What you want for handlebars is 4130 steel (cro-mo) structural tubing, in .063" or .080" wall thickness. This alloy is easy to braze and weld with all methods, you can bend it with an electrical conduit bender and it also doesn't lose stiffness from cold-bending as long as you don't kink the tube.
You could probably use mild steel structural tube but you want it at least .080" wall thickness.
Others may have used them, but I would not recommend copper pipe, electrical conduit, black iron pipe or any kind of aluminum.