Welding is a fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool causing fusion.  A filler material is typically added to the joint to form a pool of molten material that cools to form a joint part, based on weld configuration, can be stronger than the base material. 

The four main welding processes are:

1. Arc Welding (Fusion Welding)

• Weld metal melted from the edges are to be joined and allowed to solidify from the liquid state and usually below the recrystallization temperature without any applied deformation. 
• Most extensively used to join metal parts by fusion.

2. Gas Welding

• Burning of combustible gas with air or oxygen in a concentrated flame of high temperature. 
• Used to weld most common materials.

3. Gas Metal Arc Welding (MIG)

• A metal rod is used as one electrode, while the work being welded is used as another electrode.
• It is a gas shielded metal arc welding which uses high heat of an electric arc between a continuously fed, consumable electrode wire and the material to be welded.

4. Gas Tungsten Arc Welding (TIG)

• Similar to MIG welding except this arc welding process uses the intense heat of an electric arc between a non consumable tungsten electrode and the material to be welded. 
• In this process the electrode is not consumable during welding and gas is used to protect the weld area from the atmospheric air.