Characteristics of the EDMed Part
4.3.3 Effects of Using CTT during EDM: Tool Wear and Tool Shape Retention Capability
While comparing micro-hardness of the cryogenically treated Copper tool to that of normal tool electrode; it has been found that cryogenic treatment has improved hardness of the tool material due to substantial gain refinement and internal stress relief (refer to
Table 4.1). Results have indicated that ‘normal’ Copper has shown micro-hardness values
falling in the range ~ from 94.1 HV to 99.3 HV; whilst cryogenically treated Copper electrode has exhibited micro-hardness values falling in the range ~ from 114.2 HV to 119.8 HV. This has been found in good agreement of the comment made by (Lal et al.,
2001; Leskovsek et al., 2006; Molinari et al., 2001) that CT of metals/alloys improves
their hardness and consequently the wear resistance. Thus, tool wear of lesser extent is expected for the case of EDM using cryogenically treated tool.
Macroscopic view of the edge of the tool electrode (NTT and CTT, both) after EDM operations on Inconel 825 has been depicted in Fig. 4.7. Carbon deposition has been observed at the bottom surface as well as around the edge of the tool electrode during EDM operation.
This can be explained by the fact that during electrical discharge, pyrolysis of the dielectric medium takes place. Due to the pyrolysis of dielectric fluid (also called dielectric cracking), Carbon atoms come out and get deposited onto the bottom/edge of the tool (and also on the work surface) forming a blackish layer. However, as compared to normal tool, cryogenically treated tool electrode has exhibited presence of very thin layer of the deposited Carbon. As observed under optical microscope [Model No: OM-19;
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Make: Radical Instruments; Country: India], the thickness of the deposited layer (after EDM experiments using NTT) has appeared as ~0.4871mm (Fig. 4.8a); whilst the deposited layer has been found to be very tiny (~0.1203 mm) after EDM operations with CTT (Fig. 4.8b).
Fig. 4.7:Macroscopic view of the edge of (a) NTT (average thickness of deposited layer Cw= 0.4871mm), and (b) CTT (average thickness of deposited layerCw= 0.1203mm) after EDM
operation on Inconel 825 specimen
Fig. 4.8:EDS elemental spectra revealing chemical composition at the bottom surface of tool electrode: (a) NTT, and (b) CTT after EDM operation on Inconel 825 specimen
Cryogenic treatment of tool electrode has thus found advantageous due to less Carbon deposition (may be deposited material is in the form of Copper Carbide) at the bottom surface as well as along the edge of the electrode. This may be due to the fact that through cryogenic treatment, the electrical and thermal properties of tool material are substantially improved (Amini et al., 2012). Hence, electrode material can dissipate heat at a faster
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case (than in the case of normal electrode) which in turn suppresses the tendency of the Carbon atoms to be deposited and thus restricts the formation of carbides. The improved thermal and electrical conductivity of the tool material is expected to improve tool life and thus to reduce electrode wear. Moreover, lesser extent of deposited Carbon layer along the electrode edge has been found favorable from the viewpoint of shape retention capability of the tool electrode. On the contrary, relatively thick Carbon layer has been found set down both at the bottom surface and also along the edge of NTT after EDM. Formation of such layer creates a barrier for the heat to be transmitted through the electrode material and thus more heat is required to execute the same for the progress of EDM operation. It imposes untoward effect on the tool electrode with excessive tool wear and reduced tool life. Shape retention capability of the tool electrode is also adversely affected. The phenomenon of formation of thin carbon layer at the bottom surface and edge of the cryogenically treated tool electrode has also been supported by the information (wt% of C) obtained from the EDS elemental spectra of the bottom surface of the tool electrodes (Fig. 4.8). It has clearly been noticed that as compared to NTT which has corresponded to 39.37 wt% Carbon at the bottom surface; the bottom surface of the CTT has corresponded to lesser extent of carbon content (35.51 wt%) owing to the formation of very thin deposited layer of Carbon (or possibly carbides).
4.4 Conclusions
The conclusions drawn from the aforesaid research have been summarized below.
