ALLOY

Tey are alloy numbers designated by some international organization. I am not sure which one. They are both standard alloys used for welding aluminum.

5356 works better in standard MIG machines because it is a harder alloy. Therefgore it makes a stiffer wire so it is easier to push through a MIG gun liner. 4043 is more often used in spoolguns or push-pull feeders because it is too soft to push reliably.

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I think this web site will answer your question:-

Regards

John