You may care to try a search on ** mokume-gane ** which seems to be the
name of the process you mention.

Just, if I read it right, a combination of heat (until it glows red and
sweats (?)) and pressure (bolted between a couple of heavy steel
plates) according to Tim McCreight's The Complete Metalsmith. Sounds
too simple but worth a try. Offhand, I suspect that bright shiny clean
and unoxidized metal may have the best chance of fusing o diffusing.