and thermal shock resistance, whereas Sialon ceramics had excellent chemical stability and wear resistance. With the concept of the graded material, the advantages of these two ceramic materials can be complemented. Application of hot pressing sintering process and the Sialon/Si3N4 graded ceramic tool was developed, which was designed by constructing a graded structure model at the macro level and adding micro-nano-composite particles at the micro level. Aiming at nickel-based super alloy Inconel 718, the cutting performance of the graded tool was studied at high speed and ultra high speed cutting conditions. Through the research of this work, it is expected to provide optimum cutting parameters for HSM and high quality machining of nickelbased super alloy, and to popularize this graded tool.

2  Design and fabrication of the Sialon/Si3N4 graded ceramic tool 2.1  Design model and raw materials The main idea of this tool system design is to introduce residual compressive stresses on the tool surface by means of the properly graded structures. In this way, the stress-relieving effect can be achieved in the cutting process, thus improving the wear resistance of the tool. Fig. 2.1 presents the model and inserts of the graded ceramic material. The tool has five layers with symmetrical structure. According to the model, both the first and fifth layer surfaces of the tool can be

FIGURE 2.1  Model and inserts of the graded ceramic material.

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Sialon/Si3N4 graded ceramic tools at high speed machining Chapter | 2

TABLE 2.1 Compositions of the MNCM of ceramic tools. Sintering additives

Compositions Si3N4

Al2O3

AlN

TiC0.7N0.3

Al2O3

Y2O3

Grain size (µm)

0.5

0.02

0.1

0.5

0.5

0.5

Purity (%)

>99

>99

>99.6

>99

>99

>99.6

>99.99

Composites (vol.%) SAAT10

53.25

17.75

10

5

10

0

4.0

ST10

61.50

20.50

0

0

10

3.2

4.8

ST15

57.75

19.25

0

0

15

3.2

4.8

ST20

54.00

18.00

0

0

20

3.2

4.8

used as a rake face. In the present work, the thickness ratio h1/h2 = h2/h3 = 0.3 was adopted, which can lead to the optimum mechanical properties [12]. The basic material system was Sialon material with high hardness and Si3N4 material with high strength [13–15]. Therefore Si3N4 ceramic was selected in the inside layer to ensure the high antidamage capability, whereas Sialon ceramic was selected in the surface layer to ensure the high wear resistance. Table 2.1 shows the compositions of the micro-nano-composite material (MNCM) of ceramic tools. Under the appropriate temperature conditions, the solid reaction process was used to synthetize β-Sialon. The synthesis formula is as follows [16,17]:

(6 − z )Si3N 4 + zAl2 O 3 + zAlN = 3Si6− z Alz O z N 8− z

( 0 < z ≤ 4.2) .

(2.1)

Based on the thermodynamic analysis, the free energy of Sialon with 0 < z ≤ 4.2 was less than zero. Therefore the β-Sialon with different z values can be synthetized by controlling the suitable compositions of the major phases Si3N4, Al2O3, and AlN at high temperature. Based on the tool model in Fig. 2.1, Table 2.2 lists the design scheme of the ceramic tool materials. The thermal expansion coefficient of TiC0.7N0.3 was 8.6 × 10−6 K−1, while it was 3.2 × 10−6 K−1 for Si3N4. If the low volume content of TiC0.7N0.3 was put in the first layer, the compressive stress could be introduced. The first and fifth layer surfaces could be used as rake face due to the symmetrical structure (Fig. 2.1).

2.2  Fabrication process The Si3N4 nanoparticles and Al2O3 nanoparticles were dispersed by the dispersant (polyethene glycol) and dispersing medium (ethanol). And then the

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TABLE 2.2 Design scheme of the ceramic tool materials. Tool code

Structural composites

GSS1

ST10/ ST15/ ST20/ ST15/ ST10 (five layers)

GSS2

SAAT10/ ST15/ ST20/ ST15/ SAAT10 (five layers)

ST10

ST10

SAAT10

SAAT10

FIGURE 2.2  Process curve of hot-pressing sintering (1750°C, 35 MPa, 60 min).

micropowders were mixed with the well uniform suspension nanoparticles of the same composite (Table 2.1). After that, the ball-milling process (about 48 h) and the vacuum drying process (at 100°C) were successively carried out. Finally, a 120-mesh sieve was selected to sieve the mixed powder. According to the thickness ratio of 0.3 (Fig. 2.1) and material design of structural composites (Table 2.2), a graphite mold was adopted to stack the composite powders with different mixture ratios. A hot pressing sintering furnace with the type of ZRC85-25T was used to sinter the specimens (its size referred to Fig. 2.1). The vacuity was controlled at 6.75 × 10−2 Pa during sintering. Fig. 2.2 shows the process curve of hot-pressing sintering at 1750°C and 35 MPa for 60 min. And then the specimen was cut into some strips or inserts by an automatic internal circular slicing machine (Model J5060E1, China). After rough grinding, finish grinding, lapping, and polishing, the samples of the strips or inserts were cleaned by ultrasonic wave, then packaged and readied for use.

2.3  Mechanical properties and microstructure A VHX-600E optical microscope was selected to detect the surface topography of the samples. Fig. 2.3 exhibits the cross-sectional surfaces of GSS2, from which it can be seen that the layered structure was clearly presented and was

Sialon/Si3N4 graded ceramic tools at high speed machining Chapter | 2

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FIGURE 2.3  Cross-sectional surfaces of GSS2 [12].

