Laser-Assisted High-speed Finish Turning of Super Alloy Inconel 718 Under Dry Conditions

CIRP Annals - Manufacturing Technology 59 (2010) 83–88

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CIRP Annals - Manufacturing Technology

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Laser-assisted high-speed finish turning of superalloy Inconel 718under dry conditions

H. Attia (2)a,b,*, S. Tavakoli b, R. Vargas a, V. Thomson b

a Aerospace Manufacturing technology Centre, National Research Council of Canada, Montreal, Quebec, Canadab Department of Mechanical Engineering, McGill University, Montreal Quebec, Canada

A R T I C L E I N F O

Keywords:

Cutting

Laser

Machinability

A B S T R A C T

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.

� 2010 CIRP.

1. Introduction

Inconel 718 (IN718) and other nickel-based superalloys arewidely used in the aerospace and nuclear industries due to theirsuperior high temperature strength, toughness, and corrosionresistance. These alloys are difficult to machine due to their lowthermal conductivity and diffusivity, which cause steep tempera-ture gradient at the tool edge and the shift the location of themaximum temperature towards the tool tip. As a result, excessivetool wear, premature cracking and built-up edge formation areobserved. Other factors that contribute to the poor machinabilityof IN718 include the strong tendency to strain hardening duringmachining, the adhesion to the tool material, and the presence ofhard abrasive carbides and intermetallic phases in its micro-structure. The main strengthening mechanism of IN718 is agehardening due to the presence of fine uniform metastable g00

precipitates distributed throughout the matrix. At temperaturesabove 540 8C, the deformation is homogenously distributed and iscomprised of uniformly tangled dislocations. Above 700 8C, theprecipitations reach their limit of stability, causing a significantdrop in the material yield strength [1]. In laser-assisted machining(LAM), which is well suited for difficult-to-machine materials, theworkpiece is subjected to localized heating through a focused laserbeam. This heating improves the machinability through softeningthe workpiece material and reducing tool wear, without causingsubsurface damage [2].

* Corresponding author.

E-mail address: [email protected] (H. Attia).

0007-8506/$ – see front matter � 2010 CIRP.

doi:10.1016/j.cirp.2010.03.093

HSS and cemented WC tools are widely used at cutting speedsbelow 30 m/min. Although PVD multi-layer coated tools allowedhigher cutting speeds above 50 m/min [3], they are susceptible tochipping, edge fracture and nose wear [4]. CVD coated tools alsofail by severe notching and flank wear when used to machineIN718 [3]. CBN and ceramic tool materials are increasingly used tomachine Ni-based alloys in the cutting speed range of 120–300 m/min. CBN tools are, however, sensitive to notching, adhesive wearand excessive diffusion wear [5]. Despite their superior hothardness, ceramic tools showed large flank and solution wear inthe conventional machining of Ni-based alloys [5]. The toolgeometry also plays an important role in determining the tool life.Proper selection of the included and approach angles improves thetool life through their effect on the edge strength, the tool-chipcontact area, and the chip curvature [6]. Compared to rhomboidinserts, round ceramic inserts with negative rake angle weresuccessful for high-speed machining (HSM) of IN718 [7]. While dryHSM of Ni-based alloys is limited by the premature tool wear, theuse of high-pressure cooling causes a reduction in tool life due tothe decrease in the chip–tool contact length and the increase incontact stresses [6].

The main objectives of the present study are (i) to optimize thelaser-assisted high-speed finish turning of IN718, in terms of toollife, surface integrity, and productivity, and (ii) to assess the use ofsilicon nitride ceramic tools in dry HSM to minimize theenvironmental impact and to reduce cost.

2. Experimental setup

The machining tests were performed on a 6-axis BoehringerNG200, CNC turning center (36 kW main spindle and 4000 rpm

Fig. 1. LAM experimental setup: (a) a schematic diagram, and (b) a close-up.

