Journal of Materials Processing Technology 153154 (2004) 10451050

Effect of high-pressure coolant supply when machining nickel-base, Inconel 718, alloy with coated carbide tools
E.O. Ezugwu , J. Bonney
Machining Research Centre, Faculty of Engineering Science and Technology, South Bank University, London SE1 0AA, UK

Abstract Inconel 718 was machined with a triple PVD coated (TiCN/Al2 O3 /TiN) carbide tool at speeds up to 50 m min1 using conventional and various high coolant pressures, up to 203 bar. Tool life, surface roughness (Ra), tool wear and component forces were recorded. The test results show that acceptable surface nish and improved tool life can be achieved when machining Inconel 718 with high coolant pressures. Compared to conventional coolant supplies, tool life improved as much as 740%, when machining at 203 bar coolant pressure at a speed of 50 m min1 . Tool life generally increased with increasing coolant supply pressure. This can be attributed to the ability of the high-pressure coolant to lift the chip and gain access closer to the cutting interface. This action leads to a reduction of the seizure region, thus, lowering the friction coefcient which in turn results in reduction in cutting temperature and component forces. Chip breakability during machining is dependent on the depth of cut, feed rate and cutting speed employed as well as on the coolant pressure employed. Machining Inconel 718 with lower coolant pressures did not produce chip segmentation. Tool wear increased gradually with prolong machining with high coolant pressures. Nose wear was the dominating tool failure mode due probably to a reduction in the toolchip and toolworkpiece contact length/area. 2004 Elsevier B.V. All rights reserved.
Keywords: Tool life; Nose wear; Critical coolant pressure; Cutting temperature

1. Introduction Advanced materials, such as nickel-base and titanium alloys as well as composites are widely used in the aerospace and power industries. These materials are designed for high temperature applications and at the same time maintain very high strength to weight ratios. Nickel-based alloys have high creep and corrosion resistance as well as the ability of maintaining high strength-to-weight ratio, essential for the economic exploitation of aerospace engines. Machining of nickel-based alloys generate high temperatures at the tool cutting edge which impair their performance, as they are subjected to high compressive stresses acting on the tool tip leading to plastic deformation of the tool edge, severe notching and ank wear. The poor thermal conductivity of nickel-based alloys raises temperature at the toolworkpiece interface during machining, thus, accelerating tool wear. Coolants play a signicant role in improving lubrication as well as minimising temperature at the toolchip and
Corresponding author. Present address: South Bank University, School Engineering Systems and Design, 103 Borough Road, London SE1 0AA, UK. Fax: +44 171 815 7699. E-mail address: ezugwueo@sbu.ac.uk (E.O. Ezugwu).

toolworkpiece interfaces, consequently, minimising seizure during machining. Flood cooling is not effective in terms of lowering cutting temperature when machining exotic materials. The coolant do not readily access the toolworkpiece and toolchip interfaces that are under seizure condition as it is vaporised by the high temperature generated close to the tool edge. Machining of nickel-based alloys at high-speed conditions can therefore, be achieved by a combination of the appropriate tool material, machining technique and the choice of a suitable cooling technology [1]. High-pressure assisted cooling is one of the preferred technologies, currently, under exploitation especially in the aerospace and power plant industries for machining exotic materials. The credibility of high-pressure coolant assisted machining had been thoroughly investigated over the years [25]. This system, not only provides adequate cooling at the toolworkpiece interface but also provides an effective removal (ushing) of chips from the cutting area. The coolant jet under such high-pressure is capable of creating a hydraulic wedge between the tool and the workpiece, penetrating the interface deeply with a speed exceeding that necessary even for very high-speed machining. This phenomenon also changes the chip ow conditions [6]. The penetration of the high-energy jet at the toolchip interface reduces the temperature gradi-

