During the welding process, the workpiece and solder melt to form a molten area, and after the molten pool cools and solidifies, it forms a connection between the materials. During this process, pressure is usually applied. There are many sources of energy for welding, including gas flames, electric arcs, lasers, electron beams, friction, and ultrasonic waves. Before the end of the 19th century, the only welding technique used by blacksmiths for hundreds of years was metal forging welding. The earliest modern welding techniques appeared in the late 19th century, first arc welding and oxygen gas welding, and later resistance welding. In the early 20th century, with the outbreak of World Wars I and II, there was a great demand for cheap and reliable connection methods for military equipment, which promoted the development of welding technology. Today, with the widespread use of welding robots in industrial applications, researchers are still delving into the essence of welding and continuing to develop new welding methods to further improve welding quality.