Straightening warped aluminum weldment

Worse yet it is a weldment of *cast* aluminum pieces. Any tips or caveats?
Envision a large rectangular (single bowl) kitchen sink made of 3/8"
thick cast aluminum. Only each end a separate casting and TIG welded together. Quite successfully other than the use of a stupid looking backer bar. Since removed as this is a bat house mold not a sink...
Was attempting to build up a pad on the out side so I could machine a shallow grove on the inside centered on the original weld. Far less familiar with aluminum MIG than TIG. Had a steel backer plate during welding that was inadequately clamped. The crack which alerted me to stop should be trivial to fix once I get as much of the warp as possible out.
Have modified the backer plate so I can clamp it much tighter. And will have one day only access to much better equipped shop than I have at home. Oxy acetylene with a rose bud, TIG, infrared thermometer, etc... Probably this Thursday though it may slip to next week if I don't get all my ducks in a row first.
--
William

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The rules of welding aluminum castings: 1. Degrease as much as possible. 2. Grind out the original crack with carbide burrs, not grinding wheels. 3. Clamp to a strong back if possible. 4. Preheat to at least 600 degF, or until machine oil smokes if dripped on it. 5. Weld using TIG with 4047 filler rod if possible, or 4043 if you can't find 4047. If MIG has to be used, use 4043 filler. 6. Maintain the heat while welding and allow to cool very slowly by wrapping in heavy welding blankets or burying in vermiculite insulation.
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On Tue, 17 Sep 2013 21:03:34 -0700, Ernie Leimkuhler

What is the maximum safe temperature if 600 degF is not sufficient to flatten the warpage?

Original TIG V grove was mostly done with 4043. Think I did manage to source some 4047 towards the end... More recent boondoggle with the MIG pad was with 5356. Which probably made the situation worse? Do have one almost full roll (13 Lbs) of 4043 that came with my welder but it is .047 and I'm set up for .035.

Will look for both while I'm out running errands today.
Thanks!
--
William

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Aluminum melts at around 1200 degF. 800 is safe. 1000 is getting dangerous.

4047 has twice the silicon as 4043 so it has less shrinkage, lower melting point and better flow. It can be tricky to find in small quantities.

Vermiculite is wonderful stuff, but the EPA beat the hell out of it over the last 20 years. They are worried about the crystalline dust particles. They used to sell it at garden centers in huge bags, but now it is harder to find and costs a lot more.
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On Thu, 19 Sep 2013 05:30:54 -0700, Ernie Leimkuhler

Snips

Ended up buying both yesterday. Two 8 quart (1.2 Lb) bags for about $4 each at Lowe's. Did not even open them due to the potential mess while using a shop as a visitor.
Did use the blanket not only to cover the weld area between welds but also as a pad to rest my arms while welding. An 6' x 8' and folded up about 8 layers thick I could rest my arms directly over the last bead when switching sides. Should have bought one years ago!
Sadly nether the straightening or the crack repair went well:-( Plan B is to order some 1/2 x 8" bar stock and cut the entire built up area out. Was able to keep the original V grove weld flat without a heavy duty fixture. Should be able to do two 8" apart...
Good news is that all the remaining problems with my shallow slots not meeting in the corners will be eliminated. (rec.crafts.metalworking Subject: Improvised milling machine Message-ID:
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William

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