A combined experimental and optimization model to improve the machinability of Nimonic C-263 superalloy

Nickel based superalloys finds extensive usage in manufacturing of intricate part shapes in gas turbine, aircraft, submarine, and chemical industries owing their excellent mechanica l property and heat resistant abilities. However, machining of such difficult-to-machine alloys up to the desired accuracy and preciseness is a complex task owing to a rapid tool wear and failure. In view of this, present work proposes an experimental investigation and optimization of process parameters of the cryogenic assisted turning process during machining of Nimonic C263 super alloy with a multilayer CVD insert. Taguchi’s L-27 orthogonal array is used plan the experiments. Effect of input parameters viz. cutting speed (N), cutting feed (f), depth of cut (d) are studied on responses viz. surface roughness (SR), nose wear (NW) and cutting forces (F) under hybrid cryogenic (direct+indirect) machining environment. A scanning electron microscope (SEM) analysis is carried out to explore the post-machining outcomes on the performance measures. The multiple responses are converted in to single response and ranked according to Taguchi based gray relational grade (TGRG). Feed rate (f) is found to be the most influential parameter from the analysis of variance of GRG. The means of GRG for each level of process parameters are used to improve the optimal process parameters further. Finally, the confirmative experiment is performed with these optimal set of process parameters which showed an improvement of 9.34% in the value of GRG. The proposed work can be beneficia l to choose ideal process conditions to enhance the performance of turning operation.

studied the effect of cryogenically treated inserts (indirect technique) with different soaking duration at different temperatures to achieve improved machined surface quality [7,24,26,29]. Similarly, few studies report impingement of cryogenic liquid on the machining zone (direct technique) can enhance the performance of the process [8][9][10]. However, report to analyze the machinability of process with the hybrid technique i.e. combination of both direct and indirect cryogenic approach is extremely scarce in the literature. Combination of both the beneficial techniques may further enhance the performance of turning operation. Thus, the novelty and prime motive of this study is to investigate the machinability of Nimonic C-263 work piece under a hybrid cryogenic machining approach i.e. Taguchi-based grey relational analysis (TGRA) is amongst the simplest and systematic technique for dealing with multi-objective problems and several researchers have already attempted this to their machining study conducted at various environments [34,36,39,41]. However, the available literature of TGRA lacks applicability of this method to combined direct and indirect cryogenic approach and hence, TGRA method is adopted to rank the responses in the present study.
The optimum machining condition obtained through the approach is validated by conducting a confirmative test.
Complex shapes can be machined to desired dimensional accuracy with turning process. There are various factors in the turning which influences the performance measures viz. surface roughness, tool wear, cutting force etc. In a case of cutting insert, parameters like nose radius, rake angle, tool point angle, tool materials, etc. are significant whereas, for the workpiece, material properties, hardness and cutting conditions are significant [11,12]. To enhance the performance of turning process, numerous studies have been reported by researchers are discussed in this section. Rotationa l turning and hard turning have been compared with respect to the surface quality and there were no significant differences observed between the two techniques however, rotational turning showed improved results in terms of wear of tool, while hard turning performed better with respect to the surface integrity [13][14][15]. Therefore, the hard turning is more often used. Batish et al. explored the surface topography and concluded that machining forces depend upon various factors like cutting speed, feed rate and depth of cut whereas surface roughness largely depends upon the feed rate [11].
Different features of surface integrity for AISI D2 steel that has been coated with carbide inserts have been compared by Srithar et al. and Thakur et al. in their studies [12,16]. It is observed that there is a reduction in the surface integrity as spindle speed and the feed rate increases. However, surface roughness is mostly influenced by the speed and least affected by the depth of cut [14,17]. Thakur et al. has explored the tool wear, surface integrity and the tool life by comparing with taper turning [18].
In recent years, numerous studies reported application of nickel-based superalloys which finds extensive applications in manufacturing critical parts used in aerospace, defense and nuclear industries etc. Out of many superalloys, Nimonic C-263 is amongst the difficult to machine superalloy due to its inherent characteristics like lower thermal conductivity, large strain rate sensitivity, greater chemical affinity, and higher range of hot hardness [7]. These features of Nimonic alloys results into high friction and generation of excess heat at machining zone. In this aspect, Ezilarasan et al. have machined Nimonic C-263 superalloy and analyzed various responses viz; cutting force, surface roughness, tool wear rate [5]. The same researcher also used whisker-reinforced ceramic insert in other study where he observed higher range of residual stresses with whiskerreinforced ceramic inserts [19]. Koyilada [16,21]. The cutting tools must have the properties viz. good balance of hardness and toughness, anti-friction and chemical inertness properties, excellent thermal stability along with hot hardness. In order to achieve the above features, various kind of coated and uncoated material have been used in both multilayer and single layer configuration by earlier researchers. Furthermore, CVD coated tool established great features during the machining of various types of Nickel based superalloys [22,23].
Numerous machining approaches have been implemented by various researchers for improving the efficiency of the turning process. Use of lubricants and coolants on the tool-workpiece interaction zone reduces the cutting temperature considerably. To achieve effective lubrication and cooling, several methods have been employed. Lubricant which are commonly used in various industries are water based soluble oils or mineral oils. These oils are ecological threat as they have many harmful effects on workers' health. To overcome these harmful impacts, a sustainab le machining strategy such as cryogenic machining (direct cryogenics) have been claimed by researchers [24,25,29] who examined the effects of machining environment like high pressure cooling and direct cryogenic cooling on the life of tool and surface integrity throughout the machining of workpiece. The experimental results have shown that the suggested higher-press ure cooling approach carried significant growth of tool life of 133% for PVD double layered coated tool and microcrystalline uncoated carbide inserts have maximum of 275%. Apart from this, few researchers presented the report on cryogenic treatment of inserts i.e. indirect cryogenics to achieve better tool life [26][27][28]. Chetan et al. is the only researcher who has carried out turning process on Nimonic work piece with cryo-treated inserts till date [29]. However, tool wear has not been considered as a response in the study. Few studies also claimed that the lowering temperature can enhance the performance of the turning process on the machined surface keeping soaking duration constant [30,31].
In addition to these studies, several researchers have also opted for application of various optimization techniques to achieve optimum level of machining condition for turning process [5] in which they have presented a multiple objective optimization of the cutting features by considering desirability functionality towards obtaining the lower values of the mentioned responses and obtained the optimum parametric combination with feed rate of 0.05 mm/rev, cutting-speed of 210 m/min and a depth of 0.5 mm to optimize the multi-objective turning problems effectively in combination with a desirable accuracy. Few researchers carried out an optimization with genetic algorithm for the turning process to get the optimum set of machining parameters [11,15]. Neseli et al. implemented the response surface methodology and optimized various cutting tool geometry attributes [32]. Grey relational analysis was carried out for optimizing input parameters to minimize the values of residual stresses and surface roughness [33].
Extensive study of past literature indicates that good amount of work has been reported worldwide to enhance the performance of turning process by the researchers. Literature reports abundant works on indirect cryogenic and direct cryogenics approaches to achieve improve machinability on various combination of work-tool pair individually [24][25][26][27][28][29]. However, to the best of our knowledge, a report that evaluates the machinability of the turning process combining the hybrid strategy of both direct and indirect is extremely scarce in the literature. Moreover, it is also observed that limited number of studies have explored the machinability of Nimonic 263 work piece in turning process. This superalloy finds abundant usage in manufacturing complex parts in gas turbine, aerospace and automotive industries. Therefore, attempt must be made to explore the machinability of Nimonic C 263 work piece in the hybrid cryogenic machining environme nt. Therefore, the proposed work attempts to evaluate the combined effect of hybrid cryogenic machining strategy i.e. combination of direct and indirect cryogenic machining on Nimonic C263 work piece. Taguchi based gray relational grade (TGRA) is used to convert multiple responses viz.
surface roughness, cutting forces and tool wear in to equiva lent gray relational grade (GRG). The GRG is further optimized and validated by conducting a confirmation test.  Table 1.

