I'm working on this project and I have to glue together a 4mm thick aluminum and a 5mm aluminum. It's a 17mm wide hexagon.
METHOD OF JOINING ALUMINUM TO ALUMINUM A welded joint having good electrical and mechanical characteristics is obtained between aluminum members by using a copper or copper alloy cloth or screen between the members to the joint. Heat and pressure are applied through resistance electrodes to melt the small wires of the screen and cause the aluminum metal to bond together.
Aluminum does not glue very easily. Welding is better, but if it's not for something structural any kind of Crazy Glue or Super Glue will hold it in position. It won't hold together under any kind of stress though. Alternately you could use machine screws, or self tapping screws.
