The two pieces of metal were permanently joined by a rivet passing through them.

I assume the rivet you are asking about is a "tinners" rivet and not a more common "blind" rivet (commonly known as a "pop" rivet). If this the case the tool you want is called a "rivet set".

upset -------------------------------------------------------------------------------- Upsetting the rivet is to compress the rivet during the installation process. That can be done with a rivet gun to "hammer" the rivet against a bucking bar, or to use a rivet squeezer to compress the rivet.

a rivet gun is a gun

A DD rivet is recognised by 2 dashes on the head of the rivet