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Capricious Laser Soot and Weld Penetration

A twofer:

Why is it that soot can suddenly appear in the midst of welding, a
seam for example? First few spots, no soot, then a couple sooty
welds, then goes back to clean welds.

What IS soot? What specifically causes it? Sometimes the soot can
alter how the next laser shots ‘respond’, suddenly the next welds can
act as if you cranked up the power settings, that spitting sound with
resultant porosity. And then it goes back to normal.

Is there a reference for weld penetration at different power
levels/apertures/materials or is it all seat of the pants?

What IS soot? What specifically causes it? Sometimes the soot can
alter how the next laser shots 'respond', suddenly the next welds
can act as if you cranked up the power settings, that spitting
sound with resultant porosity. And then it goes back to normal. 

That “soot” isn’t soot (carbon, as in from an imcompletely burned
fuel). It’s condensed metal. When you weld, above a certain power
level, a small portion of the metal not only melts, it vaporizes.
When that vapor hits anything cold (air, or a surface around it) it
condenses again. the resulting particles of metal are very very
small, and thus give you the same color that the same metal would
give to, for example, a polishing buff. Black. Looks like soot, but
it’s condensed metal vapor.

Power levels are not the only thing at play when you weld. The
thermal conductivity of the mass you’re welding determines some of
that happens, since you have to pump energy into the metal faster
than it can conduct it away, in order to heat it enough to melt. That
thermal conductivity is not just a function of the type of metal, but
also it’s mass. Small areas are not as good a heat sink, so melt
quicker. Similarly, if there is porosity in the metal, that spongy
metal won’t conduct heat as well, so that area will melt quicker and
get hotter. The porosity not only decreases the metal available to
conduct heat, but it also means you’re heating less metal, and that
metal is partially isolated from the rest of the mass by that
insulating porous area. So porous areas can appear to
explode/pop/crackle under the laser, melting more than you were
getting with the main mass of the metal. And when that happens, you
also get more vaporization of the metal, so more black “smoke”. Also,
of course, if you’re welding an area that’s been covered by that
black smoke, that color increases the absorbtion of the beam, just as
an ink mark would do.

Additionally, with most metals, there is an increase in absorbtion
of the laser beam once the metal melts. Less is reflected, more
absorbed by the molten surface, and thus a threshold between not yet
melting and melting where the effect of the beam energy jumps
disproportionately to the increase in power level.

All in all, any given welding job is somewhat unique, even when the
metal type is known and “usual” power settings can be used as
starting points. But after that, unless everything is equal to the
last time, you always have to fine tune the required power settings
to get the desired results, and things like porosity, hidden solder
seams, or other discontinuities in the metal can always throw you for
a bit of a loop…

As to publishing a list of power settings or something, each laser
machine will be at least slightly unique. Different laser brands, age
and condition of the flash lamp and optics, and slight differences in
operator technique mean that any suggesting power setting is only a
starting point. You’re probably best off keeping a record, either
written or mental (or use your laser’s memory settings if it has
such) of the settings that most commonly work for your uses…

Peter Rowe

Black soot will definitely cause a “hotter” pulse, and deeper
penetration. You use a marker( black, blue, red) to darken silver for
the reason that the darkened metal absorbs more of the energy of the
laser pulse, and it also temporarily eliminates the high reflectivity
of the silver, which wants to bounce that energy away (and into your
fingers!). I’ve always assumed that the soot came from dirt or
grease, which sometimes seems to be inside some pitting your beam has
penetrated to. Since I am doing mostly repairs I find myself “flying
by the seat of my pants” a lot when starting each job. I have
parameters that I use for each metal, but as soon as I get going on a
particular piece of jewelry I find myself dialing up and down in
voltage, beam diameter, etc, till I find just the right combination
for this piece of metal. I am sure that someone using the laser for
production work or with consistent, known metals would have exact
parameters they would go right to, but I can never be sure exactly
what a metal’s characteristics are, or what metals another jeweler
has used before me for assembly or repair. It does keep things

The other thing that I notice is that the lasered seam seems to be
more dense than the rest of the (cast) metal. This may be why laser
joints are brittle.

I would like to see a variable speed pedal (to increase the hertz)
of the beam or maybe two stage. (like passing gear on your car).
That way you wouldn’t have to stop to reconfigure the machine before
hammering a joint.

Stanley Bright
A&M Jewelers
Baltimore, MD