wrote in message
...
On Sat, 23 Apr 2011 12:00:48 -0700 (PDT),
whit3rd
wrote:

On Friday, April 22, 2011 7:51:24 PM UTC-7,
Califbill wrote:
I am doing some rebuild on my aluminum river
jet boat. Going to weld closed
some old bolt holes in the gunnel and dash.
What is a good backing plate

Copper contamination would be bad (it
accelerates aluminum corrosion),
so I'd consider... ceramic. A brick, a coffee
cup... something like that.

You don't melt the copper. All the aluminum
welders I know use copper
backers when welding gaps in aluminum.

Awl riight.... just dug out an old welding
reference book:

Welding Kaiser Aluminum second edition copyright
1978 page 19-3.

Carbon or stainless steel backup bars, contoured
to fit one surface
are recommended, especially in repair of thin
materials, or where
poor accessibility makes welding from both sides
impractical. Such
bars should have a groove of suitable dimensions
immediately under
the joint or opening to be welded.

And on the next page:

Copper is sometimes deposited in a weld bead from
a
copper backup bar where penetration is too great,
or from a
burnback from the contact tube in MIG welding. The
presence of copper may be deduced from the
presence of
cracking and may be confirmed by radiography.

So, for most work it may not matter much but for
work on marine
equipment this could be serious. I found this too
important to not
say anything about it. Be safe out there, eih?
phil k.