Hard turning: AISI 4340 high strength low alloy steel and AISI D2 cold work tool steel

Hard turning: AISI 4340 high strength low alloy steel and AISI D2 cold work tool steel

Journal of Materials Processing Technology 169 (2005) 388–395 Hard turning: AISI 4340 high strength low alloy steel and AISI D2 cold work tool steel ...

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Journal of Materials Processing Technology 169 (2005) 388–395

Hard turning: AISI 4340 high strength low alloy steel and AISI D2 cold work tool steel a , A.M. Abr˜ ´ J.G. Lima a , R.F. Avila ao a , M. Faustino b , J. Paulo Davim b,∗ a

Department of Mechanical Engineering, Federal University of Minas Gerais, Ava Ant´onio Carlos, 31270-901, Belo Horizonte-MG, Brazil b Department of Mechanical Engineering, University of Aveiro, Campus Santiago, 3810-193 Aveiro, Portugal Received 6 July 2004; accepted 14 April 2005

Abstract The aim of this paper is to evaluate the machinability of hardened steels at different levels of hardness and using a range of cutting tool materials. More specifically, the work was focused on the machinability of hardened AISI 4340 high strength low alloy steel and AISI D2 cold work tool steel. The tests involving the AISI 4340 steel were performed using two hardness values: 42 and 48 HRC; in the former, a coated carbide insert was used as cutting tool, whereas in the latter a polycrystalline cubic boron nitride insert was employed. The machining tests on the AISI D2 steel hardened to 58 HRC were conducted using a mixed alumina-cutting tool. Machining forces, surface roughness, tool life and wear mechanisms were assessed. The results indicated that when turning AISI 4340 steel using low feed rates and depths of cut, the forces were higher when machining the softer steel and that surface roughness of the machined part was improved as cutting speed was elevated and deteriorated with feed rate. Abrasion was the principal wear mechanism acting when turning the 42 HRC steel, whereas diffusion was present when machining the 50 HRC steel. Turning AISI D2 steel (58 HRC) with mixed alumina inserts allowed a surface finish as good as that produced by cylindrical grinding. The flank wear of the mixed alumina tool increased with cutting speed and depth of cut, presenting a considerably higher tool wear rate when using at a cutting speed of 220 m/min and feed rate of 0.15 mm/rev, which resulted in tool failure by spalling. © 2005 Elsevier B.V. All rights reserved. Keywords: Hard turning; Cutting forces; Tool wear; Surface roughness

1. Introduction The principal properties expected from any tool material are high hot hardness, toughness and chemical stability. Despite the fact that cemented carbide presents these properties at an acceptable level, it is not suitable when hard materials are machined at high cutting speeds, due to the fact that these properties deteriorate rapidly at high temperatures [1]. Therefore, mixed alumina and polycrystalline cubic nitride tool materials should be employed, particularly when machining hardened steels. Since the addition of titanium carbide (TiC) to the aluminium oxide (Al2 O3 ) in the 1970s (which increased its resistance to thermal and mechanical shocks), to the use of ∗

Corresponding author. Tel.: +351 234 401 566; fax: +351 234 370 953. E-mail address: [email protected] (J.P. Davim).

0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.04.082

the ion implantation technique in order to produce coatings on mixed alumina tools with improved resistance to crack initiation and propagation [2], the properties of this tool material grade have improved considerably. These characteristics enable mixed alumina tools to satisfactorily machine hardened steels, especially when finish and continuously cutting. For heavier cutting, polycrystalline cubic boron nitride (PCBN) is recommended owing to its high hot hardness and fracture toughness, in addition to the chemical stability at temperatures below 1200 ◦ C, thus being indicated for continuous and interrupted cutting of steels and cast irons with hardness values ranging from 50 to 65 HRC, under finishing and roughing conditions. The tool material composition and properties are crucial to the behaviour of machining forces, which in turn affect tool life and surface roughness [3]. Poulachon et al. [4], assert that the microstructure of hardened steels possesses a critical influence on the tool wear

