Role of Surface Cracking and Recast Layer Deposition …
87
Fig. 1 Morphology of EDMed surface produced by using Cu electrode
the release of entrapped gases. With an increase in discharge current, intense heat
energy is released, which in turn causes more melting of work material; debris, thus
produced, is carried away by the dielectric medium. However, due to inadequate
flushing pressure, the entire melted material cannot be carried away by dielectric
circulation. Residues of molten debris deposits over the machined surface, which
forms a recast layer [19]. Globular/nodular-shaped debris is formed due to the sudden
cooling of expelled molten material [20].
Figure 2 shows the variation of surface crack density with respect to varied peak
current for different electrodes. Crack density is obtained as the total crack length
divided by the area of the micrograph.
It is observed that the EDMed surface produced by using the Cu–Ni electrode
exhibits higher values of crack density when compared to the conventional Cu electrode. This is due to lower thermal conductivity Cu–Ni than Cu. Therefore, the Cu–
Ni electrode causes excessive heat accumulation within the discharge gap since the
transfer of heat through the electrode is somewhat restricted due to its lower thermal
conductivity. Therefore, the EDMed surface, produced by using Cu–Ni electrode,
experiences thermal stresses of higher magnitude. Cu electrodes, possessing higher
thermal conductivity, can easily transfer heat through the bulk of the electrode material at a faster rate than Cu–Ni. This in turn reduces the severity of surface cracking
in the case of Cu electrode. According to Dewangan et al. [21], uniform distribution
of deposited molten metal over the machined surface truncates crack density value.
87
Fig. 1 Morphology of EDMed surface produced by using Cu electrode
the release of entrapped gases. With an increase in discharge current, intense heat
energy is released, which in turn causes more melting of work material; debris, thus
produced, is carried away by the dielectric medium. However, due to inadequate
flushing pressure, the entire melted material cannot be carried away by dielectric
circulation. Residues of molten debris deposits over the machined surface, which
forms a recast layer [19]. Globular/nodular-shaped debris is formed due to the sudden
cooling of expelled molten material [20].
Figure 2 shows the variation of surface crack density with respect to varied peak
current for different electrodes. Crack density is obtained as the total crack length
divided by the area of the micrograph.
It is observed that the EDMed surface produced by using the Cu–Ni electrode
exhibits higher values of crack density when compared to the conventional Cu electrode. This is due to lower thermal conductivity Cu–Ni than Cu. Therefore, the Cu–
Ni electrode causes excessive heat accumulation within the discharge gap since the
transfer of heat through the electrode is somewhat restricted due to its lower thermal
conductivity. Therefore, the EDMed surface, produced by using Cu–Ni electrode,
experiences thermal stresses of higher magnitude. Cu electrodes, possessing higher
thermal conductivity, can easily transfer heat through the bulk of the electrode material at a faster rate than Cu–Ni. This in turn reduces the severity of surface cracking
in the case of Cu electrode. According to Dewangan et al. [21], uniform distribution
of deposited molten metal over the machined surface truncates crack density value.