Laser-assisted machining of Inconel 718 with carbide and ceramic ...

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1.3 Experimental equipmentA numerically controlled lathe (REALMECA RT-5)equipped with a 2.5 kW ROFIN YAG laser was usedin this work. The laser nozzle can be controlled usingthree translations and two rotations. A numericalcontrol system (NUM 1060) allows the control of theseven independent degrees of freedom. The highpowerlaser beam is delivered through a fibre opticcable to the lathe chamber, where it is focused on theworkpiece surface. During machining, a gas underpressure (air) is forced through the laser nozzle toprotect the focusing lens from being damaged bychips. Three components of the cutting force aremeasured by a Kistler piezoelectric dynamometer.1.4 Material investigatedThe material under investigation is the nickel alloyNiCr19FeNb which is known commercially asInconel 718. It has been structurally hardened via aheat treatment consisting of quenching (from 940 to1010 ° C) and structural hardening (720 ° C for 8 hrfollowed by 620 ° C for 8 h). The microstructure isaustenitic with a grain size index of G=10. Thehardness of the material is uniform and has anaverage value of 482 HV (46 HRc).1.5 Cutting parameters used with the carbide insertTests were carried out with the cutting parametersrecommended by the CETIM for this type of insertand with several laser Powers: 0 Watts (conventionalmachining), 1165 and 1975 Watts. The cuttingparameters recommended are: Carbide tool insertKC5525 (Kennametal) ref. CNMG 120412 RP;advance, f = 0.2 mm/rev; cutting depth, ap = 2.5mm; cutting speed, Vc = 30 m/min; withoutlubrication. Material removal rate of 15 cm 3 .min -1 . Anew cutting edge was used for each test. The laserbeam was focused on the chamfered cut surfaceapproximately 5 mm from the cutting tool.The three components of the cutting force, thesurface integrity (roughness, residual stresses), andthe tool life were measured for each test.1.6 Parameters used with the ceramic insertSimilarly, the cutting parameters used for theceramic insert are those recommended by theCETIM: CC670 ceramic insert (Sandvik) ref. RNGN090300; advance, f = 0.18 mm/rev; cutting depth, ap= 1.5 mm; cutting speed, Vc = 220 m/min; withoutlubrication. Material removal rate of 59.4 cm 3 .min -1 .The tests were carried out without laser assistanceand with an assistance of 1500 Watts. The evolutionof the tool wear, the three components of the cuttingforce, and the surface roughness, were measured foreach test. The tool life and residual surface stresseswere also determined.2 RESULTS OBTAINED WITH THE CARBIDEINSERT2.1 Cutting force (carbide insert)The results show a decrease magnitude of the cuttingforce with the laser power (Figure.3). Specifically,there is a decrease of 6.5% with a laser power of1165 W and 10.8% with a power of 1975 W.Force intensity (N)180017001600150014001300120011001000900800700600500400Axiale forceTangentiale forceRadiale forceCutting force (norme)0 500 1000 1500 2000Laser power (W)Fig 3: Evolution of the three components and the magnitude ofthe cutting force as a function of the laser power2.2 Surface Integrity (carbide insert)The impact of laser assistance on the surfaceintegrity has been quantified by the evolution ofseveral roughness criteria and the residual stresseson the cut surface. The three surface roughnesscriteria used are: the arithmetic average (Ra), themaximum height peak/valley (Rmax) and theaverage height peak/valley (Rz). The followingfigure (Figure 4) shows that laser assistance caninfluence the surface roughness of the workpiece. Ahigh laser power results in a slight improvement ofthe roughness criteria.Roughness (µm)6543210Withoutassistance1165 W 1975 WLaser power (W)RaRmaxRzFig 4: Evolution of the surface roughness as a function of laserpowerThe residual stresses were determined by X-raydiffraction. The analysis was carried out for the testwithout assistance (conventional machining - CM),
and with the maximum laser power. The valuesobtained are summarised in the following table(Table1).Table 1. Residual Stresses values on the cut surfaceStress (MPa)CMLAM1975 WAxial direction 740 ± 25 520 ± 30Tangential direction 435 ± 45 440 ± 45The table shows that the residual stresses on thesurface are positive (tensile stresses) for the twotests. The laser assistance decreases the residualstress in the axial direction.2.3 Tool wear (carbide insert)These initial results demonstrate an improvement inthe machinability of Inconel 718 with laserassistance. However, the improvement is modest andthere is a significant drop in tool life with the use oflaser assistance. Indeed, the tool life is only 50secwith the laser assistance, compared to approximately2min 50sec in conventional machining. In bothcases, the insert deteriorates very quickly with analmost instant failure of the cutting edge.3 RESULTS OBTAINED WITH THE CERAMICINSERT3.1 Cutting force (ceramic insert)The figure below (Figure 5) shows the evolution ofthe magnitude of the cutting force as a function ofthe distance machined for both conventionalmachining and LAM (1500 Watts). The cuttingforce in LAM is lower than in CM. Depending ontool wear, the reduction in the cutting force in UALis between 40% and 20%.Cutting force (N)240019001400900400LAM (1500 W)Without Laser (CM)0 200 400 600 800 1000 1200 1400 1600 1800Length machined (m)Fig 5: Evolution of the magnitude of the cutting force, in CMand in LAM, as a function of the distance machinedInserts have machined respectively 1500m in LAMand 1650m in conventional machining prior to thecollapse of the cutting edge. The beginning of thedeteriorated area appears more rapidly in LAM(1200 m) than CM (1400 m). This deteriorationresults in an increase of the radial and axialcomponents of the cutting force, but not thetangential component.3.2 Surface Integrity (ceramic insert)The surface integrity was quantified by themeasurement of surface roughness (three criteria:Ra, Rmax and Rz), by the determination of surfaceresidual stresses and by examination of the cutsurface with a scanning electron microscope (SEM).The three roughness criteria show the same trend.Consequently only the Ra criterion is presented infigure 6.Roughness, Ra (µm)3,53,02,52,01,51,00,5LAM (1500W)Without Laser (CM)0,00 200 400 600 800 1000 1200 1400 1600 1800Length machined (m)Fig 6: Evolution of the surface roughness (Ra), in CM andLAM, as a function of the distance machinedIt can be seen that the evolutions of Ra, in CM andLAM, are similar up to a machined distance ofapproximately 800 m. After this distance, theroughness is lower and less dispersed for LAM. Forconventional machining after approximately 800 m,the roughness increase significantly and the data iswidely scattered (scratches due to chips).The analysis of residual stress was performed usinga PROTO X-ray diffraction machine. The followingtable (Table 2) summarises the values of residualstress at the surface for the two types of machining.Table 2: Surface residual stressesStress (MPa)CMLAM1500 WAxial direction 160 ± 35 260 ± 35Tangential direction 270 ± 30 555 ± 45These values indicate that these cutting parametersresult in positive or tensile residual stresses in bothdirections. In addition, LAM generates higherstresses due to thermal effects of laser heating.These values confirm the results obtained in othermaterials [9] and those found during the tests withcarbide inserts.The machined surfaces were observed with ascanning electron microscope. The figure 7 belowshows pictures of the machined surfaces in CM (a)
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