Optimization of EDM Parameters for Al – TiC Composites Prepared through Powder Metallurgy Route

The conventional monolithic materials available in the market does not find its place for the critical applications like Automotive sector and Aerospace industry which requires the properties such as lighter weight with better wear resistance. Hence a new variety of material i.e. Metal Matrix Composite (MMC) is developed by employing various techniques like Stir casting, Powder Metallurgy etc., Each one has its merit over the other. It is very difficult to machine MMC due to the abrasive nature of the base metal Aluminum and the intermittent presence of reinforcements in the MMC. Moreover, the tool wear is high with the conventional tools [1-4]. Among the nonconventional machining process EDM is one of the effective processes of removing metal from harder surfaces. In the EDM process formation of mechanical stress at the machined surface is eliminated due to the absence of contact between the tool and the work material [5]. Prabu Basanna Choudri in his review and future scope article concluded that most of the EDM work has been carried out on Steel as the work material with copper rod in cylindrical shape as the most common tool material. Pulse on/off time, current are the more common influencing factor and focus on MRR, TWR, SR was given by many researchers [6]. Kathiresan [7] et al. has worked on EDM studies on aluminium alloy-silicon carbide composites developed by vortex technique and pressure die casting. The results reveal that material removal rate and surface finish are greatly affected by current and reinforcement percentage. Kannan et al. reported [8] in their study that on machining of hard AMC with 5% TiC by using liquid casting technique, the processing parameters of EDM like discharge current, pulse on time and flushing pressure are optimized using Taguchi Techniques for minimum Tool wear rate(TWR). Various optimization techniques such as Taguchi, DOE, ANNOVA, RSM are adopted to find the optimized parameter value while machining of different harder materials with EDM process. During analysis Metal Removal Rate and surface roughness value reveals that Pulse on Time, Current, Voltage has their impact on the response variables. The important parameters affecting the performance of the EDM process differ from one research to the other. Majority of the researchers proposed the impact of Peak current on the performance of EDM [9-14.] All the above observations reveal that the parametric optimization of aluminum TiC composites prepared through powder metallurgy route needs much attention. Especially evaluation of machining parameters for non-conventional machining process such as EDM has to be addressed in the present scenario in order to excel in the field of precision machining. Therefore, in this work, electric discharge machining of Aluminum (AP50) reinforced with various weight percentage of TiC particles prepared through powder metallurgy route have been studied and analyzed. The effect of parameters like Peak current, Pulse on time and Pulse off time on the machinability characteristics of Al with TiC MMC is evaluated. The same testing methodology is employed for testing the specimen with varying reinforcement percentage i.e., 2.5%, 5% and 7.5% TiC.


Introduction
The conventional monolithic materials available in the market does not find its place for the critical applications like Automotive sector and Aerospace industry which requires the properties such as lighter weight with better wear resistance.Hence a new variety of material i.e.Metal Matrix Composite (MMC) is developed by employing various techniques like Stir casting, Powder Metallurgy etc., Each one has its merit over the other.It is very difficult to machine MMC due to the abrasive nature of the base metal Aluminum and the intermittent presence of reinforcements in the MMC.Moreover, the tool wear is high with the conventional tools [1][2][3][4].
Among the nonconventional machining process EDM is one of the effective processes of removing metal from harder surfaces.In the EDM process formation of mechanical stress at the machined surface is eliminated due to the absence of contact between the tool and the work material [5].Prabu Basanna Choudri in his review and future scope article concluded that most of the EDM work has been carried out on Steel as the work material with copper rod in cylindrical shape as the most common tool material.Pulse on/off time, current are the more common influencing factor and focus on MRR, TWR, SR was given by many researchers [6].
Kathiresan [7] et al. has worked on EDM studies on aluminium alloy-silicon carbide composites developed by vortex technique and pressure die casting.The results reveal that material removal rate and surface finish are greatly affected by current and reinforcement percentage.
Kannan et al. reported [8] in their study that on machining of hard AMC with 5% TiC by using liquid casting technique, the processing parameters of EDM like discharge current, pulse on time and flushing pressure are optimized using Taguchi Techniques for minimum Tool wear rate(TWR).
Various optimization techniques such as Taguchi, DOE, ANNOVA, RSM are adopted to find the optimized parameter value while machining of different harder materials with EDM process.During analysis Metal Removal Rate and surface roughness value reveals that Pulse on Time, Current, Voltage has their impact on the response variables.The important parameters affecting the performance of the EDM process differ from one research to the other.Majority of the researchers proposed the impact of Peak current on the performance of EDM [9-14.]All the above observations reveal that the parametric optimization of aluminum TiC composites prepared through powder metallurgy route needs much attention.Es-pecially evaluation of machining parameters for non-conventional machining process such as EDM has to be addressed in the present scenario in order to excel in the field of precision machining.Therefore, in this work, electric discharge machining of Aluminum (AP50) reinforced with various weight percentage of TiC particles prepared through powder metallurgy route have been studied and analyzed.The effect of parameters like Peak current, Pulse on time and Pulse off time on the machinability characteristics of Al with TiC MMC is evaluated.The same testing methodology is employed for testing the specimen with varying reinforcement percentage i.e., 2.5%, 5% and 7.5% TiC.

