RESPONSE SURFACE METHODOLOGY IN FINISH TURNING INCONEL 718

RESPONSE SURFACE METHODOLOGY IN FINISH TURNING INCONEL 718 M. Aruna 1 Department of Mechanical Engineering, Velammal College of Engineering and Technology, Madurai, India. Dr. V. Dhanalakshmi 2 Department of Mechanical Engineering, Thiagarajar College of Engineering, Madurai, India. Abstract Machining of hard materials used in aerospace applications require hard and tough cutting tools. Ceramic tools and cermets are used in machining of nickel alloys for such applications. In this study finish turning of Inconel 718 is carried out with cermet tools. Cutting parameters are designed using Taguchi s DOE and the experiments are conducted for the designed parameters. The surface finish measurement is carried for the various conditions and data obtained are used to build up the mathematical surface model using response surface methodology. The adequacy of the developed mathematical model is proved by ANOVA. The findings of this study show new results and the second order model was quite adequate. Keywords: ANOVA, Inconel 718, Response Surface Methodology (RSM). 1. Introduction Nickel based super alloys are the one of the most difficult material to machine because of their high hardness, high strength at high temperatures, affinity to react with tool materials and low thermal diffusivity. Of these nickel based alloys, Inconel 718 is widely used in the fabrication of critical components of turbine engines in aero space applications. These alloys have excellent mechanical properties at elevated temperatures and good corrosion resistance. To machine Inconel 718 alloy, hard and tough cutting tools such as HSS tools, cemented carbide tools, CBN tools, ceramics and cermets tools are used. HSS tools are usually employed for intermittent cutting operation such as milling, drilling and so on. For continuous cutting operations namely turning and boring, cemented carbide, ceramics, CBN tools and cermets are used. In this study, finish turning of Inconel 718 is carried out using cermet tools. Surface finish obtained at different cutting conditions designed by, Design Of Experiments (DOE) is found out. The Response Surface Methodology (RSM) is used to validate the results of surface finish measurement. 2. Literature Review Increasing the productivity and the quality of the machined parts are the main challenges of manufacturing industry, in particular for heat resistant super alloys employed in aeronautic and aerospace applications. This requires better management of the machining system corresponding to cutting tool-machine tool- study piece combination to go towards more rapid metal removal rate. Exploring higher cutting speed depends to a greater extend on the cutting tool materials [Ezuguw(2005)]. Coated carbide and ceramics are generally used for high speed machining of Inconel 718. For the coated carbide tools the right selection of the tungsten carbide-cobalt alloy as the substrate, the associated coating procedure and the selection of cutting are the major problems [Ezuguw (2005), Dudzinski et al (2004)]. Cemented carbide tools are still largely used for machining the nickelbased super alloys, especially for the Inconel 718 [Subash et al( 2000a), (2000b), Rahman et al(1997)]. The difficulty in machining Inconel 718 resolves itself into two basic problems: Namely short tool life and severe surface abuse of machined surface [Ezuguw et al (1999)]. ISSN: 0975-5462 4292

All the cutting parameters, including tool and coating materials choice, tool geometry, machining strategy, cutting speed, feed rate, depth-of-cut, lubrication must be controlled in order to achieve acceptable tool life and correct surface integrity for the machined parts [Ezuguw et al (1995), Arunachalam et al (2004)]. Taguchi s Design Of Experiment (DOE) method incorporate Orthogonal Arrays (OA) to minimize the number of experiments required to determine the effect of process factors upon performance characteristics. Taguchi experiment can be accomplished in a timely manner and at a reduced cost with results comparable to a full factorial experiment [Philip(1989)]. 3. Problem definition From the literature review, it is clear that mostly carbide tools and ceramics tool materials are used for high speed machining of hard materials. Hence in this study, it is proposed to study the performance of cermet inserts in high speed finish turning of Inconel 718 and to find the best combination of cutting parameters such as cutting speed and feed which result in better surface finish. In this study, wet machining and high cutting speeds are considered to study the behavior of cermet tools during orthogonal cutting tests. 3.1 Experimental study In this study, Inconel 718 cylindrical rod is considered as the study piece material. Titanium carbide based cermet inserts (Triangular) are used as the cutting tool. Cermets are one of the best kept secrets in the cutting tool industry. They provide the user with increased productivity and profitability through higher cutting speeds and extended tool life. Cermets have small, well controlled grain structures. Hence, they show higher wear resistance. In addition, cermets maintain a sharp edge longer than carbide. Cermets have superior resistance to built-up edge. Less affinity with the study piece results in superior micro-finishes. The specification of the cutting tool is shown in Table 1. Table 1. Cutting tool specification Tool Approximate Insert material composition Specification Cermet 85% TiC 2 Binder Ni TNMA 160408 Finish turning operation is carried out for the above cutting conditions using soluble oil as the cutting fluid. Surface roughness is measured in terms of Roughness average (Ra) using Talysurf coder. Few numbers of trials are carried out for each cutting conditions and the mean value of Ra is found out. 3.2 Mathematical model using RSM For developing models based on experimental data, careful planning of experimentation is essential. The factors, considered for experimentation and analysis are cutting speed and feed rate. In this study, mathematical models have been developed using RSM based on experimental results. The purpose of developing mathematical models relating to the machining response and their factors is to facilitate the optimization of the machining process. Factors which affect the tool life in turning operations are considered. However, for a particular study tool combination, the tool life in turning is assumed to be a function of the primary independent variables such as: Surface roughness = f (V, f) Where V = Cutting speed (m min 1 ) f = Feed (mm rev 1 ) The mathematical models commonly used are represented by R a = C V f Where R a = Surface roughness value in microns C = A constant ISSN: 0975-5462 4293

