Is this the only time you're going to be welding? Mig with aluminum that thin might be a problem. And a Tig machine is expensive.

There are 3m structural adhesives that would to the trick for a lot less money than buying a welder. Properly applied, the aluminum will tear before the adhesive bond fails.

Can you describe the kind of use (or specific stresses) the joined plates will be subjected to?

 

What are license plates made of, aluminum? TIG may still be an option if you rent one. I would think MIG would handle it fine as long as you not using flux.

 

Thanks, pretty much zero stress as they are just pieces being welded together then hung on the wall.