Laser-assisted high speed machining of Inconel 718 alloy

Laser-assisted high speed machining of Inconel 718 alloy

Chapter 9 Laser-assisted high speed machining of Inconel 718 alloy Vignesh Ma and Ramanujam Ra,b a Department of Manufacturing Engineering, School o...

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Chapter 9

Laser-assisted high speed machining of Inconel 718 alloy Vignesh Ma and Ramanujam Ra,b a

Department of Manufacturing Engineering, School of Mechanical Engineering, Vellore Institute of Technology (VIT), Vellore, India; bCentre for Innovative Manufacturing Research, Vellore Institute of Technology (VIT), Vellore, India

Chapter outline 1 Introduction 243 1.1 Brief outline on hardto-machine materials 243 2 High speed machining— an overview 244 3 Hybrid machining 245 3.1 Definition and classification 245 4 Heat-assisted hybrid machining 247 5 Laser-assisted machining (LAM) 248 5.1 Principle of LAM 248 5.2 Material removal mechanism in LAM 249 5.3 Machinability study of various LAM processes 249

6

Case study on laser-assisted high speed machining of Inconel 718 alloy (LAHSM) 253 6.1 Experimental details 253 6.2 Materials and methods 254 6.3 Influence on cutting force 255 6.4 Influence on surface roughness (SR) 256 6.5 Influence on tool wear (TW) 256 6.6 Chip morphology study in LAHSM 258 6.7 Benefits of LAHSM over conventional machining 259

1 Introduction 1.1  Brief outline on hard-to-machine materials The material is termed as hard-to-machine if it possesses any of the following properties, such as higher toughness, increased resistance to corrosion and fatigue, low thermal conductivity, high hardness and brittleness, and high strength to weight ratio. On extreme machining conditions these characteristics lead to increased tool wear, higher heat generation, increased cutting energy, poor surface finish, and difficulty in chip formation. Hard-to-machine materials are classified into three broad categories: hard materials, nonhomogeneous materials, and ductile materials as shown in Fig. 9.1. The reason for considering hard High Speed Machining. http://dx.doi.org/10.1016/B978-0-12-815020-7.00009-6 Copyright © 2020 Elsevier Inc. All rights reserved.

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FIGURE 9.1  Classification of hard-to-machine materials.

material as hard-to-machine material is because of the higher strength, hardness along with poor thermal conductivity that would end up in shorter tool life, poor surface quality, and lower productivity. Because of high ductility, strain rate, and nonuniform chip formation, the materials like low carbon steels, polymers, etc., are also categorized as hard-to-machine material. The nonhomogenous material distribution present in the composite material makes it hard-to-machine that results in lesser tool life and poor surface finish [1].

2  High speed machining—an overview High speed machining (HSM) is commonly defined as the machining process that considerably uses higher range of cutting speed and feed rate when compared to conventional machining. Further, the definition of HSM is very complex, because the actual cutting speed used for a particular process varies according to the type of the work material, type of machining operation, the type of tool used for this machining operation, etc. During HSM, the relative decrease of the cutting temperature of the tool at the cutting edge is responsible for the improvement in machinability. The reduction of the cutting tool temperature occurs at different temperature for different materials. Depending on the tool, work material, and its tool life requirements, the cutting speed of the HSM process varies between 2 and 50 times more than the conventional machining techniques. In the present-day advanced machine tool technology, there are lots of advancements for HSM. But due to the shortcoming of tool wear criterion during machining of hard-to-machine materials, such as super alloys and titanium alloy, some alloy steel grades make the cutting speed restriction to some minimal level.

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FIGURE 9.2  Cutting speed range in HSM of different materials [3].

