To join two aluminum parts, you could, in theory, place one atop the other and wait 100 years. The two will eventually form a solid-state bond. Metals naturally want to join with each other.
This is the basis behind solid-state welding processes, which rely on heat and pressure to join metal parts without melting the materials. At one end of the spectrum is diffusion bonding. Axial force is applied to the parts as they are heated to 50 to 75 percent of their melting temperatures. Over time, atoms in each part intersperse, creating a solid-state bond. There is no relative motion between the parts, but the process can take anywhere from a few minutes to a few hours.