Estimation of wire strength based on residual stresses induced and parametric studies
5.4 Measurement and analysis of experimental results .1 Study of erosion on the molybdenum wire surface
5.3.4 Measurement of workpiece surface roughness using optical profilometer
The surface Ra of machined components after every single cut was measured using a non- contact optical profilometer. Figure 5.5 depicts the measurement methodology of Ra of machined components using the optical profilometer. It is a non-contact type, high precision white light interference microscope with an objective lens of 20X magnification and focal distance of 4.7 mm. The head of the profilometer consisting the objective lens was moved along the z-axis until the light is focused on the workpiece surface. The Ra values were obtained using the 3-D analysis software TalyMap. Surface roughness measurements were carried out at five different locations for a single workpiece, and the average value was considered for further analysis.
Figure 5.5 Measurement methodology of surface roughness of machined components
5.4 Measurement and analysis of experimental results
material and attached workpiece material on the wire surface during machining. EDX analysis further shows the presence of carbon (C) in the spectra, which is due to the use of carbon tape during the preparation of samples for the analysis.
Figure 5.6 FESEM image of unused molybdenum wire
An increase in wire temperature, interaction with the dielectric, generation of debris in the machining zone, and the formation of undesired sparks degrade the surface integrity of the wire. The machining debris is attached to the wire surface due to insufficient flushing time and improper flushing pressure (Figures 5.8a, 5.8b). It decreases the interelectrode gap, which may cause undesired arc formation resulting in abrupt temperature rise. Thus, the wire gets subjected to violent thermal shock leading to surface degradation in the form of craters, pits, microholes, and microcracks. In certain conditions, the material eroded during the discharge duration is resolidified and redeposited on the wire surface due to sudden cooling by the continuously flowing dielectric and insufficient flushing pressure (Figure 5.8c). At lower pulse off-times, when the discharges occur at short intervals, the accumulated debris is not washed away due to insufficient flushing time. Instead, the melted material forms a pool of resolidified material and deposits on the wire surface. In certain regions, the splashing of the melted material on the wire surface was also observed, as shown in Figures 5.8d, 5.8e.
Figure 5.7 (a) EDX analysis and (b), (c), (d), (e), (f), (g) Elemental mapping of an eroded wire sample for the process set: V = 60 V, I = 8 A, ton = 4 μs, toff = 6 μs, v = 6 m/s
Figure 5.8 (a), (b) sticking of debris, (c) resolidified material and (d), (e) splashing of molten material on eroded wire surfaces
5.4.2 Measurement of deformed wire cross section diameters
The optical images of the cross-sections of unused molybdenum wire before and after the etching process are obtained. The unused wire is circular with a diameter of approximately 180 μm (Figure 5.9a). A uniform grain structure with very minute grains was observed after etching the polished wire sample, which appears to be a result of the wire drawing process during wire manufacturing (Figure 5.9b). The cross-sections of the eroded wire samples were prepared at various sections along the wire length and examined under the optical microscope to investigate the deterioration in wire form as the machining proceeds. Figures 5.9 (c–h) shows the cross sections of eroded wire samples at different sections for the process condition: V = 85 V, I = 4 A, ton = 16 μs, toff = 4 μs, v = 6 m/s. It was observed that the wire cross section loses its circularity and undergoes deformation during machining in two major steps, (i) an increase in wire diameter on one side at initial stage of machining and, (ii) decrease in wire diameter than the original diameter on both sides of the wire cross section as the machining proceeds. The wire suffers severe plastic degradation because of the steep temperature slope generated during the discharge phenomenon. The reason behind the change in wire shape can be attributed to a combined effect of (a) temperature rise, (b) plastic deformation when stresses originated crosses the yield point of the wire material, (c) mechanical tension applied to the wire to keep it vertically straight and (d) continuous movement of the wire electrode. This causes geometrical inaccuracy and precision error of the machined components. It further diminishes the surface quality and integrity of the products.
Minor deformation of the wire periphery starts at the initiation of sparks due to sudden temperature rise and thermal shock. As the complete diameter of the wire is inside the generated slot, sparks occur from all the sides pressing the wire as it traverses forward through the workpiece. The majority of wire damage takes place in this region, and the wire loses its original circular shape. At this stage, the wire diameter increases than the original diameter on one side and decreases on the other side (Figures 5.9c and 5.9d). Temperature rise during machining has a direct effect on the crystal structure of molybdenum, making the pattern of arrangement of atoms more regular. It reduces the number of grain boundaries, thus making the material more soft and malleable. The malleability of the molybdenum wire increases with temperature, which causes the material to flow downward, thus increasing the
diameter of the wire. The wire diameter also increases due to the sticking of debris onto the wire surface caused due to inefficient flushing and spattering of molten material. As the wire proceeds further along the feed direction, a steady decrease of wire diameter was observed due to the melting and vaporization of wire material above the melting point (Figure 5.9e).
The diameters on both sides of the wire cross-sections are found to be unequal and become smaller than the original one (Figures 5.9f, 5.9g, 5.9h). It reduces the overall cross-sectional area of the wire, thus increasing the stress in the wire cross-section. Temperature rise further causes the generation of thermal stresses in the wire electrode, which diminishes its strength.
Figure 5.9 (a) unetched and (b) etched cross-sections of unused molybdenum wire, (c), (d), (e), (f), (g), (h) Deformed wire cross-sections with unequal diameters for the process
set: V = 85 V, I = 4 A, ton = 16 μs, toff = 4 μs, v = 6 m/s
5.4.3 Measurement and analysis of surface roughness of machined workpieces
The surface roughness of the workpiece samples were measured using a non-contact optical profilometer. Three samples were cut for every process set and the Ra value was evaluated for every single workpiece. The measurements were carried out at five different locations for a single sample and the average value was considered for further analysis as listed in Table 5.2. Here, Ra1, Ra2 and Ra3 indicate the surface roughness after 1st cut, 2nd cut and 3rd cut respectively. The blank spaces in the table indicate wire breakage.
Table 5.2 Surface roughness values (Ra) of workpieces measured after the experimental sets Serial
no. Discharge
voltage (V) Discharge
current (A) Pulse on- time
(μs)
Pulse off- time
(μs)
Wire speed (m/s)
Surface roughness (μm)