EDM Effects on Fatigue Strength

[evilscott]

I'm looking to find if anyone has any background or can point me in a direction to find a resource that addresses the effects of wire and/or plunge EDM on the fatigue strength of high strength materials.

 

[CoryPad]

The factors involved can vary so much it is best to study the effect yourself for your parts and process. Having said that, the ASM Handbook shows a 63% reduction in fatigue strength for EDM parts vs. gentle grinding for Inconel 718 that has been solution treated and aged to 44 HRC.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[NickE]

EDM is one of the worst ways to prepare thin reed valves and other high-strength high-fatigue parts. This is a direct result of the HAZ that is present around the "cut". Also the jagged edges produced contain many stress concentrators that also reduce fatigue life.

EDM identifications have been known to cause failure of carbide punches and other die tooling parts.

One study my company did shows a 50% reduction in cycle life with EDM vs PhotoEtching. (resonant dwell double bend testing)


nick

 

[richardbuss]

the main problem with die tooling edm is that stress relief cutting does not take place first. in addition its recommended to cut 3 times with a reduction in current and then grind away whats left of the white layer.

this way there is no reduction in properties.

regards
richard

 

[NickE]

"the main problem with die tooling edm is that stress relief cutting does not take place first. in addition its recommended to cut 3 times with a reduction in current and then grind away whats left of the white layer."-richardbuss

Yes thats how our carbide tools are made. However at one time the identification of the details was also edm into the surface. this often times caused a critical failure of the component.

nick

 

[Maui]

The HAZ or "white layer" that is produced on the exposed surface of the part when wire EDM is performed usually runs between about 0.0002" to 0.0005" in thickness. This thin layer of material is re-austenitized by the EDM process, and it is very brittle. This makes it is an ideal initiation site for cracks. Once cracks form in this thin layer, they can then propagate into the base material, and ultimately lead to the failure of the part. So the anticipated effect would be a measurable reduction in the fatigue life. In order to quantify the effect you would need to measure it by performing fatigue tests using your material, given your specific geometry and loading conditions.

There are two practical ways to combat the effects of the EDM white layer. The method that you choose will depend upon the material in question and the limitations of your design. As suggested above, a final grind of the part after EDM will remove the white layer, and eliminate the problem. Another method that is commonly used on hardened high speed and tool steel parts that cannot be surface ground after EDM is to stress relieve the parts. This usually amounts to tempering back the white layer to give it some amount of ductility. As long as a suitable temperature is selected, it should not significantly reduce the hardness of the part.

What material are you using, and at what hardness? This information may allow us to provide you with some more detailed recommendations.


Maui

 

[evilscott]

The material is AMS6532, heat treated to HRC 52-55. There is no post edm heat treating or grinding. The surface finsih requirement is 32uin as this is a sealing surface on the real part. We actually have conducted some bending fatigue testing (primary mode) which compared ground polished to two other specimums which had very different edm schedules/settings. Both had recast layers of ~ 0.0002. Somewhat surprisedly, there appears to be no disceriable affect on the fatigue life. This was counterintuitive which is why I'm asking. The test methodology has been reviewed as well as the failed samples to insure they were valid failures.