steel/cobalt spatter using Nd:YAG laser

By concutter Date 03-05-2003 19:25

Hi, we have a process where we are fusing heat treated steel chassis to a Cobalt pad using a Nd:YAG laser. It is a precision pulsed weld process, with the pulses being about 40 Mils in diameter. We are progressively getting messier welds with a lot of spatter and pitting. We have two air knives on the system, yet spatter is still getting past them and up onto the weld lens cover. In addition we have to weld very hot (pushing all the way through the steel chassis to the back side) to keep our weld strength up.

I know we have made smooth, precision scalloped welds in the past, but I am not knowledgable enough in this topic to know why are the welds are progressively getting messier. Is it focal length? Contamination? btw, we don't use a cover gas (had used Argon in the past, but weld strength and quality seemed the same for our purpose without it). Do the air knives need better placement??

Any input would be warmly received... thanks!!

Lee
www.icsbestway.com
Leep@icsbestway.com

By arcblue (*)

Date 03-06-2003 14:27

Has the energy per pulse dropped off?
When have you last changed your flash lamps?
Cleaned the mirrors?
If the weld pool is too violent (exploding / spitting) it is over heated and the weld puddle is agitated to a point where a smooth weld fusion is not obtained. This can be due to a change in focal point.
Check out the machine settings against the original setup parameters and try to see what is different.

By concutter Date 03-06-2003 16:28

Thanks for reply, Arcblue. The energy per pulse doesn't seem to have dropped much, although our power probes give different readings (the analogs are much more stable than the digital). We replace the pulse lamps on a regular schedule, varying between OEM Lumonics lamps and Directed Light bulbs... don't really seem to notice too much of a difference.

For a little more clarification, we are using a flying head assembly with a fiber optic cable, which has given us problems in the past. We caught a cable on fire about 7 months ago, but the current one is new. We use the same laser for two different processes, and the welds come out smooth and nice on the second process... although the second process uses thicker metal.

I'm not sure about the mirrors, but I will check. We do have a trace of spatter on the lens itself (someone didn't catch the lens cover in time and it cracked), but our beam uses about 1/3 the area of the actual lens, so I don't know how much it is affected.

Hmm, so the weld pool is over heated, eh? That does make sense... we are 'blueing' our Cobalt pieces pretty severely, and we didn't see that before. The pulses are very fast, hot, and violent. As for the focal point, it is my understanding that our lens focuses down to 40mil diameter at it's tightest spot, which is about what our desired weld width is. Although, I don't foresee a problem with increasing the width if it allows smooth fusion.

The biggest problem on our end is that we don't have that much documentation on the settings. A previous engineer dialed it in, then left it... and left the company... and didn't document. So, we may have to do some experiementation.

Thank you very much for the input. I didn't know a weld puddle could get 'overheated'. I thought once it was liquified, adding more heat would just make it hotter. It makes sense though, if you blast it with too much energy, the energy dissipates in the form of melting the metal, but extra energy releases itself in the form of physical displacement.

I'll do a little experiemting with the focal length. Thanks again.