Straightening 6061 Aluminum

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I've been visiting the forums for some time now and have found alot of great information. I ran into the following problem recently and any advice would be appreciated!I have to straighten 6061 aluminum machine parts on a regular basis. I'll outline the situation so you can understand what I'm up against.. Parts are used in production machines and come in for repair. Average size would be 8" wide x 14" long by 1"+ thick. Repair usually requires welding to damaged areas (which can vary). After building up the damaged area, it is machined. It cannot be machined under original thickness.Ideally, I want the repaired piece to have under 0.005" deviation. During welding, of course, the pieces warp. I've tried various holding methods, pre-heating, post heating, etc. Least deviation so far is around 0.020". The piece can be bent but springs back. I've pulled them up to 0.250" in the opposite direction of the warp and changed nothing. I don't want to break or crack the piece. Any suggestions on straightening these parts? A controlled bend would be ideal.Thanks!
Reply:I think the best route would be to look into how heads are straightened.  I believe they are bolted to a thick *** piece of steel and then put in an oven  at a certain temp for a certain period of time.  I don't know the specifics, Castweld might be a good guy to ask.My name's not Jim....
Reply:Try clamping them with a preload in the opposite direction while you are welding . . . then let them cool completely.  I use this method when adding baffles to cast aluminum oil pans and it works well once you find the right thickness of shim.Jim
Reply:Some pictures would help.Blackbird
Reply:4sfed has the idea i would go with first.Vantage 500's LN-25's, VI-400's, cobramatics, Miller migs, synch 350 LX, Powcon inverters, XMT's, 250 Ton Acurrpress 12' brake, 1/4" 10' Atlantic shear,Koikie plasma table W/ esab plasmas. marvel & hyd-mech saws, pirrana & metal muncher punches.
Reply:Thanks Fellers.Dave, I'll try to get some pictures up today.4sfed and Dualie. Same idea I had. It reduces the twist BUT the problem is that the weld is not the same on any two peices. Short of a whole lot of trial, error and plain old experience it's hard to know how much to pull each piece. So far I've managed to get variation under the 0.020" mark but would like to get it under 0.005". At this point I figure the only way is to have a secondary straightening process. Problem with this material is it's so durned "springy". I'm not sure the best way to releive the stress t the weld joint so I can bend the piece into submission.I like Boostinjdm's idea and I think it would probably work quite well. The only problem is the time factor. From what I've seen the process to straighten a head requires several hours. I was hoping for something simpler and faster. Ahhh, but maybe thats just what I'll have to plan for. A couple big ovens capable of handling a 4 to 5 pieces at a shot.Appreciate the quick responses!Josh
Reply:You can also straighten the parts while they're hot . . . I'd suggest reheating after welding.  The yield strength of AA6061-T6 aluminum is greatly reduced at relatively higher temperatures.  At 75 degrees F, yield strength is 40 ksi with 17 percent elongation.  When heated to 300 degrees it drops to 31 ksi and elongation increases to 20 percent.  At 400 degrees it's 15 ksi and 28 percent.  I would not heat it any higher than that . . . and keep the time at temperature under 30 minutes.  This alloy is precipitation hardened by artificial aging at an elevated temperature, and over-aging will reduce its strength.     However, there's no guarantee that a part that's straight at 400 degrees will still be straight when it cools.Jim
Reply:attaching it to a strong-back and then stress relieving may be the only good option you have.  but even that isn't a very good option.  It will destroy any age hardening or other heat treatment that may have been done to the piece. (but of course the welding would have already done that)slight bends wont accomplish much,  you have to reach the yield point to accomplish anything.Also,  any non stress relieved method will still have the piece moving after the machining is done.