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"diecast (die-cast) magnesium" or "magnesium diecasting (die-casting)"
Explanation: Whether you use "magnesium" or "diecast (die-cast)" as the modifier is up to you, both are OK, "diecast (die-cast) magnesium" or "magnesium diecasting (die-casting)", whichever fits your sentence best. The use of the hyphen is a worse problem. The reputable Australian CSIRO site below does not use a hyphen for the noun or modifier. Ernst – Wörterbuch der industriellen Technik also does not use a hyphen. NODE and Langenscheidt (both technical and Muret-Saunders) do use the hyphen. Webster's Collegiate lists neither the single word nor the hyphenated form. The results of an AltaVista English search for "die-cast" yields 71,000 hits and "diecast" yields 94,000 hits, but that is not too reliable because search engines often cannot tell a hyphen from a space. I would probably go along with NODE because NODE is the nearest we have to a recognized standard.