Table 2. Cutting parameters Cutting Parameters Selected values(levels) Cutting speed (m/min) 100, 150 and 200 Feed (mm/rev) 0.04, 0.06 and 0.08 Depth of cut (mm) 0.5 Table 3. Standard deviation, mean data Std. Dev. 0.26 R-squared 0.4940 Mean -3.27 Adj R-Squared 0.1325 CV% 7.99 Pred R-Squared -1.2151 PRESS 2.09 Adeq Precision 3.9990 There are 2 input parameters and each of them has been set at three levels. Table 3 shows the standard deviation, mean data. Table 4 shows the sequential sum of squares from ANOVA, the decision making tool. ANOVA reveals that both the speed and feed rate are not significant on the response variable investigated. Table 5 reveals the design summary. The diagnostics case statistics, Table 6 reveals about the points which exceeds the limits and provides the predicted data details for further regression. Table 4. ANOVA for response surface quadratic model Source Sum of squares df Mean F p-value Prob>F Square Value Model 0.47 5 0.093 1.37 0.3406 not significant A-speed 0.24 1 0.24 3.55 0.1016 B-feed 0.066 1 0.066 0.97 0.3572 AB 1.893E-003 1 1.893E-003 0.028 0.8725 A 2 4.031E-003 1 4.031E-003 0.059 0.8151 B 2 0.14 1 0.14 2.10 0.1910 Residual 0.48 7 0.068 Lack of fit 0.24 3 0.081 1.37 0.3716 not significant Pure error 0.24 4 0.059 Cor total 0.95 12 Table 5. Design summay Response Name Ratio Units Ob s. Analysis Minimum Maximum Mean Std. Dev. Y1 Surface roughnes s microns 13 Polynomial 0.02 0.055 0.039153 0.0092806 ISSN: 0975-5462 4294

Std. order Actua l value Predi cted value Residua l Table 6. Diagnostics case statistics Levera ge Internally studentize d residual Externally studentize d residual Influence on fitted value DFFITS Cook s distanc e 1-3.17-3.12-0.047 0.625-0.296-0.276-0.356 0.024 2 2-3.35-3.51 0.160 0.625 1.012 1.014 1.309 0.284 13 3-2.90-2.98 0.084 0.625 0.522 0.493 0.637 0.076 11 4-3.00-3.29 0.290 0.625 1.830 2.347 *3.030 0.931 10 5-3.12-3.15 0.025 0.625 0.158 0.146 0.189 0.007 7 6-3.91-3.64-0.270 0.625-1.692-2.038 *-2.630 0.795 4 7-3.22-3.19-0.030 0.625-0.188-0.175-0.226 0.010 6 8-3.15-2.93-0.220 0.625-1.346-1.447-1.869 0.503 8 9-3.10-3.35 0.250 0.200 1.051 1.061 0.530 0.046 12 10-3.51-3.35-0.160 0.200-0.683-0.655-0.327 0.019 9 11-3.69-3.35-0.340 0.200-1.463-1.626-0.813 0.089 3 12-3.22-3.35 0.130 0.200 0.547 0.518 0.259 0.012 1 13-3.22 3.3 Experimental design -3.35 0.130 0.200 0.547 0.518 0.259 0.012 5 Run order The design of experimentation has a major effect on the number of the conducted experiments. Hence, it is important to have a well designed set of experiments. In this study, a 2 level factorial design also known as first order model using 3F1-factorial design is used to determine the significant factors. The most popular response surface method design is the Central Composite Design (CCD). A CCD has three groups of design points namely: Two-level factorial or fractional factorial design points Axial points (sometimes called star points) Center points Fig. 1. Normal plot of the residuals CCD s are designed to estimate the coefficients of a quadratic model. All point descriptions will be in terms of coded values of the factors. For each source of terms, the probability is examined to find out, whether it falls below 0.05. The quadratic model is the most significant one, and adding the cubic order terms will not significantly improve the fit. ISSN: 0975-5462 4295

