Is this a problem with the original casting being porous or a
problem with the alloy in the metal?
Both can be factors. Some metals don’t like laser welding as much,
and are more prone to cracking as the welds cool/shrink. The porisity
itself might be caused by voids in the weld, or too high power
settings. Try using smaller wire and lower voltage settings. But from
what I’ve seen, most of the problems originate with the casting
itself. Melting a bit of porous metal sometimes fixes a pit, and
other times, the bubble or imperfection just floats around and
remains in the metal. And cracking of a weld often seems to be an
indication that there are still voids or other imperfections in the
metal below the weld which were not reached and fixed by the weld.
Anyone have a better solution to prevent or cure the problem?
Different power settings. Sometimes a gentler, lower power setting
with smaller weld spots can forstall cracking. Other times, you need
to punch deeper into the metal to reach the deeper problem spots that
are causing the surface cracks. Argon shielding helps a good deal
with white golds in particular too. And sometimes I find that
smoothing a weld over by using low power settings as well as holding
the work so the beam hits the metal surface at a very shallow angle,
which produces a weld spot that’s a long narrow oval rather than a
round spot, can change stresses in the weld so as to reduce cracking,
as well as giving a smoother weld surface with less of the dimples
one often sees. If your laser has pulse shaping, that can go a looong
way to reducing problems too.
And another approach I sometimes find necessary is to use a filler
wire that’s not an exact match for the weld metal. Palladium white
gold or a yellower, lower nickle white gold when welding white gold
can give a more ductile weld that doesn’t crack as much. With some
higher zinc 14K yellow golds, I use a 14K that has no zinc. Higher
melting point, but welds better. Sometimes, I even find I need to use
an 18K yellow gold to weld especially problematic 14K, since the
much more ductile 18K simply isn’t likely to crack. Wire made from
hard silver solder, or IT silver solder, works better than sterling
silver for welding sterling. And so it goes…
Peter