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Laser-assisted high-speed finish turning of superalloy Inconel 718 under dry conditions
CIRP Annals - Manufacturing Technology 59 (2010) 83–88
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er. com/ci rp/ def a ult . asp
Laser-assisted high-speed finish turning of superalloy Inconel 718 under dry conditions H. Attia (2)a,b,*, S. Tavakoli b, R. Vargas a, V. Thomson b a b
Aerospace Manufacturing technology Centre, National Research Council of Canada, Montreal, Quebec, Canada Department of Mechanical Engineering, McGill University, Montreal Quebec, Canada
A R T I C L E I N F O
A B S T R A C T
Keywords: Cutting Laser Machinability
Inconel 718 (IN718) is used in aerospace applications due to its superior mechanical properties. This study investigates the high-speed machinability of this material under laser-assisted machining (LAM) and dry conditions. Finish turning tests were performed for cutting speeds up 500 m/min and feeds up to 0.5 mm/rev, using focused Nd:YAG laser beam and ceramic tool (SiAlON). At optimum machining conditions, nearly eight-fold increase in material removal rate and significant improvement in the tool life and surface finish were achieved, compared to conventional machining. The mechanisms of tool failure were identified. SEM analysis and microstructure examination of machined surfaces revealed the improvement in the surface integrity under LAM conditions. ß 2010 CIRP.
1. Introduction Inconel 718 (IN718) and other nickel-based superalloys are widely used in the aerospace and nuclear industries due to their superior high temperature strength, toughness, and corrosion resistance. These alloys are difficult to machine due to their low thermal conductivity and diffusivity, which cause steep temperature gradient at the tool edge and the shift the location of the maximum temperature towards the tool tip. As a result, excessive tool wear, premature cracking and built-up edge formation are observed. Other factors that contribute to the poor machinability of IN718 include the strong tendency to strain hardening during machining, the adhesion to the tool material, and the presence of hard abrasive carbides and intermetallic phases in its microstructure. The main strengthening mechanism of IN718 is age hardening due to the presence of fine uniform metastable g00 precipitates distributed throughout the matrix. At temperatures above 540 8C, the deformation is homogenously distributed and is comprised of uniformly tangled dislocations. Above 700 8C, the precipitations reach their limit of stability, causing a significant drop in the material yield strength [1]. In laser-assisted machining (LAM), which is well suited for difficult-to-machine materials, the workpiece is subjected to localized heating through a focused laser beam. This heating improves the machinability through softening the workpiece material and reducing tool wear, without causing subsurface damage [2].
HSS and cemented WC tools are widely used at cutting speeds below 30 m/min. Although PVD multi-layer coated tools allowed higher cutting speeds above 50 m/min [3], they are susceptible to chipping, edge fracture and nose wear [4]. CVD coated tools also fail by severe notching and flank wear when used to machine IN718 [3]. CBN and ceramic tool materials are increasingly used to machine Ni-based alloys in the cutting speed range of 120–300 m/ min. CBN tools are, however, sensitive to notching, adhesive wear and excessive diffusion wear [5]. Despite their superior hot hardness, ceramic tools showed large flank and solution wear in the conventional machining of Ni-based alloys [5]. The tool geometry also plays an important role in determining the tool life. Proper selection of the included and approach angles improves the tool life through their effect on the edge strength, the tool-chip contact area, and the chip curvature [6]. Compared to rhomboid inserts, round ceramic inserts with negative rake angle were successful for high-speed machining (HSM) of IN718 [7]. While dry HSM of Ni-based alloys is limited by the premature tool wear, the use of high-pressure cooling causes a reduction in tool life due to the decrease in the chip–tool contact length and the increase in contact stresses [6]. The main objectives of the present study are (i) to optimize the laser-assisted high-speed finish turning of IN718, in terms of tool life, surface integrity, and productivity, and (ii) to assess the use of silicon nitride ceramic tools in dry HSM to minimize the environmental impact and to reduce cost. 2. Experimental setup
* Corresponding author. E-mail address: [email protected] (H. Attia). 0007-8506/$ – see front matter ß 2010 CIRP. doi:10.1016/j.cirp.2010.03.093
The machining tests were performed on a 6-axis Boehringer NG200, CNC turning center (36 kW main spindle and 4000 rpm
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Fig. 1. LAM experimental setup: (a) a schematic diagram, and (b) a close-up.