Experimental design and procedure
[ Table 1 about here.] The tool insert material used for turning was T-style tool insert with multilayer CVD coating of TiN-MT-TiCN-Al2O3 grade KCM25. Each hard turning experiment was performed twice and mean of the result is considered for better accuracy. Figure 1 shows the pictorial view of hybrid cryogenic experimental setup (direct+indirect) and details of the experimentation are given in the Table 2 along with the level of parameters selected.
Here, the term y represents the value of the quality characteristic, k denotes the proportionality constant ; L(y) denotes the loss function.
m denotes the target value of y; σ represents a mean square deviation.

Cryogenic treatment on inserts
Cryogenic treatment is an indirect cryogenic method which comprises the process of treating the inserts at sub-zero condition (below 0°C) to enhance their mechanical properties. For this method, various gases such as CO2, liquid nitrogen and helium are generally used as working medium. In this study, cryogenic treatment on the tool inserts was done to find out its effect on the response parameters along with direct cryogenics. Liquid nitrogen was used as working medium (LN2) for this study and cryogenic temperature selected was -100°C along with soaking duration of 12 hours. Cooling rate was throughout kept constant as 2℃/min. After this sub-zero treatment, tempering was carried out at 200°C on the samples to eliminate the internal residual stresses with same rate of 2℃/min and held at that temperature for 2 hours. The inserts were then cooled down in the air slowly. The optimal specifications for cryogenic treatment were selected by referring previous literatures [26][27][28]. Figure 2 shows the timeline of the various heat treatment processes. To validate this, a full width at half maxima (FWHM) received from XRD pattern is studied.
FWHM corresponds to the enlargement of peaks which further are associated with the grain refinement. Figure 4 reveals the distinction of FWHM for untreated and treated inserts corresponding to the diffraction angle. FWHM value can be seen larger in case of cryo-treated inserts as compared to the untreated inserts. Larger FWHM value in carbide insert implies to the additional refinement of grains that resulted into an increased value of hardness. This is also supported by previous researches in which they also claimed that the refinement of grains occurred through the cryogenic treatment increases the hardness and wear resistance of tungsten carbide inserts [14,28,34].