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mechanisms involved, particularly the presence of carbides. Due to the variation in the hardness of the carbide particles, the wear of PCBN inserts may take place at different rates. The work developed by Bailey et al. [5] and Matsumoto et al. [6] described the effect of hardness during hard turning of AISI 4340 high strength low alloy steel. Strafford and Audy [7] investigated the relationship between hardness and machining forces during turning of AISI 4340 steel hardened from 29 to 57 HRC with mixed alumina tools. The results suggest that an increase of 48% in hardness leads to an increase in the machining forces from 30 to 80%. The authors reported that for hardness values between 30 and 50 HRC, continuous chips were formed and the cutting force component was reduced, however, when the workpiece hardness increased above 50 HRC, segmented chips were observed and the cutting force showed a sudden elevation. Similar results were reported by Luo et al. [8] when turning AISI 4340 steel (35–60 HRC) using mixed alumina and PCBN tools. The results indicated that the flank wear was reduced as work material hardness increased up to a critical value of 50 HRC. A further increase in the workpiece hardness accelerated the tool wear rate. The reduction in tool wear up to 50 HRC was attributed to the elevation of the cutting temperature, which reduced the shear strength of the work material, however, this effect was not observed when the hardness exceeded 50 HRC. Tool geometry is another important factor affecting machining forces, especially the feed (axial) and thrust (radial) force components [9]. When cutting hardened steels, the use of chamfered edges and negative rake and inclination angles help to increase the machining forces. In addition to that, the use of large nose radius together with low depths of cut leads to low true side cutting edge angle values (irrespective of the selected tool holder geometry), thus resulting in high thrust forces. On the other hand, large nose radius and cutting edge angle values may improve the surface finish of the machined part provided tool vibration can be avoided [10]. El-Wardany et al. [11,12] studied the quality and integrity of the surface produced and the effects of cutting parameters and tool wear on chip morphology during high-speed turning of AISI D2 cold work tool steel in its hardened state (60–62 HRC). The metallographic analysis of the surface produced illustrates the damage surface region that contains geometrical defects and changes in the sub surface metallurgical structure. The types of surface damage are dependent on the cutting parameters, tool geometry and the magnitude of the wear lands. Kishawy and Elbestawi [13] investigated the tool wear characteristics and surface integrity during high-speed turning of AISI D2 cold work tool steel. A wide range of residual stress distributions beneath the machined surface was obtained depending on the cutting parameters and edge preparation. The unfavourable tensile residual stresses were minimized at high cutting speeds and high depths of cut. This paper investigates the influence of cutting parameters (cutting speed, feed rate and depth of cut) on the machinabil-

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ity on two hard materials: AISI 4340 high strength low alloy steel and AISI D2 cold work tool steel.

2. Experimental procedure The experimental work was divided into two phases: the first phase, concerned with the machining of high strength low alloy AISI 4340 steel was carried out in the Machining and Automation Laboratory of the Federal University of Minas Gerais, Brazil. The second phase, related to the AISI D2 cold work tool steel was conducted by Machining and Tribology Research Group at the Mechanical Technology and Automation Laboratory of the University of Aveiro, Portugal. In both cases, continuous dry turning tests were performed aiming to compare the behaviour of these steels when machined in the hardened state using coated carbide, PCBN and ceramic tools. 2.1. AISI 4340 steel Bars of AISI 4340 steel with 76.5 mm diameter and 300 mm long were used as workpiece material after being heat treated (quenching and tempering) to reach two average hardness values: 42 and 50 HRC. For the machining of the AISI 4340 steel with an average hardness of 42 HRC an ISO P15-K15 coated carbide insert with a tool geometry SNMG 120408-PF GC 4015 was employed. The inserts were mounted on a PSDN 2525 M12 tool holder, which resulted in the following angles: clearance angle α0 = 6◦ , negative rake angle γ 0 = −6◦ , negative cutting edge inclination angle λs = −5◦ and cutting edge angle χr = 45◦ . When turning the AISI 4340 steel hardened to 50 HRC, high concentration (98% CBN) PCBN inserts were used. These inserts presented geometry SNMM 090316 T02020 and were mounted on a tool holder code MSDNN 1616 H09, which resulted in the same cutting angles observed in the coated carbide tool. The experimental work was carried out on a CNC lathe (5.5 kW spindle power and 3500 rpm maximum rotational speed). Three component turning forces (cutting force, feed force and thrust force) were measured using a 9257BA Kistler® piezoelectric dynamometer connected to a control unit, data acquisition board and microcomputer. Average surface roughness (Ra ) values were recorded with a Surftest 301 Mitutoyo® roughness meter set to a cut-off of 0.8 mm. The average width of flank wear (VBB ) was measured using a TM505 Mitutoyo® toolmaker’s microscope (1 ␮m resolution). The following cutting conditions were employed for measuring machining forces and surface roughness: cutting speeds (vc ) of 60–120 and 180 m/min, feed rates (f) of 0.1–0.2–0.3 and 0.4 mm/rev and depths of cut (ap ) of 0.5–1–1.5 and 2 mm. Tool wear tests were performed using vc = 120 m/min, f = 0.2 mm/rev and ap = 1 mm for predefined cutting times of 2, 5, 8 and 11 min in order to evaluate tool wear evolution. After these tests, the inserts were subjected to

