Analysis of wire wear ratio and cutting rate of Near-dry WEDM using cryogenically treated /untreated Stainless Steel are shown in Tables 4 and 5 respectively. The percentage of contribution of each parameters are calculated for the both machining conditions.
Table 4
Analysis of WWR of Near-dry WEDM using cryogenically treated and Untreated Stainless Steel
Cryogenically Untreated Stainless-Steel Material
|
Parameter
|
DF
|
Seq. SS
|
Adj. SS
|
Adj. MS
|
F
|
Percentage of contribution
|
P
|
2
|
0.3390
|
0.3390
|
0.1695
|
493.19
|
48.367
|
F
|
2
|
0.0734
|
0.0734
|
0.0367
|
106.73
|
10.467
|
C
|
2
|
0.0768
|
0.0768
|
0.0384
|
111.74
|
10.958
|
PW
|
2
|
0.2056
|
0.2056
|
0.1028
|
299.02
|
29.325
|
Residual Error
|
18
|
0.0062
|
0.0062
|
0.0003
|
-
|
0.883
|
Total
|
26
|
0.7010
|
-
|
-
|
-
|
-
|
Cryogenically Treated Stainless-Steel Material
|
P
|
2
|
0.197
|
0.197
|
0.099
|
432.250
|
42.393
|
F
|
2
|
0.031
|
0.031
|
0.015
|
67.670
|
6.637
|
C
|
2
|
0.101
|
0.101
|
0.051
|
221.490
|
21.723
|
PW
|
2
|
0.132
|
0.132
|
0.066
|
289.220
|
28.365
|
Residual Error
|
18
|
0.004
|
0.004
|
0.000
|
-
|
0.883
|
Total
|
26
|
0.465
|
-
|
-
|
-
|
-
|
Table 5
Analysis of Cutting rate of near-dry WEDM using cryogenically treated Untreated Stainless Steel
Cryogenically Untreated Stainless-Steel Material
|
Parameter
|
DF
|
Seq. SS
|
Adj. SS
|
Adj. MS
|
F
|
Percentage of contribution
|
P
|
2
|
51.415
|
51.415
|
25.7076
|
248.42
|
40.04
|
F
|
2
|
12.817
|
12.817
|
6.4085
|
61.93
|
9.98
|
C
|
2
|
17.932
|
17.932
|
8.9659
|
86.64
|
13.97
|
PW
|
2
|
44.377
|
44.377
|
22.1884
|
214.41
|
34.56
|
Residual Error
|
18
|
1.863
|
1.863
|
0.1035
|
|
1.45
|
Total
|
26
|
128.404
|
|
|
|
|
Cryogenically Treated Stainless-Steel Material
|
P
|
2
|
79.096
|
79.096
|
39.5479
|
118.78
|
31.78
|
F
|
2
|
21.609
|
21.609
|
10.8046
|
32.45
|
8.68
|
C
|
2
|
83.875
|
83.875
|
41.9377
|
125.96
|
33.70
|
PW
|
2
|
58.282
|
58.282
|
29.1408
|
87.52
|
23.42
|
Residual Error
|
18
|
5.993
|
5.993
|
0.3329
|
-
|
2.41
|
Total
|
26
|
248.855
|
-
|
|
-
|
-
|
3.1. Process parameters' effects on cutting characteristics
The effect of oxygen pressure, the flow rate of tap water, spark current, and pulse width on WWR of dry WEDM using cryogenically treated and untreated work materials are shown in Figs. 3(a), (b), (c), and (d) respectively. The WWR of the cryogenically treated material is lower than the untreated workpiece. However, the variations of all process parameters on WWR on cryogenically treated and untreated materials are comparable. It was revealed that WWR is maximum at a low pressure of oxygen due to poor dielectric strength and low flushing efficiency(Boopathi and Sivakumar 2016; Yadav et al. 2019). While increasing pressure and flow rate, the WWR is significantly minimized by increasing the spark intensity, cooling, and flushing efficiency(Saha and Choudhury 2009; Boopathi and Sivakumar 2016; Mohammed 2018; Chityal et al. 2019). While increasing C and PW, the WWR is increased by increasing the intensity of spark and depth of crater(Abdulkareem et al. 2009). While increasing PW, the WWR is increased by growing heat in the cutting zone(Valaki and Rathod 2016). It is observed that 7 bar of pressure, 16ml/min of the flow rate of mixing water, 3A of current, and 16µs pulse width are optimum values of parameters for minimum WWR (Cryogenically treated: 0.4022 ×10− 3%; Untreated: 0.4840 ×10− 3%) of dry WEDM. It is also detected that P, C, and PW are the significant factors for WWR.
