Pattern Generation using Enamel Coating in Die-sinking EDM

. Pattern generation on aluminium (Al)-6061 work surface is performed using green compacts of powder metallurgical (PM) made of copper-tungsten (Cu-W) tool. Compact load (CL) and peak current (Ip) are the variable process factors and surface roughness (Ra) and edge deviation (ED) are considered performance measures. The Ra is obtained in the range of 4.20 µm - 6.56 µm whereas ED is measured in the range of 17.29 µm - 68.98 µm. Field emission scanning electron microscope (FESEM), and energy disperse spectroscopy (EDS) are performed to validate tool particle migration on the pattern surface. It can be concluded that the performance measures depend upon the masking thickness, its resisting capacity to high temperature and the uniformity of the coating.


Introduction
In the manufacturing sector, the Electro Discharge Machining (EDM) for surface modification process has a wide range of applications.By modifying a workpiece's metallurgical structure and surface characteristics in a variety of operating environments, surface modification via EDM increases its usable life. 1 Tyagi et al. 2 , Ahmed 3 , and Mazarbhuiya et al. 4 used green compacted tools made of Powder metallurgy (PM) in EDM to improve surface quality.The C-40 steel was altered by Patowari et al. 5 using an EDM tool made of W-Cu PM sintered metal.The PM sintered tool was utilized by Patowari et al. 6 to conduct a Taguchi analysis for surface modification and characterize the deposited layer.The surface modification by EDM using a PM sintered tool was similarly described by Patowari et al.. 7 The titanium implant surfaces were modified for an antibacterial coating by Bui et al.8  Using powder-mixed EDM, Sahu et al. 9 investigated how to increase the surface integrity in powder-mixed EDM.Numerous surface characteristics, including surface texture, surface waviness and roughness, microhardness, and recast layer thickness, were examined.Utilizing a tungsten disulphide powder dispersion and a micro-EDM technique, Mohanty et al. 10 modified the surface of a Ti-alloy.3] According to Gill et al. 14 and Mazarbhuiya et al. 15, performance measures varied depending on the different parametric conditions.Apart from the surface modification, Crepe paper maskant is used to create simple shaped rectangular block pattern 16 and line pattern 17 .The author's concept to generate complex shaped patterns in EDM using the hand-lying process of enamel coating is inspired by the literature on pattern generation in EDM.By using a hand-laying manual coating process, an alternative EDM strategy must be adopted to get around this issue.This thoughtfulness inspires the authors to formulate a strategy to resist the process to generate the desired pattern on the work surface.So, this experimental study executes a parametric study to generate a complex shaped pattern with an acceptable range of surface finish and less edge deviation.

Materials and method
The experimental process uses aluminium (Al) 6061 as workpiece material and the hand laying coating process is tried to apply for pattern generation.A commercially available resilient epoxy is used as enamel coating.To carry out the experimentation, powder metallurgical (PM) green compact tool of "25%Cu-75%W" is selected.The workpieces is masked with epoxy material by selective painting process leaving a triangular area where material can be deposited to generate the pattern.However, erected patterns are generated in this experimentation.The thickness of the enamel coating is around 40 µm.The EDM sparking takes place at the non-masked area of the work surface where the eroded tool materials are deposited.The working process layout of the experimentation is shown in Fig. 1.The compact load (CL) and peak current (Ip) are chosen as process parameters as shown in Table 1.The surface roughness (Ra) and edge deviation (ED) are selected as performance measures.After conducting several pilot experiments, the input parameters are selected at different levels based on the temperature resisting capacity of the masking layer.Total of 16 experimental runs is carried out at various combinations of parameters at fixed pulse on-time (Ton) of 200 µs.

Results and Discussion
After conducting the experimentation, the performance measures are observed.The quantitative results of Ra and ED are tabulated in Table 2. Fig. 2 shows the generated pattern by the enamel coating.

