Investigation of Surface Integrity during Turning Inconel … · INVESTIGATION OF SURFACE INTEGRITY...

INVESTIGATION OF SURFACE INTEGRITY DURING TURNING INCONEL 718

M. Anthony Xavior1, M. Manohar2, Mahesh Madhukar Patil1 and P. Jeyapandiarajan11Manufacturing Department, School of Mechanical Engineering, VIT University, Vellore, India

2Materials & Mechanical Entity, VSSC-ISRO, Trivandrum, IndiaE-mail: [email protected]

Received November 2016, Accepted April 2017No. 16-CSME-124, E.I.C. Accession 4010

ABSTRACTSurface roughness and residual stress are considered to be major surface integrity issues that directly affectthe quality and life of the components. The current research work highlights surface quality and residualstresses induced while machining Inconel 718 with a range of cutting parameters and cutting environments.Further, the study aimed to determine the optimal parameters/conditions in terms of cutting speeds, toolmaterials and cutting conditions to achieve better surface quality and minimum residual stress values. Min-imum quantity lubrication resulted in minimum residual values for all cutting inserts and cutting velocities.The minimum surface roughness was obtained while machining at 100 m/min using a carbide insert underflood cooling condition.

Keywords: machining; Inconel 718; surface integrity.

ÉTUDE SUR L’INTÉGRITÉ DE LA SURFACE DURANT LE TOURNAGE DE L’INCONEL 718

RÉSUMÉLa rugosité de la surface et la contrainte résiduelle sont considérées comme étant la question majeure dansl’intégrité de la surface affectant directement la qualité de vie des composants. L’étude en cours met l’accentsur la qualité de la surface et la contrainte résiduelle causée pendant l’usinage de l’Inconel 718, avec unegamme de paramètres de coupe ainsi que l’environnement autour de la coupe. En outre, le but de l’étudeest de déterminer les paramètres et conditions optimales du point de vue de vitesse de coupe, des matériauxdes outils et des conditions de coupe pour parvenir à une meilleure qualité de la surface et des valeursde contraintes résiduelles minimales. Une quantité minimum de lubrifiant a eu pour résultat des valeursrésiduelles minimales pour toutes les plaquettes de coupe et les vitesses de la coupe. Le minimum de rugositéde la surface obtenu pendant l’usinage est 100 m/min utilisant des plaquettes en carbure et des fluidesrefroidissants.

Mots-clés : usinage; Inconel 718; intégrité de la surface.

Transactions of the Canadian Society for Mechanical Engineering, Vol. 41, No. 3, 2017 387

1. INTRODUCTION

Inconel 718 is very popular in the aerospace industry owing to its advantages over other nickel-based alloysand titanium-based alloys [1]. The key properties of Inconel 718 are high thermal resistance, strength, andtoughness at operating temperatures and also high corrosion resistance. It is known that almost half of thecomponents used in engines are composed of Inconel 718 alloy [2]. The major problems in the applicationof Inconel 718 alloy, are its low thermal conductivity which develops more heat and increases thermal effectsduring machining, its work hardening behavior and adhesive nature on the tool face [3]. Surface integritycan be divided into three main categories namely surface roughness, residual stress and microstructuraltransformation [4]. The main threat to surface integrity come from plastic deformation in the workpieceduring turning operation and it is very important to analyze the effect of such deformation. This type ofdeformation is caused and supported by many controllable and uncontrollable parameters such as cuttingparameters (cutting velocity, feed, depth of cut), cutting tool parameters (rake angle, corner radius, tool style,coatings.) and work material properties (grain size, heat treatment, chemical composition) [5]. Considerableresearch conducted to find the causes of plastic deformation report the main reasons to be localized heating(thermal effect) and high stress due to the mechanical load (mechanical effect) [6].

The factors affecting surface roughness are cutting tool geometry, depth of cut, cutting velocity, feed rate,work material chemical composition, and rigidity of machine tool [7]. During the machining process, ifthe tool material and cutting parameters are not selected properly then the cutting tool wears out quicklyor experiences catastrophic failure. A good combination of cutting parameters, tool materials and cuttingenvironments assures better surface quality [4]. Material built-up at the edge of the cutting tool due to hightemperature during turning/machining of Inconel 718 cannot be avoided completely. The rate can be min-imized by using a proper cooling/lubrication environment that help to reduce the friction between the tooland work material, and carry the chips and heat away from machining zone, which results in improved sur-face integrity to some extent [8]. While machining with carbide inserts, coatings allow an increase in cuttingspeeds up to 100 m/min. Application of coatings on the substrate is an approach to increase the efficiencyof machining Inconel 718 [9]. Typical surface alterations such as phase transformations, microhardness,and residual stresses during machining of Nickel based alloy materials are discussed and correlated with thefunctional performance of the machined products [10, 11]. The influence of process parameters on residualstresses during machining of Inconel 718 was investigated and the influence of friction on the stress profileswas reported [12]. Ulutan et al. [13] introduced a method of understanding the residual stress profile in termsof quantifiable key measures: peak tensile stress at the surface, magnitude and depth of peak compressivestress, and depth at which the residual stress becomes near-zero.

