Determing Types and Grades of Aluminum

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Hello, I would like to know how one goes about determing various types of aluminu. I need to weld many types for repair but don't have a clue as how to tell what it is. Copper is easy, so is brong, and silver. A magnet will tell if iron or steel are ferrous metals, but what do I do when I need to wld a broen part and it must be plished and wled must match?? Is there a piecec of equipment that will do the trick? Any help would be hugely apprciated. Thank You, Victor