There are different ways to link the parts together. Commonly this can be done by fasten them with rivets or bolts, solder or weld them together. If you need to do the continuous joint on long parts of joining surfaces you need welding. This is the main advantage of welding, especially for enclosed containers such as boilers, etc.
Rust, oxidation, grease, and dirt prevent a proper weld joint. Hot metal requires protection and all weld processes use one of two protection methods: flux or shielding gas. The form of a welding flux is usually liquid, paste or solid. When evaporated, it creates a small pocket of gas. This prevents oxidation. But solid flux must be removed after completing the welding. To protect welding shielding gas surrounds point of welding.