maximum speed). A high power laser beam was generated by 1006-D 4 kW Trumpf Nd:YAG laser. A schematic and a close-up of the experimental setup are shown in Fig. 1. The laser head (2) is mounted on a special fixture attached to the turret head (6) to control the orientation of the beam (Ø = 48–50 8C) and its spot size. A flat nozzle (3), capable of providing 860 kPa of air pressure, was used to protect the laser lens from excess heat and the chips. For process characterization and control, the temperature field near the cutting zone was measured using a Thermovision A20 infrared (IR) camera (5) with a wavelength detection range suitable for temperature measurement up to 900 8C. To calibrate and validate the IR measurements during cutting, two K-type thermocouples were attached to the workpiece, through a high-speed slip ring (4). A threecomponent Kistler dynamometer, type 9121, was used to measure the cutting forces. Tool wear was measured using an Olympus SZ-X12 stereoscopic microscope. Surface roughness was measured after each pass, using a portable Taylor Hobson Surtronic 3+. The measurement errors of various parameters are represented in the results shown in Section 4 as error bars. The solution heat-treated and aged IN718 workpiece material (1) has a bulk hardness of 28 HRC. The workpiece diameter was 59– 63 mm. A silicon nitride/aluminum oxide/aluminum nitride (SiAlON) round ceramic insert (7) with 58 rake angle and 6.35 mm radius (Kennametal, KY1540) was selected due to its insensitivity to notch wear. 3. Experimental approach The starting point in this study was the optimal conventional cutting conditions (v = 200 m/min and f = 0.25 mm/rev) established by the authors in [8] for finish turning of IN718, using coated carbide tools (single-layer TiAlN PVD, and triple-layer (TiCN/Al2O3/ TiN) CVD). In this study, the effect of the cutting speed (200 < v < 500 m/min) was first investigated at constant feed rate, f = 0.25 mm/rev. After the optimum cutting speed vopt was determined, this speed was kept unchanged and the feed rate was varied, 0.25 < f < 0.50 mm/rev, to determine the optimum feed fopt. To establish a reference point, a test was carried out at 200 m/ min and 0.25 mm/rev without laser heating. Following ISO 3685 [10], an average flank wear VBa of 300 mm was selected as the tool life criterion. For all tests, the depth of cut (DOC) was kept constant at 0.25 mm. The cutting length Lc was 30 mm, except for the investigation of the tool wear kinetics, where the tests were continued until the end of tool life.
4. Results analyses and discussion 4.1. Optimization of the laser heating process parameters The laser system used in this work has a focal length Lf = 80 mm, which generates a laser spot diameter dL = 0.3 mm. The dependence of dL on the distance DD from the focal length was first established. Laser heating tests were then carried out for a wide range of process variables: 100 < v < 500 m/min, and 0.1 < f < 1 mm/rev. By changing the laser power PL between 2.5 and 3 kW and the laser spot diameter, 0.3 < dL < 3 mm, the following ranges of laser power density pL and surface temperatures Ts were obtained: 0.43 < pL < 43 kW/mm2, and 220 < Ts < 880 8C, respectively. It was concluded that a power density of 0.96 W/mm2 (obtained at PL = 3 kW and dL = 2 mm) provided the desired surface temperatures of 650–700 8C. Greater power densities showed signs of plasma gas generation and surface damage. 4.2. Chip formation and morphology The microstructure of sections of the continuous ribbon-like chips was examined (Table 1). The analysis showed that chips produced by LAM exhibit more tendency to shear localization and larger strain gs in the primary shear zone due to the thermal softening effect of LAM. Increasing the cutting speed results, however, in thinner chips, i.e., lower gs. As the cutting speed increases, the cutting temperature increases but the absorption of the laser radiation heat is reduced. Fig. 2 shows that the net effect is a reduction in the surface temperature Ts. At speeds above 300 m/ min, the temperature level required to cause significant softening effect (650–700 8C) is not reached. The formation of shear localized chips (saw-tooth type) is also attributed to the favorable cutting conditions of large undeformed chip thickness, negative rake angle, and high cutting speeds that promote the fracture of the material [9]. The negative effect of the cutting speed on the shear strain, and consequently strain hardening, was further validated through the measurement of the average micro-hardness of the chip. These measurements showed a slight reduction in the hardness from HRC 41.2 to 39 as the cutting speed increases from 200 to 500 m/ min under LAM conditions. This is compared to an average hardness of HRC 45.1 at 200 m/min in the absence of laser heating. The effect of feed on LAM was found to be more significant; at v =300 m/min, an increase in the feed from 0.25 to 0.5 mm/rev resulted in an increase in HRC from 39.1 to 48.9.
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Table 1 Effect of cutting conditions on chip morphology.
