INVESTIGATION OF SURFACE INTEGRITY DURING TURNING INCONEL 718 ÉTUDE SUR L INTÉGRITÉ DE LA SURFACE DURANT LE TOURNAGE DE L INCONEL 718

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1 INVESTIGATION OF SURFACE INTEGRITY DURING TURNING INCONEL 718 M. Anthony Xavior 1, M. Manohar 2, Mahesh Madhukar Patil 1 and P. Jeyapandiarajan 1 1 Manufacturing Department, School of Mechanical Engineering, VIT University, Vellore, India 2 Materials & Mechanical Entity, VSSC-ISRO, Trivandrum, India manthonyxavior@vit.ac.in Received November 2016, Accepted April 2017 No. 16-CSME-124, E.I.C. Accession 4010 ABSTRACT Surface roughness and residual stress are considered to be major surface integrity issues that directly affect the quality and life of the components. The current research work highlights surface quality and residual stresses 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, tool materials and cutting conditions to achieve better surface quality and minimum residual stress values. Minimum 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 under flood cooling condition. Keywords: machining; Inconel 718; surface integrity. ÉTUDE SUR L INTÉGRITÉ DE LA SURFACE DURANT LE TOURNAGE DE L INCONEL 718 RÉSUMÉ La rugosité de la surface et la contrainte résiduelle sont considérées comme étant la question majeure dans l intégrité de la surface affectant directement la qualité de vie des composants. L étude en cours met l accent sur la qualité de la surface et la contrainte résiduelle causée pendant l usinage de l Inconel 718, avec une gamme de paramètres de coupe ainsi que l environnement autour de la coupe. En outre, le but de l étude est de déterminer les paramètres et conditions optimales du point de vue de vitesse de coupe, des matériaux des outils et des conditions de coupe pour parvenir à une meilleure qualité de la surface et des valeurs de contraintes résiduelles minimales. Une quantité minimum de lubrifiant a eu pour résultat des valeurs résiduelles minimales pour toutes les plaquettes de coupe et les vitesses de la coupe. Le minimum de rugosité de la surface obtenu pendant l usinage est 100 m/min utilisant des plaquettes en carbure et des fluides refroidissants. Mots-clés : usinage; Inconel 718; intégrité de la surface. Transactions of the Canadian Society for Mechanical Engineering, Vol. 41, No. 3,

2 1. INTRODUCTION Inconel 718 is very popular in the aerospace industry owing to its advantages over other nickel-based alloys and titanium-based alloys [1]. The key properties of Inconel 718 are high thermal resistance, strength, and toughness at operating temperatures and also high corrosion resistance. It is known that almost half of the components used in engines are composed of Inconel 718 alloy [2]. The major problems in the application of Inconel 718 alloy, are its low thermal conductivity which develops more heat and increases thermal effects during machining, its work hardening behavior and adhesive nature on the tool face [3]. Surface integrity can be divided into three main categories namely surface roughness, residual stress and microstructural transformation [4]. The main threat to surface integrity come from plastic deformation in the workpiece during turning operation and it is very important to analyze the effect of such deformation. This type of deformation is caused and supported by many controllable and uncontrollable parameters such as cutting parameters (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]. Considerable research 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, if the tool material and cutting parameters are not selected properly then the cutting tool wears out quickly or experiences catastrophic failure. A good combination of cutting parameters, tool materials and cutting environments assures better surface quality [4]. Material built-up at the edge of the cutting tool due to high temperature during turning/machining of Inconel 718 cannot be avoided completely. The rate can be minimized by using a proper cooling/lubrication environment that help to reduce the friction between the tool and work material, and carry the chips and heat away from machining zone, which results in improved surface integrity to some extent [8]. While machining with carbide inserts, coatings allow an increase in cutting speeds up to 100 m/min. Application of coatings on the substrate is an approach to increase the efficiency of 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 the functional performance of the machined products [10, 11]. The influence of process parameters on residual stresses during machining of Inconel 718 was investigated and the influence of friction on the stress profiles was reported [12]. Ulutan et al. [13] introduced a method of understanding the residual stress profile in terms of quantifiable key measures: peak tensile stress at the surface, magnitude and depth of peak compressive stress, and depth at which the residual stress becomes near-zero. During machining, the work material is subjected to external loading: mechanical loading and/or thermal loading. Subsequently, after removal of this external load, the material experiences some amount of stress which is the residue of the applied load known as residual stress. This stress might be tensile or compressive in nature depending on many parameters. The residual stress can vary inside the grains because of the presence 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 residual stress forces the material grains/crystals to be pulled apart from each other. Stresses are characterized by either shear stresses (that act parallel to face of material) or normal stresses (that act perpendicular to the face of the material). At any point inside a material, there exist six independent stresses (three normal and three shear) [15]. In machining of nickel-based super alloy, surface roughness and residual stress are often considered as the significant indication of surface integrity [16]. During machining, induced tensile residual stresses are the main concern, which directly affects the fatigue life of the components. On the other 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

