The machinability of inconel 718

The machinability of inconel 718

JoumaIor ELSEVIER Materials Processing Technology Journal ofMaterials Processing Teclmology 63 (1997) 199·204 The Machinability of Incone1 718 M. R...

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Materials Processing Technology Journal ofMaterials Processing Teclmology 63 (1997) 199·204

The Machinability of Incone1 718 M. Rahman, W.K.H. Seah and T.T. Teo Dept. of Mech. & Prod. Engineering, National University of Singapore Kent Ridge, Singapore 0511, Republic of Singapore

ABSTRACT Inconel 718, a high strength, thermal resistant Nickel-based alloy, is mainly used in the aircraft industries. Due to the extreme toughness and work hardening characteristic of the alloy, the problem of machining Incone1 718 is one of ever-increasing magnitude. This paper discusses the effect of cutting conditions on the machinability of Incone1 718. Flank wear of the inserts, workpiece surface roughness and cutting forces will act as the performance indicators for tool life while machining is carried out using a CNC lathe. Two types of coated cemented carbide inserts, grades EH20Z-UP (TiN coated by physical vapour deposition) and AC25 (TiN coated by chemical vapour deposition), were used. Various combinations of side cutting edge angles (SCEAs), cutting speeds and feedrates were tested at a constant depth of cut. Cutting results indicate that SCEA, together with cutting speed and feedrate, do playa significant role in determining the tool life of an insert when machining Inconel 718.

1 INTRODUCTION

Inconel 718 is a high-strength, thermal-resistant (HSTR) Nickel-based alloy [1] that plays an increasingly important part in the development and manufacture of jet aeroengines. It is also noted for its excellent corrosion resistance. Due to its extremely tough nature, the difficulty of machining Inconel 718 resolves itself into two basic problems: (1) the inability of the tool material to give long tool lives due to the work hardening and attrition properties of the alloy and (2) the metallurgical damage to the workpiece due to the very high cutting forces which also gives rise to work hardening, surface tearing and distortion in finally machined components due to induced stresses. Machinability studies [2-6] of this material have been carried out by different researchers. In a study using C2 carbide grade tools, it was found that [2] the dominant causes of tool failure were localized groove wear on the side flank and chipping on the side cutting edge. It was further observed that the effect of depth of cut on tool wear was not so significant in comparison to the effects of speed and feed. However, it was found [3] that surface finish deteriorated with increasing speed for Incone1 718. The effect of tool nose radius and feed was not significant on surface roughness. A study [4] on the performance of machinability oflncone1 718 showed that the Elsevier Science SA

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tool life of the silicon nitride based material was mainly dependent on flank wear, whereas for the silicon carbide whisker-reinforced alumina, the tool life criterion was depth of cut notch wear. It has been found [5] that at lower speeds, ceramic tools were prone to depth of cut (DOC) notch wear with minimal damage to the tool nose and at higher speeds, there was a reduction in depth of cut notching and an increase in nose and flank wear. A recent study [6] was carried out using coated cemented carbide inserts of four different grades and a ceramic insert of grade NS13OC. It was found that the coated cemented carbide inserts performed best at the lowest speed and feedrate. The ceramic insert was unsuitable for machining Incone1 718 as the tool life was less than one and a half minutes for all the cutting conditions tested. The major failure mode of the inserts were DOC notch wear. Both the carbide and the ceramic inserts exhibited excellent resistance to crater wear. The emphasis of the previous studies on the machinability ofInconel 718 was mainly on the effects of cutting speed, feedrate, depth of cut and nose radius on the tool life of the cutting tool. The effect of tool geometry on the machining of Inconel 718 has never been investigated in detail. While cutting speed, feedrate and depth of cut are important factors to consider during any machining process, the influence of tool geometry on tool life should not be neglected.

