|
|
Posted by D Sherman [72.47.9.228] on Tuesday, July 20, 2010 at 11:54:43 :
In Reply to: O.T. pot metal repair posted by Doc Dave [24.245.101.30] on Tuesday, July 20, 2010 at 09:47:55 :
I have had some success on pot metal with aluminum brazing rod. This is the stuff that is a rather lumpy uncoated aluminum-alloy rod that you use by heating the work with a propane torch and scrubbing the not-quite-melted end of the rod into the work. The rod melts at a very slightly lower temperature than the work and it's easy to accidentally melt the work when doing this. Also, since there is no flux, a good connection relies on mechanically scrubbing the rod through the oxide film and into the hot work. It's close to being a welding process. When you can get good wetting, it is as strong as the base metal, but it's treacherously easy to have what looks like a good bead, but it in fact just laying on the surface. The aluminum brazing rod will not wick into cracks like brass would on steel, so if you can't tolerate a thick bead on the surface, you'll have to vee out the crack and then grind the surface flat. There are a lot of different alloys that fall under the common name of "pot metal", most based on aluminum and/or zinc, but also with some magnesium, copper, and possibly other metals in them.
Pure zinc can actually be soldered with 50/50 solder if it's well-cleaned and fluxed with acid flux (zinc chloride). The tin shops used to solder galvanized sheet metal all the time, though now they usually just crimp it and and use silicone if a seal is needed. In general, an old zinc-based pot metal will be more grayish and rough than an aluminum-based one. If a drop of acid fizzes on it, you'll also know it's zinc-based. It might still have some aluminum in it that would make it hard to solder, but if it appears to be zinc, it would be worth a try. Tin/lead solder isn't as strong as the aluminum brazing rod, but it will wick into cracks and is easier to work with.
It is actually possible to solder pure aluminum with tin/lead solder. The trick is the flux, which has to include hydrofluoric acid to cut through the oxide layer on the aluminum. Another way to make aluminum solderable is to wet it with mercury first. That has its own problems, though. I have not tried the aluminum-soldering flux, but I've seen parts that were tinned by using it and they solder just fine.
I have had bad luck with JB Weld on everything I've tried it on. It seems to be very picky about age and exact mixing proportions. For a metal-filled epoxy, I've done much better with Titan Super-Weld.