Cryogenic machining environment
This is also known as a direct method of cryogenics where the jet of CO2, LN2 is projected directly over the cutting zone. Many researchers have achieved increase in tool life with the help of cryogenic cooling. This method removes heat from cutting zone and at the same time, it also reduces the coefficient of friction by providing cushioning effect between chip-tool interface. Liquid nitrogen at -196º C has commonly been used as a coolant in the cryogenic machining. Liquid nitrogen is an inert gas and also it is not dangerous to the workers on the shop floor. Also, it minimizes the diffus io n wear and adhesion performance of cutting tool henceforth increases the tool life. Figure 5 is the block diagram of the machining setup prepared for the same.

Surface roughness measurement
The surface roughness (SR) testing carried out with the help of Taylor Hobson Talysurf surface roughness tester (Mitutoyo made) as shown in Figure 6. The workpiece was rotated randomly and 4 readings were taken at 90̊ angle each. The average of these readings (Ra in microns) was taken as the final value of surface roughness.

Tool wear measurement
The nose wear (NW) was carried out using Toolmakers' microscope (Mitutoyo made) shown in Figure 7. The tool insert is placed on the glass below the lens of 10X magnification power. All the readings were taken twice and average is taken for better accuracy. The readings are expressed in millimetre.

Cutting force measurement
The cutting force (CF) was measured using lathe tool dynamometer 5233A (Kistler made). The data procurement system comproses of software DynoWare software. This cutting force is the resultant of the tangential force and the feed force and expressed in newton (N). The Figure 8 shows the data acquisition system for cutting forces from DynoWare software.
[ Fig. 8 about here.] Table 3 shows the Taguchi's L-27 orthogonal array of twenty seven experimental runs along with the responses viz; surface roughness (SR), nose wear (NW) and cutting forces (CF). It also shows the results of turning operation viz; surface roughness, cutting forces and nose wear. Moreover, it also comprises Grey relational coefficients (GRC), Grey relational grade (GRG) and rank after Taguchi based grey relational analysis operations.

Results and discussion
[ Table 3 about here.]

Analysis of surface roughness
Surface roughness is one of the crucial response in hard turning process as it influences the tool From the main effect plot, it is observed that, the value of surface roughness has been reduced with increasing speed. Figure also shows the incremental trend in the values of surface roughness with an increasing cutting feed values. This is due to the fact that the cutting load increases over the tool with increasing feed rate that leads to excess frequency of vibrations of tool resulting into poor surface finish. The best surface finish is obtained at the lowest cutting feed and highest spindle speed combination. Figure also shows that if the depth of cut is increased, surface finish obtained is very poor. This is due to the excessive rubbing action and temperature rise with an increasing depth of cut that causes increase in surface roughness value.

Analysis of tool wear
Tool wear is also the most crucial response as it affects the dimensional exactness and surface integrity of the machined component. It is also an important aspect which decides the tool life which ultimately is associated with the cost of machining. In machining, the nose wear is responsible for the cutting-edge retreat and has a greater influence on the dimensional accuracy of a workpiece. Figure 10 shows the main effect plots for the nose wear obtained below the ISO 3658-1993 standards.
[ Fig. 10 about here.] For the spindle speed of 110 m/min, the value of the tool wear is least till 0.122 µ m. Further, it is also evident from the graphs that, at 110 m/min of spindle speed, the tool wear is reduced approximately by 25% as compared to the lower speed of 50 m/min.

Analysis of cutting forces
Cutting force generation influence the stability of a machining system. it may deform the workpiece with non-uniform chip formation and dimensional accuracy may be lost. Figure 11 shows the main effect plots for cutting forces which shows that at the higher cutting feed and lower spindle speed, the cutting force generation is on the peak.

