I've never done it, nor do I have any knowledge of the specifics, but from my limited general experience with bending metal tubing, I imagine you'd have to start with the metal in a relatively malleable state, heat and bend it use mandrels or something else in the tubing to keep it from collapsing or kinking during the bending process, then subject the finished piece to a hardening process using heat and quench. You'd need a pretty good knowledge of metallurgy to get the final hardening right, as too soft and the bars would be prone to bending, and too hard and they'd be brittle. Steel would be infinitely easier than aluminum, I suspect.