3 Fig. 1. Experimental setup on CNC lathe. Ni Co 1 Cr Table 1. Chemical composition for Inconel 718 (%). Mo Fe Si Mn C Al Ti Cu P 3.05 *bal B S nucleation within the component [17]. This research work is aimed to investigate the surface integrity issues under different cutting conditions (dry, minimum quantity lubrication and flood) with different cutting tool materials (carbide, ceramic and cbn). Residual stress and surface roughness data are analyzed and reported specifically. 2. EXPERIMENTAL SETUP The work material used for the experiments was Inconel 718 with dimensions of 40 mm diameter and 60 mm length with the chemical composition as shown in Table 1. The turning experiments were carried out on an ACE Micromatic CNC lathe machine with spindle speed range of rpm, as shown in Fig. 1. The experiments were carried out according to Taguchi s L18 (21 37) orthogonal array. The cutting 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 three different sets of cooling/ lubrication conditions: dry cutting condition in the ambient environment, MQL cutting condition with 80 ml/hr flow rate, and flood cooling condition with l/hr flow rate. Ester oil and semi-synthetic water soluble cutting fluid was used in MQL and flood cooling cutting conditions respectively. Machined work pieces were cut into small specimens with the help of a wire EDM process and these samples were used for residual stress measurement. Table 2 shows the cutting parameters/levels/conditions considered for the experiments. 3. RESULTS AND DISCUSSIONS Induced residual stresses in the machined specimens (on the surface and subsurface) were measured by using a Bruker D8 DISCOVER X-ray diffraction (XRD) unit. In order to ensure the accuracy of the data, Transactions of the Canadian Society for Mechanical Engineering, Vol. 41, No. 3,

4 Table 2. Cutting parameters/levels/conditions. Feed ( f ) = 0.15 mm/rev and depth of cut = 0.5 mm were kept constant throughout the experimentations. Parameters Level 1 Level 2 Level 3 1. Cutting speed (V c ) m/min Tool (insert) material Coated carbide Ceramic cbn 3. 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 Ceramic cbn MQL Carbide Ceramic cbn Flood Carbide Ceramic cbn Dry Carbide Ceramic cbn MQL Carbide Ceramic cbn Flood Carbide Ceramic cbn the measurement was repeated three times at the same data point and the average value was recorded for further analysis. For surface quality measurement a Mitutoyo surface roughness tester SJ-301 model was used. The surface quality of machined samples was measured in the horizontal direction with a constant 4 mm measuring length. The surface roughness was measured at three different places located 120 apart on the periphery of each specimen and the average value was recorded. The experimental results for surface roughness (Ra) and surface residual stresses are shown in Table Surface Roughness Analysis Surface texture or surface quality mainly depends on cutting parameters, cutting conditions and the cutting environments. Figure 2 presents the detailed analysis of surface roughness obtained for different cutting conditions, cutting inserts and cutting speeds. During the dry cutting condition, it was observed that with an increase in cutting speed the surface roughness value decreased (i.e, better surface quality observed with an increase in cutting velocity). The ceramic and cbn tools show better surface quality. For MQL cutting condition at low cutting speed (V c = 100 m/min) better surface roughness values were observed. For all cutting tool materials, a similar range of surface roughness values were observed. Using the minimum quantity lubrication (MQL) method, tiny pressurized air-suspended oil droplets enter and spread at the cutting zone easily forming a very thin lubrication layer between tool face and work material that helps reduce friction between tool and work material, preventing abrasion and formation of built-up edge [8]. 390 Transactions of the Canadian Society for Mechanical Engineering, Vol. 41, No. 3, 2017

5 Fig. 2. Analysis of surface roughness obtained for different cutting conditions and cutting inserts. MQL works more effectively at a moderate cutting speed range (V c = m/min). At this range, the air-suspended oil droplets spread more homogeneously at the cutting zone and maintain a continuous lubrication layer during machining [18]. In flood cooling cutting condition, lower surface roughness values were observed for carbide and cbn tools while higher values were observed for the ceramic tool. This may be attributed to more heating effects experienced by the work material alone as ceramic does not take any part in heat conduction during machining. For the dry cutting condition the highest surface roughness values were 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 change in cutting speeds and tool materials. The surface roughness is reduced by 10 25% in MQL when compared with dry machining Residual Stress Analysis Apart 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 residual stresses from the surface to the subsurface depth. For the carbide tool at cutting speed V c = 100 m/min peak tensile and compressive values were observed under flood cooling cutting conditions as shown in Fig. 3a. For the ceramic tool at cutting speed V c = 100 m/min higher tensile stresses were observed up to a higher depth from the machined surface under flood cooling cutting condition as shown in Fig. 3b. For the cbn tool at cutting speed V c = 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 dominant for new/fresh tool with small nose radius at low cutting speed and compressive stresses are dominant at high speed for worn tool with relatively large nose radius [6, 19]. At a high cutting speed of 150 m/min (with all kind of tool materials) the induced residual stresses become purely compressive in nature. The degree and depth 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 ultimately reduces heat generation and heat build-up time in the shear zone and, as a result, it reduces the effect of thermal loading. In such cases, mechanical loading is prominent which is more responsible for induction of compressive stresses. Ulutan and Ozel [3] reported in their study that as cutting speed increases the Transactions of the Canadian Society for Mechanical Engineering, Vol. 41, No. 3,

