The effect of spark erosion on metal surfaces

The formation of craters
In the process of spark erosion an electrical potential between workpiece and tool is discharged and a spark jumps across the gap. At the point where the work piece is hit, the metal is heated up so much that it melts and evaporates. A crater is formed. One after another innumerable such sparks are sprayed on the workpiece, and one crater is formed next to the other. The diameter of the crater on the photograph is about 200 pm.

The photographs were taken through an electronic microscope. To make the individual crater more easily visible a polished metal surface was used. The spark erosion machine was only switched on for a fraction of a second, so that the edges of the craters would not overlap.

Each of these craters has a typical edge with a thermically influenced zone.
Here a crater on a titanium work piece is shown. A profile structure, formed by the rapid solidification of the heated titanium, can be clearly recognized. Part of the liquid titanium was flung into the dielectric.
Characteristic of an eroded surface are the numerous craters with overlapping edges. In addition microscopic eroded particles can be seen sticking to the surface.
By enlarging the photograph one can clearly see the crater edges on the work piece and the microparticles sticking to them, as well as a hole in the surface of the metal.
Enlarged once more one can see microscopic cracks emanating from these holes. These cracks are signs of an overheated surface.
If the electronic microscope is used to make an even greater enlargement, these microscopic cracks become very evident. The eroded particles even begin to look 'human'. 'Eyes, ears and a mouth' can be recognized.

Surface polished by means of spark erosion
If spark erosion is used to polish a surface, the edges of the craters are largely removed. A crosssection through a work piece polished by spark erosion shows clearly that there is only a very small white layer and that the influenced zone is only about 2 pm thick.

Eroded particles

Seen with the naked eye erosion sludge looks black. If the sludge of an erosion machine that works with different materials is washed out with acetone and then put under an electronic microscope one sees a great many larger and smaller balls.
After enlargement differences between the eroded particles become evident. Thus three particles have melted together to form „triplets“, a big particle has fused with a small one (mother and child), and many small particles have gathered together on a big particle because of electrostatic or magnetic force.
Some particles look like golf balls, or like our neighbouring planet Mars with its famous Martian canals.
Other eroded particles have a textile structure...
...or have velvety surfaces like peaches.
Many particles have a cavity on the one side because of the sudden shrinkage of the metal. If tool steel is used these microscopic balls are often hollow and have sharp edges that can injure the human skin.

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