Fabricating an I-beam question

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I may have the need to fabricate a couple specialized I-beams in the future. The reason for fabbing them on my own is it's shape, it will be sort of an arch but with a flat section on top and a short leg on one end. There will be 2 matching beams about 8" tall at the narrow point and 12'+/- in length. They will be aluminum with 1/2 x 4" flanges and 1/4" web. The question is what is the best way to keep the flanges from pulling when I weld them? I will work off a table of sorts so I could tack some jig type supports to hold them, but then would have to weld all one side first then the other. I could also stand them up for welding so I can alternate sides while welding. Or I'm open to any other suggestions. I'll be welding with mig.Yeah, I know, but it'll be ok!Lincoln Square wave 255Miller Vintage mig30a spoolgunThermal Dynamics Pacmaster 100xl plasmaSmith mc torchEllis 1600 band saw
Reply:Maybe something like this? Attached ImagesIan TannerKawasaki KX450 and many other fine tools
Reply:Why aluminum?  Aluminum's as welded strength is seriously compromised when compared to unwelded mill extrusions.  Properly designed, the same shape in steel will only be marginally heavier.  If deflection is a limiting factor, steel may be lighter.The steel will be much cheaper.
Reply:Originally Posted by bentWhy aluminum?  Aluminum's as welded strength is seriously compromised when compared to unwelded mill extrusions.  Properly designed, the same shape in steel will only be marginally heavier.  If deflection is a limiting factor, steel may be lighter.The steel will be much cheaper.
Reply:Make opposing welds. Back step your welds, and skip around a lot. Feather / grind all your starts and stops. Attached ImagesDon’t pay any attention to meI’m just a hobbyist!CarlDynasty 300V350-Pro w/pulseSG Spool gun1937 IdealArc-300PowerArc 200ST3 SA-200sVantage 400
Reply:Originally Posted by CEPMake opposing welds. Back step your welds, and skip around a lot. Feather / grind all your starts and stops.
Reply:I rebuilt the neck on my semi trailer and cut 14X26lb steel main beams down to 9.75" tall by taking the center section out of the original beams. I tack welded every 12" alternating sides with each tack and then welded six inch "stiches" alternating sides and end with each weld. The beams did not pull out of square, (tolerance) and serve the purpose well. I'd do the same with aluminum if it were me. A lot of time spent but it was my own project and we know how well those pay.......SlobPurveyor of intimate unparalleled knowledge of nothing about everything.Oh yeah, also an unabashed internet "Troll" too.....
Reply:Originally Posted by SlobI rebuilt the neck on my semi trailer and cut 14X26lb steel main beams down to 9.75" tall by taking the center section out of the original beams. I tack welded every 12" alternating sides with each tack and then welded six inch "stiches" alternating sides and end with each weld. The beams did not pull out of square, (tolerance) and serve the purpose well. I'd do the same with aluminum if it were me. A lot of time spent but it was my own project and we know how well those pay.......
Reply:Originally Posted by welderjHow long should those weld be? And I suppose alternating sides as well? Should I tack the whole thing together then weld, or do shorter sections and work my way along?
Reply:Originally Posted by CEPI would tack it all up. But while welding listen to see if any tack break. I wouldn’t go more than 6-inches of weld at one time. Then jump over to the other side and weld in the opposite place. All this method, and back stepping is to help prevent distortion.Now remember, I’m just a hobbyist!
Reply:I’ve spliced more beams than build them. One time I did help build some 8-feet tall wide flange beams for a bridge in Alaska.Don’t pay any attention to meI’m just a hobbyist!CarlDynasty 300V350-Pro w/pulseSG Spool gun1937 IdealArc-300PowerArc 200ST3 SA-200sVantage 400
Reply:Then I'll follow your advice. As you know I've welded plenty of aluminum, but never made a beam before. Just want to keep the flanges as straight as I can.Yeah, I know, but it'll be ok!Lincoln Square wave 255Miller Vintage mig30a spoolgunThermal Dynamics Pacmaster 100xl plasmaSmith mc torchEllis 1600 band saw
Reply:The flanges may dip anyway. But you could always heat shrink them back into position after it cools dead to the touch. A tractor torch works real well for this task. Attached ImagesDon’t pay any attention to meI’m just a hobbyist!CarlDynasty 300V350-Pro w/pulseSG Spool gun1937 IdealArc-300PowerArc 200ST3 SA-200sVantage 400
Reply:And don't  listen to CEP.  He's about a "hobbiest" as the Pope would be if he showed up incognito to the first day of youth Catechism class.  Lincoln Power Mig 216Lincoln AC/DC-225/125Miller  625 X-Treme PlasmaMiller 211 Forney 95FI-A 301HF 91110Victor Journeyman O/PMilwaukee DaytonMakita  Baileigh NRA Life Member
Reply:@ CEP - unlike Steel, flame straightening Aluminum is not a recommended tactic.  Select Al grades are annealed at ~ 650deg F and then air-cooled.  Start to reach around 400deg F via flames on Aluminum and you risk permanently changing the metal's temper. IIRC, 6061-T6's yield strength at room temperature and 400deg F are ~ 42kis and 15ksi, respectively.  And, elongation approximately doubles.YMMV.  "Discovery is to see what everybody else has seen, and to think what nobody else has thought" - Albert Szent-Gyorgyi
Reply:Originally Posted by ManoKai@ CEP - unlike Steel, flame straightening Aluminum is not a recommended tactic.
Reply:I would cut several spacer blocks to fit between the web's. Space them around the areas you are welding on each side. Moving around when welding and the blocks will help prevent distortion.
Reply:Question for you guys. Lets say when welderj completes the fabrication of these beams, and the flanges pulled down, because of the fillet weld of the web to flanges. Could he skip weld a bead on top of the flange right down the center to curl the flange back up? Then grind those welds down?Don’t pay any attention to meI’m just a hobbyist!CarlDynasty 300V350-Pro w/pulseSG Spool gun1937 IdealArc-300PowerArc 200ST3 SA-200sVantage 400
Reply:@ CEP -  believe it really boils down to minimizing the heat flux injected to the location (centerline of flange) that you suggest.  Your proposed approach, from a practical standpoint, may work depending upon the degree of the flange defelection "smile" and "frown."  From a metallurgical standpoint, you may risk creating torsional stress at the interfce between the flanges and web. Your skip welding recommendation is def spot on.  You've welded a metric ton more beams than me and your consult on steels is always laser correct.  Been learning that even preheating Aluminum isn't cool unless, again, you stay under 400 deg F.  Exceeded that temp for a long time may require you to  conduct a full re-temper, quenching, and age hardening to achieve the acceptable Fy on select Al grades.  Aluminum loves to have the heat dumped in NOW.  Your backstep/skip welding protocol is def the route would personally use if had to weld the assemblies.Surf over to the Metallurgy sticky and checkout the last post on Aluminum design considerations from Lincoln Electric. Last edited by ManoKai; 10-16-2015 at 08:35 AM."Discovery is to see what everybody else has seen, and to think what nobody else has thought" - Albert Szent-Gyorgyi
Reply:You build girders full auto sub arc in. north america no other way by D1.5, weld procedures set for the job. Control of distortion is done by fit up tacking and proper sequence of welding.
Reply:Originally Posted by PavinsteelmanYou build girders full auto sub arc in. north america no other way by D1.5, weld procedures set for the job. Control of distortion is done by fit up tacking and proper sequence of welding.