edm aluminium pitting problems

[richjam]

I have recently been edm machining a large aluminium mold tool. on clearing away edm deposits from the bottom of the cavity i have found lots ( about 30 ) small indentations with raised edges about 0.010" 0.25mm deep. They look as though a sharp object has been pushed into the surface I know that this is not possible as i am the only person that has worked on it. Is it possible that the dielectric is attacking the material or that the material is delaminating? the origional block was 750mm by 700 by 250mm.No electrode has actually been used in the area of damage so i am very perplexed.

 

[kenvlach]

Perhaps localized galvanic corrosion, with intermetallic precipitates anodic to the aluminum matrix.

What alloy, heat treatment, any thermomechanical processing, etc.?

What EDM fluid? Nitrate EDM electrolyte is reportedly better than saline.

 

[richjam]

I am not sure of the grade of aluminium or dielectric. But we have sparked 1000's of tools before using the same dielectric with no problems. Although this particuler job has been on the machine longer than most (about a week). It is only on one flat surface that the problem has occured on at the bottom of the cavity. There may have been some small pieces of swarf on this face prior to sparking (edm). Is it possible that they could react with the surface and edm deposits?