Experimental study of hardness effects on surface roughness for nanofluid minimum quantity lubrication ( NanoMQL ) technique using Jaya algorithm

Article history: Received: May 1, 2018 Received in revised format: June 16, 2018 Accepted: August 18, 2018 Available online: August 18, 2018 The NanoMQL technique is used to overcome the limitations of wet grinding due to economic and ecological problems. The performance measure is largely influenced by the process parameters such as table speed, depth of cut, air pressure, coolant flow rate and nanofluid concentration. In this paper, the performance of NanoMQL technique in terms of surface roughness was evaluated for hard and soft EN31 steel. The Experiments were conducted by response surface methodology (RSM) using statistical software to develop regression model of surface roughness and optimization was carried out using Jaya algorithm. The result shows that lowest value of surface roughness was obtained for NanoMQL of hard steel in comparison with soft steel under grinding environments such as wet, MQL and NanoMQL. Hence to improve the performance of soft steel, the modeling and optimization of surface roughness were carried out. The significant parameters were considered for model development and validity of model determined through ANOVA (Analysis of variance). Lastly, the optimal values were determined using Jaya algorithm for minimum surface roughness and the percentage error observed to be close with the experimental test. © 2018 by the authors; licensee Growing Science, Canada.


Introduction
The proper selection of machining process is the prime requirement of manufacturing industries in today's scenario.The process should be economical and environment friendly.In most of the industries, a large amount of coolant is used to reduce the friction and temperature occurred during machining either due to improper setting of process parameters or inefficient working process.The micro-lubrication techniques are widely used to overcome the limitations of wet technique (Brinksmeier et al., 2015;Kim et al., 2016;Najiha et al., 2016).Recently machining process integrated with recent technology called minimum quantity lubrication (MQL), Nanofluid minimum quantity lubrication (NanoMQL) and application of optimizer tools for optimal values are focused to obtain better grinding performance.The reason is complexity of grinding process due to more abrasive grits contact with wheel surface for microseconds and consumption of high specific grinding energy (Tawakoli et al., 2009).The better machining performance and surface integrity is obtained using MQL in comparison to other machining environments such as dry, wet and process is more sustainable, human environment friendly (Chakule et al., 2017;Huang, et al., 2016;Mao et al., 2012;Hwang & Lee, 2016).The suspended nanoparticles in base fluid significantly change the transport and heat transfer characteristics of suspension which increases the surface volume ratio, Brownian motion of nanoparticles (Godson et al., 2010;Sharma et al., 2011).The application of NanoMQL technique reduces the friction, cutting forces, specific energy and surface roughness largely when such nanofluid is applied to grinding zone as mist.The stable nanofluids of different concentrations are considered for grinding of NanoMQL experimentation.The better results in terms of grinding process efficiency and surface quality are obtained using small amount of cutting fluid (Zhang et al., 2015;Lee et al., 2015;Setti et al., 2015;Sinha et al., 2017;Zhang et al., 2016;Wang et al., 2017;Sinha et al., 2017;Kalita et al., 2012).The traditional method of optimization for NanoMQL process cannot predict the accurate guess of initial solution and may trap into local optima.The evolutionary optimization techniques are widely applied by the researchers to optimize the machining process parameters for machining performance and surface roughness (Gupta et al., 2016;Liu et al., 2017, Pai et al., 2013;Yusup et al., 2012).
During the last few years, some of the researchers have worked on grinding performance of soft and hard steel considering the dry, wet and MQL machining environments but to the best of authors' knowledge, no one has worked on hardness effect of EN31 steel for NanoMQL technique (Fig. 1).The surface quality is considered in the present study due to importance of quality production in industries.In this paper, the performance of surface roughness is studied for material hardness of EN31 under different grinding environments.Finally, the material of poor surface roughness is identified for NanoMQL technique and then it was analyzed by modeling and optimization for better value.The derived mathematical model was used as fitness function evaluation in Jaya algorithm.

