Cooling and machining strategies for high speed milling of titanium and nickel super alloys

Chapter 5

Cooling and machining strategies for high speed milling of titanium and nickel super alloys Anthony Chukwujekwu Okafor Computer Numerical Control and Virtual Manufacturing Laboratory, Department of Mechanical and Aerospace Engineering, Missouri University of Science and Technology, Rolla, MO, United States

Chapter outline 1 Introduction 1.1 Heat and temperature in dry metal cutting 2 Cooling and lubrication strategies in machining 2.1 Conventional emulsion flood cooling and lubrication 2.2 Minimum quantity lubrication (MQL) 2.3 Cooling by liquid nitrogen (LN2) 2.4 Combined (MQL + LN2) cooling 3 Milling and milling methods 3.1 Milling 3.2 Milling methods 4 High speed machining 5 Research plan overview

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Example 1 135 6.1 Design of experiments 136 6.2 Experimental setup 137 6.3 Experimental procedure 140 6.4 Results and discussion 142 6.5 Conclusions 149 Example 2 151 7.1 Experimental plan and procedure 151 7.2 Experimental parameters and procedure 151 7.3 Experimental setup and procedure 153 7.4 Tool wear measurement 153 7.5 Measurement of surface roughness 153 7.6 Results and discussion 154 Future trends nano-fluids 159

1 Introduction This chapter will introduce the reader to latest research/case study on the effect of machining parameters, milling methods, and cooling strategies on machinability in high speed machining (HSM) of titanium alloys and nickel High Speed Machining. http://dx.doi.org/10.1016/B978-0-12-815020-7.00005-9 Copyright © 2020 Elsevier Inc. All rights reserved.

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super alloys. This chapter is organized as follows. Section 1 describes heat and temperature in dry metal cutting. In Section 2, cooling and lubrication strategies in high speed machining are discussed: subsections 2.1–2.4, conventional emulsion flood cooling and lubrication (EC), minimum quantity lubrication (MQL), cooling by liquid nitrogen (LN2), and combined (MQL+LN2) cooling. In Section 3, milling and milling methods are discussed (up-milling vs. down-milling): subsections 3.1–3.2; subsections 3.2.1–3.2.3, chip length, maximum chip thickness, and approach distance calculations. In Section 4, high speed machining ranges for machining various materials from plastics to the most difficult-to-cut nickel alloys are presented. Section 5 presents the author’s overview of research plan for investigation of cooling and machining strategies for high speed milling of titanium Ti-6Al-4V and Inconel-718. Section 6 discussed Example 1: experimental investigation of the effect of cooling strategies and machining parameters on cutting forces, temperature, and surface roughness in high speed slot-milling of Ti6Al-4V. Section 7 discussed Example 2: experimental investigation of the effect of milling methods in HSM of Inconel-718. Section 8. discussed future trends. The main objectives of the research effort presented are to investigate the effects of up-milling and down-milling methods and cooling strategies [emulsion flood cooling, MQL, LN2, and combined (MQL + LN2) cooling] on cutting force components (Fx, Fy, and Fz), resultant cutting force, Fr, tool wear, and surface roughness in slot-milling and high speed end-milling of titanium alloys and nickel super alloys, respectively. After reading this chapter, the reader would have better understanding about heat and temperature generation and extraction during machining difficult-to-cut metals, make proper selection of machining parameters, cooling strategy, and milling methods for environmentally friendly, cost-effective, and sustainable machining of Ti-6Al-4V and Inconel-718.

1.1  Heat and temperature in dry metal cutting Titanium and nickel super alloys like Ti-6Al-4V and Inconel-718, respectively, are used largely in aerospace and automotive industries to manufacture parts, such as turbine blades and disc compressors used in high heat and temperature sections of jet engines and nuclear reactors. These super alloys are suitable for hostile environment due to their excellent combination of peculiar characteristics like high strength-to-weight ratio, ability to retain their characteristics at high temperatures, high corrosion, and creep resistance [1]. These super alloys are some of the most “difficult-to-cut” metals because of the problems they pose during machining like: high heat generation at the cutting zone during machining because of high friction between the chips and cutting tool, and cutting tool and workpiece, which is not extracted rapidly as it is generated owing to their low thermal conductivity. Thus, most of the generated heat during machining stays at the cutting zone, which causes high tool wear and cutting forces. Because of the generated intense heat, some particles get welded to the cutting tool edge, acerbating tool wear. End-milling operations are used extensively in

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machining industry because of its versatility as they are used in small tool-die shop and big aerospace industry. There are some problems associated with endmilling such as large tool wear, large cutting forces, cutting zone temperature, tool chatter, and breakage. These problems are further exacerbated during machining difficult-to-cut metals when the desired quality characteristic is a major requirement.

2  Cooling and lubrication strategies in machining Cooling strategies contribute significantly to address high heat, temperature, and friction generation during machining of difficult-to-cut metals by enhancing lubrication and cooling the temperature at the cutting zone, and also blowout chips from the cutting zone.

2.1  Conventional emulsion flood cooling and lubrication The problems of high temperature and friction generated in end-milling of difficult-to-cut metals can be addressed with the application of conventional emulsion flood cooling (stabilized mixture of oil and water) at the cutting zone while machining, which serves as a coolant as it takes the heat away from the cutting zone, as well as serves as a lubricant as it reduces the friction between chip/tool and tool/workpiece interfaces, and that further helps to decrease cutting zone temperature, tool wear, cutting forces, friction between chip/tool and tool/workpiece interfaces, and improve surface finish and overall machinability. Conventional mineral oil-based emulsion flood cutting fluids (EC) used in machining industries are expensive, nonbiodegradable, cause adverse effect on the environment and the machinist, and difficult to dispose [2].

2.2  Minimum quantity lubrication (MQL) The problems posed by conventional coolants and lubricants can be addressed by application of minimum quantity lubrication (MQL) using vegetable oil as the base fluid instead of conventional emulsion flood cooling. MQL diminishes the amount of coolant used by mixing small quantity of biodegradable vegetable oil-based lubricants and pressurized air to form aerosol that is sprayed in mist form and small quantity to the cutting zone using a nozzle to lubricate and cool the cutting zone instead of EC flooding of the cutting zone. There is no disposal issue with MQL since the quantity of lubricant used is very small compared to conventional emulsion flood cooling and lubrication. Moreover, MQL using vegetable oil as base fluid is cheaper than EC, biodegradable, renewable, and does not cause any health hazard or environmental pollution, thus it is environmentally friendly. It also blows the chips away from the cutting zone. MQL cooling provides adequate lubrication at the cutting zone, which reduces the friction at the chip/tool and tool/workpiece interfaces, hence further reduces temperature at the cutting zone. Also, it has been seen from experiments

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that MQL improves machinability compared to conventional emulsion flood cooling [3,4]. Zhao et al. [5] showed that MQL improves machinability over dry machining with regard to measured cutting forces, surface roughness, and tool life in high speed milling of Ti-6Al-4V using uncoated cemented carbide inserts. Dhar et al. [6] reported that MQL cooling improves machinability over dry machining in turning AISI-1040 steel with regard to cutting temperature, cutting forces, tool wears, surface finish, and dimensional deviation. Kaynak [7] reported that MQL performed better than dry and LN2 machining at lower cutting speed of 60 m/min than at higher cutting speed of 120 m/min in machining of Inconel-718.