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ultrasonic cleansing in a acetone solution followed by measurement of the average flank wear (VBB ) in accordance to ISO 3685 (1993). Finally, the worn cutting edges were observed in the scanning electron microscope. 2.2. AISI D2 steel In the second phase of the experimental, work bars of high chromium AISI D2 cold work tool steel with the following chemical composition were used as work material: 1.55% C, 0.30% Si, 0.40% Mn, 11.80% Cr, 0.80% Mo and 0.80% V. After heat treatment (quenching in a vacuum atmosphere at 1000–1040 ◦ C) an average hardness of 58 HRC was obtained. The geometry of the workpiece allowed a fixture length of 30 mm in order to obtain a cylindrical turning length (L) of 200 mm with an initial diameter of 50 mm (D), which resulted in a L/D ratio of 4. The L/D ratio at the end of the experimental work was approximately 8 in order to assure the required stiffness of the chuck/workpiece/cutting tool system. Mixed alumina inserts ISO code CNMA 120408 T01020 CC650 mounted on a PCLNR2525 M12 tool holder were used with a cutting geometry as follows: clearance angle α0 = 6◦ , negative rake angle γ 0 = −6◦ , negative cutting edge inclination angle λs = −6◦ and cutting edge angle χr = 95◦ . Turning experiments were performed using a conventional lathe (5.5 kW spindle power and a maximum spindle speed of 2200 rpm). The following cutting parameters were used: cutting speed (vc ) of 80–150 and 220 m/min, feed rates (f) of 0.05–0.10 and 0.15 mm/rev and constant depth of cut (ap ) of 0.2 mm. Flank tool wear evaluation was conducted on a TM-500 Mitutoyo® toolmaker’s microscope (30× magnification and 1 ␮m resolution). The average flank wear land measured at the tool corner (VBC ) was established according to ISO 3685 Standard (1993). The reason for using this parameter and not VBB is due to the low value for depth of cut used (ap = 0.2 mm) compared to the tool nose radius (rε = 0.8 mm). Arithmetic average roughness (Ra ) of the machined surfaces was measured using a Hommelwerke® profilometer with a cut-off of 0.8 mm (in accordance to ISO/DIS 4287/1E).

Fig. 1. Effect of cutting speed on machining forces for f = 0.2 mm/rev and ap = 1 mm (AISI 4340 steel).

42 HRC steel, a reduction in force was noticed only for Fc , while Ff and Fp remained practically unaltered. Moreover, lower machining forces were recorded when machining the harder work material. One reason for such behaviour might be related to the larger seizure area produced when turning the 42 HRC steel. The effect of feed rate on the machining forces when turning AISI 4340 steel at vc = 120 m/min and ap = 1 mm can be seen in Fig. 2. Generally speaking, all the three components of the machining force increase with feed rate due to elevation of the shear plane area, however, while the steel hardened to 42 HRC presents a linear behaviour, the steel with 50 HRC shows relatively high values for f = 0.1 mm/rev and a sudden increase when feed rate is elevated from 0.2 to 0.3 mm/rev, remaining stable when other feed rate values are employed. Fig. 3 shows the influence of depth of cut on the machining forces for vc = 120 m/min and f = 0.2 mm/rev. It can be noticed that the three components of the machining force are higher when turning the 50 HRC material and that the difference between the forces measured at these two levels of hardness increases with depth of cut. Similarly to Fig. 1, in general, the cutting force gave higher values, followed by the thrust force and finally by the feed force.

