Welding is a fabrication process in which two pieces of metal are joined together. Unlike in soldering where they pieces being joined are not melted, welding relies on melting the filler material and the two pieces being worked on to create a bond. There are many different welding processes used today including arc welding, torch or gas welding, and laser beam welding.
Usually the filler material used in welding the two parts has properties close or similar to those of the materials being welded together to create a homogenous bond. Otherwise, the bond created is called heterogeneous.
Arc welding is the most common kind of welding carried out today. In arc welding, an electrical arc is used to melt the work materials and the filler material in between to create a bond. In arc welding, an anode wire is placed on the parent material and an electrode is placed on the filler material. When the lead is pulled away, an electric arc is created which melts the pieces together to form a bond.
The electric arc is what causes the sparks seen during welding. As the pieces continue to melt, the welder feeds the filler material in between the two pieces which takes skill. The motions used to put the filler material give the welds a distinct appearance once the joint has been done. MIG, TIG, and stick welding are all types of arc welding.
Another form of welding that is common is torch or gas welding. In gas welding, combustion of acetylene gas in a stream of oxygen gas produces a high temperature flame that is used to melt the filler and parent materials to form a bond. The welder has to control the torch or gas flame and the filler rod or welding rod during the welding process.
In laser welding which is a fairly new technology, a high energy laser beam is directed at the materials to form a bond.