2.3  Cooling by liquid nitrogen (LN2) Problems caused by conventional emulsion flood cooling and lubrication can also be addressed by application of liquid nitrogen (LN2), which is commonly called cryogenic cooling. LN2 cooling applied to the cutting zone at subzero temperature, lowers the temperature at the cutting zone, and then evaporates into the atmosphere. Hence, there is no problem of disposal at all with LN2 cooling and also it does not cause any greenhouse effect. It has been seen from the literature that the application of LN2 to the cutting zone at subzero temperature improves machinability.

2.4  Combined (MQL + LN2) cooling Another alternative to address the issues associated with conventional emulsion coolants and lubricants would be to apply combined (MQL + LN2) cooling to the cutting zone. This technique combines the benefit of both MQL and LN2 cooling strategies. MQL provides efficient lubrication to the cutting zone, while LN2 provides efficient cooling and reduction of temperature at the cutting zone, thus combined (MQL + LN2) cooling provides the ideal lubrication-cooling strategy for cutting difficult-to-cut metals like titanium and nickel alloys.

3  Milling and milling methods 3.1 Milling Milling is one of the most versatile machining processes used extensively in machining prismatic parts in automotive and aerospace industries using a multitooth tool called milling cutter that rotates along its various axes with respect to the workpiece, and produces a number of chips per revolution. Milling difficultto-cut metals such as Inconel-718 and titanium Ti-6Al-4V exacerbates the problems encountered in milling conventional metals. Due to the problems mentioned earlier, care must be taken to select appropriate machining parameters for machining difficult-to-cut metals, and to decrease temperature and friction at the cutting zone, tool wear, and cutting forces, as well as to enhance surface quality and productivity.

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3.2  Milling methods Another important machining parameter that contributes significantly to improve machinability of titanium and nickel super alloys is milling method used: up-milling versus down-milling. The mechanics of chip formation showing cutter tooth entry angle in down-milling operation and cutter tooth exit angle in up-milling operation, maximum chip thickness, and chip length can be understood from Fig. 5.1. As can be seen, the workpiece is being fed along the feed direction and the cutting tool is rotating in a clockwise direction in both up- and down-milling operations.

3.2.1  Up-milling operation In up-milling operation, the cutter rotates up-against the workpiece (table) feed direction. Also, the cutter tooth enters the workpiece at 0 degree angle with

FIGURE 5.1  Chip formation showing (A) chip formation showing cutter tooth entry angle in down-milling and cutter tooth exit angle in up-milling, (B) maximum chip thickness, hmax, and (C) chip length, Lc.

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reference to the negative x-axis and exit the workpiece with angle of Øexit with reference to the negative x-axis. The chip thickness is at a minimum, 0, at tool entry into the workpiece, and it increases to a maximum at the tool exit from the workpiece.

3.2.2  Down-milling operation In down-milling operation, the cutter rotates in the same direction of the workpiece (table) feed, and cutter tooth enters the workpiece at angle Øentry with respect to the negative x-axis and exit the workpiece at 180 degree angle with respect to the negative x-axis. The chip thickness is maximum at the tooth entry into the workpiece, whereas it is minimum when the tooth exits the workpiece. It has been shown that down-milling operation improves machinability over upmilling when machining nickel alloy at high speed [3,4]. 3.2.3  Cutting mechanism and parameters in down- and up-milling Fig. 5.1(A) shows the cutter tooth entry angle in down-milling and cutter tooth exit angle in up-milling. Fig. 5.1(B) shows the maximum chip thickness hmax, and Fig. 5.1(C) shows the chip length. From Fig. 5.1(A), the cutter tooth entry and exit angles for up- and downmilling, and the maximum chip thickness (hmax) can be calculated from the following equations, the details of which are shown in [3,4]: 3.2.3.1  For down-milling The tooth always exit the workpiece at 180 degree measured clockwise from the negative x-axis. (1) φexit = 180 degree And the entry angle, r − ar  (2) φentry = 180 − arccos   r  where r is the cutter radius and ar is the depth of cut in the radial direction. 3.2.3.2  For up-milling The cutter tooth always penetrates into the workpiece at 0 degree angle measured clockwise from the negative x-axis. (3) φentry = 0 degree Exit angle can be calculated as follows: r − ar  (4) φexit = arccos   r 

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The shear action of the tool while machining creates the chips of specific length and thickness depending upon the chip load (ft) and radial depth of cut (ar). 3.2.3.3  Maximum chip thickness The maximum chip thickness hmax is calculated using radial depth of cut ar, radius of tool r, and feed per tooth ft as follows: r − ar    (5) hmax = ft Sin arccos    r    3.2.3.4  Chip length The chip length is calculated as follows: (6) Lc = 2 rar + 2 ft 2 rar − ar2 + ft2 Chip length after each pass was also measured using digital microscope. The measured chip length was compared with calculated theoretical values for upand down-milling methods and cooling strategies investigated. 3.2.3.5  Approach distance, L1 The cutter tooth starts cutting in up- and down-milling when the center of the milling cutter is at an approach distance L1 from the edge of the workpiece as shown in Fig. 5.2.

FIGURE 5.2  Approach distance in down- and up-milling.

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The distance L1 is given as follows: L1 = r 2 − ( r − ar ) (7) 2

4  High speed machining Introduction of HSM processes into the manufacturing environment promises significant improvements in productivity, product quality, and significant concomitant reductions in manufacturing costs; therefore, HSM is considered as the most efficient manufacturing technology [8]. HSM range is dependent on the material being cut [9]. High speed cutting range for nickel alloy is lower than high speed cutting range for other materials such as aluminum or steel and titanium, because Inconel-718 has higher strength which makes it much more difficult to machine. Abele and Fröhlich [9] reported that HSM of titanium alloy reduced cutting forces and improved performance over conventional machining by increased material removal rate. Liao et al. [10] recommended high speed range from 55 to 135 m/min for peripheral milling of Inconel-718. Fig. 5.3 shows the commonly accepted cutting speed ranges for various materials including titanium and nickel alloys [11]. It is evident that HSM range depends on the machinability and strength of the workpiece material. High strength and more difficult to machine the materials require cutting at lower zone of high speed [3]. Nickel alloys have the lowest HSM range as compared to other materials like aluminum and fiber-reinforced plastics listed, since they are the most difficult-to-cut materials.