Fig. 2. IR Surface temperature measurements at different cutting speeds, for conventional and LAM machining (at feed = 0.25 mm/rev).
4.3. Effect of cutting speed on cutting forces, tool wear and roughness: Optimum conditions Fig. 3 shows the effect of the cutting speed on the cutting force components at a fixed feed rate of 0.25 mm/rev. Increasing the cutting speed up to 300 m/min results in a reduction in the cutting force due to the reduction in shear strain gs, as shown earlier. The radial and feed forces were not significantly affected by the cutting speed; yet they show a significant drop when
Fig. 3. Effect of cutting speed on cutting forces for conventional and LAM machining (feed = 0.25 mm/rev).
compared to conventional machining. As expected, round inserts produce relatively large radial forces. Above 300 m/ min, the forces show no change with the increase in the cutting speed, since the workpiece surface temperature Ts in the cutting zone is below the critical temperature range of 650–700 8C, as shown in Fig. 2. The SEM images shown in Fig. 4 for the cutting conditions of 300 m/min and 0.4 mm/rev show that the dominant tool wear modes were abrasive and adhesive flank wear. In comparison with
Fig. 4. SEM image of failed ceramic tool at 300 m/min and 0.4 mm/rev under (a) conventional and (b) LAM machining conditions.
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Fig. 5. Effect of cutting speed on flank wear and surface roughness for conventional machining and LAM, feed = 0.25 mm/rev and DOC = 0.25 mm.
conventional machining, the tool failure in laser-assisted machining exhibits less severe and more uniform flank wear, due to the reduced cutting forces and the expected favorable thermal conditions (the shift in the location of the maximum temperature away from the tool tip, and the more uniform temperature distribution on the tool rake and flank surfaces). Also with LAM, the hard intermetallic phases and carbides particles in the workpiece material become less abrasive due to the thermal effect. The improved tribological performance of the tool results in a lower surface roughness, as will be shown later. The X-ray chemical spectroscopy performed on the cutting tool used in the LAM tests confirmed the adhesion of the workpiece material to the rake face and the tool cutting edge. Edge chipping and severe crater wear, which were observed by the authors in [8] for coated carbide tools in LAM and conventional machining, were not observed here. Fig. 5 shows the dependence of flank wear and surface roughness on the cutting speed for a fixed length of cut of 30 mm. It can be seen that there is a significant drop in VBa under LAM conditions, when compared to conventional machining. Above 300 m/min, the tool wear on the flank face and the surface roughness increase with the increase in the cutting speed, since the surface temperatures are not sufficient to reach the stability temperature of the g0 and g00
phases. The lack of notching and excessive flank wear led to the low roughness of the machined surface. Additionally, the contact length of the round insert is relatively large, compared to other insert shapes. This results in feed marks of smaller heights and, consequently, better surface finish. One can thus conclude that the LAM optimum cutting speed for the SiAlON ceramic tool is 300 m/ min, from the cutting forces, surface finish and tool wear points of view. 4.4. Effect of feed on cutting forces, tool wear and roughness: optimum conditions At the optimum cutting speed vopt of 300 m/min, Fig. 6 shows that the cutting force components increase with the increase in the feed, as thicker chips are produced. Measurement of the surface temperature Ts showed that as the feed increases from 0.25 to 0.5 mm/rev, Ts is reduced from 695 to 590 8C, due to the shorter interaction time between the laser beam and the workpiece surface. Fig. 7 shows that as the feed increases from 0.25 to 0.4 mm/rev, the flank wear VBa is reduced, since thicker chips create more uniform contact pressure distribution between the chip and the
Fig. 6. Effect of feed on cutting for LAM, at a cutting speed of 300 m/min.
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Fig. 7. Effect of feed on average flank wear and surface roughness (LAM, cutting speed of 300 m/min).
