backing plates

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Which is best to use as a backing plate when welding up a hole, copper plate or aluminum plate. Thanks for your help
Reply:copper. aluminum is a bad choice.
Reply:I weld in one side, flip part over and weld in the other.  Using backing plates has always ended badly for me.  Usually lot's of grinding and then welding of the back side anyway.
Reply:i use all copper (backing) for my alum. an ssno sticking and leaves a smoother surface too
Reply:Originally Posted by El_Lloydeocopper. aluminum is a bad choice.
Reply:the aluminum would prolly melt wouldnt it?  Dont consider this an answer Snoborder.  Im just taking a stab.
Reply:Originally Posted by sn0border88Care to share why?
Reply:Makes sense, Ive used it for backing when going stainless work where a back purge isnt an option.  Im assuming as long as your not welding into the alu. its not an issue?Have we all gone mad?
Reply:Ive seen ppl use it for filling holes in struc steel. i gave it a try one time and man was it nasty. its not on the list in D1.1 either.