0924-0136/$ see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.04.329

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Table 1 Chemical composition of Inconel 718 (wt.%) C 0.08 Mn 0.35 Si 0.35 S 0.15 Cr 18.6 Fe 17.8 Mo 3.1 Nb & Ta 5.0 Ti 0.9 Al 0.5 Cu 0.3 Ni bal

Table 2 Physical properties of Inconel 718 Tensile strength (MPa) 1310 Yield strength (MPa) 1110 Elastic modulus (GPa) 206 Hardness (HV150 ) 370 Density (g cm3 ) 8.19 Melting point ( C) 1300 Thermal conductivity (W/mK) 11.2

ent and minimises the seizure effect, offering an adequate lubrication at the toolchip interface with a signicant reduction in friction. Excellent chip breakability has been reported when machining difcult-to-cut materials with high-pressure coolant supply [7,8]. This is attributed to a coolant wedge which forms between the chip and the tool forcing the chip to bend upwards giving it a desirable up curl required for segmentation. This paper investigates the effect of varying coolant pressure on tool performance when machining Inconel 718 alloy with coated carbide tools at high-speed conditions.

Table 4 Mechanical properties of coated carbide inserts Hardness (HV3 ) 2000 Grain size (m) 1.7 K1C Coating thickness (m) [MPa (m1/2 )] TiCN Al2 O3 TiN 14 4 1 0.5

The following cutting conditions were employed in this investigation: Cutting speed (m min1 ): 20, 30, 50 Feed rate (mm rev1 ): 0.25, 0.3 Depth of cut (mm): 2.53.0 ramping tool path programming Coolant concentration (%): 6 Coolant supply pressure (bar): conventional, 110, 150, 203 The tool rejection criteria for roughing operation were employed. These values were considered in relation to ISO Standard 3685 for tool life testing. A cutting tool was rejected and further machining stopped based on one or a combination of the following rejection criteria: (1) Average ank wear (mm) (2) Maximum ank wear (mm) (3) Nose wear (mm) (4) Notching at the depth of cut line (mm) (5) Surface roughness (mm) (6) Excessive chipping (aking) or catastrophic fracture of the cutting edge 0.4 mm 0.7 mm 0.5 mm 1.0 mm 6.0 m

2. Experimental procedures The machining trials were carried out on a CNC Centre Lathe with a speed range from 18 to 1800 rpm. The lathe is driven by an 11 kW stepless motor which provides a torque of 1411 Nm. Cast (200 mm diameter 300 mm long) solution treated, vacuum induction melted and electroslag remelted Inconel 718 alloy bars were used for the machining trials. The chemical composition and physical properties of the workpiece are given in Tables 1 and 2 respectively. Up to 6 mm thickness of the top surface of each bar was removed prior to actual machining trials in order to eliminate any surface defect that can adversely affect the machining result. A coolant containing alkanolamine salts of the fatty acid dicyclohexylamine, specically designed for delivery at high pressures was used in the machining trials. Conventional coolant was applied by ooding the cutting interface at an average ow rate of 5 l min1 . The high-pressure coolant was supplied at an average ood rate of 2050 l min1 and directed via a nozzle on the tool holder to the region where the chip breaks contact with the tool. PVD Coated carbide insert with ISO tool designations SNMG120412 was used for the machining trials. The nominal composition and physical properties of the inserts are given in Tables 3 and 4.
Table 3 Composition of the coated carbide inserts Co (% volume) 17.1 WC (% volume) 81 TaC (% volume) 1.2 NbC (%volume) 0.6

Cutting forces generated during the machining trials were measured using a three component piezoelectric tool post dynamometer. Tool wear was measured with a travelling microscope connected to a digital readout device at a magnication of 25. Surface roughness was measured at various intervals with a stylus type instrument.

3. Results and discussions Figs. 1 and 2 show that longer tool life was achieved when machining with PVD coated carbide tool under high-pressure coolant supplies than with conventional coolant supply. Table 5 is a summary of the percentage

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Fig. 1. Tool life recorded when machining Inconel 718 with various coolant pressures at a feed rate of 0.25 mm rev1 . (CM: Conventional Machining).

