Using aluminum as backing plate for stainless?

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Hey guys I had a quick question.I need to weld some stainless sheet metal in a lap joint.  I will be welding on some pretty expensive stainless and my boss will have my neck if I have sugaring on the back.Back in welding school we clamped the stainless against a 1/4 piece of copper with a groove for butt joints to protect the backside.I am not sure where you find a piece of copper like that, and especially one I could afford.  My plan was to use aluminum instead.  Will this work?I will be tig welding and its 18 gauge I believe.Thank you for your help!Also if this asked frequently I apologize I tried googling but I just got 1 million results comparing aluminum welding to stainless welding.
Reply:Aluminum will probably work fine, maybe not so well when doing an open root.Buy a chunk of copper, justify it like any other shop tool.  www.onlinemetals.comWelding/Fab Pics: www.UtahWeld.com
Reply:I do it all the time, no issues.  Maybe not quite as nice as copper but keep the dirt off of it and it works fine.Have we all gone mad?
Reply:Or use solar flux
Reply:Set up a back purge and do it right.I got my 1/4" copper at a scrap yard.  Cut up pieces of busbar are pretty commonplace.
Reply:Thanks for the answers, guys.
Reply:Most any decent roofer will have copper flashing material available.Easy to cut and form into a backing dam.I've also used thinwall aluminum angle to form a backing dam.  Clamp and tape in place.  In a pinch, I've used aluminum foil to make backing dams.Syncro 250 DX Dynasty 200 DXMM 251 w/30A SG XMT 304 w/714 Feeder & Optima PulserHH187Dialarc 250 AC/DCHypertherm PM 1250Smith, Harris, Victor O/ASmith and Thermco Gas MixersAccess to a full fab shop with CNC Plasma, Water Jet, etc.
Reply:Hello stepho, everyone has given you great suggestions. Additionally, you may wish to consider a few other items. These welds being lap welds is somewhat of a benefit provided that you do some prep work before going after the welding. Try really hard to be sure that your fit-up is very precise, meaning that the parts don't have a lot of gaps between them. Use a generous number of "small" tacks without a great distance between them as this will help to maintain minimal joint separation and gaps. Use some form of back-step welding to distribute weld stresses and minimize localized heating and thus aid in controlling distortion and warpage.      Are these welds going to be done autogenously or with filler? Hopefully they are going to be done without filler (thus the importance of fit-up). If you have the option to use a machine with a pulse feature this might help additionally with heat control issues and help to minimize the risk of sugaring, relatively high pulse rates will often promote the fusing of the two joint components and help to break the surface tension of the weld puddle that sometimes occurs and causes burnback of the edge piece of the joint. From the description that you have given, travel speed could be of another great influence on minimization of sugaring as well. Slightly higher heats and travel speeds can aid in prevention of backside weld issues. Edge prep on the lap portion of the joint can have a significant influence on how well and readily the weld can be made, a clean square edge will provide the best weld performance along with the tight fit. Certainly, cleanliness can help you with your task, pay close attention to any metal processing fluids that might have contaminated the weld joints and clean with appropriate solvents/alcohol prior to fit-up and weld out. Whether you end up using aluminum or copper for a backing bar, they will help with puddle control and make for a more forgiving and quality finished weld. Good luck on your project and please try to get back on here to show some results and discuss your progress. Best regards, Allanaevald
Reply:If I have the time, material and the equipment then I try to back purge, but I will say that Solar Flux works excellent, and I find myself using it far more than back purging with Argon. Only thing I don't like is the cleanup (I weld 1-1/2" Stainless Steel tubing for our Paint Dept) but it cleans up with "wonder gel or a common pickling compound. Purge when possible but don't rule this out. I would recommend keeping some on hand.