During machining, the work material is subjected to external loading: mechanical loading and/or thermalloading. Subsequently, after removal of this external load, the material experiences some amount of stresswhich is the residue of the applied load known as residual stress. This stress might be tensile or compressivein nature depending on many parameters. The residual stress can vary inside the grains because of thepresence of inclusions, dislocations, and stacking faults. Further, residual stress can vary along the crystal(or grains) because of grain orientation and each grain having specific elastic and plastic properties [14].Compressive residual stress is induced by pushing material grains/crystals together while tensile residualstress forces the material grains/crystals to be pulled apart from each other. Stresses are characterized byeither shear stresses (that act parallel to face of material) or normal stresses (that act perpendicular to theface of the material). At any point inside a material, there exist six independent stresses (three normaland three shear) [15]. In machining of nickel-based super alloy, surface roughness and residual stress areoften considered as the significant indication of surface integrity [16]. During machining, induced tensileresidual stresses are the main concern, which directly affects the fatigue life of the components. On theother hand, compressive residual stresses are beneficial, which improves the fatigue life by preventing crack

388 Transactions of the Canadian Society for Mechanical Engineering, Vol. 41, No. 3, 2017

Fig. 1. Experimental setup on CNC lathe.

Table 1. Chemical composition for Inconel 718 (%).Ni Co Cr Mo Fe Si Mn C Al Ti Cu P B S

50-55 1 17-21 3.05 *bal. 0.35 0.35 0.8 0.6 0.9 0.3 0.015 0.006 0.015

nucleation within the component [17]. This research work is aimed to investigate the surface integrity issuesunder different cutting conditions (dry, minimum quantity lubrication and flood) with different cutting toolmaterials (carbide, ceramic and cBN). Residual stress and surface roughness data are analyzed and reportedspecifically.

2. EXPERIMENTAL SETUP

The work material used for the experiments was Inconel 718 with dimensions of 40 mm diameter and60 mm length with the chemical composition as shown in Table 1. The turning experiments were carriedout on an ACE Micromatic – CNC lathe machine with spindle speed range of 50–4000 rpm, as shown inFig. 1. The experiments were carried out according to Taguchi’s L18 (21× 37) orthogonal array. The cut-ting tool/inserts used in the experimentations were PVD AlTiN coated carbide insert – CNMG120408MS(KCU25); ceramic insert – CNGA120408T01020 (KY4300); and cBN insert – CNGA120408S01025MT(KB1630) with constant tool geometry, supplied by Kennametal. Experiments were conducted for threedifferent sets of cooling/ lubrication conditions: dry cutting condition in the ambient environment, MQLcutting condition with 80 ml/hr flow rate, and flood cooling condition with 100–110 l/hr flow rate. Ester oiland semi-synthetic water soluble cutting fluid was used in MQL and flood cooling cutting conditions respec-tively. Machined work pieces were cut into small specimens with the help of a wire EDM process and thesesamples were used for residual stress measurement. Table 2 shows the cutting parameters/levels/conditionsconsidered for the experiments.

3. RESULTS AND DISCUSSIONS

Induced residual stresses in the machined specimens (on the surface and subsurface) were measured byusing a Bruker D8 DISCOVER X-ray diffraction (XRD) unit. In order to ensure the accuracy of the data,


Table 2. Cutting parameters/levels/conditions. Feed ( f ) = 0.15 mm/rev and depth of cut = 0.5 mm were kept constantthroughout the experimentations.

Parameters Level 1 Level 2 Level 31. Cutting speed (Vc) m/min 100 150 –2. Tool (insert) material Coated carbide Ceramic cBN3. Cutting condition Dry MQL Flood cooling

Table 3. Taguchi L18 orthogonal array with experimental conditions and observed data.Cutting Speed Cutting Tool Surface Roughness Surface Residual(m/min) Cond. Material (Ra) (µm) Stress (MPa)100 Dry Carbide 1.96 716.4

Ceramic 1.21 780.0cBN 1.32 596.0

MQL Carbide 1.35 748.0Ceramic 1.26 670.0cBN 1.36 487.0

Flood Carbide 0.43 949.9Ceramic 1.72 804.1cBN 0.89 556.0

150 Dry Carbide 1.84 –185.4Ceramic 1.12 –192.3cBN 1.17 –188.4

MQL Carbide 1.29 –371.1Ceramic 1.09 –358.7cBN 1.02 –505.0

Flood Carbide 0.76 –392.2Ceramic 1.93 –447.0cBN 0.89 –213.9

the measurement was repeated three times at the same data point and the average value was recorded forfurther analysis. For surface quality measurement a Mitutoyo surface roughness tester SJ-301 model wasused. The surface quality of machined samples was measured in the horizontal direction with a constant4 mm measuring length. The surface roughness was measured at three different places located 120◦ aparton the periphery of each specimen and the average value was recorded. The experimental results for surfaceroughness (Ra) and surface residual stresses are shown in Table 3.