H. Attia et al. / CIRP Annals - Manufacturing Technology 59 (2010) 83–8884

maximum speed). A high power laser beam was generated by1006-D 4 kW Trumpf Nd:YAG laser. A schematic and a close-upof the experimental setup are shown in Fig. 1. The laser head (2)is mounted on a special fixture attached to the turret head (6) tocontrol the orientation of the beam (Ø = 48–50 8C) and its spotsize. A flat nozzle (3), capable of providing 860 kPa of airpressure, was used to protect the laser lens from excess heat andthe chips. For process characterization and control, thetemperature field near the cutting zone was measured using aThermovision A20 infrared (IR) camera (5) with a wavelengthdetection range suitable for temperature measurement up to900 8C. To calibrate and validate the IR measurements duringcutting, two K-type thermocouples were attached to theworkpiece, through a high-speed slip ring (4). A three-component Kistler dynamometer, type 9121, was used tomeasure the cutting forces. Tool wear was measured using anOlympus SZ-X12 stereoscopic microscope. Surface roughnesswas measured after each pass, using a portable Taylor HobsonSurtronic 3+. The measurement errors of various parameters arerepresented in the results shown in Section 4 as error bars. Thesolution 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 and6.35 mm radius (Kennametal, KY1540) was selected due to itsinsensitivity to notch wear.

3. Experimental approach

The starting point in this study was the optimal conventionalcutting conditions (v = 200 m/min and f = 0.25 mm/rev) estab-lished by the authors in [8] for finish turning of IN718, using coatedcarbide 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 feedrate, f = 0.25 mm/rev. After the optimum cutting speed vopt wasdetermined, this speed was kept unchanged and the feed rate wasvaried, 0.25 < f < 0.50 mm/rev, to determine the optimum feedfopt. 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 toollife criterion. For all tests, the depth of cut (DOC) was kept constantat 0.25 mm. The cutting length Lc was 30 mm, except for theinvestigation of the tool wear kinetics, where the tests werecontinued 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 dependenceof dL on the distance DD from the focal length was first established.Laser heating tests were then carried out for a wide range of processvariables: 100 < v < 500 m/min, and 0.1< f < 1 mm/rev. By chan-ging the laser power PL between 2.5 and 3 kW and the laser spotdiameter, 0.3 < dL < 3 mm, the following ranges of laser powerdensity pL and surface temperatures Ts were obtained:0.43 < pL < 43 kW/mm2, and 220 < Ts < 880 8C, respectively. Itwas concluded that a power density of 0.96 W/mm2 (obtained atPL = 3 kW and dL = 2 mm) provided the desired surface temperaturesof 650–700 8C. Greater power densities showed signs of plasma gasgeneration and surface damage.

4.2. Chip formation and morphology

The microstructure of sections of the continuous ribbon-likechips was examined (Table 1). The analysis showed that chipsproduced by LAM exhibit more tendency to shear localization andlarger strain gs in the primary shear zone due to the thermalsoftening effect of LAM. Increasing the cutting speed results,however, in thinner chips, i.e., lower gs. As the cutting speedincreases, the cutting temperature increases but the absorption ofthe laser radiation heat is reduced. Fig. 2 shows that the net effect isa reduction in the surface temperature Ts. At speeds above 300 m/min, the temperature level required to cause significant softeningeffect (650–700 8C) is not reached. The formation of shear localizedchips (saw-tooth type) is also attributed to the favorable cuttingconditions of large undeformed chip thickness, negative rakeangle, and high cutting speeds that promote the fracture of thematerial [9].

The negative effect of the cutting speed on the shear strain, andconsequently strain hardening, was further validated through themeasurement of the average micro-hardness of the chip. Thesemeasurements showed a slight reduction in the hardness fromHRC 41.2 to 39 as the cutting speed increases from 200 to 500 m/min under LAM conditions. This is compared to an averagehardness 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; atv =300 m/min, an increase in the feed from 0.25 to 0.5 mm/revresulted in an increase in HRC from 39.1 to 48.9.

Table 1Effect 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). Fig. 3. Effect of cutting speed on cutting forces for conventional and LAM machining

(feed = 0.25 mm/rev).