FIGURE 2.4  XRD patterns of the graded tools [12]. (A) surface layer of GSS1 and (B) surface layer of GSS2.

uniform. The highly dense layers were observed, which was no voids. In addition, the straight interfaces were also presented in Fig. 2.3. Fig. 2.4 exhibits the XRD patterns of the graded tools. The testing instruments were XRD with the type of D/max-RB. For GSS1 (Fig. 2.4A), β-Si3N4 and TiC0.7N0.3 were the major phases without α-Si3N4. It is indicated that the α-Si3N4 was completely transformed to β-Si3N4. For GSS2 (Fig. 2.4B), a Si4Al2O2N6 (β-Sialon) phase was formed by the chemical reaction of Si3N4, Al2O3, and AlN. Table 2.3 lists the mechanical properties of GSS1 and GSS1, whereas Table 2.4 lists the mechanical properties of the reference homogeneous tools. The method of three-point bending was adapted to obtain the flexural strength, and the method of indentation was selected to obtain the fracture toughness and hardness. The surface hardness of GSS1 was lower than that of GSS2, due to the Sialon phase with high hardness in GSS2. This can be attributed to the excellent overall performance of Sialon ceramic material, and its wear resistance and chemical stability were higher than those of Si3N4 ceramic material. However, the flexural strength of GSS1 was higher than that of GSS2, due to the Si3N4 phase with high strength. Additionally, the Vicker’s hardness, flexural strength, and fracture toughness of the graded tool were higher than those of the homogeneous tool, respectively, which can be attributed to the optimum graded compositional structure.

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TABLE 2.3 Mechanical properties of GSS1 and GSS2. First and fifth (surface layer)

Second and fourth (buffer layer)

Third (inside layer)

Vicker’s hardness HV (GPa)

16.91 ± 0.30

16.82 ± 0.37

16.78 ± 0.22

Fracture toughness KIC (MPa·m1/2)

9.54 ± 0.52

9.77 ± 0.36

9.83 ± 0.42

Layer code GSS1

Flexural strength σf (MPa) GSS2

980 ± 60

Vicker’s hardness HV (GPa)

16.98 ± 0.24

16.71 ± 0.29

16.59 ± 0.16

Fracture toughness KIC (MPa·m1/2)

9.33 ± 0.46

9.50 ± 0.31

9.54 ± 0.36

Flexural strength σf (MPa)

810 ± 30

TABLE 2.4 Mechanical properties of the reference homogeneous tools.

Tools

Producer

Vicker’s hardness HV (GPa)

Fracture toughness KIC (MPa·m1/2)

Flexural strength σf (MPa)

ST10

Self-made

16.29 ± 0.23

8.19 ± 0.91

860 ± 90

SAAT10

Self-made

16.59 ± 0.31

7.80 ± 0.55

645 ± 95

KY1540

Kennametal

18.24 ± 0.25

7.45 ± 0.61

KY4300

Kennametal

22.93 ± 0.66

6.90 ± 0.60

Note: The properties of KY1540 and KY4300 were obtained by Vickers indentation test.

The scanning electron microscopy (SEM) with the type of JSM-6380LA and the transmission electron microscopy with the type of Hitachi H-800 were selected to examine the microstructures. Fig. 2.5 presents the micrographs of the GSS2 specimen (surface layer). The β-Sialon grain with an interlocked duplex microstructure was presented due to the different starting sizes of Si3N4 (Fig. 2.5A). Much of the enhancement of fracture toughness and flexural strength was due to this interlocked duplex microstructure [14,15]. As can be seen in Fig. 2.5B, the smaller TiC0.7N0.3 grain was trapped in the β-Sialon grain, whereas the larger one was located at grain boundary. The grain size of the intragranular grains was smaller, compared with the intergranular spherical ones.

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FIGURE 2.5  Micrographs of the GSS2 specimen (surface layer) [18]. (A) SEM and (B) TEM.

Therefore the main reason for the enhanced fracture toughness and flexural strength was the combined action between micro- and nanoparticles.

3  High speed turning of Inconel 718 3.1  Experimental method A commercially available tool (the matrix was sialon, the grade was KY1540, Kennametal), two homogeneous tools ST10 (the matrix was Si3N4), and SAAT10 (the matrix was sialon), and two graded tools GSS1 and GSS2 were used in the high speed turning experiments to discuss those cutting performance. The mechanical properties of the tools are listed in Tables 2.3 and 2.4. Table 2.5 lists the tool geometry parameters in high speed turning. A CNC turning center with the type of PUMA200MA was selected, and its maximum spindle speed was 6000 r/min. The initial size of workpiece material was 380 mm in long and 120 mm in diameter. Tables 2.6 and 2.7 exhibit the chemical compositions and mechanical properties of Inconel 718, respectively. Table 2.8 presents the cutting parameters in high speed turning. In particular, the inserts with the type of SNGN120408T01020 were used at vc = 80∼200 m/min, whereas the type of RNGN120700E was used at vc = 270 m/min, due to the high shock resistance of the round insert. In the turning experiments, the tool life criterion was the flank wear VBave = 0.30 mm, which was measured by an optical microscope. The chip features and the tool worn surface were examined by the SEM with the type TABLE 2.5 Tool geometry parameters in high speed turning. Rake angle γ0

Clearance angle α0

Inclination angle λs

Side cutting edge angle kr

−5 degree

5 degree

0 degree

45 degree

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TABLE 2.6 Chemical composition of Inconel 718 (wt.%). C

Si

Mn

Ni

Co

B

Cr

Cu

0.031

0.18

0.05

51.50

<1.0

0.0032

19.16

0.05

Al

Ti

Mo

Fe

S

P

Nb

0.58

0.97

3.07

Bal

0.0055

0.0057

5.06

TABLE 2.7 Mechanical properties of Inconel 718. Yield strength (MPa)

Tensile strength (MPa)

Elongation (%)

Reduction of area (%)

Hardness HRC

1076

1400

19.5

34.6

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TABLE 2.8 Cutting parameters in high speed turning. Cutting speed vc (m/min)

Feed rate f (mm/rev)

Depth of cut ap (mm)

80, 120, 160, 200, 270

0.1, 0.2, 0.3

0.1, 0.2, 0.3

of JSM-6380LA and JSM-6510LV (Japan) equipped with an EDS. A surface roughness tester with the type of TR200 was selected to measure the surface roughness Ra.