FIGURE 5.3  Cutting speed ranges for machining various materials including titanium and nickel alloys [11].

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FIGURE 5.4  Overview of research plan for investigating machining and cooling strategies for high speed milling of titanium Ti-6Al-4V and Inconel-718 super alloys.

5  Research plan overview The overview of current research plan for investigating machining and cooling strategies for HSM titanium and nickel alloys is shown in Fig. 5.4. The effects of conventional emulsion cooling, MQL, cryogenic cooling (LN2), combined (LN2 + MQL) cooling and lubrication strategies, milling methods, and tool coatings on machinability of Inconel-718 and titanium Ti-6Al-4V are investigated under varying machining parameters (cutting speed and feed rate) using solid carbide helical bullnose end-mills used in aerospace manufacturing. The machinability parameters investigated are the cutting forces, cutting temperature, tool wear, tool life, and surface finish. The research details are presented in [12] and Okafor and Jasra [3,4].

6  Example 1 This section presents the experimental investigation of the effects of machining parameters and cooling strategies on cutting forces, temperature, and surface roughness in high speed slot-milling of Ti-6Al-4V. This example discusses the results of experimental evaluation of emulsion cooling, MQL, and LN2 cooling, three spindle speeds, 1000, 1500, and 2000 rpm, and three feed rates, 150, 300, and 450 ipmm (6, 12, and 18 ipm) on cutting forces, Fx, Fy and Fz, cutting temperature, tool wear, and surface rough-

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ness in slot-milling of titanium Ti-6Al-4V, using design of experiment and analysis of variance (ANOVA). A one-third fractional factorial design of experiment was used to study these three factors each at three levels. Also the results of a comparative investigation of MQL, LN2, and combined (MQL + LN2) cooling strategies at determined optimum spindle speed of 2000 rpm and feed rate of 150 mmpm (6 ipm) are also presented. End-milling experiments were conducted using four-flute 0.5 in. diameter uncoated solid carbide bullnose endmill on vertical machining center (Cincinnati Milacron Sabre 750), equipped with Acramatic 2100 controller. An ungrounded K-type thermocouple probe of 0.125 in. diameter was inserted into holes drilled into the workpiece to measure temperature of the workpiece close to the cutting zone. A cryogenic flow line was designed to regulate the flow and temperature of the cryogenic LN2 from −196 °C to −15 °C at the cutting zone, while an MQL applicator was used to supply microdroplets of Accu-lube LB-2000 biodegradable vegetable oil to the cutting zone. The workpiece was mounted on Kistler 9272 4-component force dynamometer that was clamped on the machine vice, and used to acquire signals of the cutting force component. Tool wear was indirectly measured from maximum cutting force variation for each slot from the first pass to the eighth pass. ANOVA shows that cooling method, spindle speed, and feed rate all have significant effect on cutting forces (Fx, Fy, and Fz) and that there is only one significant interaction effect of cooling method and spindle speed on perpendicular to the feed force, Fx. Among three cooling strategies investigated, MQL was found to be the best cooling/lubrication strategy that generated the lowest cutting force, followed by LN2, while the highest cutting force components were generated by emulsion cooling. LN2 cooling was the best cooling strategy that generated the lowest cutting temperature and indirect tool wear as measured by cutting force components. MQL, at 2000 rpm spindle speed, and 150 mmpm (6 ipm) feed rate are the optimum cooling strategy and machining parameters that generated the lowest cutting force components (Fx, Fy, and Fz) for slot-milling of titanium alloy Ti-6Al-4V. Cooling strategy is the dominant factor controlling cutting/workpiece temperature, followed by feed rate. LN2 cooling, 1000 rpm spindle speed, and 450 mmpm (18 ipm) feed rate are the optimum cooling strategy and machining parameters that generated lowest maximum cutting/workpiece temperature. Feed force, Fy, is the most responsive to tool wear as it increases more rapidly than the other two cutting force components Fx and Fz, thus it is recommended to be used for indirect tool wear monitoring. LN2 cooling, 2000 rpm spindle speed, and 150 mmpm (6 ipm) feed rate are the optimum cooling strategy and machining parameters that generated lowest cutting/workpiece temperature and tool wear. Combined (MQL + LN2) cooling strategy generated the lowest cutting force over that by stand-alone MQL and LN2 cooling strategy.

6.1  Design of experiments Three factors at three-level, 33, full factorial experimental design requires 27 experimental runs to be conducted. With replication, this will require 54 test runs.

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The objectives of the experiment are to study the effects of machining parameters and cooling strategies on cutting forces, cutting temperature, tool wear, and surface finish in slot-milling of titanium, Ti-6Al-4V, and determine optimum machining parameters and cooling methods to reduce cutting forces, workpiece/ cutting temperature, tool wear, residual stresses, and surface finish during slotmilling of titanium alloy Ti-6Al-4V, using uncoated bullnose solid carbide helical end-mill. Three levels of spindle speed (1000, 1500, and 2000 rpm), three levels of feed rate (150, 300, and 450 ipmm) (6, 12, and 18 ipm), and three levels of cooling strategies (conventional emulsion, MQL, and LN2) were the three factors selected for evaluation as shown in Table 5.1. Axial and radial depth of cut were kept constant at 3.125, 12.5 mm (0.125 and 0.5 in.), respectively, and also corner radius of 7.5 mm (0.3 in.) was kept constant for all the bullnosed end-mills used throughout the experiments. These machining parameters chosen were based on optimum machining conditions in a previous work done by Okafor and Aramalla [13]. Each factor is investigated at three levels to determine the optimum setting for the slot-milling process. The one-third fraction of the 27 treatment combinations is shown in Table 5.2.