Materials
In the present investigation Aluminium and TiC are used to form the MMC.All the elements are taken in their weight percentage.TiC with 44 μm particle size and Aluminium (Grade AP50) of mesh size 72μm particle sizes with 3 proportions of TiC i.e., 2.5%, 5%, 7.5% by weight is used for the preparation of three varieties of MMC specimen.

Fabrication of MMC
Powder metallurgy technique is used for the fabrication of MMC in our study.The cylindrical shaped specimen of 25mm diameter and 25mm in length is fabricated for conducting the experiments.Aluminium powder and TiC powder of required proportion by weight are blended thoroughly and kept in Ball mill unit for 4 hours to obtain uniform mix.Then it is compacted to the required shape in the press by applying pressure and sintered in electric furnace by maintaining at a temperature of 425ºC.

Mechanism of EDM
In the EDM Process material is removed from the work piece by the spark formed between the electrode (Tool) and the work piece.Due to the formation of spark, heavy pressure is developed at the junction and higher temperature is prevailed.This leads to localized vaporization and melting.

Machining of MMC
Electronika4 axes CNC controlled EDM machine was used to carry out the experiments.Copper rod of diameter 12 mm was used as electrode.The machining parameters such as Electrode feed rate, dielectric fluid pressure and applied voltage were kept constant.Peak Current, Pulse on time and Pulse off time were considered as variable parameters in the present investigation.The EDM setup is shown in Fig. 1.

Fig. 1 Experimental set up of EDM process
The fabricated cylindrical MMC specimen is fixed in the fixture (Work holding device) which is mounted over the machine table and circular hole is drilled by feeding the electrode at a uniform rate.The time duration for machining each hole was recorded.The Metal Removal Rate can be determined. 12 where: W1 (g) is the initial weight of the specimen, W2 (g) is the final weight of the specimen after machining and  is the machining time in min.Machined MMC were collected and surface roughness is checked by using Mitutoyo make Surface Roughness tester and values were recorded.
SEM images of the machined surface were taken to study the nature of surface modification after machining.The process parameter values and levels are given in the Table 1.
Table 1 Process parameter values and its levels

Analysis of experiments
The input parameters (Peak Current, Pulse on time and Pulse off Time) that are affecting the machining process were varied to analyse their effect over the response values such as MRR and Ra.
The main objective of any machining process is to remove the required material in a shorter duration, hence for MRR Larger the better concept is preferred i.e., output value must be higher.Similarly, surface should be smoother for the aesthetic appearance and longer service life of the component.Hence for Surface roughness value smaller the better concept is best suited i.e., output value must be lower.
Orthogonal array concept recommended by Taguchi is used in the design of Experiment.Percentage contribution of each parameter against stated level of confidence is achieved by using the statistical tool Analysis of Variance (ANOVA).Signal to noise ratio is a statistical measure to measure the deviation of the performance characteristics from the required values.

Experimental detail
Values were assigned to three input parameters to form an L9 orthogonal array.Individual experiments were conducted for each set of values on 9 components for each variety.Hence a total of 27 components were machined in this experimental set up.MRR can be calculated using the Eq. ( 1).The prepared composites after machining by EDM process are shown in Fig. 2. The stylus of the surface roughness tester is made to follow the machined surface and the average surface roughness Ra value is measured.The response values are shown in Table 2,

Results and discussion
From the production point of view to obtain better economy in an industry higher MRR is the most essential requirement.The next expectation from the customer is its aesthetic appearance and better life which necessitates the property fine Surface finish.First we are focussing our attention on the MRR.Minitab 16 Software is used to calculate the S/N ratio and the mean of mean values for Design of Experiment with respect to MRR.By using the experimental data, values of the Response table of MRR for 2.5%TiC, 5% TiC & 7.5% TiC were calculated and presented in Table 5.
The amount of electrical energy available for metal removal is called Spark energy (discharge energy) (Em): From the above Eq.( 2), it is very clear that the spark energy depends mainly on the factors Peak current, pulse on time and voltage applied.when the peak current increased, the number of anions striking the surface of the work piece gets increased.This in turn improves the concentration of higher spark energy and hence the temperature at the location of the spark is increased.This increased spark energy makes the temperature of the work piece to rise and melts more volume of the work piece that leads to higher metal removal rate.
This increased spark energy makes the temperature of the work piece to rise and melts more volume of the work piece that leads to higher metal removal rate.
The main effects plot shown in Figs. 3 (a, b, c) depicts the variation of S/N ratio of MRR with process parameters i.e., pulse on, pulse off and peak current.
The X axis of the Plot indicates the value of each process parameters at their levels and the Y axis of the plot indicates the mean value of S/N ratio of the response i.e., MRR.
Table 5 Response From the Fig. 3 (a) it is evident that the MRR value is gradually increased with the increase in Peak Current up to certain limit.Then the MRR declines slightly.his is due to the fact that when the current applied is gradually increased, the number of the anions formed is also increased and hence the temperature responsible for melting the work piece is raised rapidly.After attaining a stage, if the current is increased further the anions formed have to penetrate further to melt the underlying material below the fused material.Hence a light decline is observed in the later stage.The same scenario is observed in Figs.

