There are many ways to connect the parts together. The most usual are fastening by bolts or rivets, brazing and welding. If you want to do the continuous joint on long parts of joining surfaces you need welding. This is the main advantage of welding, especially for enclosed cans such as boilers, etc.
The design of joint, filler, base material and flux influence the quality of the weld. The strength of material and weld depends on the welding method and concentration of energy input, type of the flux and filler, and weldability of the base material. The weld quality also much depends on the heat-affected zone. To test the quality of the weld either destructive or nondestructive methods are used. Gas and non-metallic inclusions, cracks, incomplete penetration, distortions, lack of fusion and lamellar tearing are common defects of welds.
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. Welding flux is used in solid, paste or liquid form. It creates a small pocket of gas while evaporating. This prevents oxidation. But solid flux must be removed after completing the welding. Shielding gas protects weld directly by surrounding it.
The original fusion technique is known from the earliest uses of iron. People heated small pieces of iron to welding temperature and then hammered or pressed them to create larger useful pieces. Oxyacetylene welding, arc welding, friction welding, electron-beam welding, laser welding, resistance and other welding are the most common up-to-date techniques. The largest total volume of welding is shielded metal-arc welding (related post – best welding caps). An electric arc is created between materials leading to electrode’s metal melting and transferring it to the joint in this process. Another technique is gas (usually oxyacetylene) welding. The source of heat in this process is acetylene burned in the atmosphere of oxygen. The resistance welding is a process when the required heat is generated from electrical resistance of the joint. This welding technique is made by using high-current and low-voltage power sources with pressure added to the joint. A dense stream of high-velocity electrons bombarding the joint is a source of heat for electron-beam welding. If you use the friction of moving parts, this is called the friction welding. Laser welding is accomplished when materials are fused together by heat generated from a laser source.
Carbon and low-alloy steels are the most frequently used materials in welded constructions. The amount of carbon or alloying content influences the weldability of steels widely. Aluminium alloys are also generally weldable. Copper and its alloys are also weldable, but the high thermal conductivity makes welding difficult. As a rule, you can easily weld plastic or glass. The most common technique to weld them is heating to melting range and adding pressure on surfaces.