3. Results and discussion 3.1. AISI 4340 high strength low alloy steel Fig. 1 shows the effect of cutting speed on the turning forces when cutting AISI 4340 steel hardened to 42 and 50 HRC using a feed rate f = 0.2 mm/rev and a depth of cut ap = 1 mm. Average cutting force (Fc ), feed force (Ff ) and thrust force (Fp ) are presented. It can be seen that as cutting speed is increased, the three components of the machining forces are reduced for the 50 HRC steel. This reduction in the forces is probably due to the temperature increase in the shear plane area, which resulted in a reduction in the shear strength of the material. However, when observing the results for the

Fig. 2. Effect of feed rate on machining forces for vc = 120 m/min and ap = 1 mm (AISI 4340 steel).

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Fig. 3. Effect of depth of cut on machining forces for vc = 120 m/min and f = 0.2 mm/rev (AISI 4340 steel).

Fig. 4 presents the effect of cutting speed on surface roughness (Ra ) for f = 0.2 mm/rev and ap = 1 mm. It can be seen that lower Ra values are obtained at higher cutting speeds due to the lower forces generated and that best surface roughness is obtained when machining the harder steel. The reason for that resides in the fact that the PCBN tool used when turning the AISI 4340 steel hardened to 50 HRC possesses a nose radius of 1.6 mm, against 0.8 mm for the coated carbide tool. The effect of feed rate on surface roughness for vc = 120 m/min and ap = 1 mm is shown in Fig. 5. In addition to the experimental data, theoretical values are presented. As expected, an increase in feed rate resulted in poorer surface roughness and the higher nose radius of the PCBN tool provided lower Ra values. The experimental results were very close to the theoretical values, and occasionally even better. Fig. 6 shows the effect of depth of cut on Ra when turning AISI 4340 steel at vc = 120 m/min and f = 0.2 mm/rev. Except when machining the 50 HRC steel using a depth of cut of 1 mm, depth of cut presented a negligible influence on surface roughness up to ap = 1.5 mm. Fig. 7 shows the influence on cutting time no average flak wear land (VBB ) after turning AISI 4340 steel for 2,

Fig. 4. Effect of cutting speed on surface roughness for f = 0.2 mm/rev and ap = 1 mm (AISI 4340 steel).

Fig. 5. Effect of feed rate on surface roughness for vc = 120 m/min and ap = 1 mm (AISI 4340 steel).

Fig. 6. Effect of depth of cut on surface roughness for vc = 120 m/min and f = 0.2 mm/rev (AISI 4340 steel).

5, 8 and 11 min under the following cutting conditions: vc = 120 m/min, f = 0.2 mm/rev and ap = 1 mm. The measurement of VBB ear was not cumulative, i.e., a fresh cutting edge was used for each cutting time. It can be noticed that when

Fig. 7. Tool wear against cutting time for vc = 120 m/min, f = 0.2 mm/rev and ap = 1mm (AISI 4340 steel).

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Fig. 8. (a–d) Coated carbide cutting edges after turning AISI 4340 steel (42 HRC) at vc = 120 m/min, f = 0.2 mm/rev and ap = 1 mm. (a) t = 2 min; (b) t = 5 min; (c) t = 8 min; (d) t = 11 min.

turning AISI 4340 steel hardened to 42 HRC using the coated carbide insert, flank wear increased smoothly as cutting time elapsed. The same trend is observed in the first 8 min of cut of the same material hardened to 50 HRC with the PCBN compact, however, tool wear did not change in the last 3 min of the test. In spite of the difference in the properties of the

tool materials tested, the higher workpiece hardness value was responsible for the accelerated wear rate of the PCBN tool. The worn areas of coated carbide and PCBN tools after turning AISI 4340 steel hardened to 42 and 50 HRC for 2, 5, 8 and 11 min are presented, respectively, in Figs. 8 and 9. These

Fig. 9. (a–d) PCBN cutting edges after turning AISI 4340 steel (50 HRC) at vc = 120 m/min, f = 0.2 mm/rev and ap = 1 mm. (a) t = 2 min; (b) t = 5 min; (c) t = 8 min; (d) t = 11 min.