The different cutting speed ranges in HSM of different work materials are shown in Fig. 9.2. In HSM, for easy-to-machine materials like copper and aluminum, the cutting speed may vary between 4000 and 6000 m/min. For hardto-machine materials like nickel alloys and titanium alloys, the cutting speed range may vary between 60 and 100 m/min [2]. The advantages of this HSM are the increased material removal, lower production time, and excellent surface finish; this in turn results in reduced manufacturing cost of the process over conventional machining resulting in high productive process. In addition to this higher productivity, it also has other improved properties like better surface finish, reduction of surface damage, decreased burr formation, better chip disposal, and stability in machining. The applications of HSM have become more prevailing in present manufacturing industries, such as aerospace, defense, die and mold-making, and automotive industries, because of the availability of HSM spindles and more refractory tool materials. HSM is often associated with dry machining because at higher cutting speeds, the effect of cutting fluid is not found effective and evident at the cutting zone. HSM at dry machining conditions reduces the disposal and the associated energy consumption resulting in sustainable machining. In recent decades, in addition to HSM there are many other hybrid innovative machining methods that are developed to overcome the problems in traditional machining of hard-to-machine materials.

3  Hybrid machining 3.1  Definition and classification In machining domain, many researchers have contributed different definitions for the hybrid machining. According to Rajurkar et al., a combination of more

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FIGURE 9.3  Classification of hybrid machining processes.

than two machining techniques to produce components with good machining efficiency than the individual processes is called hybrid machining [4]. As per Aspinwall et al.'s statement, an application of more than two machining processes independently on a single machine, or two or more processes is utilized simultaneously as an assisted machining approach is called hybrid machining [5]. Whereas, Curtis et al. [6] came out with a simple definition that simultaneous working of two or more material removal processes is called hybrid machining process. To avoid these many confusions in hybrid machining terminology, CIRP proposed a open definition like two or more established processes combined together so that the advantages of each individual processes can be exploited over another synergistically. The hybrid machining processes are broadly classified into two groups, such as assisted and combined hybrid machining process as listed in Fig 9.3. In assisted hybrid processes, one process facilitates the other process for material removal without directly involved in the material removal. For instance, in heatassisted machining, the workpiece is heated by heat source prior to machining. In combined hybrid machining processes, the combination of both the process energies acts simultaneously in material removal mechanism. For instance, in

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FIGURE 9.4  Effect of tensile strength on the increase in temperature [8].

electric discharge grinding process, the electrical discharge is utilized in material removal during grinding of the material.

4  Heat-assisted hybrid machining The heat-assisted hybrid machining process uses heat sources externally to increase the shear zone temperature of the cutting area and to enable the material processing. The main motive of heating the workpiece is to increase the plastic deformation of the work material and to reduce its mechanical strength. ­Figure  9.4 depicts the reduction of mechanical strength as the cutting temperature increases. Thus, the heat-assisted hybrid machining process helps in assisting the cutting tool for smooth removal of material. The heat sources need to have certain desirable characteristics such as easy size and location control of the heated areas, increased heat energy density for quick preheating, reasonable cost and easy application to the conventional machines and safety, etc. [7]. Some of the external heat sources are laser, induction coil, gas flame, and plasma. In induction coil-assisted machining, the heating is provided on the electrically conductive material by electromagnetic induction generated by a high frequency alternating current. In flame-assisted machining, the heat is provided by the burning of petroleum gas and oxygen. In plasma-assisted hybrid machining, the plasma is provided by direct current, transferred arc that generates thermal plasma. This plasma is made to flow on the work surface of the material and

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TABLE 9.1 Heat sources in heat-assisted hybrid machining [9]. Heat source

Advantages

Disadvantages

Induction coil

• Easy to handle • Increased preheating capacity

• Not possible for increased preheating concentration • Limited tool accessibility

