It is not always possible or necessary to make metal objects by shaping and machining. Often it is better to construct them by joining several pieces together. This may be done by welding.
In fusion welding the welder first heats the edges of metal pieces with a high temperature from an oxy-acetylene torch until the surfaces melt. Then he usually adds more molten metal from a welding rod to fill the gap between the two pieces. The filler metal and the melted edges fuse together. When the metal cools and solidifies : a strong joint forms.
Instead of a gas flame, the welder may use the heat produced by an electric arc to melt the metal. This is known as arc welding.
Another form of electric welding is called resistance welding. When an electric current is passed through two overlapping metal parts, they heat up. A little pressure is applied to join the parts together. Spot welding and seam welding are two forms of resistance welding.
In friction welding, the parts are rubbed together under pressure until they soften and fuse.
The most recent method of welding is by laser beam, Laser welding is often done where delicate parts must be joined very accurately.