It is common misconception by many people that welding is a process that requires heat or electric arc. In reality heat is secondary product or some time it is the something that can aid welding. Any process where two separate parts penetrate into each other and forming bond that is as strong as surrounding material is welding. As example plastic pipes that used for water or drainage have elbows, t pieces and flanges that plumber ,,glue,, together. Correction WELD,s together, the glue designed not to stick to the pipe but to melt pipe and neighboring object so when you join them together their surfaced are welded.
Here is a video that shows this process in details.
Depends on application and material type different processes are used. Right enough avast majority of them utilize heat to produce welds and quite often quite a lot of it. Even plastics can be welded with heat.