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This paper discusses the machinability of Inconel 718 subjected to various machining parameters including tool geometry, cutting speed and feedrate. Flank wear of the inserts, workpiece surface roughness and the cutting force components (tangential, axial and radial forces) have been considered as the. performance indicators for tool life. 2 EXPERIMENTAL METHOD

DESIGN,

MATERIALS

AND

There were a total of four input parameters, namely: SCEA (approach angle) of the cutting tool, cutting speed, federate and duration of cut. The depth of cut was maintained at 2mm throughout the cutting test. With the exception of duration of cut, three approach angles, three cutting speeds and three feedrates were used as the independent variables. Three tool holders with a different SCEA or approach angle each were used to hold the inserts. These values are presented in Table 1.

Three types of Hertel Tool Holders [8] were used. They were: (1) PCLN R 2525 M12 (SCEA of _5°), (2) PCBN R 2525 M12

(SCEAof 15°) and (3) PCSN R 2525 M12 (SCEA of 45°) Four cutting edges per insert were used in the machining of Inconel 718. The failure criteria applied to the inserts were: (I) flank wear greater than O.3mm, (ii) depth of cut notch wear greater than 0.8mm, (iii) flank face flaking and (iv) total edge fracture The workpiece was a heat-resistant alloy Inconel 718, 0300 x 450mm. The chemical composition of Inconel 718 is shown in Table 3. Table 3 Nominal Chemical Composition of Inconel 718, % wt. Cr 18.8 Mn 0.07

tb !Mo h'i 5.27 2.99 1.02 ~u ~ P 10.07 10.03 10.01

Table 1 Levels of input parameters for machining Inconel 718

Elements tNi %wt. 153.4 Elements Si % wt. 10.12

Cuttinj!. Parameters SCEA (0) Cutting Speed (m/min) Feedrate (mm/rev) Depth of Cut (mm)

3. RESULTS AND DISCUSSION

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3.1 SIDE CUTTING EDGE ANGLE (SCEA)

The experiment was conducted on a CNC lathe under wet conditions. The duration of cut was timed at regular intervals (ie: 2 min) for each set of cutting conditions. At each interval, the machining was stopped, allowing flank wear of the insert, surface roughness of the workpiece and the three cutting forces (tangential, axial and radial forces) to be measured. Flank wear was measured using an optical toolmaker's microscope, surface roughness with a Taylor Hobson Surtronic 10 and cutting forces with a 3-component dynamometer. Two types of inserts supplied by Sumitomo Electric [7] were used and they were: (1) K type substrate, TiN physical vapour deposition (PVD) coated cemented carbide grade EH20Z-UP and (2) Multi AI203 chemical vapour deposition (CVD) coated cemented carbide grade AC25. The properties of the main compositions for the cemented carbide inserts are shown in Table 2. Table 2 Features of main materials for cemented carbide Material Specific Hardness Young's Gravity (H v ) Modulus (xI0 3kg/ mm 2 ) 70 15.6 2150 WC 26 5.43 2000 TiN 42 AI'}O'l 3.98 3000 10-18 8.9 Co --

AI 0.50 Fe Bal

The effects of SCEA on tool life are plotted in Figs. 13 for AC25 inserts. It is observed that tool life increases as the side cutting edge angle or approach angle increase from _5° to 45 0. These trends are generally similar for EH20Z-UP inserts also.

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Figure 1 Effect of SCEA on tool life at a cutting speed of 30m/min, based on different feedrates using AC25

5.1 9.2 8.5 12.3

~rom Figure 4, it can be seen that the SCEA actually determines the true feed or thickness of the uncut chip layer, t, perpendicular to the cutting edge, as well as the width of the deformed chip. At the same feed (f) and depth of cut, an

2900 2950 2050 1495

M. Rahman et al. / Journal of Materials Processing Technology 63 (1997) 199-204

increase in SCEA will reduce the true feed and increase the width of the chip. The tool-chip interface temperature, 8, is related to the true feed, t, according to: 8 oc t1/2

Hence, the interface temperature will be less with a reduced true feed because of an increased SCEA. Further, heat generated during the cutting process will be distributed over a greater length of the cutting edge. This improves heat removal from the cutting edge, distribute the cutting forces over a larger portion of the cutting edge, reduces tool notching, and substantially improves tool life.