Gas flame

• Very low cost

• Not possible for increased preheating concentration

Plasma

• Increased concentration of heat • No need for electrical conduction

• Impossible to control precisely

Laser

• Increased concentration of heat • Control of heat source is easier

• High cost process • Rate of absorption is different for different material

thus supports material removal. Because of the ineffective power density, these processes do not come out with the good results. One more heat-assisted process is the laser-assisted machining (LAM) process which uses laser as the heat source which helps in improvement in the workability and material removal mechanism. The advantages and disadvantages of various preheat sources in heat-assisted hybrid machining are listed in Table 9.1. Based on the preheating requirements, laser is found to be more effective due to its instantaneous heating capability with focused beam. The types of laser heat sources used for preheating are diode, CO2, and Nd:YAG laser. Diode laser source has high laser output and improved energy efficiency which results in easy beam profile control and transfer. CO2 laser is ideally suitable for ceramics due to its good absorption rate with a higher wavelength of 10.6 µm. Nd:YAG laser source has low wavelength and has high energy density, thus used for effective heating of metals with minimal deep hardening depth.

5  Laser-assisted machining (LAM) 5.1  Principle of LAM LAM is one of the heat-assisted machining processes that uses the laser beam to heat the workpiece to an elevated working temperature, thereby making the surface to get softened prior to machining. When compared to conventional machining, LAM process saves more energy by reducing the cutting force and making the process as a green machining one. The laser heat source is applied prior to the cutting tool enhancing the material removal temperature. This makes the material softer and machining process an easier one by reducing the flow stress and strain hardening of the material without compromising the microstructural changes and properties of the material [10]. These properties facilitate

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FIGURE 9.5  Schematic of laser-assisted machining [8].

the easy removal of material even on hard-to-machine materials. The schematic of laser-assisted hybrid machining process is shown in Fig. 9.5.

5.2  Material removal mechanism in LAM In LAM process, the heat energy is applied at the shear zone of the work material resulting in increased heat generation at that point. The generated heat is carried away by the chip partially. Hence, the selection of laser heat source should be done properly by fulfilling few conditions: [11] 1. The selected power source should not cause thermal damage to the work material substrate due to its increased power density and higher temperature generation. 2. Control of laser spot diameter to be done effectively. 3. The selected power source should be low cost, harmless, and easily adjustable by the manufacturing industries. The assistance of laser during machining results in benefits like reduced cutting forces, minimal tool wear, lesser power consumption, better surface quality, increased tool life, higher possibility to machine brittle material without cracking, and higher production rate [12]. LAM process is highly recommended for the materials like engineering ceramics, high temperature material like titanium and nickel alloys, higher abrasive constituent (silicon content) material like aluminum alloys, substantial strain hardening material like austenitic stainless steels, etc. [13].

5.3  Machinability study of various LAM processes Depending on application of the component, the type of LAM process is selected. Some of the conventional LAM processes are laser-assisted turning, laser-assisted milling, laser-assisted grinding, and laser-assisted turn-mill. The

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FIGURE 9.6  Schematic of (A) LAM turning, (B) LAM milling, and (C) LAM grinding [7].

nonconventional LAM processes are laser-assisted electrochemical machining, laser-assisted electro discharge machining, laser-assisted water-jet machining, etc. Laser-assisted turning process is commercially used and studied by many researchers, especially for machining hard ceramics like silicon nitride, alumina, and obtained encouraging results [14]. Laser-assisted milling can be applied to any work material that needs to be machined for its complex shape and structure. Milling in LAM process is yet to commercialize due to the difficulty in controlling the heat source and tool path during machining. Hence, research and development on LAM are still expanding [15]. The schematic representation of LAM turning, LAM milling, and LAM grinding is shown in Fig. 9.6. LAM process is highly recommended for machining ceramics because of its very high hardness and brittleness. During LAM, the brittle material gets plastically deformed by reducing its yield strength. Because of this, the life of the tool is extended and can be used for a longer machining time. Thus, this process is found to be a best alternative for machining of ceramics when compared to conventional machining. Chang and Kuo [16] used the CO2 laser source as the thermal assistance during conventional machining of Al2O3 ceramics. Here, the purpose is to conduct the experiments by heating the workpiece prior to machining zone and makes it to reach the glass-transition temperature for the easier removal of material. The increased temperature reduces the force that is required for material removal of the ceramics. In addition to reduced forces generated during machining, the surface quality of the machined components was also found better, with least tool wear criterion than conventional machining [17]. In addition to Al2O3 ceramics, another group of researchers, Rozzi et al. [18], used silicon nitride (Si3N4) ceramic as a workpiece material in LAM. The