cutting edge. Above a feed of 0.4 mm/rev, however, the reduced thermal softening effect caused by lower radiation heat absorption, causes the tool wear to increase. Fig. 7 shows also that increasing the feed from 0.25 to 0.40 mm/rev reduces the surface roughness by 25%, since laser heating facilitates the material removal and avoids smearing and surface tearing, as will be seen later. In addition, the relatively large size of the laser beam spot (2 mm) reduces the sharpness of the feed marks on the machined surface [2]. By increasing the feed from 0.4 to 0.5 mm/rev, the roughness increases to Ra = 0.64 mm. This pattern follows the change in tool wear with feed, and explains the deviation from the theoretical predictions, where surface roughness is proportional to the square of the feed [9]. As a conclusion, the optimum LAM finishing cutting conditions using SiAlON ceramic insert (vopt = 300 m/min and fopt = 0.4 mm/rev, at depth of cut of 0.25 mm) produced practically the lowest surface roughness, tool wear and cutting forces. It is also estimated that the material removal rate (MRR) increases by approximately 800%. Additional tests were carried out to compare LAM to conventional machining, in terms of the evolution of the surface roughness and tool wear, at the optimum cutting conditions
(v = 300 m/min, f = 0.4 mm/rev). The results showed that the end of tool life (VBa = 300 mm) is reached at a cutting length Lc = 310 mm in conventional turning. Under LAM conditions, the tool life is increased by 40% to Lc = 430 mm. It was also observed that at Lc = 150–160 mm, the surface roughness reaches a minimum value of Ra = 0.44 and 0.31 mm, for conventional and laser-assisted machining, respectively. At the end of tool life, the surface roughness increases to 0.92 and 0.88 mm, respectively. 4.5. Surface integrity Fig. 8 shows the microstructure of the machined surface produced at the optimum cutting conditions of 300 m/min and 0.4 mm/rev, at the end of the tool life. Only the surface produced by conventional machining showed heavy smearing (Fig. 8(a)). Under LAM conditions, the plastically deformed surface layer is deeper (dp = 75 mm, vs. 63 mm for conventional machining) and more uniform. The absence of smeared material and the increased plastic deformation zone are indicative of the favorable compressive residual stresses that are promoted by round inserts due to ploughing and the negative rake angle [11]. The SEM images (insets, Fig. 8) show no change in precipitate size within the matrix,
Fig. 8. Optical micrograph of the machined surface (300 m/min, 0.4 mm/rev) under: (a) conventional and (b) LAM machining conditions.
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Fig. 9. X-ray chemical spectroscopy of the machined surface (300 m/min, 0.4 mm/rev) under: (a) conventional and (b) LAM machining conditions.
in comparison to the bulk material. Although the grain boundary precipitates were elongated due to the plastic deformation, they did not show a loss of coherency. In the conventional and LAM operations, there was no sign of overheating/burning, macro- and micro-cracks, cavities, or microdefects. X-ray chemical spectroscopy showed that there is no significant change in the chemical constituents of the surfaces generated by laser-assisted machining (Fig. 9). Comparison with the bulk material also indicated that no phase change took place within the machined surface. Micro-hardness measurement of the plastically deformed surface layer showed that higher hardness levels (40 HRC) were obtained on the surface and decreases to the bulk material nominal hardness value at a depth of 150 mm. This hard surface layer is a result of the strain hardening of the workpiece material under the high pressure and temperatures generated during machining, and the cold working action when the machined surface rapidly cools down. The micro-hardness profile in the surface sub- layer was practically the same for LAM and conventional machining. 5. Concluding remarks The optimum conditions for laser-assisted finish turning of IN718 using SiAlON ceramic tool were established. Under these conditions, a significant drop in the cutting forces is achieved when compared to conventional machining. The surface finish is also improved by more than 25%, and the material removal rate is increased by approximately 800%. In terms of surface integrity, the optimum laser-assisted machining conditions did not introduce phase change overheating, or microdefects. The absence of smeared material, which was observed in conventional machining, and the increased plastic deformation zone are indicative of the favorable compressive residual stresses in the subsurface layer of the machined surface.
Acknowledgements The authors acknowledge the support of the Aerospace Manufacturing Technology Centre (AMTC), Institute for Aerospace Research (IAR), National Research Council Canada (NRC), where the experimental work was carried out. The partial financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC) is greatly appreciated.
