No, it's the alloy (and the casting, to some extent). They'll be something like A320 or A356, silicon based alloy. Depending on how it's cast (high pressure injection?), there may be gas inclusions as well (which makes it notoriously hard to weld..).
Silicon alloys are strong in part by the usual additions (Cu, Mg, Si, etc.), but also by the excess silicon precipitating as crystals in the metal matrix. Which, being rather hard and strong, tend to frustrate the metal pushing and sliding around -- it's strengthened, harder, but also less ductile.
Which for a casting, is a good thing. You can't forge or roll or draw it, it's gotta be net shape. So it's not a big deal that it's not very ductile.
Unless you want to do something very different with it, which... will be a bit of a problem.
So, consider some kind of cutting and bonding process instead. If you have to start with the diecast box*, cut whatever openings you need, and, say, bolt, rivet or epoxy your widgets to it.
*Since the alternative would probably be something like a sheet metal box, seam welded to get the same sealed box. Which... ain't gonna be cheap.
The other way is, if you can find deep-draw boxes. I don't recall running across these at the usual suspects, but maybe there's something out there. You'll only have five sides not six, like the diecast box, gotta have some kind of flange, seal and lid. But as you can guess from the method, it's wrought alloy. Now, again, it's
already been wrought, it's not dead soft, it might not bend very far -- you're lucky to bend a 6061-T6 plate past 90 degrees -- so if you want to cut open a flap or something, fire may be needed (get it somewhere around "black hot", 400-500C something like that*, this will anneal the alloy).
*Typical test is to use a sooty flame to blacken it, then heat until it burns off.
Tim
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