Fig. 2. Tool life recorded when machining Inconel 718 with various coolant pressures at a feed rate of 0.3 mm rev1 . (CM: Conventional Machining).

increase in tool life when machining at various coolant pressures relative to conventional coolant supply. Up to 740% improvement in tool life was achieved when machining with carbide inserts most aggressive conditions (50 m min1 ) using 203 bar coolant pressure. Increase in coolant pressure, generally increased in tool life when machining at speeds in excess of 20 m min1 . The major cause of tool rejection when machining Inconel 718 is high temperature generation at the toolchip and toolworkpiece interfaces. The temperature is signicantly reduced by administering coolant under high pressures directly to the cutting interface. This could therefore, minimise and/or completely eliminate thermally related wear mechanisms. Therefore, tool performance tend to be primarily dependent on mechanical wear phenomena. It can also be seen in Figs. 1 and 2 that increasing coolant pressure did not in-

Table 5 Percentage improvement in tool life relative to conventional coolant supply after machining Inconel 718 with coated carbide tool Speed (m min1 ) 20 30 50 20 30 50 Feed rate (mm rev1 ) 0.25 0.25 0.25 0.3 0.3 0.3 110 bar 8 87.7 335.0 8.6 27.05 517.6 150 bar 9.8 50.6 411.1 11.5 95.2 647.2 203 bar 33.8 64.1 462.8 43.9 104.5 739.8

crease tool life in cutting conditions investigated. This is evident from Table 5, where, a drop in tool life as much as 44% is recorded when machining with 203 bar coolant pressure at a speed of 20 m min1 and a feed rate of 0.3 mm rev1 . It has been established that at any speed condition, the toolchip interface temperature initially decreased with an increase in jet pressure, up to a critical pressure, above which it rose to a relatively constant value for pressures in excess of the critical pressure [2]. Cutting tools operate within a safety temperature zone with minimal tool wear when machining at the critical coolant pressure as thermal stresses are kept to a minimum thereby prolonging tool life [1]. It is clear form this that 203 bar coolant pressure is above the critical pressure for machining at a speed of 20 m min1 , hence, increased toolchip contact temperature resulting in accelerated tool wear and, hence, higher cutting forces. Fig. 3 shows variation in component forces when machining Inconel 718 with PVD coated tool at various cutting speeds and coolant pressures. The cutting forces generally decreased with increasing cutting speed as expected. Reduction in cutting forces when machining at high coolant pressure suggests that high-pressure jet is able to penetrate the cutting interface, thus, providing efcient cooling as well as lubrication. The coolant water wedge created at the toolchip interface reduces toolchip contact length and also lowers the coefcient of friction and consequently lowers cutting forces. Fig. 3 also shows that higher cutting forces were, however, recorded when machining at a speed

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Fig. 3. Cutting forces recorded when machining Inconel 718 at various coolant supplies and machining conditions. (CM: Conventional Machining).

of 20 m min1 with 203 bar coolant pressure. This increase in cutting force is due to increased friction coefcient and higher nose wear rate generated by higher temperatures induced by the hyper-critical coolant pressure (203 bar) at the cutting interface. Nose wear is the dominant tool failure mode observed when machining Inconel 718 at the conditions investigated. Fig. 4 shows the nose wear plot when machining at a speed of 20 m min1 with various coolant pressures and feed rates.
CM (f=0.25) 150 bar (f=0.25) 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 5 10 15 CM (f=0.3) 150 bar (f=0.3)

It can be seen from the graph that the nose wear rate increased steadily with prolong machining. Rapid increases in nose wear rate occurred when machining with 203 bar coolant pressure at both feed rates, hence, the lower tool life recorded. Fig. 5a shows a typical worn tool illustrating nose, ank and rake face wears. The uniform ank wear observed may be due to the low wear rate caused by temperature reduction at the cutting interface when machining
110 bar (f=0.25) 203 bar (f=0.25) 110 bar (f=0.3) 203 bar (f=0.3)

Nose wear (mm)

20

25

30

35

40

Cutting time (min)

Fig. 4. Nose wear curves when machining Inconel 718 at a speed of 20 m min1 under different coolant pressures and feed rate conditions. (CM: Conventional Machining).