References [1] Leshok CE, Kim J, Shin YC (2001) Plasma Enhanced Machining of Inconel 718: Modeling of Workpiece Temperature with Plasma Heating and Experimental Results. International Journal of Machine Tools and Manufacture 41:877–897. [2] Anderson M, Patwa R, Shin Y (2006) Laser-Assisted Machining of Inconel 718 with an Economic Analysis. International Journal of Machine Tools and Manufacture 46:1879–1891. [3] Devillez A, Schneider F, Dominiak S, Dudzinski D, Larrouquere D (2007) Cutting Forces and Wear in Dry Machining of Inconel 718 with Coated Carbide Tools. Wear 262:931–942. [4] Ezugwu EO, Bonny J (2005) Finish Machining of Nickel-Base Inconel 718 Alloy with Coated Carbide Tool under Conventional and High-Pressure Coolant Supplies. Tribology Transactions 48:76–81. [5] Richards N, Aspinwall D (1989) Use of Ceramic Tools for Machining Ni-based Alloys. International Journal of Machine Tools and Manufacture 29:575–588. [6] Ezugwu EO, Wang ZM, Machado AR (1999) The Machinability of Ni-Based Alloys: A Review. Journal of Materials Processing Technology 86:1–16. [7] Ezugwu EO, Tang SH (1995) Surface Abuse When Machining Cast Iron (G-17) and Nickel-Base Superalloy (Inconel 718) with Ceramic Tools. Journal of Materials Processing Technology 55:63–69. [8] Bhatt A, Attia MH, Vargas R, Thomson V (2010) Wear Mechanisms of WC Coated and Uncoated Tools in Finish Turning of Inconel 718. Journal of Tribology International 43:1113–1121. [9] Shaw MC (2005) Metal Cutting Principles. second ed. Oxford Press. [10] International Standard ISO 3685 (1993), Tool Life Testing in Single-Point Turning Tools, second ed. 1993-11-15. [11] Arunachalam RM, Mannan MA, Spowage AC (2004) Surface Integrity When Machining Age Hardened Inconel 718 with Coated Carbide Cutting Tools. International Journal of Machine Tools and Manufacture 44:1481–1491.
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er. com/ci rp/ def a ult . asp
Laser-assisted high-speed finish turning of superalloy Inconel 718 under dry conditions H. Attia (2)a,b,*, S. Tavakoli b, R. Vargas a, V. Thomson b a b
Aerospace Manufacturing technology Centre, National Research Council of Canada, Montreal, Quebec, Canada Department of Mechanical Engineering, McGill University, Montreal Quebec, Canada
A R T I C L E I N F O
A B S T R A C T
Keywords: Cutting Laser Machinability
Inconel 718 (IN718) is used in aerospace applications due to its superior mechanical properties. This study investigates the high-speed machinability of this material under laser-assisted machining (LAM) and dry conditions. Finish turning tests were performed for cutting speeds up 500 m/min and feeds up to 0.5 mm/rev, using focused Nd:YAG laser beam and ceramic tool (SiAlON). At optimum machining conditions, nearly eight-fold increase in material removal rate and significant improvement in the tool life and surface finish were achieved, compared to conventional machining. The mechanisms of tool failure were identified. SEM analysis and microstructure examination of machined surfaces revealed the improvement in the surface integrity under LAM conditions. ß 2010 CIRP.
1. Introduction Inconel 718 (IN718) and other nickel-based superalloys are widely used in the aerospace and nuclear industries due to their superior high temperature strength, toughness, and corrosion resistance. These alloys are difficult to machine due to their low thermal conductivity and diffusivity, which cause steep temperature gradient at the tool edge and the shift the location of the maximum temperature towards the tool tip. As a result, excessive tool wear, premature cracking and built-up edge formation are observed. Other factors that contribute to the poor machinability of IN718 include the strong tendency to strain hardening during machining, the adhesion to the tool material, and the presence of hard abrasive carbides and intermetallic phases in its microstructure. The main strengthening mechanism of IN718 is age hardening due to the presence of fine uniform metastable g00 precipitates distributed throughout the matrix. At temperatures above 540 8C, the deformation is homogenously distributed and is comprised of uniformly tangled dislocations. Above 700 8C, the precipitations reach their limit of stability, causing a significant drop in the material yield strength [1]. In laser-assisted machining (LAM), which is well suited for difficult-to-machine materials, the workpiece is subjected to localized heating through a focused laser beam. This heating improves the machinability through softening the workpiece material and reducing tool wear, without causing subsurface damage [2].
HSS and cemented WC tools are widely used at cutting speeds below 30 m/min. Although PVD multi-layer coated tools allowed higher cutting speeds above 50 m/min [3], they are susceptible to chipping, edge fracture and nose wear [4]. CVD coated tools also fail by severe notching and flank wear when used to machine IN718 [3]. CBN and ceramic tool materials are increasingly used to machine Ni-based alloys in the cutting speed range of 120–300 m/ min. CBN tools are, however, sensitive to notching, adhesive wear and excessive diffusion wear [5]. Despite their superior hot hardness, ceramic tools showed large flank and solution wear in the conventional machining of Ni-based alloys [5]. The tool geometry also plays an important role in determining the tool life. Proper selection of the included and approach angles improves the tool life through their effect on the edge strength, the tool-chip contact area, and the chip curvature [6]. Compared to rhomboid inserts, round ceramic inserts with negative rake angle were successful for high-speed machining (HSM) of IN718 [7]. While dry HSM of Ni-based alloys is limited by the premature tool wear, the use of high-pressure cooling causes a reduction in tool life due to the decrease in the chip–tool contact length and the increase in contact stresses [6]. The main objectives of the present study are (i) to optimize the laser-assisted high-speed finish turning of IN718, in terms of tool life, surface integrity, and productivity, and (ii) to assess the use of silicon nitride ceramic tools in dry HSM to minimize the environmental impact and to reduce cost. 2. Experimental setup
* Corresponding author. E-mail address: [email protected] (H. Attia). 0007-8506/$ – see front matter ß 2010 CIRP. doi:10.1016/j.cirp.2010.03.093
The machining tests were performed on a 6-axis Boehringer NG200, CNC turning center (36 kW main spindle and 4000 rpm
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Fig. 1. LAM experimental setup: (a) a schematic diagram, and (b) a close-up.
