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If the fiber lasers have enough power, they can cut through reflective substances. They can cut any kind of metal, provided they have enough power. They will be limited in the thickness that they can cut.
If you’re cutting metals that are highly reflective, like aluminum, copper, or brass, and they are in the thicker side of cutting, then a plasma will be ideal. Lasers machines usually struggle with cutting reflective metals, especially if they are on thicker side. You’ll see that if you want to cut reflective metal, then you’ll need a higher amount of power to do so. And more power means more $$$.