A Study on Surface Roughness and Cutting Width for Circular Contour Machining of Stir Cast AA6063/SiC Composites in WEDM

6524 | P a g e F e b r u a r y 2 0 1 7 w w w . c i r w o r l d . c o m A Study on Surface Roughness and Cutting Width for Circular Contour Machining of Stir Cast AA6063/SiC Composites in WEDM Dr. I.Balasubramanian, Dr. R. Maheswaraan, Dr. Ranganath M Singari, S Velayutha Ganesh 1 Assistant professor, Department of Mechanical Engineering, VV College of Engineering, Tisaiyanvilai, India. balacim@vvcoe.org 2 Professor, Department of Mechanical Engineering, MEPCO Schlenk Engineering College, Sivakasi, India. rmaheshin@yahoo.com 3 Associate professor, Department of Mechanical and Production Engineering, Delhi Technological University, New Delhi, India. ranganathdce@gmail.com 4 Lecturer, Department of Mechanical Engineering, Ramana Polytechnic College, Tirunelveli, India. Ganesh270286@gmail.com Abstract

Metal Matrix Composites (MMCs) have evoked a keen interest in recent times for potential applications in aerospace and automotive industries owing to their superior strength to weight ratio and high temperature resistance.For many decades, Silicon Carbide (SiC), a ceramic material having higher hardness is being used as a reinforcement material in lightweight Aluminium metal matrix [1].Advanced composites with different composition of aluminium and silicon carbide, which have enhanced hardness and high temperature resistance properties, are under study [2].In this field, many research works have been carried out on AA6063/SiC Composites which were prepared by a casting process by melting the constituent alloy AA6063 and incorporating the SiC particles in the solidifying metallic slurry of AA6063 [3].Stir casting is the widely used method of processing where the aluminum matrix is completely melted and ceramic particles (SiC) are added into the molten metal in a vortex created by a mechanical stirrer [4].Generally a stir casting setup has been integrated by an electric resistance muffle induction furnace with a melting zone of separate preheating chamber, and a mild steel stirrer rotated by a coupling motor.AA6603/SiC composites are stir casted by melting AA6603 aluminum ingots in melting zone and preheating SiC in the preheating chamber and mixing using the stirrer in the molten stage and solidify in a reform die with required dimensions [5][6][7][8][9][10][11][12].Stir cast AA6063/SiC is an electrically conductive material that can be machined by nontraditional WEDM process independent of their hardness, shape and toughness [13][14][15][16][17][18][19].
In WEDM process, the material is eroded from AA6063/SiC composite work piece by a series of continuous discrete sparks occurring between the work piece and the electrode wire separated by a stream of deionized dielectric fluid [20].Due to the abrasive nature of reinforcement traditional machines are very limited for machining composites.High tool wear and very low performance of traditional machinery to machine hard composite materials cause way to non-traditional methods like water jet and laser cutting but these processes are limited to linear cutting only.Garg et al (2010) suggested that Wire Electrical discharge machining (WEDM) shows higher capability for cutting complex shapes in composites [21].The effect of WEDM parameters such as pulse on time (Ton), pulse off time (Toff), peak current (IP) wire feed (WF), servo voltage(V), flushing pressure (FP), wire tension (WT), open voltage (OV), servo voltage (SV), and dielectric pressure (DP) on the performance measure like surface roughness (Ra) and cutting width (Kerf) and metal removal rate (MRR) are investigated by many researchers [22][23][24].Yan et al (2005) found that Ton has the significant effect on Ra during rough machining by WEDM.For finish cutting, cutting feed rate has the significant effect on Ra [25].Nilesh et al (2010) found that the volume fraction of ceramic reinforcement in the composite, Ton and Toff were found to be significant on metal cutting, surface finish and Kerf [26].Nihat Tosun (2003) proved experimentally that increasing pulse duration, open circuit voltage, wire speed and dielectric fluid pressure increase the cutting speed.The surface quality of the work piece increases with the decreasing pulse duration, open circuit voltage and wire speed, and with increasing dielectric fluid pressure by [27].Plaza et al (2009) results in machining show that part thickness and taper angle are the most influencing variables in taper cutting through WEDM [28].Mahapatra et al (2007) discusses factors like discharge current, pulse duration, and dielectric flow rate and their interactions have been found to play a significant role in rough cutting operations for maximizations of MRR, minimization of Ra and minimization of Kerf [29].In the investigation of Kapilkumar et al (2012) informs that IP, Ton, Toff, WF, WT and FP effect on the material removal rate and surface finish in a significant way [30].A significant amount of research has been going to find the different methodologies of achieving the ultimate WEDM goals of optimizing the numerous process parameters by analysis with the total elimination of the wire breakages, Ra and maximum Kerf [31][32][33].DOI : 10.24297/jac.v13i10.5829In this work it has been proposed to remove a semi cylindrical piece from a stir cast rectangular AA6063/SiC composite plate with different compositions by circular contour machining in WEDM.Then the effect of parameters such as Ton, Toff and IP were studied with performance measures like Ra and Kerf.The results of this work would help to select various WEDM parameters with respect to different levels provided to ensure appropriate WEDM process parameters to machine circular contour in AA6063/SiC composite at various proportions.

