Welding refers to the uniting or fusing of pieces by using heat and/or compression so that the pieces form a continuum. The source of heat in welding is usually an arc flame produced by the electricity of the welding power supply. Arc-based welding is called arc welding.

The fusing of the pieces can occur solely based on the heat produced by the arc so that the welding pieces melt together. This method can be used in TIG welding, for example.

Usually a filler metal is, however, melted into the welding seam, or weld, either using a wire feeder through the welding gun (MIG/MAG welding) or by using a manual-feed welding rod. In this scenario, the filler metal must have approximately the same melting point as the material welded.

Before beginning with the welding, the edges of the weld pieces are shaped into a suitable welding groove, for example a V groove. As the welding progresses, the arc fuses together the edges of the groove and the filler, creating a molten weld pool.

For the weld to be durable, the molten weld pool must be protected from oxygenation and effects of the surrounding air, for example with shielding gases or slag. The shielding gas is fed into the molten weld pool with the welding torch. The welding electrode is also coated with a material that produces shielding gas and slag over the molten weld pool.

The most commonly welded materials are metals, such as aluminium, mild steel and stainless steel. Also plastics can be welded. In plastic welding, the heat source is hot air or an electric resistor.