maximum speed). A high power laser beam was generated by 1006-D 4 kW Trumpf Nd:YAG laser. A schematic and a close-up of the experimental setup are shown in Fig. 1. The laser head (2) is mounted on a special fixture attached to the turret head (6) to control the orientation of the beam (Ø = 48–50 8C) and its spot size. A flat nozzle (3), capable of providing 860 kPa of air pressure, was used to protect the laser lens from excess heat and the chips. For process characterization and control, the temperature field near the cutting zone was measured using a Thermovision A20 infrared (IR) camera (5) with a wavelength detection range suitable for temperature measurement up to 900 8C. To calibrate and validate the IR measurements during cutting, two K-type thermocouples were attached to the workpiece, through a high-speed slip ring (4). A threecomponent Kistler dynamometer, type 9121, was used to measure the cutting forces. Tool wear was measured using an Olympus SZ-X12 stereoscopic microscope. Surface roughness was measured after each pass, using a portable Taylor Hobson Surtronic 3+. The measurement errors of various parameters are represented in the results shown in Section 4 as error bars. The solution heat-treated and aged IN718 workpiece material (1) has a bulk hardness of 28 HRC. The workpiece diameter was 59– 63 mm. A silicon nitride/aluminum oxide/aluminum nitride (SiAlON) round ceramic insert (7) with 58 rake angle and 6.35 mm radius (Kennametal, KY1540) was selected due to its insensitivity to notch wear. 3. Experimental approach The starting point in this study was the optimal conventional cutting conditions (v = 200 m/min and f = 0.25 mm/rev) established by the authors in [8] for finish turning of IN718, using coated carbide tools (single-layer TiAlN PVD, and triple-layer (TiCN/Al2O3/ TiN) CVD). In this study, the effect of the cutting speed (200 < v < 500 m/min) was first investigated at constant feed rate, f = 0.25 mm/rev. After the optimum cutting speed vopt was determined, this speed was kept unchanged and the feed rate was varied, 0.25 < f < 0.50 mm/rev, to determine the optimum feed fopt. To establish a reference point, a test was carried out at 200 m/ min and 0.25 mm/rev without laser heating. Following ISO 3685 [10], an average flank wear VBa of 300 mm was selected as the tool life criterion. For all tests, the depth of cut (DOC) was kept constant at 0.25 mm. The cutting length Lc was 30 mm, except for the investigation of the tool wear kinetics, where the tests were continued until the end of tool life.
4. Results analyses and discussion 4.1. Optimization of the laser heating process parameters The laser system used in this work has a focal length Lf = 80 mm, which generates a laser spot diameter dL = 0.3 mm. The dependence of dL on the distance DD from the focal length was first established. Laser heating tests were then carried out for a wide range of process variables: 100 < v < 500 m/min, and 0.1 < f < 1 mm/rev. By changing the laser power PL between 2.5 and 3 kW and the laser spot diameter, 0.3 < dL < 3 mm, the following ranges of laser power density pL and surface temperatures Ts were obtained: 0.43 < pL < 43 kW/mm2, and 220 < Ts < 880 8C, respectively. It was concluded that a power density of 0.96 W/mm2 (obtained at PL = 3 kW and dL = 2 mm) provided the desired surface temperatures of 650–700 8C. Greater power densities showed signs of plasma gas generation and surface damage. 4.2. Chip formation and morphology The microstructure of sections of the continuous ribbon-like chips was examined (Table 1). The analysis showed that chips produced by LAM exhibit more tendency to shear localization and larger strain gs in the primary shear zone due to the thermal softening effect of LAM. Increasing the cutting speed results, however, in thinner chips, i.e., lower gs. As the cutting speed increases, the cutting temperature increases but the absorption of the laser radiation heat is reduced. Fig. 2 shows that the net effect is a reduction in the surface temperature Ts. At speeds above 300 m/ min, the temperature level required to cause significant softening effect (650–700 8C) is not reached. The formation of shear localized chips (saw-tooth type) is also attributed to the favorable cutting conditions of large undeformed chip thickness, negative rake angle, and high cutting speeds that promote the fracture of the material [9]. The negative effect of the cutting speed on the shear strain, and consequently strain hardening, was further validated through the measurement of the average micro-hardness of the chip. These measurements showed a slight reduction in the hardness from HRC 41.2 to 39 as the cutting speed increases from 200 to 500 m/ min under LAM conditions. This is compared to an average hardness of HRC 45.1 at 200 m/min in the absence of laser heating. The effect of feed on LAM was found to be more significant; at v =300 m/min, an increase in the feed from 0.25 to 0.5 mm/rev resulted in an increase in HRC from 39.1 to 48.9.