6 Fig. 3. Residual stress, (a) Carbide tool with V c = 100 m/min, (b) Ceramic tool with V c = 100 m/min, (c) cbn tool with V c = 100 m/min, (d) Carbide tool with V c = 150 m/min, (e) Ceramic tool with V c = 150 m/min, (f) cbn tool with V c = 150 m/min. compressive stresses become more dominant. Bushlya et al. [20] reported in their study that at high cutting speed (V c = 300 m/min) the induced stresses were purely compressive in nature. The current research work indicates that there is an increase in compressive stress values as the cutting speed increases, which is in line with the findings reported by Ulutan and Ozel [3] and Bushlya et al. [20]. 392 Transactions of the Canadian Society for Mechanical Engineering, Vol. 41, No. 3, 2017

7 4. CONCLUSIONS The temperature generation during high-speed turning of Inconel 718 is found to play a major role in tool wear, which ultimately affects the surface quality of the machined component. The cutting parameters, tool materials, and cutting environments also play a major role in surface roughness changes. It is found that cutting velocity is the major factor that influences the induced residual stresses. At the low cutting velocities the residual stresses are tensile on the machined surface and become compressive in subsurface depths. At higher cutting speeds, the induced residual stresses are found to be purely compressive. The main factors responsible for the generation of compressive residual stresses are cutting velocity and tool geometry. With an increase in cutting speeds the stresses are more compressive in nature. Carbide inserts under flood cooling at a cutting speed of 100 m/min results in minimum surface roughness while cbn under the same cutting conditions at 100 m/min, induces a minimum surface residual stress value. Therefore a cutting speed of 100 m/min and flood cooling can be considered as the optimum parameters for machining Inconel 718. The MQL 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 work vide sanction order no. ISRO/RES/3/685/ REFERENCES 1. Elshwain, A.E.I., Redzuan, N. and Yusof, N.M., Machinability of nickel and titanium alloys under of gas-based coolant-lubricants (Cls) A review, International Journal of Research in Engineering Technology, Vol. 2, No. 11, pp , Borse, P.S.C., Predicting surface roughness, tool wear and MRR in machining Inconel 718: A review, International Journal of Engineering Research and General Science, Vol. 3, No. 2, pp , Ulutan, D. and Ozel, T., Machining induced surface integrity in titanium and nickel alloys: A review, International Journal of Machine Tools and Manufacture, Vol. 51, No. 3, pp , Maranhao, C. and Davim, J.P., Residual stresses in machining using FEM analysis A review, Review on Advanced Material Science, Vol. 30, No. 3, pp , Ezugwu, E.O., Wang, Z.M. and Okeke, C.I., Tool life and surface integrity when machining Inconel 718 with PVD and CVD coated tools, Tribology Transactions, Vol. 43, No. 2, pp , Sharman, A.R.C., Hughes, J.I. and Ridgway, K., An analysis of the residual stresses generated in Inconel 718 when turning, Journal of Materials Processing Technology, Vol. 173, No. 3, pp , Ince, M.A. and Asiltürk, I., Effects of cutting tool parameters on surface roughness, International Refereed Journal of Engineering and Science, Vol. 4, No. 8, pp , Behera, B. K. Chetan, Ghosh, S. and P. Venkateshwara Rao, P., Effects on forces and surface roughness during machining Inconel 718 alloy using minimum quantity lubrication, All India Manufacturing Technology, Design and Research Conference (AIMTDR), Assam, India, pp , Dec , Sharman, A.R.C., Huges, J.I. and Ridgway, K., Workpiece surface integrity and tool life issues when turning Inconel 718 Nickel based superalloy, Machining Science and Technology, Vol. 8, No. 3, pp , M Saoubi, R., Outeiro, J.C., Chandrasekaran, H., Dillon Jr., O.W. and Jawahir, I.S., A review of surface integrity in machining and its impact on functional performance and life of machined products, International Journal of Sustainable Manufacturing, Vol. 1, Nos. 1/2, pp , Guo, Y.B., Li, W. and Jawahir, I.S., Surface integrity characterization and prediction in machining of hardened and difficult-to-machine alloys: A state-of-the-art research review and analysis, Machining Science and Technology, Vol. 13, No. 4, pp , Arrazola, P.J., Kortabarria, A., Madariaga, A., Esnaola, J.A., Fernandez, E., Cappellini, C., Ulutan, D. and Ozel, T., On the machining induced residual stresses in IN718 nickel-based alloy: Experiments and predictions with finite element simulation, Simulation Modelling Practice and Theory, Vol. 41, pp , Transactions of the Canadian Society for Mechanical Engineering, Vol. 41, No. 3,

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