Today, more than 120 countries have issued bi-metallic coins and the number is growing. These coins are minted in many different combinations of precious and base metals: yellow and white gold, gold and silver, silver and titanium, silver and nickel, non-magnetic stainless steel and aluminium bronze, and combinations of copper or brass and nickel, etc. View some examples above.
Another type of bi-metallic coin is where there is a large core of one metal with a ring (of another metal) around the core.
Another type of "bi-metallic" coin is where there is a large clad core of three metals with a ring (of another metal) around the core. Although this is not actually a bi-metallic coin, they appear to be, and are probably being considered (and collected) as bi-metallic coins.
There have been other types of bi-metallic coins tried in past years, in fact past centuries in various countries. These have included clad, plug, plated / coated, shrunk and other methods.
Today’s common production of bi-metallic coins is to join bi-metal blanks. The external ring is manufactured by a multiple-die progressive tool, which pierces out the center hole prior to blanking from a strip. The raised outer edge of the blank, formed by "rimming" assists in reducing the coining pressure. The inner, or "dump" is made very much like an ordinary coin blank, except for the special milling applied to the edge. When the two components are struck by the assembling press, the outer ring deforms to flow inside the milled indentations, providing efficient anti-twist locking and increasing the strength of the bond. There are other ways of joining bi-metal blanks, with each manufacturer having their own preferred method.
If two or more sheets or strips of metal are laid on top of each other with a sheet of high explosive above and below them, and the explosive is detonated, the sheets or strips of metal are welded together and appear (from the edge) to look like a sandwich. The resultant metal sandwich is then referred to as a "clad" metal and it can be worked in similar ways to other metals, including the various processes to make coins.
Collecting bi-metallic coins is a large area of numismatic interest, so much so, there is an international club of collectors, that has been around for over 20 years. Check out the Worldwide Bi-Metallic Collectors Club (WBCC) at https://wbcc-online.com/.