201

3,3 DURATION OF CUT Cutting forces increase as feedrate and cutting time increase. It is interesting to note that the cutting force increases, on the average, by 600N as feedrate increases from O.2mm/rev to OAmm/rev. There was no significant change in cutting force as cutting speed increases from 30m/min to 40m/min. It was observed that as cutting time and feedrate increase, axial force also increases. However, no significant changes in axial forces were found when the cutting speed was increased for machining at a constant SCEA. The values of axial force only increased sharply when the inserts have reached the failure criterion.

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SCFA (deg) Figure 2 Effect of SCEA on tool life at a cutting speed of 40m/min, based on different feedrates using AC25

Figure 3 Effect of SCEA on tool life at a cutting speed of 50m/min, based on different feedrates using AC25

On the other hand, an increase in the SCEA will increase the radial component of the cutting force and may lead to chatter vibration. Figure 5 is chosen to represent this trend. It can be seen that the radial forces increased tremendously when the SCEA was increased from _5° to 45°. The radial forces at _5° were fairly constant at their respective feedrates.

3.2 CUTTING SPEED Tool life decreases as cutting speed increases. At high cutting speeds, more heat is generated due to friction. This causes the insert to fail faster, thereby reducing tool life. From Figure 6, it can be seen that at a speed of 30m/min, SCEA of 45 ° and feedrate of O.2mm/rev, the tool life was 42min. When the cutting speed was increased to 40m/min, the tool life decreased to lOmin, which was a drop of almost 75%. Figure 4 Effect of SCEA on true feed (tl>t2>t3)

M. RiJhmon et 01. / Journal of Matorials Processing Technology 63 (1997) 199-204

202

axial and radial forces. Usually at this instant, the axial force tends to surpass the tangential force, This trend can be represented by Figures 7, 8, and 9. These results obtained agree with the conclusions drawn by B. S. Yeo [5] for machining Inconel718 using cemented carbide tools.

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Figure 5 Effect of cutting time on radial force at a cutting speed of 30m/min, SCEA of 45°, based on different feedrates using AC25

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Figure 7 Effect of cutting time on tangential force at a cutting speed of 40m/min, SCEA of 45°, based on different feedrates using AC25 Flank wear rate also increases as feedrate and cutting speed increase for a constant SCEA. The wear rate of the EH20Z-UP insert is lower than the AC25 insert, hence, generating a higher tool life. Besides, the wear rate at a SCEA of 45° is also slower than that at _5°.

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3.4 PVD COATED CEMENTED CARBIDE INSERT (Grade EH20Z-UP)

While comparing the values of axial forces for the machining of Inconel 718 at different SCEA, a general increase in the forces were observed for a decrease in SCEA from 45° to 5°. The radial forces behaved in a similar fashion.

Throughout the experiment, the EH20Z-UP insert exhibited excellent resistance to depth of cut notch wear at the SCEAs of 45° and 15°. The inserts performed satisfactorily even at a high speed of 50m/min and a high feedrate of O.4mm/rev at 45° SCEA. However, at an approach angle of _5°, depth of cut notch wear became the main mode of tool failure. A transition from flank wear (SCEA of 45°) to nose wear (SCEA of IS°) and then to notch wear (SCEA of _5°) could be observed.. The insert performed best at the speed of 30m/min and feedrate of O.2mm/rev when the approach angle was 45°. The tool life was approximately 42min.

Of the three forces measured, the tangential force is the largest and the smallest is the radial force. However, when the inserts begin to fail (ie: the insert's flank wear approaches O.3mm), all the three forces increased sharply, especially for the

Although the tool life increased tremendously with a higher SCEA of 45°, the surface finish quality deteriorated by a substantial amount due to the generation of chatter. Generally, crater wear was less significant than flank wear. E.M. Trent [9],

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Figure 6 Effect of cutting speed on tool life at a SCEA of 45°, based on different feedrates using EH20Z·UP

M. Rahman et al. / Journal of Materials Processing Technology 63 (19971 199-204

has a similar conclusion. The outermost coating (TiN), which has a low coefficient of friction, reduces the friction between the chip and the rake face. The penultimate coating (AI203)' has the characteristic of reducing the crater wear on the tool. These two coatings greatly contribute to the carbide tool resistance to crater wear.

