The effect of spark erosion on the work piece

Abhängigkeit der Oberflächenrauheit von der Brennzeit.

Spark erosion has a completely different effect on working material than customary methods of processing. The electrical spark hitting the work piece heats up the outer layer of the steel so much (about 10,000° C) that the material evaporates. The metal gases formed then condense in the dielectric, usually in the form of hollow balls, open on one side and having a sharp edge. In the work piece itself depressions, shaped like craters, are formed. How great is the danger for the working material to be so unfavourably affected on the surface, that the serviceability of the tool suffers? And what about tool life, resistance to wear and buffability? Figures 1, 2 and 3 show surface roughness, electrode wear and metal removal in relation to the firing period.

Electrode wear relative value

Abhängigkeit des Elektrodenverschleißes von der Brennzeit.
Abhängigkeit der Abtragleistung von der Brennzeit.

Apart from metal removal, surface roughness and electrode wear, the effect on the surface quality of the working material is of utmost importance. In most cases it was shown that there was no effect on the functioning of the tool. In some cases, e.g. in a cutting tool, it even became more resistant to wear, in others, however, tools broke prematurely.

All changes that could be detected were due to the high temperatures that were produced on the rim. In this rim the structure, hardness, state of stresses and carbon content of the steel are influenced. The next picture shows a section of a surface that has been roughened down by spark erosion, showing the various structural changes, which are typical of such a rim.

Schnitt durch eine funkenerosiv bearbeitete Oberfläche mit Gefügeänderungen. Werkstoff: UHB Rigor, auf 57 HRC gehärtet.

The melted zone shows clearly that it has solidified very quickly. Columnated crystals have grown vertically up out of the metal surface during solidification. A crack that has formed in this layer runs inward along the line of crystals. The melted layer is usually about 15 – 30 μm thick after normal rough work. In the hardened zone the temperature rose above that needed for hardening. A hard and brittle martensite has formed.

In the annealed zone the temperature was not so high as to harden the steel. It has only been tempered. Underneath is the unaffected core. The thickness of the various layers appears to be unrelated to the type of steel used and the electrode material. However, there is a very clear difference between hardened and softened materials. In softened steel the layers are thinner and there are fewer cracks.

The brittle, hardened layer is almost non-existent. During rough work the thickness of the layers varies much more than during finishing. The longer the firing period, the thicker the melted and hardened layers become. Further research has shown that the strength of current has basically the same effect as the length of the firing period.

Steel with a high carbon content gets the most cracks. Steel with a low carbon content only develops few cracks in the melted layer. About 20% of the cracks extend into the hardened zone and only a few reach the core. In the core there are seldom cracks longer than 10 μm. These cracks in the core are usually found in high alloy tool steel and in high alloy high-speed steel.

Anzahl Risse/cm 1) (in der geschmolzenen Zone) 2) (in der gehärteten Zone) 3) (in der Kernmasse)

The cracks are caused by stresses, which result from the repeated, rapid chilling of the work material by the dielectric, as well as from the differences in volume between the various structural parts in the different layers. If erosion is properly done and includes the final finishing process, the surface errors that result from rough work can largely be corrected. Where finishing is not possible, the following procedures may be used:

a) stressfree annealing at about 15° C less than before. This decreases the hardness of the surface without influencing the core.

b) softening and renewed hardening and annealing leads to an almost complete restoration of the structure (cracks however remain)

c) grinding or scouring removes the surface structure together with the cracks. The rate of cut is important here, and should be about 5 – 10 μm.

In summary it may be said that the structural faults caused by rough work can be corrected during the normal process of spark erosion, which includes rough work and finishing. A certain amount of structural changes will, of course, always remain. However, in most cases they are of little importance. There are even instances in which the great hardness of the hardened layer improves the tool‘s resistance to wear. In others, the craters on the surface of the work piece provide a better hold for lubricants, which also increases the service life of the tool.

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