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figures show the clearance face (left), the cutting edge (centre) and the rake face (right) of the tools tested. Fig. 8 shows that as cutting time progresses, abrasion takes place particularly at the rake face of the coated carbide insert, resulting in the flank wear values presented in Fig. 7. Crater wear also presents signs of moderate abrasion. Evidence of other wear mechanisms, such as adhesive wear, notch wear or plastic deformation, were not observed. Fig. 9 shows that abrasion is also an important wear mechanism when using a PCBN cutting tool, giving a significant contribution to flank wear, probably owing to the presence of hard carbide particles. In contrast to Fig. 8, the crater appears in the first minutes of cutting, however, its polished texture suggests the occurrence of diffusion wear. 3.2. AISI D2 cold work tool steel Table 1 presents the cutting conditions (cutting speed, feed rate and cutting time) and the corresponding experimental results regarding surface roughness (Ra ) and flank wear (VBC ) after turning AISI D2 steel (58 HRC). The results presented in Table 1 are plotted in Fig. 10, where the effect of cutting speed and feed rate on surface roughness along cutting time can be more clearly seen. It can be noticed that the Ra values increase with feed rate and are reduced as cutting speed is elevated, i.e., best surface roughness is obtained when using high cutting speeds and low feed rates. Comparing Fig. 10(a) to (b and c) it can be seen Table 1 Surface roughness (Ra ) and flank wear (VBC ) results (AISI D2 steel) Test

vc (m/min)

f (mm/rev)

time (min)

Ra (␮m)

VBC (mm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

80 80 80 80 80 80 80 80 80 150 150 150 150 150 150 150 150 150 220 220 220 220 220 220 220 220 220

0.05 0.05 0.05 0.1 0.1 0.1 0.15 0.15 0.15 0.05 0.05 0.05 0.1 0.1 0.1 0.15 0.15 0.15 0.05 0.05 0.05 0.1 0.1 0.1 0.15 0.15 0.15

5 10 15 5 10 15 5 10 15 5 10 15 5 10 15 5 10 15 5 10 15 5 10 15 5 10 15

0.47 0.42 0.4 0.66 0.82 0.76 1.04 0.93 1.09 0.34 0.36 0.37 0.5 0.55 0.58 1.00 0.93 0.81 0.31 0.33 0.34 0.52 0.5 0.54 0.41 0.37 0.7

0.10 0.11 0.13 0.09 0.12 0.14 0.09 0.12 0.15 0.10 0.13 0.16 0.10 0.14 0.18 0.10 0.13 0.18 0.10 0.16 0.21 0.10 0.14 0.20 0.13 0.21 0.29

Fig. 10. Effect of cutting time on surface roughness for: (a) f = 0.05 mm/rev; (b) f = 0.1 mm/rev; (c) f = 0.15 mm/rev (AISI D2 steel).

that tighter scatter in the Ra values is produced at the lowest feed rate. However, the influence of cutting time is not clear, probably due to the fact that alterations in the cutting edge due to wear may improve or damage surface finish, depending on the acting shape of the cutting edge. Comparing Fig. 10(b and c) with Figs. 5 and 4, respectively, it can be seen that the Ra values measured in the AISI D2 steel and AISI 4340 steel (50 HRC) were within the same range when approximate cutting parameters were tested, despite the differences in hardness of the workpiece and tool nose radius. With the results concerning the surface roughness given in Table 1, it is possible to apply a multiple linear regression model and find an equation which correlates Ra values to

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J.G. Lima et al. / Journal of Materials Processing Technology 169 (2005) 388–395 Table 2 Tool life for VBC = 0.15 mm (AISI D2 steel) vc (m/min)

f (mm/rev)

Cutting time (min) for VBC = 0.15 mm

80 80 80 150 150 150 220 220 220

0.05 0.1 0.15 0.05 0.1 0.15 0.05 0.1 0.15

18.4 16.1 15.4 13.2 11.5 11.7 9.6 10.8 6.6

Using the data presented in Table 1 (related to the flank wear VBC ), a multiple linear regression model can be employed in order to correlate VBC to the machining parameters: cutting speed (vc ), feed rate (f) and cutting time (t), with R2 = 0.89, as given by Equation (2): VBC = 0.00038vc + 0.22f + 0.0083t − 0.0217

