Dielectric during EDM may get polluted due to erosion of tool and workpiece, and decomposition of the dielectric itself. Dielectric contamination has been reported [Erden and Bilgin, 1980] to influence breakdown, time lag (or ignition delay), short circuit, machined surface characteristics, spark gap and machining rate (or machining performance in general).
150 \Work piece
JS
Fig. 7.14 Comparison o f EDM performance while using water and mineral oil as dielectric. Dielectric pressure = 70 psi (tap water); = 50 psi
(mineral oil); Tool polarity -ve (tap water) and +ve (mineral oil)
[Godinho and Noble].
The impurity particles tend to gather around surface irregularities due to elec
tric field concentration around such points (Fig. 7.15). It has theoretically been shown [Erdin and Bilgin, 1980] that the larger impurity particle size results in shorter time lag and vice versa. Experiments have also been conducted to investi gate the effects of dielectric impurity on the performance of the EDM process. For this purpose, commercially pure metal powders (market grade) of Cu, Al, Fe and C were usfcd. Electrodes are ground and polished prior to machining to minimize
possible surface effects. Presence of above impurities decreases the time-lag and
some of the open circuit pulses are replaced by the effective discharges, thus improving the EDM performance. However, such effects are not observed when copper is used as electrode material because copper, by its own virtue, gives short time lag, and absence o f short circuit and open circuit pulses.
E = E 0 E = Eb
N = N(oo) N = Ncr
Fig. 7.15 Schematic diagram o f concentration o f impurity particles (Cu, Al, Fe, or C) around surface irregularities [Erdin and Bilgin, 1980], Fig. 7.16a shows the effect of impurity concentration on workpiece erosion rate (or machining rate) for different types of impurities in powder form, and Fig. 7.16b shows the same effect for different pulse times (ts). The particles suspended in the dielectric liquid aid in the breakdown because the total time spent in erosion is increased due to decrease in the time-lag. This fact is further supported by the results shown in Fig. 7.16b where the effect o f impurity concentration on machining rate is more pronounced at longer pulse times. In case of short pulses the voltage pulse applied across the 1EG ends before the breakdown conditions are satisfied. It is reverse in case o f long pulses that is why the number of pulses with effective discharges is significantly increased leading to higher MRR. Fur ther, IEG is reported to increase by an increase in concentration of the impurity in dielectric liquid. Machining rate is appreciably increased at low concentration of impure particles but remained almost constant for higher values of impurity con centration.
As explained above, tool wear rate is also influenced by the kind and concentration of impurity in the dielectric, Fig. 7.16c.
Microcrack length has been found to depend on the discharge pow er in gen eral, and on the pulse duration in particular. Longer pulse durations (> 800 |xs) result in abnormally long cracks which may extend into the HAZ.
W O R K P IE C E ERO SIO N R A T E (m g /p u ts e ) x lO 6 3 0 0 200 100 0 12 2 4 3 6 4 8 On* 0 5 1.0 1.5 2 .0 C A 0 .5 1.0 1 .5 2 .0 Al O 4 6 12 16 F* o IM P U R IT Y C O N T E N T (g /V ) Pul*c = 8 0 0 > ii 0 0 . 2 5 0 5 0 . 7 5 1.0 1 .2 5 1.5 1 .7 5 2 . 0 A l * 6 12 18 2 4 3 8 3 6 4 2 4 8 C u ° 2 4 6 8 10 12 14 16 F « & 0 . 2 5 0 . 5 0 . 7 5 1.0 1 .2 5 1.6 1 . 7 5 2 0 C a IMPURITY CONTENK g / l ) (c)
Fig. 7.16 Effect of impurity concentration on (a) workpiece erosion rate for various kinds of impurities, (b) change in workpiece erosion rate for different pulse times, (c) tool erosion rate for various kinds of impurities [ Erdin and Bilgin, 1980].
P R O C E S S C H A R A C T E R IS T IC S
Physical, mechanical and metallurgical properties o f the workpiece do not sig nificantly influence the performance of the process. EDM can be em ployed for machining any electrically conductive material irrespective o f its hardness and other mechanical and physical properties. It can perform different kinds of opera tions, viz drilling, slotting, multiple hole drilling, etc. It gives high degree of repeatability and high accuracy of the order o f ± 0.025 to ± 0.127 mm. It can give tolerances as good as ± 2.5|am. When deep and accurate holes are to be drilled, use of separate tools for roughing and finishing passes is recommended so that the taper can be minimized. Taper ranges from 0.005 to 0.050 mm/cm depending upon the values o f machining parameters employed. Aspect ratio of 100:1 during drilling of small holes can be achieved if special care about flushing o f gap is observed.
Volumetric material removal rate (M RRV) achieved during EDM is quite low (0.1 to 10 mm3/min-A). Actual value of M RRV depends upon the machining con ditions employed.
Surface integrity deals basically with two issues, e g surface topography and surface metallurgy (i e possible alterations in the surface layers after machining). Surface integrity greatly affects the performance, life and reliability o f the com ponent.
Microscopic study of the machined components reveals three kinds of layers, e g recast layer, heat affected zone (HAZ), and converted layer (Fig. 7.17). If molten material from the workpiece is not flushed out quickly, it will resolidify and harden (as a martensite) due to cooling effect of the dielectric, and gets adhered to the machined surface. This thin layer (say, about 2.5 to 50 jim or so) is known as “ re-cast layer” . It is extremely hard (65 HRC) and brittle. The surface is porous and may contain microcracks. Such surfaces should be removed before using these products. The layer next to fhe recast layer is called “ heat affected zone” (HAZ which is approximately 25 jam thick). Heating, cooling and diffused material are responsible for the presence of this zone. Thermal residual stresses, grain boundary weaknesses, and grain boundary cracks are some o f the character istics of this zone. Conversion zone (or converted layer) is identified below the HAZ and is characterized by a change in grain structure from the original structure.
Fig. 7.17 Schematic diagram of three kinds o f layers on an ED M ’d component.
Excessive local thermal expansion and subsequent contraction may result in residual tensile stresses in the eroded layer. Such experiments were conducted
[Barash, 1959] on mild steel and hard rolled brass. It was found that the thickness
o f the affected layer was less than 25p.m.
The “ su rface fin ish ” achieved (0.8-3.1 pm ) during EDM is also influenced by the chosen machining conditions. New generation EDM machines are capable to produce surface finish as good as 0.18-0.25 pm . Surface finish is mainly gov erned by pulse frequency and energy per spark. The texture of eroded surface has been analyzed by Kahng and Rajurkar [1977], and a schematic illustration of roughing and subsequent finishing is given in Fig. 7.18. It is observed that the application of higher discharge energy results in deeper HAZ and subsequently deeper cracks. Spark eroded surfaces have been examined [Barash, 1959] to study
FIN ISHING
Fig. 7.18 Schematic diagram of roughing and subsequent finishing operation
[Kahng and Rajurkar, 1977].