3.1 Influence of EMCFP on tool lifetime
Figure 5 [17] presents images of P10 cemented carbide tool flank faces at the four standard stages of wear: initial worn, normal worn, acutely worn and tool failure. Because the flank face is predominantly subject to wear during the cutting process, the ISO 8688-1 standard defines tool wear states according to flank face wear (VB) values, and assigns VB values of 0.00–0.10 mm to the initial worn stage, VB values of 0.10–0.20 mm to the normal worn stage, VB values of 0.20–0.30 mm to the acutely worn stage, and VB values greater than 0.3 mm as the tool failure limit.
The VB values obtained for three untreated and three EMCFP-treated P10 cemented carbide tools are presented in Fig. 6. We note that all tool lifetime curves obtained conform to a standard tool wear mechanism, where the value of VB increased rapidly in the initial worn stage, increased at a lower rate in an approximately linear fashion during the normal worn stage, and finally increased rapidly again in the acutely worn stage as the tool approached failure. The results indicate that EMCFP treatment increased the average tool lifetime significantly by a factor of 1.92.
3.2 Influence of EMCFP on workpiece surface roughness
The workpiece topographies obtained at cutting lengths of 900 mm and 1800 mm when machining with untreated and EMCFP-treated P10 cemented carbide tools are presented in Figs. 7(a) and (b), respectively. The results indicate that the surface roughness of the workpieces was significantly reduced when machined by the EMCFP-treated cutting tool owing to the enhanced surface smoothness of the tool.
The workpiece topographies obtained at cutting lengths of 2700 mm and 3600 mm when machining with untreated and EMCFP-treated P10 cemented carbide tools are presented in Figs. 8(a) and (b), respectively. These results obtained at greater cutting lengths very clearly demonstrate the benefits of employing the proposed EMCFP treatment, where the cutting furrows of the workpiece surface machined by the untreated tool are particularly deep and jagged because the VB value of the tool is greater than 0.2 mm, which is indicative of an acutely worn tool with a very rough surface.
Sections of the actual machined workpiece surfaces subjected to cutting lengths of 3600 mm were removed for three-dimensional (3D) imaging using a VHX-2000 optical microscope (Kenyence Corp. of America). The 3D images and corresponding topologies obtained are presented in Figs. 9(a) and (b), respectively. The results clearly reveal that the surface roughness of the machined workpiece was greatly reduced after subjecting the cutting tool to EMCFP treatment, and the profile height decreased by 22.81% to 22 μm relative to the conditions obtained with the untreated tool.
The Ra values (μm) of the machined workpieces and the corresponding VB values (mm) of the untreated and EMCFP-treated tools are listed in Table 2 for different cutting lengths. The results indicate that the Ra values of the workpiece machined by the EMCFP-treated tool decreased significantly, particularly at cutting lengths of 1800 mm and 2700 mm. In addition, while the changes in the corresponding VB values of the tools are much less than those of the Ra values of the workpiece at cutting lengths of 1800 mm and 2700 mm, these changes are much greater than those of the Ra values at cutting lengths of 900 mm and 3600 mm. Accordingly, we can conclude that the decreased tool wear leading to an enhanced tool lifetime contributes strongly to the reduced surface roughness of the machined workpieces.
Table 2. Comparison of the average surface roughness (Ra) values (nm) of the machined workpieces and the corresponding flank face wear (VB) values (mm) of the untreated and EMCFP-treated tools.
Cutting length (mm)
|
Ra-Untreated
|
Ra-EMCFP
|
Difference
|
VB-Untreated
|
VB-EMCFP
|
Difference
|
900 mm
|
1051.999 nm
|
887.989 nm
|
15.590%↓
|
0.100 mm
|
0.06 mm
|
40%↓
|
1800 mm
|
1408.746 nm
|
913.863 nm
|
35.139%↓
|
0.201 mm
|
0.16 mm
|
20.40%↓
|
2700 mm
|
1853.088 nm
|
1125.766 nm
|
39.249%↓
|
0.203 mm
|
0.18 mm
|
11.33%↓
|
3600 mm
|
2076.265 nm
|
1538.121 nm
|
25.919%↓
|
0.290 mm
|
0.20 mm
|
31.03%↓
|
The surface waviness observed over a lateral distance perpendicular to the cutting direction is another important index reflecting the surface roughness of a machined surface. This condition is illustrated schematically in Fig. 10. The waveform in Fig. 10 (b) was obtained from Fig. 10 (a). The extracted wave profiles indicative of wave width W and wave height R for workpieces subjected to machined by untreated and EMCFP-treated P10 cemented carbide tools are presented in Fig. 11. Comparing the wave width W and wave height R in Fig. 11, the wave width W obtained with the untreated tool and the treated tool were the same. However, wave height R is different, especially in Fig. 11 (b) and Fig. 11 (c).
Additional surface waviness features, including the root mean square deviation (Rq), maximum profile peak height (Rp), and the maximum profile valley depth (Rv) along the lateral x direction are illustrated in Fig. 12, and their corresponding calculation formulas based on the average position of the surface Z(x) over the lateral space considered are presented as well. The calculated values of Rq, Rp, and Rv obtained for workpieces machined by untreated and EMCFP-treated tools are listed in Table 3. The results indicate that the values of all features decreased after subjecting the cutting tool to EMCFP treatment, particularly Rv. Accordingly, we can conclude that the waviness of the microstructure of the machined surface was decreased to a large extent after EMCFP, and the cutting process became more stable.
