lemel_man wrote:
Sam Soltan wrote:

(...)
I wonder if I could use the rod I saw advertised on TV that "welds"
(seems like soldering to me) using a gas torch.

Has anyone experience with this stuff? Please comment pro or con.

I've used it quite a bit. It works, sort of, but I'm pretty sure it
won't do your job.
The process is more like welding than soldering, except that the stuff
itself is technically a solder.
Welding refers to the process of joining (usually metal) by locally
melting the joint - and usually adding a filler of the same material.
Soldering is the process of joining with a material of a lower melting
point than the items to be joined; the items themselves do not melt.
When soldering, the molten solder wets the joint and flows into it by
capillary attraction, and when it freezes the joint is made. For
soldering to be successful the molten solder must wet the joint, and
this is where the problems start, it won't wet a dirty joint. And for
most metals the very act of heating the joint makes it dirty - the
oxygen in the air reacts with the hot metal to create oxide which
prevents the solder from wetting and flowing. There are only two ways
around this: solder in an oxygen free environment, or use a barrier to
prevent the oxygen from reaching the joint. The latter is the most
convenient method, and the barrier is called a "flux".

Aluminium is particularly difficult to solder because the oxide forms
almost immediately at room temperature and there are very few fluxes
that can get rid of it.

The "welding" rods you mention are not made of aluminium, so they don't
actually weld, they also have a lower melting point than aluminium. They
are used to "tin" the components of the join before bringing them
together. You first clean the joint area with a stainless steel brush
then heat it until the tip of a rod melts when it touches it. You then
spread the molten rod material over the joint area with a thin stainless
steel rod, making sure to scratch the joint - this removes some oxide
and the molten solder itself prevents further oxidisation. Keep
scratching through the molten solder until you're happy you've removed
all the oxide, then remove the heat and allow the solder to freeze. You
do this for all the components of the joint.
You then bring the components together and reheat the joint until the
solder melts again, remove the heat and the joint is made. A flux is not
used because the solder itself doesn't readily oxidise.

You can now appreciate that this method will not allow the solder to
flow into a crack. The solder itself is quite strong - stronger than
most aluminium alloys, so you could use it to fill a crack that has been
"prepped" into a V-groove. But there is an additional problem: a lot of
aluminium alloys are heat treated and the act of heating to melt the
solder is likely to anneal (soften) it so much as to make it no use for
its intended purpose.

I hope this has helped with your decision.

--
Regards, Gary Wooding
(To reply by email, change feet to foot in my address)


Hey Scott,

I hereby nominate Gary's explanation as FAQ - worthy because it is the
clearest most accessible explanation of those aluminum 'welding rods'
I have ever seen.

--Winston