Deep cryogenic treatment of Copper tool electrode has attributed decrease in crystallite size resulting relatively more refined grain structure as compared to that of ‘non-treated’ Copper. Cryogenic treatment of the electrode material has resulted reduced (~12%) crystallite size and increased (~28%) dislocation density as compared to NTT material. Moreover, cryogenic treatment has resulted reduced residual stress and crystal imperfections; thus ensuring improved tool life and improved tool shape retention capability; and also reduced tool wear.
Top surface morphology of the EDMed work surface of Inconel 825 has exhibited presence of crater mark, globules of debris, spherical deposition, pock marks (or chimneys), and surface cracks. However, the intensity of aforesaid surface irregularities has been found relatively less for the case of EDM with CTT as compared to the case NTT. Reduced crack density (and also the crack opening width)
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for the case of EDM using CTT could be found beneficial for the fatigue life of the EDMed part component whilst subjected to service. For a constant setting of process parameters: [IP=10A; Ton=100µs; τ=85%], surface crack density has been found relatively less (~73%) for the EDMed Inconel 825 work surface obtained by using CTT, as compared to the case of NTT.
The white layer has been found relatively thick onto the top surface of the EDMed Inconel 825 specimen obtained by using CTT. Increased heat conduction rate and thereby reduced tool wear has resulted decrease in energy density at the discharge gap. Due to smooth deposition of molten material, thicker white layer has been formed. Results have indicated that relatively thick white layer (~26%) has been attributed to the EDMed Inconel 825 specimen obtained by using CTT, as compared to the case of NTT for a common parameters setting: [IP=6A; Ton=300µs; τ=85%]. As compared to ‘normal’ Inconel 825 parent material, EDS elemental spectra of
EDMed work surface has exhibited higher Carbon content (wt%). This has been attributed due to the Carbon enrichment onto the work surface during pyrolysis of dielectric fluid. However, for the case of EDM using CTT, Carbon enrichment on the work surface has been found relatively less as compared to the case of EDM using NTT.
As compared to ‘normal’ Inconel 825 work material, the average micro-hardness (obtained at the transverse-cut section of the EDMed specimen; approximately at the mid-depth of the thickness of while layer measured from the top surface) has been found more for EDM using NTT and CTT both. This may be explained due to thermo-electrical effect of EDM process which has resulted considerable grain refinement (decease in crystallite size) within the work material. However, for the case of the EDMed specimen obtained using CTT, average micro-hardness has been found the highest. As compared to the EDMed work surface of Inconel 825 obtained by using NTT (at parameters setting: IP=10A; Ton=300µs; τ=85%), the EDMed Inconel 825 work surface obtained by CTT has exhibited relatively less crystallite size (~70% reduced) due to the formation of more refined grain structure. Hence, the EDMed Inconel 825 specimen obtained by using CTT has exhibited higher hardness values as compared to the case of NTT.
As compared to ‘normal’ Inconel 825 work material, the residual stress has been found more for the EDMed specimen obtained using NTT and CTT both. However, for the case of EDM with CTT, evolution of residual stress within the EDMed
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specimen has been found relatively less in magnitude as compared to the case of EDM using NTT.
XRD spectra of EDMed Inconel 825 work surfaces have identified presence of Nickel-Iron solid solution with precipitates of carbides of varied extent. As compared to the EDMed surface obtained by using NTT; use of CTT has caused relatively more grain refinement. This has further been found in good agreement to the decrease in crystallite size and consequently the increase in dislocation density.
In comparison with NTT, use of CTT has resulted relatively tiny layer of deposited Carbon at the bottom as well as edge of the tool electrode. This in turn has facilitated increased rate of heat transfer through the bulk of the electrode material. This is expected to cause reduction of tool wear, and hence, substantial improvements of tool shape retention capability. As compared to NTT, deposited Carbon (possibly carbides) layer of relatively less thickness (~75%) has been observed at the edge of the tool electrode for CTT, after execution of EDM operation.
During electro-discharge machining operations, stresses are induced within the work material resulting in internal stresses, strains, voids and dislocations. The combined effect of those creates a barrier (resistance) to heat transfer through the bulk of the work material. This in turn, results in decreased rate of heat transfer through the work material; as a result, EDM performance is adversely affected. During cryogenic treatment of the work material, internal stresses and strains are substantially reduced with refinement of the grain structure. These effects are expected to favourably improve thermal conductivity of the work material thereby improving performance of the EDM process.
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