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Table 1 Effect of cutting conditions on chip morphology.
Fig. 2. IR Surface temperature measurements at different cutting speeds, for conventional and LAM machining (at feed = 0.25 mm/rev).
4.3. Effect of cutting speed on cutting forces, tool wear and roughness: Optimum conditions Fig. 3 shows the effect of the cutting speed on the cutting force components at a fixed feed rate of 0.25 mm/rev. Increasing the cutting speed up to 300 m/min results in a reduction in the cutting force due to the reduction in shear strain gs, as shown earlier. The radial and feed forces were not significantly affected by the cutting speed; yet they show a significant drop when
Fig. 3. Effect of cutting speed on cutting forces for conventional and LAM machining (feed = 0.25 mm/rev).
compared to conventional machining. As expected, round inserts produce relatively large radial forces. Above 300 m/ min, the forces show no change with the increase in the cutting speed, since the workpiece surface temperature Ts in the cutting zone is below the critical temperature range of 650–700 8C, as shown in Fig. 2. The SEM images shown in Fig. 4 for the cutting conditions of 300 m/min and 0.4 mm/rev show that the dominant tool wear modes were abrasive and adhesive flank wear. In comparison with
Fig. 4. SEM image of failed ceramic tool at 300 m/min and 0.4 mm/rev under (a) conventional and (b) LAM machining conditions.
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Fig. 5. Effect of cutting speed on flank wear and surface roughness for conventional machining and LAM, feed = 0.25 mm/rev and DOC = 0.25 mm.
conventional machining, the tool failure in laser-assisted machining exhibits less severe and more uniform flank wear, due to the reduced cutting forces and the expected favorable thermal conditions (the shift in the location of the maximum temperature away from the tool tip, and the more uniform temperature distribution on the tool rake and flank surfaces). Also with LAM, the hard intermetallic phases and carbides particles in the workpiece material become less abrasive due to the thermal effect. The improved tribological performance of the tool results in a lower surface roughness, as will be shown later. The X-ray chemical spectroscopy performed on the cutting tool used in the LAM tests confirmed the adhesion of the workpiece material to the rake face and the tool cutting edge. Edge chipping and severe crater wear, which were observed by the authors in [8] for coated carbide tools in LAM and conventional machining, were not observed here. Fig. 5 shows the dependence of flank wear and surface roughness on the cutting speed for a fixed length of cut of 30 mm. It can be seen that there is a significant drop in VBa under LAM conditions, when compared to conventional machining. Above 300 m/min, the tool wear on the flank face and the surface roughness increase with the increase in the cutting speed, since the surface temperatures are not sufficient to reach the stability temperature of the g0 and g00
phases. The lack of notching and excessive flank wear led to the low roughness of the machined surface. Additionally, the contact length of the round insert is relatively large, compared to other insert shapes. This results in feed marks of smaller heights and, consequently, better surface finish. One can thus conclude that the LAM optimum cutting speed for the SiAlON ceramic tool is 300 m/ min, from the cutting forces, surface finish and tool wear points of view. 4.4. Effect of feed on cutting forces, tool wear and roughness: optimum conditions At the optimum cutting speed vopt of 300 m/min, Fig. 6 shows that the cutting force components increase with the increase in the feed, as thicker chips are produced. Measurement of the surface temperature Ts showed that as the feed increases from 0.25 to 0.5 mm/rev, Ts is reduced from 695 to 590 8C, due to the shorter interaction time between the laser beam and the workpiece surface. Fig. 7 shows that as the feed increases from 0.25 to 0.4 mm/rev, the flank wear VBa is reduced, since thicker chips create more uniform contact pressure distribution between the chip and the
Fig. 6. Effect of feed on cutting for LAM, at a cutting speed of 300 m/min.
