Welding technology is mainly applied to metal base materials. Commonly used are arc welding, argon arc welding, CO2 shielded welding, oxygen-acetylene welding, laser welding, electroslag pressure welding, etc., and non-metallic materials such as plastics can also be welded. There are more than 40 metal welding methods, which are mainly divided into three categories: fusion welding, pressure welding and brazing.
Fusion welding is a method in which the joint of the workpiece is heated to a molten state during the welding process, and the welding is completed without pressure. During fusion welding, the heat source rapidly heats and melts the joint of the two workpieces to be welded to form a molten pool. The molten pool moves forward with the heat source and forms a continuous weld after cooling to connect the two workpieces into one.
In the fusion welding process, if the atmosphere is in direct contact with the high-temperature molten pool, the oxygen in the atmosphere will oxidize metals and various alloying elements. Atmospheric nitrogen, water vapor, etc. enter the molten pool, and will also form pores, slag inclusions, cracks and other defects in the weld during the subsequent cooling process, deteriorating the quality and performance of the weld.
Pressure welding is to make two workpieces realize interatomic bonding in a solid state under pressure, which is also called solid state welding. The commonly used pressure welding process is resistance butt welding. When the current passes through the connecting end of the two workpieces, the temperature rises due to the large resistance. When heated to a plastic state, the connection becomes a whole under the action of axial pressure.
The common feature of various pressure welding methods is to apply pressure without adding filler material during the welding process. Most pressure welding methods such as diffusion welding, high frequency welding, cold pressure welding, etc. have no melting process, so there is no problem of beneficial alloy elements burning like fusion welding and harmful elements intruding into the weld, thus simplifying the welding process and also Improved welding safety and sanitation conditions. At the same time, because the heating temperature is lower than that of fusion welding and the heating time is shorter, the heat-affected zone is small. Many materials that are difficult to be welded by fusion welding can often be welded by pressure welding to form a high-quality joint with the same strength as the base material.
Brazing is to use a metal material with a lower melting point than the workpiece as the solder, heat the workpiece and the solder to a temperature higher than the melting point of the solder and lower than the melting point of the workpiece, and use the liquid solder to wet the workpiece, fill the interface gap and realize it with the workpiece A method of mutual diffusion between atoms to achieve welding.
The joint that connects the two connected bodies formed during welding is called a weld. The two sides of the weld will be subjected to welding heat during welding, and the structure and performance will change. This area is called the heat-affected zone. During welding, due to different workpiece materials, welding materials, welding current, etc., overheating, embrittlement, hardening or softening may occur in the weld and heat-affected zone after welding, which also reduces the performance of the weldment and deteriorates the weldability. This requires adjustment of welding conditions, preheating the joints of the weldment before welding, heat preservation during welding and post-weld heat treatment can improve the welding quality of the weldment.