Experimental details
The workpiece selected in the present experimental study was EN31 steel of hard and soft type.For hard material the hardness was maintained up to 58 HRC and the same material without hardening was considered here as a soft steel of hardness 28 HRC.The experiments were conducted for both steel materials of rectangular plate of size 100×50×25 mm using horizontal surface grinding machine for different machining conditions such as wet, MQL and NanoMQL.The experiments were carried out with Al2O3 white corundum vitrified bond grinding wheel of size 500×50×203.2mm and specification A-46-3-L5-V8.The stable Al2O3 nanofluid of concentrations 0.15 and 0.30 vol.% for NanoMQL and 5% concentration of soluble oil in MQL were used for experimentation.The concentration (0) means the experiments are performed for MQL.The wheel dressing, nozzle position, parameter level selections were considered properly during experimentation as it significantly influences on the results.The importance of nozzle position for penetrating the cutting fluid into contact zone to obtain better surface finish was stated in paper (Mao et al. 2013).The homemade MQL setup was used for precise flow delivery in mist form at contact interface of wheel abrasive and workpiece surface.The quadratic model of surface roughness was formulated for selected range of parameters.The computer code was developed for Jaya algorithm to find optimal values for minimizing the surface roughness.The input parameters were coded as table speed (TS), depth of cut (DOC), air pressure (AP), coolant flow rate (CFR), nanofluid concentration (NC) whereas surface roughness (Ra) is considered as response parameter.The average value of response was considered for analysis.The levels of parameters are shown in Table 1.The results of surface roughness of hard and soft steel under different working environments such as wet, MQL and NanoMQL are shown in Fig. 2. The surface roughness of hard steel obtained under wet and MQL grinding environments are 0.234 µm and 0.2 µm respectively whereas the lowest value of surface roughness 0.17 µm obtained in NanoMQL at 0.30 vol.% concentration.Similarly for soft steel the lowest value of surface roughness 0.18 µm was obtained in NanoMQL whereas for wet and MQL the surface roughness values obtained were 0.262 µm and 0.21 µm, respectively.The poor surface quality of soft material is due to ductile fracture mechanism of higher material malleability and elongation before fracture which gives high material deformation and surface defects.For hardened steel, the brittle chip mechanism of material removal was observed.The trend of responses of hard and soft steel materials under dry, wet and MQL working conditions were discussed by Rabiei et al. (2015).The NanoMQL grinding gives the lowest surface roughness value compared to wet and MQL due to tribofilm of nanoparticles under extreme pressure on workpiece surface which reduces the friction and maintained cutting grits sharpness.The lubricity and cooling effects are more dominant in NanoMQL due to better lubricity properties of nanofluid and cooling effects by compressed air.In wet and MQL grinding, the soluble oil was used as cutting fluid and having poor lubricity.Again in wet grinding, the poor penetration of cutting fluid into high hydrodynamic pressure-grinding zone occurred due to high rotation of grinding wheel.Thus the poor lubricity and inefficient penetration of cutting fluid generates high value of surface roughness in wet grinding whereas the better penetration and removal of debris from cutting zone was possible in MQL due to high air pressure which gives better surface finish.The generation of lower forces due to better heat transfer characteristics of nanofluid using MQL may be the reason to obtain better surface finish in NanoMQL.The mode of material removal is more shearing in NanoMQL of hard steel whereas more deformation and ploughing in case of wet grinding.In order to improve the surface finish, the process parameters of soft steel are optimized using Jaya algorithm.

Modeling and Optimization
The regression equation was developed by RSM to improve the performance of NanoMQL grinding process of soft steel material.The mathematical model of surface roughness in terms of significant independent parameters is represented in Eq. ( 1).The independent parameters are coded as:

