Use stainless steel wires for stainless steel, aluminum wires for aluminum, and steel wires for steel and nickel alloy rods for nickel alloys. (cast iron is welded with nickel wire but that’s a story for another day)
This might seem like a no brainer but I guarantee you sometime somewhere, someone has said, "Hey why don’t we try welding this aluminum tree stand with this mig gun and mild steel wire?!"
Don’t laugh, I have heard of this firsthand.
There are many reasons why you want to match filler wire with base metal.
Mainly just because it makes good sense. Most of us welders know that you can weld mild steel with stainless rod. (Heck, I have worked at shops where Hastelloy W was used on everything from coffee pots to tool box frames and everything in between).
But welding mild steel with stainless is not always a good idea. Especially if the weld will be subjected to thermal cycling... so its just a good practice to know what you are welding and use a matching filler metal. Not to mention the fact that mild steel wire is the cheapest wire of all. So if its better for the application, and cheaper, why not use it?
Another exception is welding 4130 chromoly. E70s2 is usually a better choice than 4130 filler metal for ductile welds.
read more mig welding tip articles