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FIGURE 9.7  Cutting force ratio by LAM and conventional machining for fused silica [20].

workpiece surface is heated to nearly 1000 °C, deforming it plastically without causing any fracture in the work material. At the glass-transition temperature of 920–970 °C, YSiAlON is formed and results in visco-plastic flow of the material, which softens the material. This produces the semicontinuous or continuous chip formation during LAM. Because of all these conditions the amount of specific cutting energy generated during machining is found to be very low, with increased material removal [19]. Song et al. [20] used CO2 laser as a heat source to locally heat the fused silica that has the improved properties such as high strength and hardness, poor plastic deformation, and lower fracture toughness. These properties make the material as hard-to-machine under conventional machining category. The combination of brittle fracture and quasi-plastic deformation makes the material easy to machine through LAM process. This causes the increased material removal with better tool wear, minimal cutting forces generated, and a very good surface finish of the machined component. Fig. 9.7 shows the cutting force ratio by LAM and conventional machining for fused silica. Rebro et al. [21] machined reaction sintered mullite ceramic at a high spindle rotational speed of 800 rpm. Mullite material possesses very poor thermal diffusivity and tensile strength. Hence, a severe care needs to be taken while preheating the work material for removal without inducing any cracks on surface and subsurface of the material. With a laser power of 170–190 W produces better material output parameters, say, lower cutting force, continuous chip formation, minimal tool wear, etc. Pfefferkorn et al. [22] machined magnesia partially stabilized zirconia using polycrystalline cubic boron nitride (PCBN) tool with the assistance of laser source. The assistance of laser increases the tool life of 121 min for PCBN tool and specific cutting energy by 2.5 times the conventional machining of

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FIGURE 9.8  Influence of responses with increasing cutting speed [23].

partially stabilized zirconia. During the material removal, it experiences the mixed, ductile, and brittle nature behavior. In addition to ceramic material, the metallic material identified as hard-to-machine can also be machined using this novel LAM hybrid machining process. The principle of material removal remains the same as that of ceramic material, since the melting and recrystallization temperature of these metals is not much lower than ceramic material. Hence, during laser parameter selection an utmost care needs to be taken. The materials which are categorized as the hard-to-machine are nickel-based alloys, titanium alloys, cobalt alloys, tool steels, etc. Many researchers used laser as preheat source to assist during hybrid machining of this kind hardto-machine material. The various types of laser sources used for LAM hybrid machining are Nd:YAG laser, diode laser, CO2 laser, etc. Attia et al. [23] used SiAlON ceramic tool for finish turning of Inconel 718 with a speed range of 100–500 m/min with Nd:YAG laser as a heat source during machining. They reported that, due to laser assistance during machining, there is an increased plastic deformation zone, which resulted in reduced cutting force generation, with 800% improvement in rate of material removal and 25% surface finish improvement. The predominant formation of smearing of material during conventional machining is found to be absent during this LAM process resulting in compressive residual stress formation at the subsurface of the machined component. These favorable outputs are obtained at the optimal machining level of cutting speed at 300 m/min, 0.25 mm of depth of cut, and feed rate at 0.4 mm/ rev. The influence of forces, tool wear, and surface roughness with the increase in cutting speed is given in Fig. 9.8. Traditionally, machining of hard materials is processed using conventional machining and grinding operation. But the amount of material removal is very negligible in grinding operation and the amount of force generation in conventional machining is found to be very high with increased tool wear. To overcome these issues, Ding and Shin [24] used LAM process for machining AISI 4130 steel. The laser source used was CO2 and Nd:YAG laser. This produces good surface finish of less than 0.3 µm roughness value with uniform hardness distribution at the surface, 20% reduction in the cutting force, and zero micro-