H. Attia et al. / CIRP Annals - Manufacturing Technology 59 (2010) 83–88 85

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 cuttingforce components at a fixed feed rate of 0.25 mm/rev. Increasingthe cutting speed up to 300 m/min results in a reduction in thecutting force due to the reduction in shear strain gs, as shownearlier. The radial and feed forces were not significantly affectedby the cutting speed; yet they show a significant drop when

Fig. 4. SEM image of failed ceramic tool at 300 m/min and 0.4 mm/

compared to conventional machining. As expected, roundinserts produce relatively large radial forces. Above 300 m/min, the forces show no change with the increase in the cuttingspeed, since the workpiece surface temperature Ts in the cuttingzone is below the critical temperature range of 650–700 8C, asshown in Fig. 2.

The SEM images shown in Fig. 4 for the cutting conditions of300 m/min and 0.4 mm/rev show that the dominant tool wearmodes were abrasive and adhesive flank wear. In comparison with

rev under (a) conventional and (b) LAM machining conditions.

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.

H. Attia et al. / CIRP Annals - Manufacturing Technology 59 (2010) 83–8886

conventional machining, the tool failure in laser-assisted machin-ing exhibits less severe and more uniform flank wear, due to thereduced cutting forces and the expected favorable thermalconditions (the shift in the location of the maximum temperatureaway from the tool tip, and the more uniform temperaturedistribution on the tool rake and flank surfaces). Also with LAM, thehard intermetallic phases and carbides particles in the workpiecematerial become less abrasive due to the thermal effect. Theimproved tribological performance of the tool results in a lowersurface roughness, as will be shown later. The X-ray chemicalspectroscopy performed on the cutting tool used in the LAM testsconfirmed the adhesion of the workpiece material to the rake faceand the tool cutting edge. Edge chipping and severe crater wear,which were observed by the authors in [8] for coated carbide toolsin LAM and conventional machining, were not observed here. Fig. 5shows the dependence of flank wear and surface roughness on thecutting speed for a fixed length of cut of 30 mm. It can be seen thatthere is a significant drop in VBa under LAM conditions, whencompared to conventional machining. Above 300 m/min, the toolwear on the flank face and the surface roughness increase with theincrease in the cutting speed, since the surface temperatures arenot sufficient to reach the stability temperature of the g0 and g00

Fig. 6. Effect of feed on cutting for LAM

phases. The lack of notching and excessive flank wear led to the lowroughness of the machined surface. Additionally, the contactlength of the round insert is relatively large, compared to otherinsert shapes. This results in feed marks of smaller heights and,consequently, better surface finish. One can thus conclude that theLAM optimum cutting speed for the SiAlON ceramic tool is 300 m/min, from the cutting forces, surface finish and tool wear points ofview.

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 showsthat the cutting force components increase with the increase in thefeed, as thicker chips are produced. Measurement of the surfacetemperature Ts showed that as the feed increases from 0.25 to0.5 mm/rev, Ts is reduced from 695 to 590 8C, due to the shorterinteraction time between the laser beam and the workpiecesurface.

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 moreuniform contact pressure distribution between the chip and the

, at a cutting speed of 300 m/min.

Fig. 7. Effect of feed on average flank wear and surface roughness (LAM, cutting speed of 300 m/min).

H. Attia et al. / CIRP Annals - Manufacturing Technology 59 (2010) 83–88 87

cutting edge. Above a feed of 0.4 mm/rev, however, the reducedthermal softening effect caused by lower radiation heatabsorption, causes the tool wear to increase. Fig. 7 shows alsothat increasing the feed from 0.25 to 0.40 mm/rev reduces thesurface roughness by 25%, since laser heating facilitates thematerial removal and avoids smearing and surface tearing, aswill be seen later. In addition, the relatively large size of the laserbeam spot (2 mm) reduces the sharpness of the feed marks onthe machined surface [2]. By increasing the feed from 0.4 to0.5 mm/rev, the roughness increases to Ra = 0.64 mm. Thispattern follows the change in tool wear with feed, and explainsthe deviation from the theoretical predictions, where surfaceroughness is proportional to the square of the feed [9]. As aconclusion, the optimum LAM finishing cutting conditions usingSiAlON ceramic insert (vopt = 300 m/min and fopt = 0.4 mm/rev,at depth of cut of 0.25 mm) produced practically the lowestsurface roughness, tool wear and cutting forces. It is alsoestimated that the material removal rate (MRR) increases byapproximately 800%.

