During the welding process, the workpiece and solder melt into the molten area, and the molten pool cools and solidifies to form a connection between the materials. In this process, it is often necessary to exert pressure. There are many sources of welding energy, including gas flame, arc, laser, electron beam, friction and ultrasonic wave. By the end of 19th century, the only welding process was the blacksmith's hundreds of-year metal Forge. The earliest modern welding technology appeared at the end of 19th century, first arc welding and oxygen gas welding, and later the Resistance welding appeared. In the early 20th century, with the first and Second World War, the cheap and reliable connection method of military equipment was very demanding, so it promoted the development of welding technology. Today, with the wide application of welding robots in industrial applications, researchers are still deeply studying the nature of welding, and continue to develop new welding methods to further improve the quality of welding.