8.6.1 Effects of process parameters on wire surface wear
Successive and random discharges during WEDM raise the temperature as high as 10000 °C or more due to the high plasma energy of the spark, which removes material from both the electrodes. The discharge energy increases along with increasing voltage and current causing
critical damage on the wire surface. The damages on the wire surface are minor at low levels of current due to low discharge energy (Figure 8.8a). At higher current levels, deep and wide craters are formed on the wire surface (Figure 8.8c). It was observed that the surface integrity of the wire deteriorates at higher levels of current due to generation of higher temperatures.
Longer pulse duration produces heat flux for a longer time causing severe erosion from the wire electrode due to excessive temperature rise. The thermally affected region due to spark formation is more significant at higher pulse on-time (8 µs), as shown in Figure 8.9b. It increases the generation of debris and unwanted arcs in the machining zone, which degrades the wire surface quality and increases the probability of wire failure.
The wire speed also plays a crucial role in determining the nature and intensity of wire surface wear. When the wire speed is quite low, there is a localized temperature rise because the heat flux traverses over the wire surface at a low velocity. As a result, localized material ablation takes place, causing the formation of deep craters in a particular region of the wire surface (Figure 8.10a). At higher wire speed, the wire quickly traverses the workpiece getting less time to form deep craters on the wire surface (Figure 8.10b). Thus, the wire wear at higher speed is less intense as compared to that of lesser wire velocity.
Figure 8.8 FESEM wire image after WEDM of Ti-6Al-4V at varying currents:
(a) 4 A, (b) 6A, (c) 8 A
Figure 8.9 FESEM wire image after WEDM of Ti-6Al-4V at varying discharge durations:
(a) 4 μs, (b) 8 μs
Figure 8.10 FESEM wire image after WEDM of Ti-6Al-4V at varying wire speeds:
(a) 3 m/s, (b) 9 m/s
8.6.2 Effects of process parameters on workpiece surface roughness
The contribution of various input parameters viz. discharge voltage, discharge current, pulse on-time, pulse off-time and wire velocity on the response variable i.e workpiece Ra was evaluated using the ANOVA analysis. It was observed from Table 8.2 that all the considered input parameters have significant influence on the response variable variation. Figure 8.11 shows the interaction plots of workpiece surface response variation with respect to the significant interaction terms
I t on, I v t , ontoff
. It is noted that Ra value rises with an increase in voltage and current due to a rise in discharge energy. Higher spark energy causes deep and wide craters on the workpiece surface, thus increasing the surface Ra with successive discharges. Discharges with similar nature form overlapping craters on the irradiated region. A slight decrease in Ra values were observed with increasing pulse off- time. The decrease in the pulse off-time leads to an increase of the duty factor. This factor allows the formation of smaller and fewer gas bubbles containing lesser energy. When the discharge stops, these small gas bubbles will collapse which result in finer craters, thus decreasing the surface Ra of the machined surface. Also, it has been observed from Figure 8.11 that with an increase in wire velocity, the workpiece surface quality improves. This is because with an increase in wire speed, the heat flux moves at a higher speed, thus providing very less time for temperature rise on the wire surface as well as on the workpiece surface.Less temperature rise causes less material melting and eventually causes lesser damage on the machined surface, which reduces the surface roughness. Higher values of wire speed further causes less localized damage on the workpiece surface instead forming micro craters over a larger surface thus improving the surface finish.
Figure 8.11 Analysis plots for surface roughness: (a) interaction of voltage and current, (b) interaction of current and pulse on-time, (c) interaction of current and wire velocity, (d)
interaction of pulse on-time and pulse off-time 8.7 Comparison between molybdenum wire and zinc coated brass wire
Extensive experimental investigations were carried out for both the molybdenum wire and zinc coated brass wire to understand the wire erosion mechanism and minimize the frequency of wire failure. FESEM images were collected to carefully examine the damages undergone
by both the types of wire surfaces. Visual observation of the images showed that the zinc coated brass wire suffered severe damages as compared to the molybdenum wire. The reason behind this is because the melting point of zinc (693 K) and brass (1203 K) are lower than the melting point of molybdenum (2896 K). The zinc coating initially acts as a protection to the brass core against the violent spark discharges. The outer coating finally wears off and erosion starts occurring from the brass core, which degrades the wire strength. A wire surface quality index for the eroded wire samples was evaluated to quantify the extent of erosion undergone by the wire tool. It was noted that for the same set of process conditions, the histogram mean values for the zinc coated brass wire were lower as compared to the values of molybdenum wire. This indicates that the intensity of erosion for the coated wire is critical as compared to the molybdenum wire. The zinc coated brass wire has lower tensile strength, which further causes higher deformation than the molybdenum wire. The workpiece Ra values were found to be higher while machining with the coated wire in comparison to the molybdenum wire. Thus, apart from the process conditions used, the wire material also plays an important role in determining the surface quality of the machined components. Therefore, the performance of the molybdenum wire can be considered better in terms of wire health and workpiece surface quality. The disadvantage associated with molybdenum wire is its high cost and hence industries may prefer zinc-coated brass wire for bulk machining of products and rough cutting operations for a set of optimized process conditions.