3.2  Chip patterns The macroscopic and microscopic patterns of the chips at vc = 80 ∼ 270 m/min are illustrated in Fig. 2.6. Within the range of cutting speeds, the macroscopic morphology of the chips was similar. The bottom of the chip was smooth. The chip was a typical continuous chip. Particularly, the width of the chip was slightly larger due to the use of round insert at vc = 270 m/min. The flowing features of the chip can lead to the rake wear in high speed turning process, such as abrasive wear. As displayed in Fig. 2.6, the characteristic of the serrated profile along its edges was presented in the microscopic morphology of the chips. The rake face could be abraded by these serrated chips. As vc was enhanced from 80  to 270 m/min, plastic flow of the chips was improved, and then led to plastic deformation. That is to say, the serration tendency of the chip was improved, as the cutting speed enhanced. In high speed cutting, the catastrophic thermoplastic shear theory [19] and the periodic crack initiation theory [20] can be used to explain the formation of

Sialon/Si3N4 graded ceramic tools at high speed machining Chapter | 2

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FIGURE 2.6  Macroscopic and microscopic patterns of the chips at vc = 80∼270 m/min at f = 0.1 mm/rev and ap = 0.1 mm (GSS1).

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High speed machining

the serrated chips. The shear hands could be formed when the thermal softening was large enough to reach the strain hardening in the primary shear zone. Moreover, the workpiece high temperature was obtained in the cutting process, because the thermal conductivity of Inconel 718 was low. So the plastic deformation was severely because of the high cutting heat and cutting temperature at high cutting speed. So the typical serrated chip was formed. This was the catastrophic thermoplastic shear theory. On the other hand, due to the high stress, the periodic crack initiation could result in the serrated chip [20]. The separation surface of material was caused by the propagation of cracks in the first deformation zone. Finally, the high friction heat and high pressure could weld the two surfaces, and the serrated chip was formed.

3.3  Cutting force and cutting temperature The orthogonal method with three factors and three levels and the single factor method were adopted. Based on the fitting orthogonal test data, the empirical formula of cutting temperature (θ ) and cutting force (F ) are obtained as follows:

θ = 325 ⋅ vc0.0873 ⋅ f 0.1082 ⋅ a 0.0139 ( º C) and p F = 381 ⋅ vc0.5140 ⋅ f 0.6297 ⋅ a 0.9264 p

(N).

(2.2) (2.3)

With the enhancement of cutting parameters, both cutting temperature and cutting force were increased [Eqs. (2.2) and (2.3)]. Figs. 2.7 and 2.8 exhibit the changes of cutting force and cutting temperature with cutting parameters, respectively. The cutting temperature and cutting force of the graded tool GSS1 was enhanced, as the cutting parameters improved. This conclusion was consistent with that drawn from Eqs. (2.2) and (2.3). As exhibited in Fig. 2.7A, the cutting force of the three homogeneous tools (ST10, SAAT10, and KY1540) was also improved with cutting speed. Moreover, the homogeneous tools had a near cutting force. However, at the same turning conditions, the homogeneous tool ST10 (SAAT10) depicted a higher cutting force,

FIGURE 2.7  Changes of cutting force with cutting parameters. (A) f = 0.1 mm/rev and ap = 0.1 mm; (B) vc = 120 m/min and ap = 0.1 mm; and (C) vc = 120 m/min and f = 0.1 mm/rev.

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FIGURE 2.8  Changes of cutting temperature with cutting parameters. (A) f = 0.1 mm/rev and ap = 0.1 mm; (B) vc = 120 m/min and ap = 0.1 mm; (C) vc = 120 m/min and f = 0.1 mm/rev.

FIGURE 2.9  Residual stress in the graded tool GSS1 material [21].

compared to that of the graded tool GSS1 (GSS2). As exhibited in Fig. 2.8, the same conclusion can be drawn from the change of cutting temperature with cutting parameters, compared with the change of cutting force. Based on the design of the graded structure, in the cooling stage of the fabrication process, the residual thermal stress can be induced in the graded ceramic material with the changes of temperature (from sintering temperature to room temperature). The result of FEM indicated that the tensile stress was distributed on the center layer whereas the compressive stress was distributed on the surface layer (Fig. 2.9 [21]). During the high speed turning process, the high friction force and the high cutting heat could result in the tensile stress at the cutting surface. On the tool surface, the residual compressive stress could offset a portion of this tensile stress that was induced by cutting heat and force. This can lead to the smaller of the temperature and force. Therefore, at the same turning conditions, the homogeneous tool had a higher temperature and force, compared with the graded tool (Figs. 2.7A and 2.8A).

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3.4  Tool life Fig. 2.10 displays the flank wear VBave with cutting time at vc = 80∼270 m/min. The tool life of the graded tool GSS1 (GSS2) was longer than that of the reference tool ST10 (SAAT10), especially at vc = 80∼120 m/min. Moreover, the commercially tool KY1540 exhibited a shorter cutting time at vc = 80∼120 m/ min, compared with the graded tool GSS1. So the GSS1 exhibited high cutting performance. However, the homogeneous tool SAAT10 displayed the shortest tool life due to its relatively low mechanical properties (Tables 2.3 and 2.4) As depicted in Fig. 2.10, the cutting speed influenced on the cutting tool life. At vc = 120 m/min, a relative long cutting life was obtained for the five tools. Due to the relatively high force of the five tools, all the tool life was not very long at vc = 80 m/min. It is indicated that the ceramic tools were not recommended to cut of Ni-based alloys at lower cutting speeds. Along the cutting edge, the high cutting temperature was obtained at the high cutting speed, which can weaken its strength. Thus when vc passed 200 m/min, the tool lives were also short (Fig. 2.10C and D). As also depicted in Fig. 2.10, whether at low cutting speed or high cutting speed, the cutting life of GSS1 was greater than that of the corresponding homogeneous tool ST10. For GSs1, the possible one reason was a high hardness (surface layer) because of the proper structure, as can be seen in Table 2.3. The plastic properties and the elastic properties affected the hardness of the material. Also those properties were related to the tool stress. The residual compressive stress on the graded tool surface layer were just benefit for the improvement of the hardness [22,23]. So the reduced wear rate of the graded composite resulted

FIGURE 2.10  Flank wear VBave with cutting time at (A) vc = 80 m/min; (B) vc = 120 m/min; (C) vc = 200 m/min; and (D) vc = 270 m/min, f = 0.1 mm/rev, and ap = 0.1 mm.

