ArtMetal

I'm not familiar with manganese bronze, but I use a fair amount of silicon bronze and love it. You can forge it, weld it, machine it, do almost anything with it.

Using a TIG welder, you can create invisible seams, and it doesn't outgas fumes like brass and some other bronzes. I forges either hot or cold (very easily at a low red heat), and it has a very pleasing brownish hue that really glows with handling. Tough stuff, too. High tensile strength, good section modulus, etc.

The only drawback is the price. Sheet is running around $15/lb, some bar stock the same, some a bit lower. Wonderful stuff, even if spendy!