Gas / Soot problem laser welding 316L

[cljohns]

Looking for ideas on root-cause for laser-weld defect:

Currently, we are seeing small voids approximately 5-10% of a 0.016" weld spot. The process involves welding several SS parts together; the issue is not porous, as we only see one, two at most, voids in one 0.4" weld pass. The voids do not protrude through the weld but rather look like a location where trapped gas was emitted. There is no discernable pattern in when the voids arise either. We will run 50 weld operations per day and maybe see one, none, or 4, and not always in a row or in the same location on the part; they do however seem to appear near a joint in the part. The upstream processes are constant and are producing parts per normal build, so I am starting to rule out the "pass-the-buck" option : ) and am wondering if this could be a gas / vacuum issue at our process? There is a considerable amount of soot produced in the weld and the vacuum is designed to remove the soot during the weld (operates near 15-20 SCFH), but the same location is flooded with Argon at close to 28 SCFH...?

Thanks!

 

[TVP]

Why is there soot? It sounds like the parts are not clean, and this is why porosity and soot develops.

 

[cljohns]

The weld surface is pretty clean; the parts come directly from a punch process and there is generally no contamination in the area. Soot builds up on fixturing that is in close contact with the weld zone. I have tried to simulate the problem with process related contamination but have not been able to consistently recreate the small void. Shouldn't there be more pores than just one and wouldn't they occur in localized groups on the weld? We only see one void, for one part, once in a while (one part has 4 off-set weld passes). So the occurance of this defect is minimal, which has made it harder to troubleshoot. Is it possible that the soot, which is circling the weld zone, could get trapped/mixed into the weld process, heated up, and cause a small out-gas bubble to form? If that were the case though, I would imagine we would be seeing more of these....?

 

[EdStainless]

My guess is that there are a lot more, under the surface. Only the big ones are open.
If you have soot you are not clean. There is nothing in the steel or your gases to give you this.
If the voids are clean and smooth then gas is prob the issue. Any dirt/residue on the surface will trap moisture and result in gas.

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