6.2  Experimental setup A photograph of the complete experimental setup showing the vertical machining center, cutting force data acquisition system, MQL, and LN2 cooling systems is shown in Fig. 5.5. The slot-milling experiments were conducted on a Vertical Machining Center (VMC) (Cincinnati Milacron Sabre 750 VMC) with Acramatic 2100 controller. The workpiece materials used are two rectangular blocks of titanium alloy Ti-6Al-4V, 6 in. long × 3 in. wide × 1.5 in. thick (152.4 × 76.2 × 38.1 mm). Two sets of experiments were conducted on each block. For the first block, Block A, all the experimental runs using conventional emulsion and MQL cooling strategies were carried out on it with three slots on one half of the block for conventional emulsion cooling (experimental runs 1, 2, and 5) and another three slots on the second half of the block for MQL cooling strategy (experimental runs 3, 6, and 7), all at different combinations of machining parameters as shown in Table 5.2. The second

TABLE 5.1 Experimental factors and levels. Factors Level

Cooling methods

Speed (RPM)

Feed (IPM) [mm]

0

Emulsion

1000

6 [150]

1

MQL

1500

12 [300]

2

LN2

2000

18 [450]

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TABLE 5.2 One-third fractional factorial experimental design. Three factors at three levels Experimental runs

Treatment combinations

Cooling methods

Speed (RPM)

Feed (IPM) [mmpm]

1

(0,0,0)

Emulsion

1000

6 [150]

2

(0,1,2)

Emulsion

1500

18 [450]

3

(1,0,1)

MQL

1000

12 [300]

4

(2,0,2)

LN2

1000

18 [450]

5

(0,2,1)

Emulsion

2000

12 [300]

6

(1,1,0)

MQL

1500

6 [150]

7

(1,2,2)

MQL

2000

18 [450]

8

(2,1,1)

LN2

1500

12 [300]

9

(2,2,0)

LN2

2000

6 [150]

block, Block B, has the experimental runs (slots) for LN2 cooling strategy (experimental runs 4, 8, and 9) on one half of the block and on the second half of the block three slots were machined for a comparative evaluation of MQL, LN2, and combined (MQL + LN2) cooling strategies using the identified optimum machining parameters from the ANOVA of fractional factorial experimental design. The workpiece geometry and dimensions are shown in Fig. 5.6. Four-flutes uncoated solid carbide helical end-mills of 0.5 in. (12.7 mm) diameter, 0.5 in. (12.7 mm) shank diameter, 1 in. (25.4 mm) flute length, and 3 in. (76.2 mm) overall length with 0.03 in. (0.76 mm) corner radius were used. A corner radius of 0.03 in. was chosen to give minimum cutting force values as reported by Okafor and Aramalla [13]. New endmills were used for each experimental run (slot) to exclude tool wear effect, and a total of 12 end-mills were used for all experiments. The spindle speed range and feed rate range were selected to suitably cover the recommended range of machining conditions for titanium alloy, while cooling methods and cryogenic temperature of −15 °C were selected from recommendations in the published literature. The cutting conditions chosen for the experiments were spindle speeds: 1000, 1500, and 2000 rpm (39.9, 59.85, and 79.8 m/min), feed rate: 6, 12, and 18 ipm (150, 300, and 450 mm/min), axial depth of cut: 0.125 in. (3.125 mm), and radial depth of cut: 0.5 in. (12.5 mm). Experimental setup for the cooling strategies investigated consists of cryogenic LN2 flow line built in-house at Missouri S&T for LN2 cooling, conventional emulsion flood coolant supplied from the VMC coolant tank, and Acculube MQL precision box applicator for MQL cooling using Accu-lube LB2000 vegetable oil coolant.

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FIGURE 5.5  Photograph of complete experimental setup showing VMC, force data acquisition system, MQL, and LN2 cooling systems. (A) Photograph of complete experimental setup, data acquisition system, MQL, and cryogenic flow line, and (B) picture of MQL, LN2, and conventional cooling supplied to the cutting zone.

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Before the slot end-milling tests, workpiece materials were prepared by drilling two holes with two counter bores. The holes were tapped to allow tight clamping of the workpiece to the KISTLER force dynamometer with threaded bolts. Clamping the workpiece on the dynamometer through threaded holes on the workpiece will prevent play and extraneous noise effects being superimposed on the acquired force and temperature signals.

6.3  Experimental procedure Slot-milling experimental runs were performed at cutting speeds, feed rates, and cooling strategies discussed in Section 6.2. The table feed was in the negative yaxis, applying the right-hand rule. A total of eight machining passes were made for each machined slot, for the 1 in. (25.4 mm) deep slots.

6.3.1  Cutting force acquisition and surface roughness measurement For each slot-milling pass, the voltage proportional signals obtained by the dynamometer were separated through a Kistler type 5405A breakout junction box into three cutting components (Fx, Fy, and Fz) signals, and then passed through three separate Kistler type 5010B amplifiers, then through low pass filters set at 680 Hz to remove extraneous noise from the slot-milling. The filtered cutting force signals were sent to a digitizing oscilloscope (Tektronix TDS 420A) and digitized at a 2.5 KHz sampling frequency, Fs, using 5000 sampling points (N) per signal, which gives a record length T = 2 s. The acquired cutting force signals were further processed and analyzed. The cutting force components were acquired at every pass when the end-mill has traveled about 1 in. (25.4 mm) into the slot. A new end-mill was used per slot and forces acquired at pass numbers 1 and 2, and 7 and 8 were treated as replicates for purpose of ANOVA and error estimation for sharp and worn tools, respectively. Forces acquired for pass numbers 1, 2, 4, 5, 7, and 8 were used for indirect tool wear monitoring per machined slot and cooling strategy. 6.3.2  Workpiece temperature measurements The design of the workpiece block and slots is shown in Fig. 5.6. Eight 0.125 in. (3.175 mm) diameter, 1 in (25 mm) deep holes were drilled on the front sides of all the titanium Ti-6Al-4V blocks at the middle of each slot at 0.25 in. (6.35 mm) from the bottom of the block. An ungrounded K-type thermocouple probe of 0.125 in. (3.125 mm) diameter was inserted into the drilled holes to ensure snug fit. The thermocouple probe was connected to a National Instrument NI USB-9211A Data Acquisition Device (DAQ) for thermocouple and the DAQ device was connected to a desktop computer. Temperature signals were acquired using LabVIEW Software. The maximum temperature experienced at the cutting tool–workpiece interface was recorded for every machining pass at every experimental run. The workpiece was allowed to cool for 24 h before starting the next experimental run.

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FIGURE 5.6  Workpiece design for slot end-milling experiments. (A) 3D view of workpiece with machined eight slots and eight drilled thermocouple holes, and (B) top and side views of workpiece with dimensions and hatched section.

6.3.3  Comparative evaluation for optimum machining parameters Based on the results of the design of experiment investigation that established optimum cooling strategies and machining parameters, convention emulsion cooling was eliminated and a comparative investigation of MQL, LN2, and combined (MQL + LN2) cooling strategies was conducted at determined optimum machining parameters of 2000 rpm spindle speed, and feed rate of 150 mmpm (6 ipm), to further determine the overall optimum cooling strategy.