Grey Relational Analysis
Taguchi

a) Data set normalization
As the motive is to estimate an optimum range of input variables yielding minimal values of surface roughness, nose wear and cutting forces, lower-is-better model is implemented as per Equation (2). Normalized value Xi*(j) of Xi (j) is obtained by equation (2) Xi * (j) = max j.Xi ( j ) −Xi ( j ) max j.Xi ( j ) −min j.Xi ( j ) Where, max j. Xi (j) is the highest j value and min j. Xi (j) is the lowest j value.

d) Assigning the Rank to a data
The highest value of GRG is assumed to be the most influential combination and are assigned with rank accordingly. GRG was estimated for all set of data values and it is used to identify the most significant levels of input parameters at which improved results are obtained. Table 3 shows GRC, GRG and rank for the experimental values whereas. Table 4 shows mean of GRG values for each level.
The larger GRG value implies the degree of closeness of experimental response to the ideal value.
From the Table 4, it is observed that second experimental combination shows the largest amonst all the 27 experiments. The GRG value is further analysed by using ANOVA to analyse the influence of individual parameter on the value of grey relation grade.

Analysis of variance
The motive of ANOVA is to find out the most significant machining parameters that influe nces machining process. It provides the data such as degrees of freedom, sum of squares, mean squares, Fvalue, P-value and % contribution of parameters. Table 5 illustrates the ANOVA results which clearly shows that a cutting feed is the most significant attribute in cryogenic hard turning of Nimonic C-263 superalloy that contributes 48.33%. Past research also found the same trend of results [37]. It can also be seen that the spindle speed has 14.90% contribution whereas, depth of cut has 36.77 % contributio n on the performance of hard turning.

Confirmation experiments
The trial experiments were conducted as a confirmation test considering optimum combinatio n of machining parameters to validate the TGRA. The predicted Grey relational grade Ѓprojected is estimated using equation (5).
Where, Ѓm implies the mean of total GRG Ѓ0 implies average of the optimal levels GRG j is total no. of the input parameters.
The predicted GRG for optimum parameters is computed from equation (5) and were tabulated in the Table 6. Experimental trial carried out as confirmation test and their results were compared with the values of first ranked set i.e. 2 nd orthogonal array. [ Table 6 about here.] After selection of the optimum machining parameters, 14.42 % reduction was observed in the cutting forces in comparision to the machining parameters of 2 nd orthogonal array. This is due to the fact that the excessive temperature generated at higher spindle speeds caused thermal softening of the workpiece, moreover, the LN2 over machining zone exhibited the cushioning effect. This resulted into the reduced machining zone friction which ultimately decreased the tool-tip shearing area. At the selected optimum machining parameters, surface finish was observed to be improved by 8% compared to the 2 nd orthogonal array. Figure 12 shows the scanning electron microscope (SEM) morpholoies of machined surface in which 2 nd orthogonal array image is shown in fig 12(a) whereas, image obtained with optimal process parameters is illustrated in fig 12 (b). This clearly shows the micro cavities on the surface for 2 nd orthogonal array. On other hand, optimal machining parameters show comparative ly better surface morphology. This is primarily due to the fact that the region in the vicinity of chip-tool interface is thermally softened at higher temperatures which leads into the minimization of surface flaws and discontinuities at the machining zone. This is also supported by the previous researchers [39,40].
[ Fig. 12 about here.] Figure 13 (a) and 13 (b) shows the SEM images of the tool morphology after machining with 2 nd orthogonal array and optimised machining parameters respectively. Rapid tool wear is observed at the morphology obtained with 2 nd orthogonal array machining parameters. The reason of this tool wear is primarily due to the abrasion and weakening of the section. On other hand, less damage is observed in the morphology obtained with optimal machining parameters.

Conclusions
The combined use of direct and indirect cryogenics implemetation along with multi-objec tive optimization of machining parameters for turning of Nimonic C-263 superalloy have been presented in this work. The cryogenic machining strategy has proven to be an environment friendly and work friendly machining approach as it doesn't have any ecological hazards. Moreover, there is no concern of leakage of coolant waste, as LN2 is disperesed in an atmosphere immediately after contact with a machining zone. It has also been found that with use of LN2 as a coolant, there is no attempt required for disposal of the same. The key findings obtained from the study are discused below.
• Taguchi based grey relational analysis is amongst the simplest technique to treat multi response optimisation problems and involves least computation steps.
• The ideal combination for cryogenic hard-turning machining attributes obtained as spindle speed of 110 m/min, cutting feed of 0.08 mm/rev and depth of cut as 0.4 mm.
• Feed rate has found to be the most crucial parameter which influences the hard turning upto 48.33% .
• On machining the Nimonic C 263 workpiece again with optimum combination of process parameters obtained from TGRA , cutting force found to be decreased from 188.3 N to 161.14 N, surface roughness reduced from 0.911 µ m to 0.837 µ m whereas, tool nose wear reduced from 0.07 mm to 0.06 mm.
• The values obtained from TGRA are again validated by carrying out a confirmative test predicting an improvement of 9.34 % in between the experimental and otimization results.    The improvement in GRG= 0.0802

List of Tables
The percentage improvement in GRG =9.34%