Additional tests were carried out to compare LAM to conven-tional machining, in terms of the evolution of the surfaceroughness and tool wear, at the optimum cutting conditions

Fig. 8. Optical micrograph of the machined surface (300 m/min, 0.4 mm

(v = 300 m/min, f = 0.4 mm/rev). The results showed that the end oftool life (VBa = 300 mm) is reached at a cutting length Lc = 310 mmin conventional turning. Under LAM conditions, the tool life isincreased by 40% to Lc = 430 mm. It was also observed that atLc = 150–160 mm, the surface roughness reaches a minimum valueof Ra = 0.44 and 0.31 mm, for conventional and laser-assistedmachining, respectively. At the end of tool life, the surfaceroughness increases to 0.92 and 0.88 mm, respectively.

4.5. Surface integrity

Fig. 8 shows the microstructure of the machined surfaceproduced at the optimum cutting conditions of 300 m/min and0.4 mm/rev, at the end of the tool life. Only the surface produced byconventional machining showed heavy smearing (Fig. 8(a)). UnderLAM conditions, the plastically deformed surface layer is deeper(dp = 75 mm, vs. 63 mm for conventional machining) and moreuniform. The absence of smeared material and the increasedplastic deformation zone are indicative of the favorable compres-sive residual stresses that are promoted by round inserts due toploughing and the negative rake angle [11]. The SEM images(insets, Fig. 8) show no change in precipitate size within the matrix,

/rev) under: (a) conventional and (b) LAM machining conditions.

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.

H. Attia et al. / CIRP Annals - Manufacturing Technology 59 (2010) 83–8888

in comparison to the bulk material. Although the grain boundaryprecipitates were elongated due to the plastic deformation, theydid not show a loss of coherency. In the conventional and LAMoperations, there was no sign of overheating/burning, macro- andmicro-cracks, cavities, or microdefects.

X-ray chemical spectroscopy showed that there is no significantchange in the chemical constituents of the surfaces generated bylaser-assisted machining (Fig. 9). Comparison with the bulkmaterial also indicated that no phase change took place withinthe machined surface. Micro-hardness measurement of theplastically deformed surface layer showed that higher hardnesslevels (40 HRC) were obtained on the surface and decreases to thebulk material nominal hardness value at a depth of �150 mm. Thishard surface layer is a result of the strain hardening of theworkpiece material under the high pressure and temperaturesgenerated during machining, and the cold working action when themachined surface rapidly cools down. The micro-hardness profilein the surface sub- layer was practically the same for LAM andconventional machining.

5. Concluding remarks

The optimum conditions for laser-assisted finish turning ofIN718 using SiAlON ceramic tool were established. Under theseconditions, a significant drop in the cutting forces is achieved whencompared to conventional machining. The surface finish is alsoimproved by more than 25%, and the material removal rate isincreased by approximately 800%. In terms of surface integrity, theoptimum laser-assisted machining conditions did not introducephase change overheating, or microdefects. The absence ofsmeared material, which was observed in conventional machining,and the increased plastic deformation zone are indicative of thefavorable compressive residual stresses in the subsurface layer ofthe machined surface.

Acknowledgements

The authors acknowledge the support of the AerospaceManufacturing Technology Centre (AMTC), Institute for AerospaceResearch (IAR), National Research Council Canada (NRC), wherethe experimental work was carried out. The partial financialsupport of the Natural Sciences and Engineering Research Councilof Canada (NSERC) is greatly appreciated.

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