(2)

Establishing a tool life criterion VBC = 0.15 mm for the results presented in Fig. 11, it is possible to relate the cutting time required to reach the above criterion under each cutting condition, as indicated in Table 2. Using the same procedure (multiple linear regression) one can obtain Equation (3), which gives the time required to reach VBC = 0.15 mm as a function of cutting speed (vc ) and feed rate (f) with R2 = 0.93: T = 23.27 − 0.055vc − 25f

Fig. 11. Effect of cutting time on flank wear for: (a) f = 0.05 mm/rev; (b) f = 0.1 mm/rev; (c) f = 0.15 mm/rev (AISI D2 steel).

cutting speed (vc ), feed rate (f) and cutting time (t), see Equation (1), which R-squared is R2 = 0.89. Ra = −0.002vc + 4.38f + 0.0038t + 0.425

The vc –T curves for VBC = 0.15 mm and cutting speeds from 80 to 220 m/min and feed rates from 0.05 to 0.15 mm/rev are presented in Fig. 12, where it can be seen that the higher the cutting speed and feed rate, the lower the tool life. Moreover, for vc = 220 m/min the use of a feed rate of 0.15 mm/rev resulted in a considerably shorter tool life. Fig. 13 shows tool fracture at the cutting edge of the mixed alumina insert observed under the optical microscope. In addition to the low fracture toughness of this tool material, the cutting conditions employed (vc = 220 m/min, f = 0.15 mm/rev and ap = 0.2 mm) probably were too severe and resulted in higher shear stress than this tool material could withstand. As a consequence, the cutting tool failed by frac-

(1)

Fig. 11 shows the flank wear (VBC ) evolution as cutting time elapses for distinct cutting speed and feed rate values. As expected, VBC increases with cutting time, cutting speed and feed rate, however, a drastic increase in tool wear is observed when turning at a cutting speed of 220 m/min and a feed rate of 0.15 mm/rev. Additionally, for a feed rate of 0.15 mm/rev, VBC presented considerably higher values at vc = 220 m/min when compared to 80 and 150 m/min.

(3)

Fig. 12. vc –T curves for VBC = 0.15 mm (AISI D2 steel).

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Fig. 13. Fractured mixed alumina insert after turning AISI D2 steel (58 HRC) at vc = 220 m/min, f = 0.15 mm/rev and ap = 0.2 mm.

ture, in contrast to the gradual wear observed in the coated carbide and PCBN tools tested against the AISI 4340 steel.

4. Conclusions The following conclusions can be drawn from this work: • After turning AISI 4340 steel hardened to 42 HRC and 50 HRC using coated carbide and PCBN tools, respectively, the machining forces were reduced as cutting speed was increased and increase with feed rate and depth of cut, however, for low feed tares and depths of cut the forces observed when turning AISI 4340 steel hardened to 42 HRC were higher than when machining the 50 HRC steel. The surface finish of the machined part was improved as cutting speed was elevated and deteriorated with feed rate. Depth of cut presented little effect on the Ra values. Best surface finish was produced by the cutting tool with larger nose radius (PCBN). When turning the 42 HRC steel with the coated carbide insert, tool wear rate increase smoothly and the wear mechanism was probably abrasion. In the case of the 50 HRC steel, higher wear rates were observed in the PCBN tool, which presented signs of diffusive wear. • Turning of AISI D2 steel (58 HRC) with mixed alumina inserts allowed a surface finish as good as that produced by cylindrical grinding. In addition to that, Ra values increased with feed rate and were reduced as cutting speed was elevated, ranging from 0.28 to 1.12 ␮m. The flank wear of the mixed alumina tool increased with cutting speed and depth of cut, presenting a considerably higher tool wear rate when using a cutting speed of 220 m/min and feed rate of 0.15 mm/rev. In contrast to the coated carbide and PCBN tools tested against the AISI 4340 steel, when machining using heavier cutting conditions the mixed alumina tool failed by spalling.

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