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Fig. 7. Effect of feed on average flank wear and surface roughness (LAM, cutting speed of 300 m/min).
cutting edge. Above a feed of 0.4 mm/rev, however, the reduced thermal softening effect caused by lower radiation heat absorption, causes the tool wear to increase. Fig. 7 shows also that increasing the feed from 0.25 to 0.40 mm/rev reduces the surface roughness by 25%, since laser heating facilitates the material removal and avoids smearing and surface tearing, as will be seen later. In addition, the relatively large size of the laser beam spot (2 mm) reduces the sharpness of the feed marks on the machined surface [2]. By increasing the feed from 0.4 to 0.5 mm/rev, the roughness increases to Ra = 0.64 mm. This pattern follows the change in tool wear with feed, and explains the deviation from the theoretical predictions, where surface roughness is proportional to the square of the feed [9]. As a conclusion, the optimum LAM finishing cutting conditions using SiAlON ceramic insert (vopt = 300 m/min and fopt = 0.4 mm/rev, at depth of cut of 0.25 mm) produced practically the lowest surface roughness, tool wear and cutting forces. It is also estimated that the material removal rate (MRR) increases by approximately 800%. Additional tests were carried out to compare LAM to conventional machining, in terms of the evolution of the surface roughness and tool wear, at the optimum cutting conditions
(v = 300 m/min, f = 0.4 mm/rev). The results showed that the end of tool life (VBa = 300 mm) is reached at a cutting length Lc = 310 mm in conventional turning. Under LAM conditions, the tool life is increased by 40% to Lc = 430 mm. It was also observed that at Lc = 150–160 mm, the surface roughness reaches a minimum value of Ra = 0.44 and 0.31 mm, for conventional and laser-assisted machining, respectively. At the end of tool life, the surface roughness increases to 0.92 and 0.88 mm, respectively. 4.5. Surface integrity Fig. 8 shows the microstructure of the machined surface produced at the optimum cutting conditions of 300 m/min and 0.4 mm/rev, at the end of the tool life. Only the surface produced by conventional machining showed heavy smearing (Fig. 8(a)). Under LAM conditions, the plastically deformed surface layer is deeper (dp = 75 mm, vs. 63 mm for conventional machining) and more uniform. The absence of smeared material and the increased plastic deformation zone are indicative of the favorable compressive residual stresses that are promoted by round inserts due to ploughing and the negative rake angle [11]. The SEM images (insets, Fig. 8) show no change in precipitate size within the matrix,
Fig. 8. Optical micrograph of the machined surface (300 m/min, 0.4 mm/rev) under: (a) conventional and (b) LAM machining conditions.
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Fig. 9. X-ray chemical spectroscopy of the machined surface (300 m/min, 0.4 mm/rev) under: (a) conventional and (b) LAM machining conditions.
in comparison to the bulk material. Although the grain boundary precipitates were elongated due to the plastic deformation, they did not show a loss of coherency. In the conventional and LAM operations, there was no sign of overheating/burning, macro- and micro-cracks, cavities, or microdefects. X-ray chemical spectroscopy showed that there is no significant change in the chemical constituents of the surfaces generated by laser-assisted machining (Fig. 9). Comparison with the bulk material also indicated that no phase change took place within the machined surface. Micro-hardness measurement of the plastically deformed surface layer showed that higher hardness levels (40 HRC) were obtained on the surface and decreases to the bulk material nominal hardness value at a depth of 150 mm. This hard surface layer is a result of the strain hardening of the workpiece material under the high pressure and temperatures generated during machining, and the cold working action when the machined surface rapidly cools down. The micro-hardness profile in the surface sub- layer was practically the same for LAM and conventional machining. 5. Concluding remarks The optimum conditions for laser-assisted finish turning of IN718 using SiAlON ceramic tool were established. Under these conditions, a significant drop in the cutting forces is achieved when compared to conventional machining. The surface finish is also improved by more than 25%, and the material removal rate is increased by approximately 800%. In terms of surface integrity, the optimum laser-assisted machining conditions did not introduce phase change overheating, or microdefects. The absence of smeared material, which was observed in conventional machining, and the increased plastic deformation zone are indicative of the favorable compressive residual stresses in the subsurface layer of the machined surface.
Acknowledgements The authors acknowledge the support of the Aerospace Manufacturing Technology Centre (AMTC), Institute for Aerospace Research (IAR), National Research Council Canada (NRC), where the experimental work was carried out. The partial financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC) is greatly appreciated.
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