Have any of you had better luck repairing this sort of really severe spongy >porosity? If so, how, and with what sort of welding paramaters? (voltage, pulse length, focus diameter, etc)."
I don’t have any laser welding experience at all, but I’ve
“succeeded” (the porosity remains inside, so it really isn’t a
success) with combination of lower tech methods that should be
helpful for craftspeople with somewhat less than a laser.
Additionally, this method doesn’t require heat so it doesn’t
cause firescale - covers firescale infact - and the added metal
is of higher purity than the metal you’re repairing, so it’s
still legitimate. (Better than flooding with low carat alloy, or
cadmium solder with silver)
As Peter described, peen and burnish to close the pores near the
surface as much as possible, but then don’t sand or buff it quite
yet. Electroform a skin of new metal over it.
Actually you might start by electroforming before the first
burnishing. Chemically clean the metal with a strong pickle, then
ultrasonically clean out the pickle with water. Rinse in Hot
water or steam it, dry, paint nail polish on the areas that you
don’t want to electroplate. Keep the metal scrupulously clean of
oils from finger prints etc (wear rubber gloves with cyanide
plating solutions, avoid mixing pickle and cyanide, maintain
ventilation etc., etc. )
I use pure gold or fine silver annodes, but it should be
possible to electroplate an alloy, I just haven’t bothered.
Set the electrical source - I use a cheap Radio Shack� power
supply set to 3 or 6 volts, DC, and a reostat with a couple of
meters on a strip of plywood - cost, about $20 - to the
appropriate voltage for the area you’re working on (this can be
determined by formula, but trial and error works for me) and
build up a deposit on the surface of the porous casting. It won’t
fill in the pores because it will preferentially plate on the
high spots, but that’s ok because the next step is to burnish
that raised newly plated metal down into the microscopic holes,
Repeat several times until the burnishing fills the pores and the
plating grows together to form a continuous skin and then plate
for a longer while, over night if necessary. Smooth the surface
with burnishing and 1400 grit wet/dry sandpaper.
Of course by then you’ve had to repaint the nail polish several
times as it peels or blisters. Removing nail polish is easy, hot
water and the ultrasonic cleaner will usually lift an edge that
you can grab with a tweezer and peel. For a complete removal,
soak in acetone.
None of this is fast, admittedly, but it’s at least a way to
save an otherwise lost piece, and most of the plating time can be
unattended. I’m in the process of doing this right now with a
special silver watch case that entails enormous labor; I will NOT
GIVE UP! I find that I can produce what appears to be a perfect
surface . . . with lots of elbow grease and hand sanding. Plus,
in silver, the color is whiter and doesn’t tarnish as fast as
sterling.
Alan Heugh
http://www.nas.com/~aheugh/
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