Optimizing of surface roughness
From literature studies it was observed that all the evolutionary algorithms are probabilistic and requires common controlling parameters like population size and the number of iterations.Again different algorithms require their own algorithm-specific control parameters.The improper tuning of algorithm-specific parameters either increases the computational effort or yields the local optimal solution.In this research work the developed model is optimized using coded Jaya algorithm to obtain optimal values of parameters for minimizing the response using Eq. ( 1).The reason is that Jaya algorithm is free from such common controlling and algorithm specific parameters.The Jaya algorithm is simple and is able to get the optimal solutions with fewer number of function evaluations and memory requirement (Rao et al., 2017;Rao et al., 2016;Rao, 2017;Rao, 2016).The regression equation developed is subject to constraint such as 7000≤TS≤13000, 20≤DOC≤40, 250≤CFR≤750 and 0≤NC≤0.30.The algorithm is based on the concept that the final solution should be the best one and avoid the worst solution.The algorithm always tries to move closer to success (i.e.reaching the best solution) and avoid failure (i.e.moving away from the worst solution).The f(x) is the objective function to be minimized.
where Xj,best,i is the value of the variable j for the best candidate and Xj,worst,i is the value of the variable j for the worst candidate.X ' j,k,i is the updated value of Xj,k,i and r1,j,i and r2,j,i are the two random numbers for the j th variable during the i th iteration in the range (0, 1).The term ''r1,j,i (Xj,best,i _ |Xj,k,i|)" indicates the tendency of the solution to move closer to the best solution and the term ''-r2,j,i(Xj,worst,i _ |Xj,k,i|)" indicates the tendency of the solution to avoid the worst solution.X ' j,k,i is accepted if it gives better function value.All the accepted function values at the end of iteration are maintained and become the input to the next iteration.The random numbers r1 and r2 ensure good exploration of the search space.The absolute value of the candidate solution (|Xj,k,i|) considered in Eq. ( 2) further enhances the exploration ability of the algorithm.The convergence curve to optimize the surface roughness of soft steel is shown in Fig. (5).It shows that the convergence graph lower continuously until it reaches the lowest value of surface roughness and then remains stable.This shows that the Jaya algorithm is robust and does not get trapped in local optima.
The optimal values, elapsed time and convergence values after number of generations are summarized in Table 3.The number of generations and population selected are 50 and 100, respectively.The experimental tests were conducted at optimal feasible values of parameters to validate the results obtained from Jaya algorithm.The minimum surface roughness of 0.1466 µm predicted by Jaya was in good agreement with result of experiment test (0.138 µm).The optimal values obtained from Jaya algorithm is really beneficial to improve the performance of grinding of soft steel with NanoMQL technique.

Conclusions
The hardness effect of EN31 steel under different grinding environments such as wet, MQL and Nano-MQL for minimizing the surface roughness was studied.Further the Jaya algorithm is used for optimization of soft steel to obtain optimal values for enhancing the surface quality.The results obtained in this study are summarized as follows:  The lowest surface roughness is obtained in NanoMQL technique of hard steel.The reduction in surface roughness value of hard steel are obtained by 5.88% in NanoMQL at 0.30 vol.% concentration, 5 % in MQL and 11.96% in wet grinding over the soft steel material.The reasons are less friction at contact interface in comparison to other cutting environments due to efficient lubrication and cooling effects.The surface quality is improved in fewer amounts of cutting fluid compared to wet and MQL grinding environments.In hard material the brittle fracture occurred, due to which surface damage and defects are comparatively less than soft steel. To improve the surface quality of soft steel, the modeling and optimization of surface roughness is performed.The quadratic model of surface roughness was formulated for significant parameters and its adequacy was checked by ANOVA to predict the surface roughness of soft steel at 95% confidence interval.The optimal values obtained from Jaya algorithm are TS (7000 mm/min) DOC (20 µm), CFR (750 ml/hr) and NC (0.186 vol.%).The predicted value of surface roughness obtained from modeling is 0.1466 µm.The experimental tests were conducted to confirm the optimal values determined from Jaya algorithm.The surface roughness obtained through experimental test is 0.138 µm shows the percentage error of 5.87 %.The good agreement in results of surface roughness obtained with predicted and experimental value. The convergence accuracy and speed of Jaya algorithm is very high.The better grinding results are obtained at optimal values with less function evaluations and memory requirement.The results obtained from Jaya algorithm are really beneficial to improve the surface finish of Nano-MQL grinding process especially of soft steel.

Fig. 2 .
Fig. 2. Surface roughness under different cutting environments of (a) hard and (b) soft steel.

Fig. 3 .
Fig.3.Normal probability plot of surface roughness of EN31 soft steel

table speed
2 value of surface roughness is 0.9475 which is close to 1, whereas pred R-squared value 0.9172 is in good agreement with adj R-squared value of 0.9349.This value indicates that the model was well fitted by the data.The P-value of parameters less than 0.05 indicates that the model terms were significant.The F-value of table speed means table speed is influential parameter in response.The ANOVA table of surface roughness of soft steel is shown in Table2.

Table 3
Optimal values and confirmation experiments of soft steel using Jaya algorithm