Welding, or the fusion of two materials (usually metallic in structure), has been around for over 1500 years! Although the methods have changed quite substantially, the goal remains the same.
Welding is quite a broad term with hundreds, if not, thousands of techniques used for different jobs and materials, each featuring unique equipment and methods.
The most common practise for welding used today are:
Shielded Metal Arc Welding
Gas Tungsten Arc Welding
Gas Metal Arc Welding
Flux-Cored Arc Welding
Submerged Are Welding
Each of these processes is designed to suit different metals. Each material has its very own melting point, which is necessary for fusion to occur via some sort of energy source.
The problem that most welders face is when welding dissimilar metals. For example, aluminium with steel. In the event that a fabricator would want to do this, usually, a filler metal is used which is compatible with both kinds of metal. In the event that you would want to weld aluminium with steel, usually, zinc would be used as the filler metal to bind both of these together and ensure longevity and durability.
Take a look at the video below by the YouTube channel, Concerning Reality, on How Does Welding Work?