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from the improved hardness [18]. The much higher cutting life of the graded tool can be ascribed to the enhanced antifriction performance, compared with that of the homogeneous tool.

3.5  Failure characteristics and failure mechanisms Fig. 2.11 shows the worn surface of the graded tools at vc = 120 m/min. As exhibited in Fig. 2.11A, the notch wear of GSS1 took place in the direction of the depth of cut, besides flank wear and crater wear. Additionally, the microcrack on the rake face and build-up layer on flank face was found for GSS1 in Fig. 2.11B and D. The graded tool GSS2 was less sensitive to the notch wear, as depicted in Fig. 2.11F. Along the chip flow direction, the plastic deformation was presented on the rake face (Fig. 2.11C), which was caused by the high stress of the tool–chip interface. As displayed in Fig. 2.11C and E, on rake face and flank face, the build-up layer, and the abrasive trace were expressed, which were the characteristics of the adhesive wear and abrasive wear. At the initial and steady wear stage, on the tool surface, a protective action was possessed because the built-up layer could reduce the further expansion of the grooves. So the tool wear was slowed down, and the tool life was improved. At the rapid wear stage, however, the tool wear rate could be aggravated by the bonding workpiece material, because the element diffusion between the tool material and the chip enhanced their affinity. Fig. 2.12 presents the EDS analyses of Point 1 in Fig. 2.11C [24]. The elements of Inconel 718 were found in Fig. 2.12, including Ni, Cr, and Fe. The relevant research showed that the element of Ni likely diffused to the ceramic tool surface [25]. Due to the low melting point, the diffusion of the Fe, Ni, and Cr can weaken the surface hardness of the ceramic tool, which accelerated the tool wear. As exhibited in Fig. 2.11D, a microcrack was presented on the worn surface of GSS1. During the high speed turning process, the high stress can be generated, and its distribution affected the tool failure. A fracture will occur when the tensile stress exceeds the tensile strength. For GSS1, at the tool surface, because of the residual compressive stress, a portion of the tensile stress required for the propagation of the crack could be hindered. Therefore there was not severe fracture on the worn surface, which can be attributed to the residual compressive stress. Fig. 2.13 exhibits the worn surface of the graded tools at vc = 270 m/min. On the rake face and near the nose, the flaking and the chipping were depicted (Fig. 2.13A, B, D, and E), because of the relatively lower thermal shock resistance and fracture toughness of the ceramic materials. Also the adhered workpiece materials and some scratches were presented on worn surface (Fig. 2.13A and E). So, adhesive wear and abrasive wear were the major wear mechanisms of the graded tool. As illustrated in Fig. 2.13E and F, some microcrack were found on the worn rake face and flank face. The chipping and flaking could appear when the crack continued to expand, as can be seen in Fig. 2.13B, E,

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FIGURE 2.11  Worn surface of the graded tools GSS1 (7.8 min) and GSS2 (4.7 min) at vc = 120 m/min, f = 0.1 mm/rev, and ap = 0.1 mm.

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FIGURE 2.12  EDS analysis of the point 1 in Fig. 2.11 C [24].

and D. For the graded tool, at the surface layer, the enhancement of the fracture resistance could be obtained because of the residual compressive stresses. Fig. 2.14 exhibits the worn surface of the homogeneous ceramic tool ST10 at vc = 120 m/min and vc = 270 m/min, from which it can be indicated that flaking, chipping, abrasion, and adhesion were the primary failure mechanisms of ST10. As illustrated in Fig. 2.14A, a larger groove was presented on the rake worn surface, because ST10 had a relatively low hardness (Tables 2.3 and 2.4). On the flank worn surface, a microcrack was also evident (Fig. 2.14F). For the homogeneous ceramic tool ST10, the existence of such microcrack can aggravate tool wear. Compared with GSS1, the homogeneous ceramic tool ST10 was worn more serious. Therefore the ceramic tool wear was mainly caused by adhesion, abrasion, oxidation, and diffusion, accompanied with a little peeling and chipping at higher turning speed. Additionally, the thermal stress relieving the effect of the graded tool has significantly improved its wear resistance, according to the worn micrographs.

3.6  Machined surface roughness The surface roughness Ra obtained at initial wear stage was analyzed to avoid the influence of cutting tool wear. The cutting speed was an importance factor, which influenced on the surface roughness. Fig. 2.15 illustrates the influence of cutting speed on surface roughness Ra. With the improvement of vc from 80 m/min to 200 m/min, the surface roughness Ra reduced. The high temperature and force were achieved at high cutting speed, which was not conducive

42 High speed machining

FIGURE 2.13  Worn surface of the graded tools at vc = 270 m/min, f = 0.1 mm/rev, and ap = 0.1 mm {(C)∼(E) [18]}.

Sialon/Si3N4 graded ceramic tools at high speed machining Chapter | 2

FIGURE 2.14  Worn surface of the homogeneous ceramic tool ST10 at (A)∼(C) vc = 120 m/min and (D)∼(F) vc = 270 m/min, f = 0.1 mm/rev, and ap = 0.1 mm {(B) [24], (B) [26], (E)∼(F) [18]}.