Composition of Samples
AA6063/SiC composites are formulated by the process of stir casting which is a liquid phase transformation method, with different weight ratio of AA6063 and SiC as proposed by different researchers [34][35].In this study, the raw materials were prepared with four different weight ratios as given in Table 1. Tab.

WEDM Process Parameters
In WEDM, the amount of material melts and evaporates depend on the duration of sparks produced, which is represented by pulse on time (Ton).The melted part of the AA6063/SiC composite plate form as a small crater in the machining zone.
The crater material was removed by the flow of deionized dielectric fluid and the remaining part resolidifies rapidly on the surfaces of the brass wire for a period known as pulse off time (Toff).Application of consecutive pulses with high frequencies together with the forward movement of the brass wire towards the AA6063/SiC composite plate resulted in a form of a circular contour shape.The breakdown in the dielectric medium during material removal process is due to high local electric field represented by peak current (IP).In this paper, the effect of all the above three electric parameters Ton, Toff, and IP plays significant role in performance measures Ra and Kerf for machining circular contour in AA6063/SiC composites were studied.

Fig.2. Specimen used for WEDM 2.5 Performance Measures for WEDM
The performance like surface roughness (Ra) and cutting width (Kerf) were measured as follows.

Surface roughness (R a )
The Ra was measured by using Mitutoyo Surf Test machine SJ410.The average values of Ra were measured at various machined area by using the sensible probe for a travelling distance of 8 mm of the machine.At each experiment roughness values were measured in different location of circular contour machined by WEDM and the average value was taken as final Ra value in microns.The reciprocal value of Ra will normally give the value for surface finish.

Cutting width (Kerf)
To observe cutting width of the WEDM process Mitutoyo Crystaplus M443 Co-ordinate measuring machine (CMM) was used.Normally for measuring the cutting width of linear contour image processing system is used.In this investigation cutting width of circular contour was calculated by using a formula Kerf = R-r.The radius of the cut region in the work piece was considered as R and the radius of the cut piece is r as shown in Figure 3.