Analysis for Surface Roughness
The Ra is one of the major output parameters of the experimentation.The surface characteristics depend upon the amount of roughness of the surface.The newly generated surface characteristics depend on the process variables.Each parameter has its own effect on the Ra under different working conditions.The effects of CL and Ip on Ra are observed by graphical plots and ANOVA analysis.The Ra value decreases with increased CL settings.On the contrary, Ra increases along with increased Ip settings.At constant Ton of 200 µs, CL of 5 ton and Ip of 6 A, maximum Ra value of 6.56 μm is recorded.A minimum Ra of 4.20 μm is recorded at CL of 20 ton and Ip of 3 A. Higher settings of Ip results in a higher amount of tool wear and its subsequent deposition.Thus, the formation of big sized crater increases the Ra.ANOVA results presented in Table 3(a) show the highest contribution of Ip on Ra followed by CL in the experiment.The contribution of Ip and CL is found to be 49.78% and 47.21% respectively.Fig. 3(a) shows the decreasing trend of Ra against the increased settings of CL.However, the opposite behaviour is observed for Ip.Higher Ip results in tool erosion with large volume of material followed by arbitrary deposition.At high CL, particles are deposited uniformly leaving craters of small size.

Analysis for Edge Deviation
The deviations of edges of the pattern are measured at different consecutive locations under optical microscope.However, the average value is recorded as the ED of the generated pattern.It is observed that the shape and size of the pattern have distorted due to damage of masking during the process.Because the masking could not withstand the high temperature during the process.Therefore, a parametric analysis is performed to reduce the pattern deviation.

Energy Dispersive Spectroscopy Analysis
Energy Dispersive Spectroscopy (EDS) analysis is performed to check the presence of tool materials on the deposited surface of the patterns.The peak of W and Cu confirms the migration of tool constituents.The peak of C is observed due to carbon particles liberated from disintegrated EDM oil.17Similarly, the peak of Al also existed due to base material.The weight and atomic percentage of the elements are shown in Fig. 5

Conclusion
From the experimentation, it can be concluded that the enamel coating is applied successfully as a masking material for the generation of user desired pattern on the Al-6061 work surface by migrating the tool particles.The pattern is generated by uniform material deposition on the Al-6061 surface.The Ra is obtained in the range of 4.20 µm -6.56 µm whereas ED is measured in the range of 17.29 µm -68.98 µm.The performance measures depend upon the masking thickness, its resisting capacity to high temperature and the uniformity of the coating.It is difficult to obtained desired results due to uneven thickness of coating done by manual painting process.

Figure 2 .
Figure 2. Generated pattern on work surface

Fig. 3 (
b) depicts the influence of parameters on ED.The ANOVA shows the significant contribution (58.52%) of Ip followed by CL (33.07%).The main effect plot indicates that ED increases due to increased Ip whereas, the increased CL dominates the ED of the pattern.At CL of 5 ton and Ip of 6 A, the maximum ED of 68.98 μm is recorded.The minimum ED of 17.29 μm is recorded at CL of 20 ton and Ip of 3 A. The tool fabricated at 20 ton of CL experiences high density as well as high molecular binding energy.Hence, the low rate of erosion and uniform deposition produces less ED of the pattern.The higher settings of Ip increase the spark energy and therefore it leads to a higher deviation of the edge of the pattern.The high energy exposes the area for unwanted deposition due to thermal damage.Thus, it enhances the amount of ED.
Field emission scanning electron microscope (FESEM) images show the migration of the tool particles to the desired area of the work surface.However, the material deposition differs depending on the parametric conditions.Fig.4(a) and 4 (b) show the material deposition at the combination of "CL = 5 ton and Ip = 6 A" & "CL = 20 ton and Ip = 3 A" respectively at constant Ton of 200 µs.Due to low Ip, the particles are deposited in small globular forms.17At 6 A of Ip big sized craters and at 3A of Ip small spherical particles are observed on the surface.Due to the formation of big sized crater, the surface roughness is increased.(a) CL = 5 ton, Ip = 6A (b) CL = 20 ton, Ip = 3A

Table 1 .
Process parameters and fixed parameters

Table 2 .
Experimental results

Table 3 .
ANOVA for (a) Ra and (b) ED