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6.4  Results and discussion 6.4.1  Cutting force and workpiece temperature data processing and analyses The acquired cutting force signals, Fx, Fy, and Fz, were processed by applying 10-point moving average to filter out some of the random noises and bring out the major patterns in the time domain signal. Fig. 5.7 shows the processed cutting force signals, Fx, Fy, and Fz of 2 s record length for experimental run number 3 (MQL). Absolute maximum cutting force values were calculated for every machining pass. The absolute maximum cutting force values for pass numbers 1 and 2 were used as first and second replicates to investigate the effects of speed, feed, and cooling methods on cutting force, temperature, and indirect tool wear through ANOVA. The acquired cutting force component data for the seventh and eighth passes of each experimental run were used

FIGURE 5.7  Measured cutting force components (Fx, Fy, and Fz) for experimental run #3 (MQL) @ n = 1000 rpm, F = 12 ipm. (A) Graph of cutting force component Fx for run 3 (MQL, n = 1000 rpm, F = 12 ipm), (B) graph of cutting force component Fy for run 3 (MQL, n = 1000 rpm, F = 12 ipm), and (C) graph of cutting force component Fz for run 3 (MQL, n = 1000 rpm, F = 12 ipm).

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to analyze indirect tool wear. The seventh and eighth passes are treated as first and second replicates, respectively, for indirect tool wear analysis using ANOVA.

6.4.2  Statistical analysis for cutting force (Fx, Fy, and Fz) ANOVA, Pareto chart, marginal means plots, and surface/contour desirability plots were used to statistically analyze the main and interaction effect of the speed, feed, and cooling methods on cutting forces. ANOVA: ANOVA tables were created which show the main effects, the measurable two-factor linear by linear interactions, the P-values, P, for each factor level and interactions, along with the sum of square effects, SS, degree of freedom, DF, mean square, MS, and F-values, F. A low P-value, that is, P < 0.05, indicates that the factor has statistically significant effect on the respective response. The 0.05 indicates 5% significant level. Pareto charts: The corresponding Pareto charts for the ANOVA tables are given in Fig. 5.8 (A–C) for Fx, Fy, and Fz. The effects of the investigated factors are divided into linear and quadratic main effect, and linear x linear two-factor interactions, with all effects listed in their order of significance. The ANOVA table for cutting force components, Fx, Fy, and Fz, shows that main effect of spindle speed, cooling method, and feed rate all have significant

FIGURE 5.8  Pareto charts for Fx, Fy, and Fz. (A) Pareto chart for Fx using force data for the first and second passes.

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FIGURE 5.8 (Cont.) (B) Pareto chart for Fy using force data for the first and second passes, and (C) Pareto chart for Fz using force data for the first and second passes.

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effect on Fx, Fy, and Fz at 5% significant level. The interaction effect of cooling method and spindle speed is significant only on the feed force, Fy. Desirability surface/contour plot: Desirability surface/contour plots are shown in Fig. 5.9, which shows the levels of the main effects of cooling strategy, spindle speed, and feed rate that will produce the most desirable responses on cutting force components. The first plot of Fig. 5.9 shows the range of desirable values and the effects of cutting speed and cooling strategy on cutting force components. It is seen that desirable low (optimum) cutting force components are produced by cooling method level of MQL in combination with high spindle speed of 2000 rpm. The next plot of feed rate and cooling method shows that low feed level of 150 mm (6 ipm) in combination with either cooling method level of MQL or LN2 is the optimum combination of feed rate and cooling method that produce the most desirable low level of cutting force components. The third plot shows the surface plot of feed rate and spindle speed. It is seen that the desired optimum combination of feed rate and spindle speed that produces the most desirable low level of cutting force components was at low feed rate level of 150 mmpm (6 ipm) in combination with high spindle speed level of 2000 rpm.

FIGURE 5.9  Desirability surface/contour plots for cutting force components.

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6.4.3  Comparative evaluation between cooling strategies Fig. 5.10 presents the bar charts of absolute maximum cutting force components, Fx, Fy and Fz, obtained during comparative slot-milling at the identified optimum spindle speed of 2000 rpm, feed rate of 150 mmpm (6 ipm), MQL, LN2 cooling, and combined (MQL + LN2) cooling strategies for the first and eighth passes during slot-milling of titanium alloy Ti-6Al-4V. The chart shows that stand-alone LN2 cooling method generated the highest cutting forces (Fx, Fy, and Fz) and thus highest indirect tool wear. Combined (MQL + LN2) cooling generated the least cutting forces (Fx, Fy, and Fz) and indirect tool wear, while stand-alone MQL came second best in generating lower cutting force components indirect tool wear. The demonstration that combined (MQL + LN2) cooling generates lowest cutting forces is one of the novel contributions of this research. 6.4.4  Measurement of surface roughness Portable Brown and Sharpe pocket surf with a measuring range of 0.3–6.35 µm was used to measure surface roughness of machined slots. Measurements were made for a traverse of length of 5 mm at 5.08 mm/s speed. The roughness measurements were made along the direction of feed on the web at three different locations that are of 25 mm (1 in.) space from each other at each experimental run/slot. The average of the three measured surface roughness values was recorded and used for further analysis. For surface roughness, the measurements

FIGURE 5.10  Bar charts of absolute maximum cutting forces, Fx, Fy, and Fz for optimum machining parameters [spindle speed of 2000 rpm, feed rate of 150 mmpm (6 ipm)], optimum cooling strategy (MQL and LN2), and combined (MQL + LN2) cooling for the first and eighth passes.

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were on the left rib walls and on the web (looking from the side of the drilled holes for thermocouple probes).

6.4.5  Analysis of surface roughness Average measured surface roughness of the machined slot was analyzed using fractional design of experiment to obtain optimum machining conditions. Results of the fractional factorial design of experiment and comparative analysis of the optimum machining parameters and cooling strategies are presented. 6.4.6  Effects of machining parameters and cooling methods on surface roughness In this study, the slot rib and web surface roughness were measured. The average surface rough values on the ribs were found to be significantly higher than on the web; for this reason, attention was focused on the rib to find the best way to improve surface finish on the rib using ANOVA table, Pareto charts, and marginal means of effects plot from fractional factorial design of experiment. The surface roughness desirability plot in Fig. 5.11 shows the three-dimensional plots of surface roughness on the vertical scale (Z plane) versus two factors on the horizontal (X-Y plane) scale. From the desirability surface contour plot of Fig. 5.11, the first plot shows the combined effects of cooling methods and spindle speed on average surface

FIGURE 5.11  Surface roughness desirability surface/contour plots.