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FIGURE 2.15  Influence of cutting speed on surface roughness Ra at f = 0.1 mm/rev and ap = 0.1 mm.

FIGURE 2.16  Changes of surface roughness Ra with cutting time at vc = 120 m/min, f = 0.1 mm/rev and ap = 0.1 mm (GSS1) [24].

to obtaining low surface roughness Ra. The low-level natural frequency of the machine had a great influence on the surface roughness in the cutting system. As vc improved, the difference between the operating frequency and the low-level natural frequency was enlarged. So the low surface roughness was achieved at the higher speed machining. At vc ≥ 200 m/min, the great machined surface quality (low surface roughness Ra) can be obtained. For the surface roughness, the tool wear was another important influence factor. The changes in surface roughness Ra with cutting time at vc = 120 m/min was presented in Fig. 2.16. Between the tool surface and the chips/workpiece, there was a wear-in process in the early cutting stage (<1 min). Some microcracks or burrs were presented on the new tool surface. Therefore, in the initial cutting stage, the higher surface roughness was achieved. For the force, its fluctuation was reduced in the steady wear stage. So a lower surface roughness

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was achieved due to the relative stable cutting process. However, the improved cutting force caused to the sharp flank wear that influenced and increased the surface roughness Ra in the rapid wear stage (>7 min).

4  High speed milling of Inconel 718 4.1  Experimental method Inconel 718 was also selected as the workpiece material. The initial size of Inconel was 100 mm in length, 50 mm in height and 100 mm in width. Its chemical composition and mechanical properties were the same as those used in turning experiments (Tables 2.6 and 2.7). The graded tool GSS1, homogeneous tool ST10 and KY4300 (SiC whisker reinforced, Kennametal) were adopted in the high speed milling experiments. Those mechanical properties are present in Tables 2.3 and 2.4. A CNC machining center with the type of DAEWOO ACE-V500 was selected for high speed milling experiments. Its maximum spindle speed was 10,000 r/ min. Fig. 2.17 illustrates the experimental setup for high speed milling and scene picture. The method of up-milling and a dry cutting condition were carried out in the high speed milling experiments. Fig. 2.18 exhibits the schematic of up-milling. To avoid inserts interaction, just one insert was used in the milling process. The cutting parameters in high speed milling are shown in Table 2.9. The tool wear of the milling inserts was observed by a VHX-600E optical microscope. A NEC TH5104R infrared thermal imaging system and a KISTLER 9275A dynamometer fixed on the table were adapted to measure cutting temperature and cutting force, respectively. The cutting forces included axial force (Fz), feed force (Fy) and normal force (Fx), as exhibited in Fig. 2.17.

FIGURE 2.17  Experimental setup for high speed machining and scene picture.

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FIGURE 2.18  Schematic of up milling.

TABLE 2.9 Cutting parameters in high speed milling. Cutting speed vc (m/min)

Feed per tooth fz (mm/z)

Axial depth of cut ap (mm)

Radial depth of cut ae (mm)

500, 700, 900, 1000, 1100, 1300, 1500, 1600, 1900, 2300, 2500, 2700, 3000

0.07

1.5

12, 19

Additionally, in the initial cutting stage, chips were collected. The morphology of the chips, failure surfaces of the cutting tool were observed with a SEM with the type of JSM-6510LV equipped with EDS. A white light interferometer with the type of Wyko NT9300 was applied for measuring of the surface roughness of workpiece.

4.2  Chip patterns Fig. 2.19 Macroscopic and microscopic patterns of the chips at vc = 500∼3000 m/ min. The fan-shaped chips were presented at the macro level (vc  ≤ 1500 m/ min). As the cutting speed enhanced, the temperature and shear strain rate were improved. The deep color chips and the light color chips appeared successively, as vc improved from 500 m/min to 3000 m/min. The transition from ductileness to brittleness was exhibited at v > 1500 m/min, as can be seen from the chip color and the chip sharpening. So, at the relatively high cutting speed, the chip was crisp and brittle, which exhibited sparse (like “straw mat”) and small.

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FIGURE 2.19  Macroscopic and microscopic patterns of the chips at cutting speed from 500 m/min to 3000 m/min at fz = 0.07 mm/z, ap = 1.5 mm, and ae = 12 mm (GSS1).

On the other hand, the microscopic morphologies of the chips at v = 500∼3000 m/min were all serrated shapes. Because of the high cutting heat, the shear band was formed when the thermal softening reached the strain hardening. Additionally, the dispersion of heat was restricted because the thermal conductivity of the workpiece was low. At the tool–chip contact interface, the temperature improved highly. So the plastic deformation was found due to the high temperature and high heat. Therefore a typical serrated chip was formed (Fig. 2.19) when the thermal softening of Inconel 718 played a major role. Additionally, the degree of the plastic deformation of the chips was enhanced at the ultra high cutting speed. The serrated chip exhibited a characteristic of segment separation, because of the improvement of the brittleness at v > 1500 m/min.

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4.3  Cutting force and cutting power The resultant force F was defined as F = (Fx2 + Fy2 + Fz2)1/2, according to the experimental setup in Fig. 2.17. Fig. 2.20 displays the cutting force wave at v = 1500 m/min, from which it can be described that Fz was the highest cutting force in the three components. So the resultant force F can be expressed in Fz. Fig. 2.21 shows the axial cutting force (Fz) at vc = 500 m/min and vc = 3000 m/min. The time-varying chip thickness and the discontinuous contact of the workpiece-cutting edge were exhibited in the milling process, because a high speed milling was an interrupted cutting process. Especially, in the up-milling process (Fig. 2.18), the thickness of the chip was improved from zero to maximum. The cutting force was periodical fluctuations with the changes of thickness (Fig. 2.20). Therefore the cyclical change of the cutting force resulted from

FIGURE 2.20  Cutting force wave with cutting time at v = 1500 m/min, fz = 0.07 mm/z, ap = 0.5 mm, and ae = 12 mm (GSS1).