3.4 Model graphs The adequacy of the underlying model should be checked before the conclusions from the analysis of variance are adopted. Figure 1 illustrates the graph for the normal probability of the residuals for the surface roughness. The entire points lie near the center line which depicts that all the points lie within the limits. Perturbation, in the normal path or state can be visualized from Fig. 2. It shows how the response changes, as each factor moves from the chosen reference point, with other points held constant at the reference value. The reference point is set at the middle of the design space that is coded zero level of each factor. Figure 3 shows the Box-Cox plot for power transformations which clearly marked the shift of lambda to 1.The values are as follows from the plot: Best = 1.9, Low C.I = - 0.79 and High C.I = 4.7 Fig. 2. Perturbation Fig. 3. Box-Cox plot 3.5. Results Factorial design and response surface analysis are used to study and optimize the cutting conditions for turning Inconel 718. The optimum conditions as stated by further numerical analysis of the responses using design expert software reveals that the most influencing factor is the feed rate. Using Analysis of Variance for obtaining the significant factors, it is distinguished that all main factors and some interactions have insignificant effect on surface roughness. Figure 4 represents the 2-D contour plot. Figure 5 represents the response surface for the transformed variable, by which the points above the level can be taken care. The optimal value of each variable is clearly represented in this 3-D display of surface plot. Surface roughness increases as a function of speed and feed rate. Fig. 4. Contour plot Fig. 5. 3-D surface plot ISSN: 0975-5462 4296

The reduced quadratic models developed using RSM are reasonably accurate and can be used for prediction within the limits of the factors investigated. Based on the graphical results, it was found that in order to achieve higher tool life and surface finish, the cutting conditions should be maintained at medium range for all the parameters investigated. 3.5. Conclusion Factorial design and response surface analysis were used to study and optimize the conditions for machining Inconel 718. The optimum conditions as stated by further numerical analysis of the responses using the Design Expert software reveal that the significant parameter is feed. This study presents the findings of an experimental investigation of the effect of feed rate, cutting speed and depth of cut on the surface roughness when turning Inconel 718. From the analysis of Table 4, it is apparent that the F calculated value is greater than the F table value and hence the second order response function developed is quite adequate. From the ANOVA table, it is inferred that the tool type and feed rate have significant effect on the surface roughness at 95% confidence level. References [1] Arunachalam, R M.; Mannan, M A.; Spowage, C. (2004): Surface integrity when machining age hardened Inconel 718 with coated cutting tools. Int. J. Mach. Tool Manufact., 44: 1481-1491. [2] Dudzinski, D.; Moufki, D.; Larrouquere, J.; Vigneau.; Zerrouki,V. ( 2004): A review of development towards dry and high speed machining of Inconel 718 alloy. Int. J. Mach. Tool Manufact., 4: 439-456. [3] Ezuguw, E O.; Tang, S H. (1995): Surface abuse when machining cast iron (G17) and nickel-base super alloy (Inconel 718) with ceramic tools. J. Mate. Process. Technol., 55: 63-69. [4] Ezuguw, E O.; (2005): Key improvements in machining of difficult-to cut aerospace super alloys. Int. J. Mach. Tool Manufact., 45: 1353-1367. [5] Ezuguw, E O.; Wang, Z M.; Machado, A R. (1999): The machinability of nickel-based alloys: A review. J. Mate. Process. Technol., 86: 1-16. [6] Philip, J R.; (1989): Taguchi Techniques for Quality Engineering. Mc Graw Hill Book co. [7] Rahman, M.; Seah, W K H.; Teo, T T. (1997): The machinability of Inconel 718. J. Mate. Process. Technol., 63: 199-204. [8] Subas, B K.; Ramanuja, K.; Ramachandra, H K.; Balakrishna. (2000a): Dimensional instability studies in machining of Inconel 718 nickel based super alloy as applied to aero gas turbine components. J. Eng. Gas Turbine Power Trans. ASME, 122: 55-61. [9] Subash, B K.; Ramaraja, K.; Ramachandra,; Balakrishna, H K. (2000b): Simulation optimization of machining parameters for dimensional instability control in aero gas turbine components made of Inconel 718 alloy. Trans. ASME., A 22: 586-590. ISSN: 0975-5462 4297