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roughness on the web and rib. From the first plot, MQL cooling is the best cooling strategy at all spindle speed levels considered for this experiment. For MQL cooling strategy, the best spindle speed that generates the lowest surface roughness is found to be 2000 rpm. The worst surface roughness is produced by the combination LN2 at high spindle speed of 2000 rpm. The second plot on Fig. 5.11 shows the effects of different levels of cooling method and feed rate on measured surface roughness on the web and rib. From the plot, MQL cooling strategy at low feed rate of 150 mmpm (6 ipm) produced the best surface finish, next was MQL cooling strategy at high feed rate of 450 mmpm (18 ipm). Emulsion cooling and LN2 cooling at medium feed rate of 300 mmpm (12 ipm) are the worst combinations. The third plot shows that low feed rate of 150 mmpm (6 ipm) at both low and high spindle speed of 1000 rpm and 2000 rpm, respectively, are the best combination for best surface finish on slot-milling Ti-6Al-4V. Also, feed rate of 450 mmpm (18 ipm) at low speed gave a better surface finish. Medium feed rate of 300 mmpm (12 ipm) at all speed levels gave the worst surface finish. Fig. 5.12 presents bar charts of measured average surface roughness generated at constant spindle speed of 2000 rpm and constant feed of 150 mmpm (6 ipm) for MQL, LN2, and the combined (LN2 + MQL) cooling methods. The fourth comparative plot is for emulsion cooling at 2000 rpm spindle speed but using a feed rate of 12 ipm instead of 6 ipm. This is because fractional factorial design of experiment was used, which does not have the experimental run at low level of cooling using high level of speed and low level of feed rate, and the closest in the list of experimental run in this experimental design is low level of cooling (emulsion) at medium feed rate and high spindle speed. We have plotted these charts for comparative evaluation of the effects of three cooling methods

FIGURE 5.12  Average surface roughness values for optimized cutting and cooling conditions.

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on surface roughness generated on the rib and web of machined slots in titanium Ti-6Al-4V block using constant spindle speed and feed rate. The bar charts show that combined (MQL + LN2) cooling generated the best surface finish both on the web and on the rib below 0.5 and 1.25, respectively. MQL generated the poorest surface finish both on the ribs and webs; next to MQL was LN2 that generated significantly low average surface roughness both on the web and on the ribs compared to MQL. The charts show that generated surface roughness on the rib and web using LN2 is less than 1.5 µm and 1 µm, respectively. When the average surface roughness values of the optimized cutting and cooling conditions were compared with cutting conditions of 12 ipm and 2000 rpm using emulsion cooling, the surface roughness average on the web was higher than those for both LN2 and combined (MQL + LN2), but appeared to give a good surface finish on the rib. Therefore, it may be safe to focus attention on surface roughness on the rib for improvement of surface finish as the experiment has indicated that there is significant increase in surface roughness average on the rib for all conditions. This also indicates that the rib will be part of the slot with possible high-induced residual stresses.

6.5 Conclusions 6.5.1  Cutting forces From the results of the investigation, the following conclusions can be drawn: 1. A cryogenic flow line was designed, built, and calibrated at Missouri S&T that is used to regulate the temperature of cryogenic LN2 from a temperature of −196°C to −15°C at the exit of the nozzle during slot-milling of difficultto-cut metals Ti-6Al-4V using bullnose solid carbide end-mill. 2. ANOVA shows that cooling strategy, spindle speed, and feed rate all have significant effects on cutting forces, Fx, Fy, and Fz, and that interaction effect of cooling method and spindle speed has significant effect only on the feed force Fx. 3. Among the three cooling strategies investigated, MQL is the best cooling and lubrication method that generates lowest cutting force components, followed by LN2, while emulsion cooling generates highest cutting forces. 4. LN2 is the best cooling method that generates lowest temperature, thus reduces tool wear in slot-milling titanium alloy Ti-6Al-4V. 5. MQL at 2000 rpm spindle speed and 150 mmpm (6 ipm) feed are identified as optimum machining parameters and cooling strategy that generate lowest cutting forces, Fx, Fy, and Fz, but this combination generates the highest cutting/workpiece temperature than LN2 cooling and combined (MQL + LN2) cooling throughout the length of cut for slot-milling of Ti-6Al-4V. 6. Perpendicular to feed force, Fx, is higher than the other cutting force components Fy and Fz, while the axial force Fz is the least cutting force components and remains fairly stable for all the cutting passes.

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7. Cooling strategy is the dominant and statistically most significant factor that affects workpiece temperature, followed by feed rate. 8. LN2 cooling at 1000 rpm spindle speed and 450 mmpm (18 ipm) feed rate are the optimum cooling strategy and machining parameters that generate the lowest maximum cutting temperature. 9. LN2 cooling at 2000 rpm spindle speed and 150 mmpm (6 ipm) feed rate are the optimum cooling strategy and machining parameters that generate lowest workpiece/cutting temperature and indirect tool wear, followed by combined (MQL + LN2) cooling. 10. Comparative evaluation of MQL, LN2, and combined (MQL + LN2) cooling strategies at optimum machining parameters of 2000 rpm and 6 ipm show that combined (MQL + LN2) cooling strategy generated lowest cutting forces, Fx, Fy, and Fz, and lowest workpiece/cutting temperature for the first pass (76.2 mm length of cut) and last passes (609.6 mm length of cut) in slot-milling of Ti-6Al-4V.

6.5.2  Surface roughness 1. Emulsion cooling produced the highest average surface roughness on the web, while MQL produced the lowest average surface roughness value on the web, and LN2 produced the next best average surface roughness on the web. On the rib, the same was observed with MQL being preferred over LN2 and emulsion cooling, but the differences between the average surface roughness values measured on the rib using the different cooling strategies are not much. 2. Low speed of 1000 rpm gave the highest average surface roughness values on the web, while medium and high speed of 1500 rpm and 2000 rpm, respectively, are the best spindle speed levels for low surface roughness on the web. On the rib, low speed of 1000 rpm produced the lowest average surface roughness value, while medium and high speed of 1500 and 2000 rpm, respectively, produced the worst surface roughness. 3. Low feed rate of 150 mmpm (6 ipm) produced the best average surface roughness on the web, while high feed rate of 450 mmpm (18 ipm) produced the worst surface finish on the web. On the rib, low feed rate of 150 mmpm (6 ipm) and high feed rate of 450 mmpm (18 ipm) produced the best surface finish, while medium feed rate level of 300 mmpm (12 ipm) produced the worst surface finish on the rib. 4. Combined (MQL + LN2) cooling produced the best surface finish both on the web and on the rib. On the web, the average surface roughness was less than 0.5 µm, while on the rib the average surface roughness is slightly lower when compared with the value from both MQL and LN2 cooling strategies. Attention should be focused on surface roughness on the rib for improvement slot wall, as the experiment has indicated that there is significant increase in surface roughness on the rib for all conditions.