The influences of each process parameter (oxygen pressure, the flow rate of tap water, spark current, and pulse width) on the cutting rate of dry WEDM using cryogenically treated / untreated materials are shown in Figs. 4(a), (b), (c), and (d). While increasing oxygen gas pressure, the Cutting rate is improved from 3 bar to 5 bar and decreased from 5 to 7 bar(Boopathi and Sivakumar 2013, 2016). The maximum CR has been obtained at moderate pressure (5 bar). The spark transfer efficiency between tool and work materials has been interrupted by the high velocity of oxygen-mist at a high-pressure level(Boopathi et al. 2012; Boopathi and Sivakumar 2013, 2016). The cutting rate is improved by increasing the flow rate of mixing water. The flushing efficiency and dielectric strength in the plasma zone While increasing water flow rate with oxygen gas (Boopathi and Sivakumar 2014; Boopathi and Myilsamy 2021). While increasing pulse width and spark current, the cutting rate has been changed by high heat and spark intensity produced in the cutting zone (Abdulkareem et al. 2009; Boopathi and Sivakumar 2013; Garg et al. 2017). It was revealed that the overall cutting rate of dry WEDM has been improved by cryogenically treated work material. However, the impact of each process parameter on the CR of both processes is comparable. It is revealed that 5 bar of pressure, 16ml/min of the flow rate of mixing water, 5A of current, and 28µs pulse width are the best values of parameters for maximizing cutting rate ( Cryogenically treated: 17.6719 mm3/min; Untreated: 14.6112 mm3/min) of dry WEDM. It is also noticed that P and PW are significant factors for maximizing the cutting rate.
3.2. Comparative Analysis
It was also revealed from Table 6 that the WWR of oxygen-mist dry WEDM using cryogenically treated Stainless Steel is 20.31% higher than untreated material. The SEM 400X enlargement images of wire tools used in dry WEDM processes for both treated and untreated work materials are shown in Figs. 5 (a) and (b) respectively. It was revealed that the wire wear has occurred along with the traveling (longitudinal) direction. The wire tool damages in the cutting of cryogenically treated material are lower than wire damages during the untreated work material because of high heat dissipation capacity and electric conductivity of treated material(Kennedy 2001; Kalsi et al. 2010; Fard et al. 2013; Akincioğlu et al. 2015; Mohanty et al. 2018). The heat dissipation capacity of the work material is increased by increasing the thermal conductivity of cryogenically treated Stainless Steel. The thermal conductivity of the work material is also improved by increasing electrical conductivity (Mohanty et al. 2018). The cutting rate of the dry WEDM using cryogenically treated work material is 22.32% higher than the cutting rate of untreated work material. It is improved by increasing electric and thermal conductivities by the cryogenic treatment process and wide cutting path in the cutting zone(Akincioğlu et al. 2015; Garg et al. 2017). The flammable oxygen-mist dielectric is also increasing the cutting rate due to accelerating the spark erosion process by ionization, heating, and melting of work material. The microstructure of the machined surfaces of cryogenically treated / untreated stainless steel of Near-dry WEDM is exposed in Figs. 6 (a) and 6(b). The specimens for the microstructural analysis has been prepared using optimum cutting rate condition(P2F3C3PW3). It was observed that the surface roughness of the cryogenically treated materials(surface roughness: 2.71 µm) is lower than untreated steel(surface roughness: 3.06 µm).
The confirmation tests were supported to authorize the results predicted by the Taguchi technique using optimum parameters settings. The maximum cutting rate of dry WEDM using cryogenically treated work material and untreated work materials are predicted as 17.8719 mm3/min and 14.6112 mm3/min respectively. Similarly, the optimum WWR using cryogenically treated and untreated Stainless Steel is obtained as 0.4022×10− 3% and 0.4840×10− 3% respectively.
Table 6
Comparison of dry WEDM performances using Cryogenically treated and untreated Stainless Steel.
Response
|
Material
|
Optimum parameters
|
Prediction
|
Confirmation Experiments
|
CR (mm3/min)
|
Cryogenic treated Stainless Steel
|
P2-F3-C3-PW3
|
14.6112
|
14.52
|
Untreated Stainless Steel
|
17.8719
|
17.65
|
WWR (%)
|
Cryogenic treated Stainless Steel
|
P3-F3-C1PW1
|
0.4022
|
0.41
|
Untreated Stainless Steel
|
0.4840
|
0.48
|