Welding achieves the purpose of joining through the following three pathways:

1. Fusion welding - heating the workpieces to be joined to cause local melting to form a molten pool, which is then joined after cooling and solidification. If necessary, a filler material can be added to assist. It is suitable for welding various metals and alloys without requiring pressure;

2. Pressure welding - the welding process requires the application of pressure to the weldment, which belongs to the processing of various metal materials and some non-metal materials;

3. Brazing - Using a metal material with a lower melting point than the base material as the brazing filler metal, the liquid brazing filler metal wets the base material, fills the joint gap, and achieves a bonded weldment through mutual diffusion with the base material. It is suitable for welding various materials and also for welding different metals or dissimilar materials. Welding technology is mainly applied to metal base materials, with commonly used techniques including arc welding, argon arc welding, CO2 shielded welding, oxyacetylene welding, laser welding, electroslag pressure welding, etc. Non-metallic materials such as plastics can also be welded.

There are over 40 metal welding methods, primarily categorized into three major types: fusion welding, pressure welding, and brazing.

Fusion welding is a method of welding in which the workpiece interface is heated to a molten state without applying pressure to complete the welding process. During fusion welding, the heat source rapidly heats and melts the interface between the two workpieces to be welded, forming a molten pool. The molten pool moves forward with the heat source and, after cooling, forms a continuous weld seam, connecting the two workpieces into one.

Pressure welding is the process of achieving interatomic bonding between two workpieces in a solid state under pressure, also known as solid-state welding. The commonly used pressure welding process is resistance butt welding. When current passes through the connecting ends of the two workpieces, the temperature rises due to high resistance at that point. When heated to a plastic state, the connection becomes integrated under axial pressure.

Brazing is a welding method that utilizes a metal material with a lower melting point than the workpiece as the brazing filler metal. The workpiece and the brazing filler metal are heated to a temperature higher than the melting point of the brazing filler metal but lower than the melting point of the workpiece. The liquid brazing filler metal wets the workpiece, fills the interface gap, and achieves interatomic diffusion with the workpiece, thereby achieving welding. The seam formed during welding that connects two connected bodies is called the weld. Both sides of the weld undergo structural and performance changes due to the heat effect of welding, and this area is known as the heat-affected zone.

During welding, due to differences in workpiece materials, welding materials, welding currents, etc., overheating, embrittlement, hardening, or softening may occur in the weld seam and heat-affected zone after welding, which can also degrade the performance of the weldment and deteriorate weldability. This requires adjusting the welding conditions. Preheating the weldment joint before welding, maintaining heat during welding, and performing post-weld heat treatment can improve the welding quality of the weldment.