EDM effects on plated surface 2

[debruch]

I have a few stainless steel tubes with a thin layer (30 um) of copper plating on the ID, for use in our accelerator project. I worked with a colleague to process and inspect some samples from these tubes to measure the finished surface roughness. When I received the samples I realized they had been cut by wire EDM, which I worry has affected the plating integrity. We viewed a few samples in our SEM and noticed a lot of pitting in the copper layer, which is atypical compared to previous projects. Does anyone have experience using wire EDM on plated surfaces, and does it indeed affect the plating integrity?

Thanks,
Daniel

 

[tbuelna]

Are you cutting the tube samples in half longitudinally and then inspecting the plated ID surface well away from the trimmed edges? Or are you cutting the samples radially and then inspecting the cross section of the tube wall and plating? Surfaces produced by wire EDM are not very nice. Wire EDM basically uses electric current to melt material from the surface of the part in close proximity to the wire and flush the debris away using a flow of DI water. The cut surface consists mostly of recast, which is metal that was melted and quickly re-solidified as slag on the cut surfaces.

Since DI water is used with most wire EDM systems, I don't think this would cause corrosion of the plating. But I have seen situations where a part being wire cut that was not properly grounded experienced pitting damage. So you might want to check for this.

 

[debruch]

Thank you tbuelna. I was worried that the current might be the culprit. We are cutting the tubes longitudinally and then inspecting at the sample's center, away from the edge. I attached a SEM picture here so you can see exactly what we discovered.

 

[CoryPad]

Those pits are so small, what about this makes you concerned? What is the function of the copper plating?

 

[debruch]

The plated tubes are used in an accelerator and will see a high-frequency electron beam. The copper plating produces less power losses and allows a higher beam output. Since we are operating at high frequencies the tube (plating) ID must be very smooth.

We have since looked at a few similar samples and are convinced that the pitting is caused by EDM. The wire EDM was performed at another facility, so it will be difficult to track down the root cause (dielectric, ground, etc) of these defects. Lesson learned; in the future we surely won't wire EDM any sample pieces!