Fig. 5. (a) Worn tool after machining Inconel 718 with 203 bar coolant pressure at a speed of 20 m/min and a feed rate of 0.3 mm/rev. (b) Enlarged view on the ank face showing abrasive wear and coating delamination of coated carbide tool.

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Fig. 6. (a) Chip generated when machining at coolant pressures up to 150 bar. (b) Segmented chips generated when machining with 203 bar coolant pressure.

CM (f=0.25) 150 bar (f=0.25) Surface Roughness (um) 6 5 4 3 2 1 0 0 5

CM (f=0.3) 150 bar (f=0.3)

110 bar (f=0.25) 203 bar (f=0.25)

110 bar (f=0.3) 203 bar (f=0.3)

10

15

20

25

30

35

40

Cutting time (min)

Fig. 7. Surface roughness values recorded when machining with various coolant pressures and feed rates at a speed of 30 m min1 . (CM: Conventional Machining).

with high coolant pressures. Fig. 5b shows that the wear mechanism is mechanically related and typical of abrasion wear. Machining Inconel 718 with lower coolant pressures, up to 150 bar, produced long continuous spiral chips (Fig. 6a), while smaller segmented chips were produced when machining with higher coolant pressure of 203 bar (Fig. 6b). Coolant supply at high-pressure tends to lift up the chip after passing through the deformation zone resulting to a reduction in the toolchip contact length/area. This tends to enhance chip segmentation as the chip curl radius is reduced signicantly, hence, maximum coolant pressure is restricted only to a smaller area on the chip. Similar observation with chip segmentation, was made while machining steel. It was observed that the power of the coolant jet and the lateral position of the point where the jet hits the line where the chip exits the tool rake face has signicant inuence on the chip segmentation process with high-pressure coolant supplies [9]. Fig. 7 is a plot of the surface roughness values recorded when machining at a cutting speed of 30 m min1 . The curves show that lower surface roughness values (hence, im-

proved nish) were generated when machining at lower feed rate of 0.25 mm rev1 while higher values were generated at a higher feed rate of 0.3 mm rev1 . The curves also show that the surface roughness values varies marginally with prolong machining with high coolant pressures. This could be attributed to the fact that the tool wear process was gradual due to the signicant reduction in temperature at the cutting interface. The tool cutting edge may have been maintained for longer periods, thereby, ensuring minimal variations in recorded surface roughness values. Surface roughness values recorded in all the cutting conditions investigated are well below the stipulated rejection criterion of 6 m. This shows that the integrity of the machined surfaces may not be affected when machining Inconel 718 alloy under high coolant pressures.

4. Conclusions 1. Machining Inconel 718 with coated carbide tools under high-pressure coolant supplies can improve tool life by up to 7-folds, especially at higher speed conditions.

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2. Tool life tend to improve with increasing coolant pressure. There is also evidence that once a critical pressure has been reached any further increase, in coolant pressure may only result to a marginal increase in tool life. 3. Chip segmentation depends on the cutting conditions employed and to a greater extent on the coolant pressure employed when machining Inconel 718. Machining with a 203 bar coolant pressure produced well segmented C-shape chips. 4. Surface roughness values generated when machining Inconel 718 alloy with the coated carbide tool vary marginally with prolong machining due probably to the gradual wear generated at the tool edge as well as temperature reduction at the cutting interface by the high coolant pressure employed.

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Acknowledgements The authors would like to thank Rolls-Royce plc and Sandvik Coromant for their support that enabled this work to be carried out.