Analysis on material removal rate
To find the significance of the parameters on response values F-test is the most reliable process.Higher F value for any parameter means more influence of that parameter on the response value.
From the Table 6, it is found that the most significant factor influencing the MRR is Peak current followed by Pulse on Time.Pulse off Time has lower impact on MRR For the 2.5% TiC variety the F Value for Peak current is 28.35 whereas for Pulse on Time the value is 6.01 and for Pulse off Time value is 1.23.This indicates that the Peak current is the dominating factor whose contribution is much higher than that of Pulse on Time.The same pattern is identified for the 5% TiC and 7.5% TiC varieties.During Analysis of variance, if the P value for any process parameter is less than 0.05, it is significant.With the above point of view also it is very clear that the Peak current is more significant parameter than Pulse on Time, whereas the Pulse of Time is not a significant Parameter.
By analysing the main effect plot and the mean response table for Metal removal rate the optimal combination of process parameters is presented in the Table 7.The optimum values are Peak current (level 3, value 20 A), PON (level 3, value 80 μs), and POFF (level 1, value 2 μs).
From the contour graphs shown in Figs. 4 (a, b, c) variation of MRR with respect to the Peak current and Pulse on Time is displayed.Fig. 4 (a) shows that for a Pulse on Time up to a value of 60 μs with peak current value between 10 A to 11.5 A the MRR value is minimum.Maximum MRR is possible with Peak current value greater than 16 A with a Pulse on Time value greater than or equal to 55 μs.This is due to the fact that higher amount of Peak current and higher value of Pulse on Time enables generation of more spark energy to melt more material in lesser time.The same range of Peak current and Pulse on Time enables more MRR for the TiC 5% and Tic 7.5%.Maximum MRR value is obtained with optimum conditions for 2.5% Tic variety is 19.80, for 5% Tic variety is 19.04 and for 7.5% Tic variety is 14.24.This decline in MRR value is entirely due to the increase in percentage of TiC particles in the above three variety of composite materials.From this, we can infer that MRR is inversely proportional to the percentage of TiC presence, which is similar to the study carried out by Kathiresan et al [7].
The surface finish is the most expected requirement next to MRR.The surface finish value is lesser when the pit formation during the EDM process is smaller in size.The pit formation due to melting is controlled by lowering the temperature during spark.Minitab 16 Software is used to estimate the S/N ratio and the mean of mean values for the design of Experiment with respect to surface roughness value Ra.From the experimental data, values for the response table of surface roughness value for the 2.5% TiC, 5% TiC & 7.5% TiC% were presented in Table 8.
The main effect for data means is shown in Figs. 5  (a, b, c).Which depicts the changes in Ra value with process parameters i.e., Pulse on Time, Pulse off Time and Peak current.
The Figs. 5 (a, b, c) displays the effect of Pulse on time, Pulse off Time and Peak current.Surface roughness increases with the increase in Peak current.In the graph it is evident that if the current is increased up to 15.The surface roughness value also increases steadily and if increased beyond that value the surface roughness value declines slightly.Hence to obtain lower value of surface roughness the Pulse on Time should be low.If Pulse on Time is increased the magnitude of Ra value increased is lower than that of the peak current.
From the Figs. 5 (a, b, c), we can infer that the increase in the Pulse off time has no effect on the Ra value.This is because increase in Pulse off Time reduces active impact of anions and hence little change in the Ra value.The significance of the parameters on response values F test is the ideal procedure.Analysis of variance of surface roughness for 2.5% TiC, 5% TiC and 7.5% TiC are shown in Table 9.For the 5% TiC variety the F Value for Peak current is 68.77whereas for Pulse on Time the value is 10.95 and for Pulse off Time the value is 2.42.This indicates that the Peak current is the more dominating factor whose contribution is more than that of Pulse on time.The same pattern is identified for the 7.5% TiC and 2.5% TiC varieties.Also the pulse of time is not a significant parameter.The optimal combination of process parameters for surface roughness is presented in the Table 10

Conclusions
From the analysis after the completion of the experiments, it is concluded that: 1. Increase in Peak current and increase in Pulse on time improves the metal removal rate whereas the increase in the value of Pulse off time has no significant effect.
2. The increase in the percentage of TiC decreases the metal removal rate.This is due to the fact that the harder TiC particles present in the specimen retards the process.Thus the metal removal rate is decreased with increase in the reinforcement material i.e., TiC 3. To obtain higher material removal rate, more peak current and more pulse on time should be ensured.4. Better Surface finish is obtained with lower Pulse on time and lower peak current.This is due to the formation of minimum pits and lesser amount of heat generation at the spark and sufficient time to flush the debris and cools the melted area. 5.The percentage of TiC present is responsible for the formation of pits around TiC particles.i.e., more TiC percentage leads to more pits and hence poor surface finish (Higher Ra value) is obtained.

Table 7
Optimal combination of process parameters for MRR