Fig.3. Cutting width (Kerf) of the process 3 Circular Contour Results
Machining of AA6063/SiC composites were performed in an Electronica 4-axis CNC Sprintcut wire electrical discharge machine and its machining zone are shown in Figure 4.A negatively polarized brass wire of 0.175 mm diameter was used as an electrode.Deionized water was used as a dielectric fluid The experiments were conducted to cut a circular contour of diameter 10mm and radius 5mm for each sample of composite.A small gap of 0.025 mm to 0.05 mm was maintained in between the wire and the work.Different machine setting parameters such as Ton, Toff and IP were varied at three levels.The machining parameters at different levels were selected based on the trial runs carried out in such a way to avoid wire breakage during machining of composite materials.According to the circular contour program fed into the control unit, the wire travel along the circular path and created a circular contour for the dimension of radius 5 mm and diameter 10 mm as shown in Figure 5.The removed piece of semicircular shape was used for finding the Kerf in CMM.The surface of the circular contour formed in AA6063/SiC plate was used for finding the Ra in Surftest machine.The values were recorded from the readings obtained from the CMM and Surftest machine as shown in Table 2.The values of Ra and Kerf for all the four samples are tabulated as shown in Table 2 and then the graphs are plotted against all the parameters and discussed.The effect of Ton on Ra is shown in Figure 6 and it was found that the Ra increases with decreases in Ton Moreover, when the Ton was lower, there was a poor surface finish of the samples.The Ra results obtained under different Toff were plotted in Figure 6.It is noted that the Ra increases as Toff increases to certain extent and then the trend changes.It was observed that 50 units of Toff provide maximum surface roughness.Further, it was confirmed that long gap between pulse left some impressions in machined surface which results in very poor surface finish.Figure 6 shows the effect of IP on surface roughness for various percentage of SiC.When the IP increases Ra also increases in sample 1, but in sample 2, 3, and 4 for 190 units of IP the Ra decreases as the sparks production decreases.It is observed from the Figure 8 that there is an increase on Ton results in increase in kerf.In composite samples the cutting width was not evenly produced, due to it uneven distribution of particles.From the Figure 7, it is observed that 40 units of Toff produces the high Kerf and also 60 units of Toff resulted in very low Kerf for all samples.From Figure 7, the effect of 50 units IP is high in producing a large size Kerf and the 210 units IP resulted in small Kerf gap.

Conclusion
In this investigation, WEDM is used for removing semi cylinder pieces from rectangular AA6063/SiC composite plates fabricated with reinforcement of SiC in 0%, 5%, 10% and 15% weight fractions in AA6063 aluminium matrix through stir casting.In these experimental investigation the influence of cutting conditions of WEDM are analyzed and the following conclusions are drawn from the analysis.
The WEDM performance measures like Ra and Kerf varies for the different composition of AA6063/SiC composite plates at same cutting conditions.
Then the effect of WEDM parameters such as Ton, Toff and IP on Ra and Kerf on different composition of AA6063/SiC were significant.
The medium values of Ton, IP and low value of Toff result in lowest Ra.The low value of Ton, medium value of Toff and high value of IP produces highest Ra.
The low values of Ton, IP and high value of Toff result in small size in Kerf.The high values of Ton, IP and low value of Toff produces large size Kerf.
u m e 1 3 N u m b e r 1 0 J o u r n a l o f A d v a n c e s i n c h e m i s t r y 6525 | P a g e F e b r u a r y 2 0 1 7 w w w .c i r w o r l d .c o m

Fig. 1 .
Fig.1.Schematic view of Circular Contour MachiningWEDM is a nontraditional, thermoelectric process which erodes material from the conductive AA6063/SiC composite work piece by a series of discrete sparks using travelling brass wire of diameter of 0.175 mm.These electrical discharges melted and vaporized minute amounts of the AA6063/SiC composite plate, which were flushed away by dielectric fluid.The sparks occur at high frequency continuously and effectively remove the AA6063/SiC composite plate material by melting and erosion.The brass wire is programmed and controlled by control unit to travel along a circular contour corresponding to the work piece table movement and also driving the wire accurately at constant tension.The machining power supply unit energizes the wire electrode.The deionized dielectric water supply was properly controlled by separate control unit.The Schematic view of WEDM is shown in Figure1.

2 . 4
u m e 1 3 N u m b e r 1 0 J o u r n a l o f A d v a n c e s i n c h e m i s t r y 6526 | P a g e F e b r u a r y 2 0 1 7 w w w .c i r w o r l d .c o m Specimen used for machining circular contour WEDM specimen was machined from the raw composite plate material casted by stir casting to a dimension of 95 × 95 × 10 mm.The specimen and the circular contours are shown in Figure 2. The circular contour is of dimensions diameter 10 mm and radius 5 mm.

Fig. 6 .
Fig.6.a) Effect of pulse on time on Ra , b) Effect of pulse off time on Ra , c) Effect of peak current on Ra

Fig. 7 .
Fig.7.a) Effect of pulse on time on Kerf , b) Effect of pulse off time on Kerf, c) Effect of peak current on Kerf