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structural change. They chose the machining levels at higher values, say, up to 300 m/min, and feed rate varies from 0.05 to 0.1 mm/rev. The reduction of yield strength of the material during LAM process is not only the only factor which contributes in the mechanism of material removal, also one more parameter that results in better machining during LAM is the lesser work hardening effect. In order to obtain better material removal during LAM process, García Navas et al. [25] found out some optimal machining conditions through experimentation. Optimal laser to tool distance produces reduced cutting forces during machining. Increased cutting speed produces lesser force generation during machining. This is because at the increased cutting speed amount of heat absorbed by the material has less time to dissipate out. This results in softening of the shear zone of the material during machining. Therefore, the selection higher cutting speed range favors the LAM process. LAM process has better surface integrity than conventional machining, because of its very low surface roughness and residual stresses formed during machining. Researchers during LAM of Inconel 718 used physical vapor deposited (PVD) and chemical vapor deposited (CVD) coated carbide cutting tool and obtained promising and better results than conventional machining [26]. The improvements in the results are found to be in agreement with the previously discussed authors. In addition, chip morphology study during LAM was also reported [27]. Metal matrix composites (MMCs) are also categorized as hard-tomachine material because of its ceramic reinforcement in the matrix base. Kong et al. proposed LAM process as a suitable method for machining of MMCs and reported some promising and useful results during machining. They used cemented carbide tool as the cutting tool material with 2.31 times of tool life improvement compared to conventional machining. The machining experiments are performed at higher cutting speed of up to 1600 rpm [28]. It is proven that LAM is a very good alternative for traditional machining of hard-to-machine materials because of its improved properties without affecting the work material surface and subsurface. It also enhances the tool life of the cutting tool and allowing carbide tools to machine high hard material. In addition to this, the higher cutting speed also supports in getting increased rate of material removal and better surface quality. As high the cutting speed, the time spent in dissipation of the heat is found to be lower resulting in reduced yield strength and enhances ductile deformation behavior of the material, causing better material removal.

6  Case study on laser-assisted high speed machining of Inconel 718 alloy (LAHSM) 6.1  Experimental details A continuous wave Nd:YAG laser with a wavelength of 1.064 µm and a maximum laser output power of 2 kW is used as a laser source in the present experimental study. Based on preliminary experimentation, the laser spot diameter of 2 mm and lead distance of 2.5 mm is maintained throughout the process

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FIGURE 9.9  Experimental setup for laser-assisted machining [30].

for effective heat absorption and reduction in the yield strength of Inconel 718 material. In addition to these laser parameters, laser approach angle plays a significant role in efficient material removal. Based on the preliminary trials, effective laser approach angle is fixed at 60 degree [27,29]. A specially designed fixture, integrated with the saddle, holds the laser head for effective laser absorbance and efficient machining. The infrared pyrometer is used to measure the temperature during machining at the cutting zone [26]. The experimental setup for LAM including the laser head and specially designed fixture is shown in Fig. 9.9. The machining trials are carried out on the high speed lathe (Make: Gedee Weiler) using CVD coated (TiCN/Al2O3/TiN) carbide inserts of grade KC5525 (Make: Kennametal). The properties of the coating are increased toughness, wear resistance, and temperature resistance. The cutting force (Fz) results are recorded using 9257B type 3-component Kistler force dynamometer coupled with data acquisition system. The cutting force (Fz) results are recorded using 9257B type 3-component Kistler force dynamometer coupled with data acquisition system. Mahr Surf GD120 surface profile meter with a measuring distance of 5.6 mm is used to measure the surface roughness (Ra) on the machined component. The tool wear is measured using Carl Zeiss optical microscope after each machining trial.