3.1. Surface Roughness AnalysisSurface texture or surface quality mainly depends on cutting parameters, cutting conditions and the cuttingenvironments. Figure 2 presents the detailed analysis of surface roughness obtained for different cuttingconditions, cutting inserts and cutting speeds. During the dry cutting condition, it was observed that withan increase in cutting speed the surface roughness value decreased (i.e, better surface quality observed withan increase in cutting velocity). The ceramic and cBN tools show better surface quality. For MQL cuttingcondition at low cutting speed (Vc = 100 m/min) better surface roughness values were observed. For all cut-ting tool materials, a similar range of surface roughness values were observed. Using the minimum quantitylubrication (MQL) method, tiny pressurized air-suspended oil droplets enter and spread at the cutting zoneeasily forming a very thin lubrication layer between tool face and work material that helps reduce frictionbetween tool and work material, preventing abrasion and formation of built-up edge [8].


Fig. 2. Analysis of surface roughness obtained for different cutting conditions and cutting inserts.

MQL works more effectively at a moderate cutting speed range (Vc = 90–120 m/min). At this range,the air-suspended oil droplets spread more homogeneously at the cutting zone and maintain a continuouslubrication layer during machining [18]. In flood cooling cutting condition, lower surface roughness valueswere observed for carbide and cBN tools while higher values were observed for the ceramic tool. This maybe attributed to more heating effects experienced by the work material alone as ceramic does not take anypart in heat conduction during machining. For the dry cutting condition the highest surface roughness valueswere observed while for the flood cooling condition overall less surface roughness values were observed.During MQL cutting condition there is no noticeable variation in the surface roughness value with changein cutting speeds and tool materials. The surface roughness is reduced by 10–25% in MQL when comparedwith dry machining.

3.2. Residual Stress AnalysisApart from surface roughness, the other main aspect of surface integrity is the induced residual stresses.Therefore, in this work, an attempt has been made to analyze the distribution and intensity of residualstresses from the surface to the subsurface depth. For the carbide tool at cutting speed Vc = 100 m/min peaktensile and compressive values were observed under flood cooling cutting conditions as shown in Fig. 3a.For the ceramic tool at cutting speed Vc = 100 m/min higher tensile stresses were observed up to a higherdepth from the machined surface under flood cooling cutting condition as shown in Fig. 3b. For the cBNtool at cutting speed Vc = 100 m/min significantly lower tensile stresses were observed. On the other hand,higher compressive stresses were observed as shown in Fig. 3c. Generally, the tensile stresses are dominantfor new/fresh tool with small nose radius at low cutting speed and compressive stresses are dominant at highspeed for worn tool with relatively large nose radius [6, 19]. At a high cutting speed of 150 m/min (with allkind of tool materials) the induced residual stresses become purely compressive in nature. The degree anddepth of compressive stresses vary with different cutting conditions and tool materials as shown in Figs. 3d,3e, and 3f.

During high-speed machining, the chip flow rate is high with shortened machining time, which ultimatelyreduces heat generation and heat build-up time in the shear zone and, as a result, it reduces the effect ofthermal loading. In such cases, mechanical loading is prominent which is more responsible for inductionof compressive stresses. Ulutan and Ozel [3] reported in their study that as cutting speed increases the


Fig. 3. Residual stress, (a) Carbide tool with Vc = 100 m/min, (b) Ceramic tool with Vc = 100 m/min, (c) cBN toolwith Vc = 100 m/min, (d) Carbide tool with Vc = 150 m/min, (e) Ceramic tool with Vc = 150 m/min, (f) cBN tool withVc = 150 m/min.

compressive stresses become more dominant. Bushlya et al. [20] reported in their study that at high cuttingspeed (Vc = 300 m/min) the induced stresses were purely compressive in nature. The current research workindicates that there is an increase in compressive stress values as the cutting speed increases, which is in linewith the findings reported by Ulutan and Ozel [3] and Bushlya et al. [20].


4. CONCLUSIONS

The temperature generation during high-speed turning of Inconel 718 is found to play a major role in toolwear, which ultimately affects the surface quality of the machined component. The cutting parameters, toolmaterials, and cutting environments also play a major role in surface roughness changes. It is found thatcutting velocity is the major factor that influences the induced residual stresses. At the low cutting velocitiesthe residual stresses are tensile on the machined surface and become compressive in subsurface depths. Athigher cutting speeds, the induced residual stresses are found to be purely compressive. The main factorsresponsible for the generation of compressive residual stresses are cutting velocity and tool geometry. Withan increase in cutting speeds the stresses are more compressive in nature. Carbide inserts under flood coolingat a cutting speed of 100 m/min results in minimum surface roughness while cBN under the same cuttingconditions at 100 m/min, induces a minimum surface residual stress value. Therefore a cutting speed of100 m/min and flood cooling can be considered as the optimum parameters for machining Inconel 718. TheMQL cutting condition showed minimum residual stress values for all tool materials and cutting velocities.This indicates that even a small amount of lubrication helps to reduce the degree of residual stresses.

ACKNOWLEDGEMENT

The authors would like to thank VSSC, Department of Space, Government of India for funding this workvide sanction order no. ISRO/RES/3/685/15-16.

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