This is from
http://www.mmsonline.com/articles/020107.html
these results would be good enough for me... are the new RAM's using AC?
A Question Of Recast
Having been in the Wire EDM business for 25 years, Don has had to deal with the subject of recast in many instances. As EDM technology advanced, it became less of an issue, however, where critical applications are concerned, such as parts used in aerospace equipment, recast is still a valid factor and must be addressed.
Don explained that they do a great deal of prototype work, such as engineering projects for one particular company heavily involved in aerospace work. That company has a stipulation that any EDM work, if it goes into an aircraft part, must have no recast whatsoever, with a guaranteed 3X metal removal of the thickness of the recast layer. However, many times Wire EDM is used as a last resort, because there is no other way to produce the part, and no positive way to remove the recast layer. And if laps are used, there is no way of knowing if more material is removed from one side or the other.
Recently, EWT received a project, and Don explained to the company that he could not guarantee removal of the recast layer according to their stipulation. However, he suggested trial EDM machined parts, with the absolute minimum of recast, then cut apart and checked by the end user's metallurgy department. The trial parts were cut on one of EWT's Wire machines that incorporates full time AC circuitry. They employed several skim cuts that would insure the best results possible, and sent the parts off for metallurgical testing. The test results showed NO recast layer, and they got the job. Don commented that it would seem improbable that EDM could cut with no recast whatsoever, however, if metallurgical experts say so, it must be true.