6.2  Materials and methods Inconel 718 alloy has wider applications in industries like marine, nuclear reactors, petrochemical plants, automotive, aerospace, aircraft, biomedical, etc. [31]. The properties like higher affinity of the tool toward work material, increased ten-

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TABLE 9.2 Experimental parametric levels. Input parameter

Unit

Level 1

Level 2

Level 3

Cutting speed (s)

m/min

60

80

100

Feed rate (f)

mm/rev

0.06

0.08

0.1

Laser power (P)

Watt

1200

1300

1400

dency of work hardening, lower thermal conductivity, and higher resistance to oxidation and corrosion make the material to fall under hard-to-machine category. In conventional machining, Inconel 718 possess increased temperature at the shear zone of the work material due to its poor thermal conductivity leading to built-up edge formation and worse tool life and surface finish. In addition to this, structural changes at the surface of the work material, micro-cracks, micro-hardness variation, and induced tensile residual stress affect the material. Due to these difficulties, machining of Inconel 718 in conventional machining process with high cost cutting tools like ceramics and cubic boron nitride tool makes the process an expensive. For the present study, Inconel 718 (one of the hard-to-machine materials) workpiece with a diameter of 25 mm and length of 300 mm is used. The experiments are conducted with full factorial design by varying three input parameters, cutting speed, feed rate, and laser power each at three levels with constant depth of cut (0.5 mm) and laser approach angle (60 degree). In this study, cutting speed is considered between 60 and 100 m/min as an HSM range for Inconel 718 alloy as reported by Fang et al. [2]. The experimental parametric levels are shown in Table 9.2. The optimal levels are analyzed using main effects plot. The benefits of laser-assisted high speed machining (LAHSM) are compared with conventional machining by conducting confirmation trials at the optimal levels.

6.3  Influence on cutting force Cutting force (Fz) is a force exerted by the cutting tool on the workpiece material along the tangential direction and it is found to be a significant parameter in deciding the quality of workpiece as long as the tool is concerned. The parametric effect on cutting force using main effects plot is shown in Fig. 9.10(A). It is evident that, as the cutting speed increases to a moderate level of 80 m/min, the cutting force decreases. With the increase in laser power up to 1300 W, the cutting force increases and then decreases at 1400 W of laser power. At lower laser power of 1200 W, the heat energy required for reduction of yield point of the material is obtained. Hence, the increase of laser power does not contribute too much in deciding the machining forces. Also, at low feed rate, the minimal cutting forces are generated and for higher feed rate it is vice-versa. The optimal parameter combination for cutting force criterion is cutting speed at 80 m/min, feed rate at 0.08 mm/rev, and laser power at 1200 W.

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FIGURE 9.10  Main effects plot for cutting force and surface roughness.

6.4  Influence on surface roughness (SR) In addition to cutting force, surface roughness is also a considerable measure to decide the machining ability of the work material. The temperature produced at the tool tip due to machining is a major player to affect tool surface resulting in edge chipping or wear at the flank surface in turn results in surface irregularities. In LAHSM process, in addition to the heat generated at the cutting zone, an additional external heat by laser is provided for efficient machining. Hence, an extreme care needs to be taken during laser assistance on the work specimen during machining and to avoid thermal degradation to the cutting tool material. The parametric effect on surface roughness is shown using main effects plot in Fig. 9.10(B). The SR value increases as the feed rate and laser power increases. The change in laser power from 1200 W to 1300 W has negligible effect on SR but further increase of laser power causes steep rise in SR. The feed rate has linear effect toward SR and its low value of 0.06 mm/rev of feed rate produces better surface finish. The optimal SR is obtained at cutting speed of 60 m/min, feed rate of 0.06 mm/rev, and laser power as 1300 W.