FIGURE 2.21  Axial cutting force (Fz) at (A) v = 500 m/min and (B) v = 3000 m/min, fz = 0.07 mm/z, ap = 0.5 mm, and ae = 12 mm.

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FIGURE 2.22  Influence of cutting speed on resultant force F and cutting power P at fz = 0.07 mm/z, ap = 0.5 mm and ae = 12 mm.

the periodical changes of the chip thickness and the periodical cutting-in and cutting-out of the cutting edge [27]. The period of one revolution was about 20 ms at vc = 1500 m/min, and the cutting time was less than 7 ms because of just one tooth used in the milling process. The cutting time ratio was fixed at 0.35 at vc = 1500 m/min (Fig. 2.20), which was defined as the ratio of the cutting time to air-cutting time. In the same way, the ratio was fixed at 0.15 at vc = 500 m/min, while it was 0.5 at vc = 3000 m/min, as depicted in Fig. 2.21. Obviously, with the enhancement of cutting speed, the cutting time ratio was improved sharply. In other words, the cutting efficiency was enhanced. In the work, the cutting power was also used to discuss the influence of cutting speeds, which was caused by the cutting force F. Fig. 2.22 depicts the influence of cutting speed on resultant force F and cutting power P at vc = 500∼3000 m/ min, from which it can be found out that the resultant force had been great influenced by the cutting speed. Because of the changes of the shear angle with vc and the softening of Inconel 718 at high temperature [28], the resultant force F reduced when vc < 1500 m/min, as displayed in Fig. 2.22A. Afterwards, with a further increase of cutting speed, the resultant force was not decreased, but there was a slight increase in trend, due to the high cutting temperature and sharply tool wear. As described in Fig. 2.22B, the cutting power increased rapidly with the enhancement of vc, especially at vc > 1500 m/min. For example, the cutting power (P) of GSS1 was 2.43 kW at vc = 500 m/min, 4.28 kW at vc = 1500 m/ min, but it was improved to 9.80 kW at vc = 3000 m/min. From energy consumption, the cutting speed of less than 1900 m/min was suggested. On the other hand, compared with other two homogeneous tools, the graded tool possessed a relatively low cutting forces and cutting power. For the graded tool material, this can be attributed to the high thermo-mechanical properties. The thermal conductivity of GSS1 tool was lower than that of the SiC whisker reinforced tool (KY4300) and the Si3N4 tool (ST10), which promoted the high temperature of the tool–chip contact interface. The low friction force was

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obtained due to the reduced strength of the workpiece at high temperature. With the enhancement of vc, the advantage of its small cutting power was more obvious. According to the smaller cutting force and less energy consumption, vc = 1000∼1900 m/min was a good choice for high speed milling of Inconel 718.

4.4  Failure characteristics Fig. 2.23 describes the influence of cutting speed on flank wear VBmax. The flank wear VBmax had a smaller value at vc  ≥ 1500 m/min. The higher the cutting speed was, the shorter cutting time it took. The low cutting force at vc ≥ 1500 m/min (Fig. 2.22A) could be another reason for the low value of tool wear. So VBmax of vc ≥ 1500 m/min was smaller than that of vc < 1500 m/min. As depicted in Fig. 2.23, the wear resistance of GSS1 was higher than that of the homogeneous tools (ST10 and KY4300). Both the high fracture toughness and flexural strength (Tables 2.3 and 2.4) and the low cutting force of GSS1 (Fig. 2.22A) resulted in the high wear resistance. At lower cutting speeds, the mechanical shock was the main factor of tool failure because the cutting temperature was not very high. For the graded tool, the thermal stress relieving was ineffective. Thus the flank wear VBmax was equivalent at vc < 1500 m/min. In addition, the high thermal stress was obtained due to the high-frequency alternation of heating and cooling at higher cutting speed. So, at vc ≥ 1500 m/ min, the thermal stress played an important role. At the tool surface of GSS1, the tensile stresses caused by the thermal load and the mechanical load can

FIGURE 2.23  Influence of cutting speed on flank wear VBmax at the metal removal volume of 600 mm3 at fz = 0.07 mm/z, ap = 0.5 mm, and ae = 12 mm.

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be resisted partly by the residual compressive stress. Thereby, the graded tool GSS1 exhibited a smaller flank wear VBmax at higher cutting speeds, compared with other two tools (Fig. 2.23). Figs. 2.24 and 2.25 describe the failure patterns of GSS1 and KY4300 after metal removal volume of 600 mm3, respectively, from which it can be found that the flaking, fracture, notch wear, and chipping were the main failure types of GSS1 and KY4300. At the same metal removal volume, the damage and the flaking of KY4300 and GSS1 were presented in flank surface and in rake surface at vc = 500 m/min, respectively. The higher force at vc = 500 m/min resulted in the periodic mechanical shocks, which led to the tool failure. However, when vc exceeded 1500 m/min, the tool displayed the smaller flank wear and rake wear, due to the enhanced cutting efficiency. As can be seen in Figs. 2.24 and 2.25, a fracture was a typical characteristic of tool failure. For the cutting tools, the alternation of heating and cooling in milling cycles can result in the tensile and compressive stresses. Under the action of the alternating stress, a fracture of the tool could be caused by the maximum tensile stress when it reached the ultimate strength. However, because the structure and properties of the graded and the homogeneous inserts were different, the influence of stress on tool fractures was also

FIGURE 2.24  Failure patterns of GSS1 after metal removal volume of 600 mm3 at fz = 0.07 mm/z, ap = 0.5 mm, and ae = 12 mm.

