Don't know about most repros. The cheap ones (and there are lots of those) often use zinc-aluminium alloy (even easier to cast - lower melting point, flows well). Possibly brass is still more common. For Medieval European repros, cast steel might be most common, and for Japanese repros, cast zinc-aluminium alloy is most common. European complex hilts are often fabricated from bar and/or sheet (or cast brass).
Whatever is used, there's a definite move away from forging compared to historical methods. It's a replacement of skill by technology.
Another reason to use brass: fancy Chinese fittings will often be carved. Brass, being softer, is easier to carve. Cheap fancy fittings can be cast and just cleaned up a little (or on cheap and nasty ones, not even cleaned up, just left with flash and all). Fancy zinc-aluminium can be too weak. Fancy is relatively easy with brass.
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