A feature of fusion welding is melting of the base metal or of the base and filler metals and producing a welded joint without application of external pressure. This involves heating to high temperatures a small region of the metal, which is surrounded by the cold metal whose volume is much larger than that of the molten metal. In fusion welding, melting of just the base metal and of the base and additional metals may take place simultaneously. Fusion welding process is a high-temperature process, it is accompanied by intensive interaction of the contacting phases. Such interacting phases in fusion welding include molten and solid metals, metal melt-gas or slag, depending on the applied welding process. The molten metal then flows togeather forming one homogeneous piece. In actuality, this process is called fusion welding.