need help with welding 3" alum plate

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My current job has me welding a 14 1/2" x 14 1/2" x 3" 6061-T651 aluminum plate with a 8" square cut out of the plate for a 8' x 8" x 1/4" 6061 aluminum tube to be welded to.  They want me to slide the tube through the plate so a half inch of the tube will be sticking through the bottom of the plate.  The machines I have to use are a Miller syncrowave 350 lx tig welder or a Lincoln power mig 300.  The gas I have for the tig welder is an argon/helium mix with 25% argon and the mig is currently set up with 100% argon.  Can someone suggest which would be better to use and also how hot I will have to preheat the plate to.
Reply:Originally Posted by grnsmilyfaceMy current job has me welding a 14 1/2" x 14 1/2" x 3" 6061-T651 aluminum plate with a 8" square cut out of the plate for a 8' x 8" x 1/4" 6061 aluminum tube to be welded to.  They want me to slide the tube through the plate so a half inch of the tube will be sticking through the bottom of the plate.  The machines I have to use are a Miller syncrowave 350 lx tig welder or a Lincoln power mig 300.  The gas I have for the tig welder is an argon/helium mix with 25% argon and the mig is currently set up with 100% argon.  Can someone suggest which would be better to use and also how hot I will have to preheat the plate to.
Reply:I would use the wire feed and MIG it.What service is it going into? What kind of a finish are you looking for?Thanks,HoboLincoln SA200's... at least 15 - 20. They come and go. Growing partial to the "Short Hoods" in my old age. Last count on Short Hoods was 13 in possession.
Reply:I would probably throw it in the oven and heat it to 250degrees just to help a puddle form. I don't think Id go much hotter if warping is a concern. I'd probably TIG it with like 40-50% helium.Welding/Fab Pics: www.UtahWeld.com
Reply:It's a base plate for a 14' tall school sign.  I don't really care how the weld looks as I'm more concerned about getting enough penetration into the plate and yes I would be using 4043if I tig it
Reply:Very strange choice of material...Welding/Fab Pics: www.UtahWeld.com
Reply:Ya I know but I don't make those choices.  I just have to make it work and that's how it usually is here.
Reply:Originally Posted by grnsmilyfaceIt's a base plate for a 14' tall school sign.  I don't really care how the weld looks as I'm more concerned about getting enough penetration into the plate and yes I would be using 4043if I tig it
Reply:TIG it.3" inches thick ?!?! Welded to 1/4" tube? These must be left over parts. If you're really worried about the penetration into the 3", just run another bead or two, to build up the fillet. Also, why do they want the tube run through the plate? Wouldn't it be stronger the other way?Ian TannerKawasaki KX450 and many other fine tools
Reply:Even though it's only 1/4" wall tubing going into it, I would preheat the plate, cause it's a heckuva heatsink.Also, if possible, I would make the weld on the bottom of the plate instead of the top, so that the HAZ is not under stress. A sign post will probably carry a lot of wind load, and the highest stresses will be where the vertical tube attaches to the plate.JohnA few weldersA lot of hammersA whole lot of C-clamps
Reply:Preheat can't hurt, but that Sync has enough grunt to do the job at 400 amps.Weld like a "WELDOR", not a wel-"DERR" MillerDynasty700DX,Dynasty350DX4ea,Dynasty200DX,Li  ncolnSW200-2ea.,MillerMatic350P,MillerMatic200w/spoolgun,MKCobraMig260,Lincoln SP-170T,PlasmaCam/Hypertherm1250,HFProTig2ea,MigMax1ea.