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7  Example 2 This section presents the investigation of cooling methods, up-milling, and down-milling methods in HSM of Inconel-718. In this section, results of experimental investigation of up-milling and downmilling process, and cooling methods [conventional emulsion, MQL, LN2, and combined (MQL + LN2)] to improve machinability and reduce cost in high speed peripheral milling of Inconel-718 are presented. Machinability parameters investigated include tool flank wear and surface roughness. Down-milling generated least amount of flank wear than up-milling for all four cooling strategies, therefore down-milling improves machinability over up-milling. Downmilling with MQL cooling strategy generated least flank wear and is therefore recommended for machining Inconel-718, while LN2 with up-milling generated greatest flank wear. The mechanism of tool wear in up-milling is adhesion and failure modes are chipping and plastic deformation, while the mechanism of tool wear in down-milling is abrasion. Up-milling produces longer chips than down-milling, which produces shorter chips. Up-milling with emulsion flood cooling generated lowest (best) surface roughness, while down-milling with emulsion, MQL, and combined (MQL + LN2) cooling strategies generated equal amount and next lower surface roughness.

7.1  Experimental plan and procedure High speed peripheral milling tests were conducted on a Vertical Milling Machine (VMC), Cincinnati Milacron Sabre 750 VMC, using 12.7 mm (0.5 in.) diameter uncoated solid carbide, four-flutes helical bullnose end-mills.

7.2  Experimental parameters and procedure The experimental plan of the investigation is shown in Table 5.3. All the experiments were performed at constant machining parameters of 1127 rpm spindle speed (45 m/min), 114.5 mm/min feed rate (4.510 ipm), axial depth of cut aa = 5.08 mm (0.2 in.), and radial depth of cut ar = 3.81 mm (0.15 in.); these are selected based on review of literature and prior research investigations. The cutting parameters, aa, ar, and feed per revolution (fr) were kept constant and selected to enhance productivity without adversely sacrificing tool life. Each experiment consists of eight radial passes, each of 76.2 mm (3 in.) length of cut. Four cooling methods [conventional emulsion, MQL, LN2, and combined (MQL + LN2)] were evaluated under up-milling and down-milling methods. A total of eight uncoated carbide end-mills and two workpiece blocks of Inconel-718 were used. Each experiment was replicated resulting to a total of 16 experiments. Fig. 5.13(A) shows workpiece 1 and experiments #1, 2, 5, and 6 performed on it. Fig. 5.13(B) shows workpiece 2 and experiments #3, 4, 7, and 8 performed on it. Milling experiments with emulsion and MQL cooling methods

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FIGURE 5.13  Schematic of experiments performed on workpieces. (A) Workpiece 1 for experiments #1, 2, 5, and 6, and (B) workpiece 2 for experiments #3, 4, 7, and 8. All dimensions in mm (inches).

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TABLE 5.3 Experimental plan of the investigation of the effect of up- and down-milling and cooling methods on machinability in high speed endmilling of Inconel-718. Milling method Cooling strategy

Depths of cut Cutting Feed per MRR UpDown- Block speed revolution Axial Radial (mm3/ milling milling no. (m/min) (mm/rev) (aa) mm (ar) mm min)

Emulsion

Exp. #1 Exp. #2 1

45

0.1016

5.08

3.81

2216.2

MQL

Exp. #3 Exp. #4 2

45

0.1016

5.08

3.81

2216.2

LN2 (−15 °C)

Exp. #5 Exp. #6 1

45

0.1016

5.08

3.81

2216.2

MQL + LN2 Exp. #7 Exp. #8 2 (−15 °C)

45

0.1016

5.08

3.81

2216.2

were performed prior to milling experiment with LN2 cooling or combined (MQL + LN2) cooling to avoid hardening effect on the workpiece and tool by LN2 cooling. The results were analyzed to find the better milling method between up-milling and down-milling, and best cooling method among conventional emulsion, MQL, LN2, and combined (MQL + LN2) cooling.

7.3  Experimental setup and procedure The prepared workpiece was clamped on a Kistler 4-component dynamometer clamped in the machine vice on VMC and eight radial milling passes were performed for each experimental run as shown in Table 5.3. Cutting forces were acquired during each pass and analyzed later. The operation was stopped after each pass, chip samples were collected for analysis, and maximum wear on the flank were measured off the VMC. Surface roughness of machined surfaces was also measured after the end of each experiment. Cutting force analysis is not under the scope of this book chapter.

7.4  Tool wear measurement After completion of each machining pass, the operation was stopped and the maximum wear on the flank was measured using Keyence VHX-5000 digital microscope.

7.5  Measurement of surface roughness Measurement of surface roughness of machined surfaces was made parallel and perpendicular to feed directions using a portable Brown and Sharpe pocket surf profilometer. For each experiment, average of three surface roughness

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measurements, Ra, at three different locations that are 25.4 mm (1 in.) apart from each other was calculated.

7.6  Results and discussion 7.6.1  Maximum flank wear for milling under emulsion cooling Fig. 5.14 shows the plot for maximum flank wear versus number of passes (machined length) for up- and down-milling methods under emulsion cooling strategy. The plot shows that maximum flank wear in emulsion under down-milling method is less than that in up-milling throughout the machining length and increases gradually with number of passes (machined length). The initial flank land and progressive flank wear after first, fifth, and eighth passes under emulsion cooling strategy for up- and down-milling methods are shown in Fig. 5.15. As can be seen from Fig. 5.15, tool wear under emulsion up-milling operation is very high as compared to emulsion with down-milling method. The formation of notch wear can also be seen from the first pass of up-milling operation. The mechanism of tool wear in up-milling is chipping of tool edge and notch wear which leads to increase of the contact area between chip and tool edge. This further increases tool wear in successive passes. During down-milling with emulsion cooling, tool wear increases gradually due to increase in tool edge and chip contact area with progression of time. Also, no chipping or notch wear was observed throughout the machining in down-milling operation and mechanism of tool wear in down-milling is abrasion. The reason of high flank wear in up-milling can be explained with the help of chip formation during up- and down-milling operation as shown in Fig. 5.16.

FIGURE 5.14  Maximum flank wear versus number of passes (machined length) for up- and down-milling under emulsion cooling strategy.

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FIGURE 5.15  Initial flank land and progressive flank wear for up- and down-milling under emulsion cooling strategy. Emulsion up-milling at (A) pass 0, (B) pass 1, (C) pass 5, and (D) pass 8, and emulsion down-milling at (E) pass 0, (F) pass 1, (G) pass 5, and (H) pass 8.

During up-milling operation, the chip thickness at the entry of the tooth into the workpiece is zero and keeps on increasing till the tooth comes out of the workpiece. During penetration of the tool into the workpiece, flank face rubs against the workpiece. This rubbing of the tool against the workpiece causes excessive adhesive tool wear during up-milling operation. Also, the rubbing at tool engagement (entry into the cut) causes an excessive work hardening of the layer of the newly exposed surface of the workpiece [14]. During rotation of tool, the next tool flute penetrates into the work-hardened layer generated by previous rotation of the tooth, further exacerbates tool wear. For down-milling operation, the thickness of the chips at tool entry into the workpiece is highest and decreases gradually till the tooth exits out of the workpiece. Although the mechanical impact in down-milling is higher at the beginning as compared to up-milling, there is no rubbing between the tooth and the workpiece. Furthermore, during rotation of the tool, the successive tooth penetrates into the fresh

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FIGURE 5.16  3D view of chip formation in up- and down-milling operations. (A) Up-milling and (B) down-milling operation.

surface of the workpiece and not into the work-hardened surface created by the previous tooth. Thus, there is no impact of surface hardening on successive tooth, created by the previous tooth during down-milling. As a result, tool wear in down-milling is lower than up-milling.