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3.5 CVD COATED CEMENTED CARBIDE INSERT (Grade AC25) The AC25 insert failed mainly by depth of cut notch wear. Flank nose wear became more prominent when the cutting speed was increased. However, it still performed well at an approach angle of 45°. The tool life for the AC25 insert at a speed of 30m/min, feedrate of 0.2mm/rev and approach angle of 45° was 30min. The chips produced were continuous chips. Only at high speeds and feedrates will the chips become discontinuous.

4. CONCLUSION The machining parameters for Inconel 718 were studied. Experiments based on variations in tool geometry (ie: SCEA or approach angles), cutting speeds, feedrates and duration of cut were performed. The performance indicators depicting tool life were flank wear of the inserts, workpiece surface roughness and cutting forces (tangential, axial and radial). A study on the results revealed the following findings: (I)

Tool life is significantly increased with an increase in the SCEAs from _5° to 15° then to 45°. For a speed of 30m/min and a feedrate of 0.2mm/rev, the tool life of the EH20Z-UP insert decreased by 64.3% when the SCEA was changed from 45° to _5°. On the whole, the inserts performed best at a SCEA of 45°.

(2)

The allowable speed and feed ranges for machining Inconel 718 are at notably low levels. Tool life of the inserts decreased when the speed or the feed was increased. This is due to the commonly experienced high cutting forces, low thermal conductivity, abrasiveness and work hardening tendencies of Inconel 718, resulting in high heat generated at the cutting edge.

Duration ofcut (min)

Figure 8 Effect of cutting time on axial force at a cutting speed of 40m/min, SCEA of 45°, based on different feedrates using AC25

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203

(3)

The tool life of the EH20Z-UP insert was mainly dependent on flank wear, whereas for the AC25 insert, the tool life criterion was DOC notch wear. For the EH20Z-UP insert, it exhibited excellent resistance to depth of cut notch wear at approach angles of 15° and 45°. For the AC25 insert, DOC notch wear was the main mode of tool failure at all three angles tested. This implied that AC25 insert might is not a suitable grade for cutting Inconel 718.

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25 REFERENCES [1]

Figure 9 Effect of cutting time on radial force at a cutting speed of 40m/min, SCEA of 45°, based on different feedrates using AC25

[2]

"Inconel Alloy 718", Inca Alloys International, 4 th Edition, 1985. Inyong Ham, "Computerized Machinability Study for Inconel 718", Pennsylvania State University, University Park, Pa. 16802.

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[3]

[4]

[5]

M. Rahman et al. / Journal of Materials Processing Technology 63 (1997) 199-204

Anil Mital & Manish Mehta, "Surface Finish Prediction Model for Fine Turning", Int. J. Prod. Res., Vol 26, No. 12, pg. 1861-1867, 1988. G. Brandt, A. Gerendas & M. Mikus, "Wear Mechanisms of Ceramic Cutting Tools When Machining Ferrous and Non-ferrous Alloys", Journal of the European Ceramic Society 6, pg. 273-290, 1990. S. F. Wayne & S. T. Buljan, "Wear of Ceramic Tools in Nickel-based Superalloy Machining", Tribology Transactions, vol 33, pg.618-626, 1990.

[6] [7] [8] [9]

"The Machinability of Inconel 718", B. S. Yeo, 19931994. Sumitomo Electric Industries Ltd., "92 Performance Cutting Tools". Hertel Industries Ltd., "Hertel Tool Holders". E.M. Trent, "Metal Cutting", Butterworth-Heinemann Ltd., 1991