6.5  Influence on tool wear (TW) Tool wear mainly depends on the material removal temperature. During machining, the interface between tool and workpiece causes intense friction at the tool and workpiece juncture and causes severe plastic deformation.

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FIGURE 9.11  Main effects plot for tool wear.

Tool wear indirectly affects other output responses like cutting force and SR. In LAHSM process, the force is exerted by the cutting tool on the work material that causes rise in temperature to some extend and starts to deteriorate. The obtained tool wear is also found to be very less even though the temperature of the introduced laser source is found to be considerably high. Hence, an optimal level of machining conditions needs to be identified for minimizing tool wear resulting in longer tool life. The parametric response plot showing the tool wear for different input parameters is shown in Fig. 9.11. From the graph it is evident that cutting speed and laser power affect the wear of the tool significantly. As laser power increases, the tool flank wear also increases. Higher tool flank wear is obtained especially at the higher cutting speed and the laser power. This cutting tool causes more heat generation in addition to laser power source, causing lesser shear deformation zone and lesser thermal softening of the material. Minimum wear is observed at low cutting speed of 60 m/min and lower laser power input of 1200 W. Based on the experimentation, the cutting speed at 60 m/min, feed rate at 0.08 mm/rev, and laser power of 1200 W are found to be optimal levels to get minimal tool wear.

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FIGURE 9.12  Types of chip in LAHSM of Inconel 718 alloy [27].

6.6  Chip morphology study in LAHSM In machining studies, it is very important to study the type of chip formed and its complete morphology. The type of chip formed completely depends on the work material, type of cutting tool and its geometry, machine settings, lubricating fluids, etc. The types of obtained chip during LAHSM of Inconel 718 are shown in Fig. 9.12. The types of chips that are formed in LAHSM are snarled ribbon type, ribbon shaped, helical washer type, and coil type (long and short). During LAHSM, a continuous regular-shaped chip with less segmentation is produced at lower cutting conditions. This is because of lesser rubbing action of the chip on the tool irrespective of the laser assistance. The entangled-type snarled chips are produced for higher cutting speed, washertype helical chips are obtained at increased feed rate, and long ribbon-type continuous chips are obtained at increased laser power, respectively. The long tubular chip is obtained at moderate cutting speed with low feed rate and laser power. In addition to this, combination of low feed rate and laser power results in long tubular chip. The geometry of chip produced in LAHSM of Inconel 718 alloy is sawtooth type and its cross-section and optical images are shown in Fig. 9.13. It is observed that more segmentation of chip is at higher laser power. The chip deformation is formed at the narrow shear bands of the sawtooth chip and forms

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FIGURE 9.13  Microscopic images of chip cross-sections in LAHSM [27].

relatively low strain due to the increased cutting temperature. Some of the geometrial parameters considered in chip analysis are chip thickness, tooth-pitch, shear band angle, etc. The reduced SR, flank wear, and cutting force are obtained due to the increased chip thickness and reduced shear band angle. These are obtained as the result of higher flow stress material at the uncut chip region caused due to laser preheating [27].

6.7  Benefits of LAHSM over conventional machining The benefits of LAHSM are analyzed by conducting experiments on conventional machining of Inconel 718 alloy under similar cutting conditions. The machining experiments are conducted at optimum machining conditions like cutting speed at 80 m/min, feed rate at 0.08 mm/rev, and laser power at 1200 W. The results are compared and plotted in Fig. 9.14 for the responses such as cutting force, SR, and tool wear. It is observed that LAHSM performed better than the conventional machining of all the selected output responses. The percentage improvements of various output responses are 24.5% of reduced cutting force, 56% decrease in SR, and 29% reduction in tool wear. These benefits in terms of improvement in machinability could be attributed because of the thermal softening effect on the work material caused by laser assistance and selection of cutting conditions.

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FIGURE 9.14  Improvement of LAHSM over conventional machining.

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