7.6.2  Comparitive evaluation between cooling strategies for up- and down-milling Fig. 5.17 shows the average surface roughness under all four cooling strategies for up- and down-milling operations. The lowest average surface roughness was generated under emulsion cooling with up-milling operation between all cooling strategies and milling methods. During up-milling operation, other cooling strategies were not as effective in chip removal and lowering the temperature of the cutting zone as emulsion cooling, thus they were not able to create as smoother surface as emulsion cooling. The application of cooling by LN2 increases the workpiece hardness and that of end-mill, exacerbating surface roughness due to increased cutting forces and generates the highest surface roughness in both up-milling and down-milling operations. Emulsion cooling, MQL, and combined (MQL + LN2) cooling methods produced next lower surface roughness as the chips were very easy to be removed from the cutting zone which made the machining smoother and, hence, lower surface roughness. Fig. 5.18 shows the plot for maximum flank wear versus machining pass (machined length) for up-milling method under all four cooling strategies. As can be seen in Fig. 5.18, only emulsion cooling was able to machine all eight passes using up-milling method. Although even emulsion cooling was also not able to remove the segmented chip from the cutting zone because it was not completely detached from the material, the application of emulsion lowered the temperature of the continuous chip along at the cutting zone. Also, chips

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FIGURE 5.17  Surface roughness versus cooling strategies and milling methods.

other than continuous chip were removed quickly using emulsion flood cooling. The continuous serrated chip obtained during each pass caused the chipping of the tool flutes under emulsion cooling also. On the other hand, LN2 cooling increased the hardness of the workpiece and tool and also was least efficient in chip removal. Due to that, the tool deformed plastically during first pass only. MQL provided sufficient lubrication to the cutting zone but, due to the presence

FIGURE 5.18  Maximum flank wear versus machining passes (machined length) for up-milling method under four cooling strategies.

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FIGURE 5.19  Maximum flank wear versus machining passes (machined length) for downmilling method under four cooling strategies.

of continuous chip and high rate of chipping, tool could not last more than four passes. Combined (MQL + LN2) cooling increased tool life from one pass in LN2 to three passes. The main causes of tool wear in up-milling operation were abrasion, chipping, and plastic deformation. Fig. 5.19 shows the plot for maximum flank wear versus machining pass (machined length) for down-milling method under four cooling strategies [V = 45 m/min (1127 rpm) and feed rate, f = 114.5 mm/min (4.510 ipm)]. The aforementioned shows that the emulsion cooling generated highest flank wear. During the first pass, LN2 cooling generated the highest flank wear but the trend changed and flank wear under emulsion cooling exceeded the flank wear under all other cooling strategies. Until sixth pass, combined (MQL + LN2) cooling provided the least value of maximum flank wear, after which MQL cooling generated the least maximum flank wear at seventh and eighth passes. In case of LN2 cooling, maximum flank wear remained second highest when machining under down-milling method. The MQL cooling strategy generated the least value of maximum flank wear followed by combined (MQL + LN2) cooling among all cooling strategies and milling methods. Tool wear in downmilling operation was mainly due to abrasion.

7.6.3 Conclusions This research investigated the effects of up- and down-milling and four cooling methods [conventional emulsion flood cooling, MQL, LN2, and combined (MQL + LN2)] on machinability of Inconel-718 alloy in high speed milling with four-flutes uncoated solid carbide bullnose helical end-mills. From the results obtained, the following conclusions can be drawn:

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1. Down-milling generates least flank wear than up-milling under emulsion, MQL, LN2, and combined (MQL + LN2) cooling strategies investigated and therefore is recommended to improve machinability of Inconel-718. 2. The mechanism of tool wear in up-milling is adhesion and failure modes were chipping and plastic deformation, while abrasion was the mechanism of tool wear in down-milling. 3. Up-milling operation generates irregular and longer chips with mostly torn edges, whereas down-milling generates uniform and shorter chips. 4. Up-milling generates a segmented chip, with sawtooth shape for each machining pass along with other chips under all cooling strategies. 5. Down-milling operation generates discontinuous serrated chips under all cooling strategies, which were easily removed from the cutting zone by each of the cooling strategy, while segmented chips are generated in up-milling operation. 6. In up-milling, emulsion cooling generated lowest surface roughness, while LN2 generated highest surface roughness. 7. Down-milling with emulsion, MQL, and combined (MQL + LN2) cooling strategies generates equal amount of surface roughness, while downmilling with LN2 generates highest surface roughness. 8. Emulsion up-milling generated lowest surface roughness, whereas LN2 upmilling generated highest surface roughness among all cooling strategies and milling methods. 9. Comparative evaluation of four cooling strategies using up-milling method determines that emulsion-cooling strategy yields the least amount of flank wear. 10. Comparative evaluation of the four cooling strategies using down-milling method determines that MQL cooling strategy generates least amount of flank wear, followed by combined (MQL + LN2) cooling method. 11. MQL can successfully replace conventional emulsion cooling leading to significant cost savings and sustainable machining. 12. Down-milling with MQL cooling method improves machinability and is recommended for milling Inconel-718 alloy.

8  Future trends nano-fluids The addition of nanoparticles to base-fluids like water or conventional emulsion coolant to form nano-fluids has attracted the attention of researchers over the past decade due to improved thermal conductivity and cooling capability over the base-fluids. Sidik et al., 2017 [15], reported that nano-particles in MQL improve its heat conductivity, reduce tool wear, and friction coefficient. Sharma et al., 2016 [16] reported that adding metallic nano-particles into conventional fluid significantly improves its thermal conductivity, but that adding nanoparticles to conventional flood coolant is not advisable due to flushing of nanoparticles and the cost of recycling during machining. Comparative investigation of various nanofluids is currently in progress.

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Acknowledgments The financial support from the National Science Foundation (NSF) under grant no. CMMI800871, and from the Intelligent Systems Center (ISC) and the Department of Mechanical and Aerospace Engineering at Missouri University of Science and Technology in the form of Graduate Research Assistantship and Graduate Teaching Assistantship are gratefully acknowledged. The author thanks former graduate students, Paras Mohan, Chukwujekwu Nnadi, and Emenike Chukwuma who performed the cooling and machining experiments and machinability data characterization.

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