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FIGURE 2.25  Failure patterns of KY4300 after metal removal volume of 600 mm3 at fz = 0.07 mm/z, ap = 0.5 mm, and ae = 12 mm.

different. A fracture can be caused by the stress distributed beneath the rake face in high speed cutting process Compared with KY4300, GSS1 had a high fracture resistance, because the surface residual compressive stress of GSS1 was induced [12,21]. In the same way, under the same cutting conditions, KY4300 had a lower notch wear resistance, compared with GSS1 (Figs. 2.24 and 2.25). The high fracture toughness of GSS1 can bring about the high resistance to notch wear.

4.5  Failure mechanisms and self-sharpening Fig. 2.26 described the failure micrographs of GSS1 at vc = 500 m/min and vc = 700 m/min. The microcrack, adhesion, abrasion, and step-shaped flaking were the primary failure mechanisms at relatively low milling speed. As depicted in Fig. 2.26A, on the tool rake surface, some of the melted materials were bonded. Workpiece material components were detected on the rake face (Fig. 2.26F), such as Fe, Ni, Cr, etc. It is indicated that the melted material was Inconel 718 (workpiece material). In other words, the tool surface was bonded to the workpiece material. After that, on the rake face, the adhesive wear may be caused. The melting temperature of Inconel 718 was about

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FIGURE 2.26  Failure micrographs of GSS1 at (A)∼(C); (F) vc = 500 m/min [29] and (D)∼(E) vc = 700 m/min [21], fz = 0.07 mm/z, ap = 0.5 mm, and ae = 19 mm.

53

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1260–1320°C. The cutting temperature may reach or even exceed the melting temperature, as can be indicated in Fig. 2.26A. As depicted in Fig. 2.26B and D, the step-shaped flaking surface was found on worn surface, accompanied with the normal wear (e.g., adhesive wear, abrasive wear). In the high speed milling process, this was mainly caused by the role of the periodic thermal–mechanical loads. So a massive flaking was another reason for the graded ceramic tool failure in the high speed milling. It is worth mentioning that this characteristic of step-shaped flaking was attributed to the self-sharpening of the graded tool [21]. As depicted in Fig. 2.26C–E, some microcracks were also found. With the increment of tool wear, the high alternating stress acted on the inserts, which caused by the alternating thermal and mechanical loads. On the tool surface, the initiation and propagation of cracks can be activated when the stress reached the tensile strength. Fig. 2.27 displays the failure micrographs of GSS1 at vc = 1500 m/min and vc = 1600 m/min. The failure types were the step-shaped flaking on the rake face, and a narrow flaking strip along the cutting edge was, accompanied with micro-crack, adhesion and build-up layer. The melted workpiece material Inconel 718 was also presented on rake face (see Point b in Fig. 2.27E), which indicated the high cutting temperature at vc = 1600 m/min. The tool was mainly subjected to mechanical loads because of small heat accumulation at the beginning of tool wear. So some microcracks may be initiation on the worn surface (e.g., Fig. 2.26B, C, and E, Fig. 2.27D and F). Some small flaking formed and extended near the cutting edge. Along the cutting edge, a small flaking strip was developed, as can be seen in Fig. 2.27B and D. The impact of the small flaking on the overall size and performance of the tool was a little. So the tool can be still used as usual. The temperature and the force were increased with the enhancement of MRV. The strip enlarged gradually forward the rake face, and some chips were bonded on the flaking surface (Fig. 2.26A and E, Fig. 2.27C and F). The further wear of the tool was caused by the flaking off of bonded materials. Then, on the rake face, a step-shaped flaking surface was formed (Points 1–5 in Fig. 2.26B, Points 1–4 in Fig. 2.26D, and Points 1–6 in Fig. 2.27A). In the formation process of the step-shaped surface, the tool was still able to meet the cutting requirements. Eventually, the failure of the tool resulted from the massive flaking. In short, a narrow flaking strip along the cutting edge and the step-shaped flaking surfaces on rake face were the failure evolution characteristics of the ceramic tool. Its emergence was a performance of self-sharpening. Both the formed residual compressive stress and the high thermal shock resistance can resulted in the self-sharpening of the graded tool. Fig. 2.28 presents the failure micrographs of GSS1 at vc = 3000 m/min. The conchoidal flaking and step-shaped flaking were exhibited failure surface, accompanied with adhesion and microcrack. For the graded ceramic cutting tool, the microcrack growth rate was slow because the residual compressive stress existed on the rake face (Fig. 2.28B and D). The flaking area increased gradually with MRV. Finally, the conchoidal flaking morphology was formed on the

Sialon/Si3N4 graded ceramic tools at high speed machining Chapter | 2

FIGURE 2.27  Failure micrographs of GSS1 at (A)∼(C) vc = 1500 m/min[21] and (D)∼(F) vc = 1600 m/min, fz = 0.07 mm/z, ap = 0.5 mm, and ae = 12 mm.

55

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FIGURE 2.28  Failure micrographs of GSS1 at vc = 3000 m/min, fz = 0.07 mm/z, ap = 0.5 mm, and ae = 12 mm [21].

rake face (Fig. 2.28A). A step-shaped surface (Points 1–4 in Fig. 2.28C) caused by microcracks and some bonded material after flaking were exhibited on the failure surface. That is to say, the emergence of these failure characteristics on the rake face, such as the conchoidal surface and step-shaped surface, suggested the self-sharpening of the graded tool in UHS milling. Fig. 2.29 shows the failure micrographs of KY4300 at vc = 500, 1600, 2700, and 3000m/min. The chipping, flaking, adhesion, and abrasion were also the major failure mechanism of KY4300 at ultra high speed milling conditions. The self-sharpening failure characteristics (step-shaped flaking from Point 1 to Point 5 in Fig. 2.29A, a narrow flaking strip in Fig. 2.29C) were also presented on failure surface at vc ≤ 1600 m/min. However, the self-sharpening characteristics were not present on the failure surface at vc > 1600 m/min. It is indicated that the self-sharpening characteristics were dependent on the cutting condition, especially cutting speed. The graded ceramic tool GSS1 had the typical self-sharpening characteristics on rake face in the high speed milling process. Synergistic mechanism of toughening and strengthening caused by the reasonable graded structure improved the mechanical properties of the tool. At the same time, the compressive residual stress was formed on the rake face. These brought about the selfsharpening cutting characteristic. And this performance was conducive to the

Sialon/Si3N4 graded ceramic tools at high speed machining Chapter | 2

FIGURE 2.29  Failure micrographs of KY4300 at (A)∼(B) vc = 500 m/min [21]; (C)∼D) vc = 1600 m/min; (E) vc = 2700 m/min; and (F) vc = 3000 m/min [21], fz = 0.07 mm/z, ap = 0.5 mm, and ae = 12 mm.

