PSI - Issue 10

E.L. Papazoglou et al. / Procedia Structural Integrity 10 (2018) 235–242

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E.L. Papazoglou et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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correlation model and the unforeseen factors during machining, the deviation between the experimental and calculated roughness parameters are acceptable, with most of the predicted values being within the confidence interval, as it can be seen in Fig. 5 and 6. The ability to predict through equations, even approximately, the surface roughness of a machined workpiece, drastically reduces the need of experiments, making the machining process more efficient by reducing its cost.

Fig. 5. Experimental and calculated average roughness (Ra)

Fig. 6. Experimental and calculated maximum roughness (Rt)

3.3. White layer formation

During EDM, only a small part of the melted cavity is ejected, with the formatted crater by the spark representing only a proportion, namely 15% - 35% of the total melted material volume, while the rest of material returns to solid state, forming a re-solidified layer (Rebelo et al. (1998); Pérez et al. (2007)). At the same time, a small portion of the vaporized material from the workpiece remains in close proximity to the spark cavity surface and when the spark is turned off, reattaches as a layer of the spark cavity surface. The layer is known as re-deposited layer, since the material was separated from the workpiece surface and then returned. The upper region has a porous structure and may contain elements of the electrode material and by-products of the dielectric fluid (Jameson (2001)). Re-solidified and re-deposited layers are often referred as white layer. High temperature and pressure variations during machining are two of the major factors which cause surface cracks, decreasing the material fatigue and corrosion resistance. Pulse duration, pulse current, work material properties as thermal conductivity and carbon content, and dielectric fluid properties are the main factors determining the cracking propability. As a rule of thumb, the density of surface cracks increases with increasing the pulse energy, although the cracking mechanisms are far more complicated; for example a severe cracking network appears on the surface when using low pulse current and high pulse-on durations (Jahan (2015)). The understanding of how discharge energy affects the formation of the white layer is important for the better usage of the capabilities of EDM. In some cases, machining requirements may need the minimization of the WL, while in other cases the potential of using the WL in order for the machined surface to acquire some desired properties, may exist. In Figs. 7 to 10, microscope images of the EDMed cross section surface for different machining parameters are listed, where the formation, the quality and the thickness of the WL are distinguished. For I P of 24 A and T on of 75 μ s (Fig. 7) there was not formed a continuous and uniform WL, but it appears to be random material surface depositions, with intense ridges and globules formations. Also, there are not any visible cracks, only a few small diameter inner cavities. For I P equal to 24 A and T on of 100 μ s (Fig. 8) the WL tends to become continuous, while maintain the anomiogenity of its thickness, having a few cracks and a couple of inner cavities. Increasing the discharge energy I P to 27 A and T on to 300 μ s (Fig. 9) the WL becomes continuous, with less variation in thickness and with visible cracks and different diameters of inner cavities. Finally, for I P and T on of 12 A and 500 μ s (Fig. 10), the WL is continuous, with a more uniform thickness and with less but more intense surface cracks. The above mentioned experimental results are in line with the conclusions of many studies about the EDMed surface quality, where researchers have concluded that the WL thickness is correlated with the per pulse discharge energy and have proposed semi-empirical correlations in order to quantify it, see Eqs. (7) and (8) (Lee et al. (1988); Rebelo et al. (1998)). The WL thickness according on above mentioned relations and the experimental results is increased with the increase of the discharge energy.