Table 3. Comparison of root mean square deviation (Rq; nm), maximum profile peak height (Rp; nm), and the maximum profile valley depth (Rv; nm) values obtained for workpieces machined by untreated and EMCFP-treated tools.
Cutting length (mm)
|
Rq-Untreated
|
Rq-EMCFP
|
Rp-Untreated
|
Rp-EMCFP
|
Rv-Untreated
|
Rv-EMCFP
|
900
|
1339.071
|
1126.44
|
4467.328
|
3207.091
|
3018.959
|
2356.863
|
1800
|
1953.701
|
1178.258
|
5628.238
|
3078.597
|
3519.171
|
2374.656
|
2700
|
2275.394
|
1384.879
|
6295.996
|
3997.206
|
4408.783
|
2586.526
|
3600
|
2495.876
|
2092.815
|
6596.894
|
6645.089
|
4901.345
|
2942.691
|
3.3 Influence of EMCFP on the friction coefficient
The results of ball-on-block reciprocating friction testing obtained for untreated and EMCFP-treated P10 cemented carbide materials are presented in Fig. 13 according to the mean friction coefficient calculated at each reciprocating cycle of testing. The average friction coefficient obtained over the full 1800 cycles for the untreated sample was 0.583, while this value decreased by 11.4% to 0.516 after EMCFP treatment. Moreover, we note from the results that the mean friction coefficient values obtained at each testing cycle for the treated sample were more uniform over the full number of cycles compared with those of the untreated sample.
3.4 Influence of EMCFP on the ERP spectra of cutting tools
The effect of EMCFP treatment on the ERP spectra of P10 cemented carbide samples is shown in Fig. 15. Here, an increased EPR signal intensity represents a larger number of unpaired electrons in the sample. We note that the peak values of the EPR spectra increased by 7.28% at the signal crest and 29% at the signal trough after EMCFP treatment. Magnetic field can increase the number of unpaired electrons which can change the state of free radical form (Singlet) S to (Triplet) T [7,19]. The combine energy of electron pair is strong in S state, but weak in T state. In T state, the force of dislocation and pinning centers is weak, even depinning and move. The dislocation multiply lead the plasticity improvement
3.5 Influence of EMCFP on the dislocation density of cutting tools
The full XRD patterns and the four individual diffraction peaks (WC (001), TiC (100), TiC (200), and WC (101)) of untreated and EMCFP-treated P10 cemented carbide samples are presented in Figs. 14(a) and (b), respectively. The four individual XRD peaks are observed to have shifted toward lower diffraction angles after EMCFP treatment, which is indicative of reduced residual stress [20]. The dislocation densities were calculated using Eqs. (1) and (2), and the results are listed in Table 4. We note that the dislocation density increased by 38% after EMCFP treatment. Under the combined impacts of the electro-plastic effect [21,22] and the magnetostriction effect [23], the EMCFP treatment increased dislocation mobility by decreasing the activation energy of dislocation movement, which promoted internal stress release. In addition, the EMCFP process provides the necessary energy for dislocation motions to occur, resulting in the homogenization of the dislocation distribution in the material [7]. Simultaneously, applied electric fields reduce entanglement between dislocations, which removes obstacles toward dislocation slippage, so that the material exhibits higher plasticity [24].
Table 4. Comparison of dislocation density values obtained for untreated and EMCFP-treated P10 cemented carbide tools
|
Untreated
|
Treated
|
Dislocation density (×1015)
|
1.4527
|
2.0101
|
3.6 Influence of EMCFP on the surface residual stress of cutting tools
The residual stress values obtained at the same three points on the flank face surfaces of P10 cemented carbide tools before and after EMCFP treatment are listed in Table 5. The results indicate that residual stress values at all three points of the flank face surfaces decreased significantly after EMCFP treatment, and the average residual stress value of the three points was reduced by 51.4% compared with that of the untreated tool. The decreased residual stress can improve the mechanical properties of tool [25] and improve the tool wear resistance, thus decrease the workpiece surface roughness. Accordingly, we can conclude that EMCFP treatment can reduce the workpiece surface roughness significantly due to reduced residual stress of the flank face surface.
Table 5. Residual stress values of three points on the flank face regions illustrated in Fig. 13 of P10 cemented carbide tools before and after EMCFP treatment.
Measurement point
|
Untreated (MPa)
|
Treated (MPa)
|
Difference
|
1
|
−129
|
−74
|
43%↓
|
2
|
−146
|
−70
|
52%↓
|
3
|
−150
|
−64
|
57%↓
|
Average value
|
−142
|
−69
|
51.4%↓
|
3.7 The relationship of tool lifetime, workpiece surface roughness, the friction coefficient, the dislocation density, and residual stress
Dislocation, as a crystal defect, produces a special stress field. EMCFP can effectively increase the dislocation density and make the dislocation more uniform. Dislocation homogenization can affect the tool friction coefficient and residual stress. Dislocation homogenization can reduce the lattice distortion and thus reduce the tool friction coefficient. The friction coefficient of tools directly affects the workpiece surface roughness. When the friction coefficient of the tool is reduced, the flank face wear can be directly reduced, thus improving the life of the tool. Dislocation homogenization can also reduce tool residual stress and improve tool life[26]. Fig. 16 showed the relationship between tool lifetime, workpiece surface roughness, the friction coefficient, the dislocation density, and residual stress.