57

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enhancement of tool life. Because of the addition of SiC whisker (Fig. 2.29B), the high strength and toughness of KY4300 were obtained, which brought about similar characteristics. However, the self-sharpening characteristics of GSS1 were not found in high speed turning at vc = 80∼270 m/min. Also the homogeneous cutting tool KY4300 did not show the characteristics in ultra high speed milling conditions (vc > 1600 m/min). In a word, these high cutting performance characteristics were in close relation to the cutting parameters and tool mechanical properties. It is indicated that the ceramic tool with excellent mechanical properties may performance a self-sharpening cutting characteristic and obtain a longer tool life, only under certain cutting conditions. Under ultra high speed cutting conditions, the microcrack, chipping, adhesive wear, abrasion wear, step-shaped flaking, and conchoidal flaking were the primary failure mechanisms of the ceramic tools. Although the tool wear rate was high, its cutting efficiency was enhanced greatly. The main factor of tool failure was the combined stress effect, including mechanical stress and thermal stress.

4.6  Machined surface roughness The machined surface topography at vc = 1100 m/min was exhibited in Fig. 2.30, in which the covering area of the machined surface sample was 99.9 µm × 200 µm. According to the analysis of the failure mechanisms, due to the high cutting force, the flaking and chipping can be caused easily in the high speed milling process. On the other hand, the adhesive wear was very severe because Inconel 718 expressed the strong adhesion at a high temperature. All of these can affect the surface roughness

FIGURE 2.30  Machined surface topography at vc = 1100 m/min, fz = 0.07 mm/z, ap = 0.5 mm, and ae = 19 mm [29].

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FIGURE 2.31  Influence of cutting speed on surface roughness Ra at fz = 0.07 mm/z, ap = 0.5 mm, and ae = 12 mm.

Fig. 2.31 depicts the influence of cutting speed on surface roughness Ra. With the enhancement of vc, the surface roughness Ra was reduced when vc was less than 1000 m/min. However, an improved Ra was obtained at vc > 1000 m/ min. At vc = 1000–1900 m/min, the relatively low values of the surface roughness Ra were obtained, as described in Fig. 2.31. Within this range of cutting speed, the relatively small cutting force (Fig. 2.22A) was obtained, and the tool wear began to decrease (Fig. 2.23), at the same metal removal volume. The low surface roughness Ra was achieved by the small vibration of the cutting system caused by the small force. Afterwards, at the same metal removal volume, the tool wear was reduced because of the high cutting efficiency at a higher cutting speed. The main reason for the higher surface roughness Ra at vc > 1900 m/ min was the improved complexity of the machined surface profile and the wore stability of the cutting system.

5 Summary 1. The Sialon/Si3N4 graded ceramic tools were developed by the concept of FGM and MNCM, and the high mechanical properties were obtained. 2. In high speed turning process, the chip was a typical continuous chip in macroscopic, and the serrated chips was characterized in microscopic. However, there was a transition tendency from ductile to brittle in high speed milling process. The fan-shaped chips were observed at v ≤ 1500 m/min in macroscopic, while it exhibited sparse (like “straw mat”) and small at v > 1500 m/min. 3. The graded tools exhibited lower cutting temperature and cutting force compared to the homogeneous tools under the same turning conditions,

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because of the stress-relieving effect. The relatively lower cutting forces and cutting power of the graded tool were also observed than those of the homogeneous tool in the milling process, because of the high thermomechanical properties. 4. The ceramic tool wear was mainly caused by adhesion, abrasion, oxidation, and diffusion, accompanied with a little peeling and chipping at higher turning speed. Additionally, the thermal stress relieving the effect of the graded tool has significantly improved its wear resistance, according to the worn micrographs. 5. The brittle damage (e.g., some micro-cracks, flaking and adhesion in a large area) was the main failure mechanical in the high speed milling process. Some narrow flaking strips along the cutting edge and the step-shaped flaking and conchoidal flaking on the rake face were the self-sharpening characteristics, which was beneficial for the cutting process. 6. As the turning speed increased, the surface roughness Ra was reduced, and the great machined surface quality can be achieved at vc ≥ 200 m/min. Additionally, in the milling speed of 1000∼1900 m/min, the relatively low Ra were achieved. 7. For the graded tool, the high cutting performance can be ascribed to the combination of the characteristics of thermal stress relief and surface compressive stress of the FGM and the strengthening and toughening mechanisms of MNCM.

Acknowledgments This work was supported by the National Natural Science Foundation of China (51505264, 51775315, 50875156, 51175310, 51475273), the National Basic Research Program of China (2009CB724402), the Specialized Research Fund for the Doctoral Program of Higher Education (20090131110030), the Scientific Research Foundation of Outstanding Youth Scientists of Shandong Province of China (BS2014ZZ005), the Zibo City—Shandong University of Technology Cooperative Projects (2017ZBXC032), the National Key Research and Development Program of China (2018YFB2001400), and the China Postdoctoral Science Foundation (2019M652439). The author was also grateful to the helps of all colleagues in School of Mechanical Engineering, Shandong University, China.

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