In the 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 alloy elements. Nitrogen and water vapor in the atmosphere enter the molten pool, and defects such as porosity, slag inclusion and cracks will also be formed in the weld during the subsequent cooling process, which will worsen the quality and performance of the weld.

In order to improve the welding quality, people have developed various protection methods. For example, gas shielded arc welding is to isolate the atmosphere with argon, carbon dioxide and other gases to protect the arc and molten pool rate during welding; For example, when welding steel, adding titanium iron powder with high affinity for oxygen to covered electrode coating for deoxidation can protect beneficial elements manganese and silicon in covered electrode from being oxidized and entering the molten pool, and obtain high-quality weld after cooling.

Pressure welding is to combine two workpieces in 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 connection end of two workpieces, the temperature there rises due to the large resistance, and when it is heated to a plastic state, it is connected into a whole under the action of axial pressure.

The common feature of various pressure welding methods is that pressure is applied during welding without filling materials. Most pressure welding methods, such as diffusion welding, high frequency welding and cold pressure welding, have no melting process, so there is no problem of beneficial alloy elements burning and harmful elements invading the weld, which simplifies the welding process and improves the welding safety and hygiene conditions. At the same time, because the heating temperature is lower than that of fusion welding, the heating time is short and the heat affected zone is small. Many materials that are difficult to weld by fusion welding can often be welded by pressure welding into high-quality joints with the same strength as the parent metal.

Brazing is a method that uses a metal material with a melting point lower than that of the workpiece as a brazing filler metal, heats the workpiece and the brazing filler metal to a temperature higher than or lower than the melting point of the workpiece, wets the workpiece with liquid brazing filler metal, fills the interface gap, and realizes mutual diffusion between atoms with the workpiece, thus realizing welding.

The weld formed when welding two connectors is called weld. When welding, both sides of the weld will be affected by welding heat, and the structure and properties will change. This area is called the heat affected zone. When welding, due to the difference of workpiece material and welding current, etc. After welding, the weld seam and heat affected zone may be overheated, brittle, hardened or softened, which will also reduce the performance of the weldment and worsen the weldability. Therefore, it is necessary to adjust the welding conditions. Preheating the interface of weldment before welding, keeping temperature during welding and heat treatment after welding can improve the welding quality of weldment.

In addition, welding is a local rapid heating and cooling process. Due to the constraints of the surrounding workpiece body, the welding zone cannot expand and contract freely, and the cooled weldment will produce welding stress and deformation. Important products need to eliminate welding stress and correct welding deformation after welding.

Modern welding technology has been able to weld welds with mechanical properties equal to or even higher than those of the connected body, and there are no internal and external defects. The mutual position of welded bodies in space is called welded joint, and the strength of the joint is not only affected by the quality of weld, but also related to its geometry, size, stress and working conditions. The basic forms of joints are butt joint, lap joint, T-joint (positive joint) and angle joint.

The cross-sectional shape of butt weld depends on the thickness of the welded body before welding and the groove form of the two edges. When welding thick steel plates, grooves of various shapes are made at the joints for penetration, so that covered electrode or welding wire can be easily fed. Groove forms include single-sided welding groove and double-sided welding groove. When choosing the groove form, in addition to ensuring the penetration depth, factors such as convenient welding, less metal filling, small welding deformation and low groove processing cost should also be considered.

When two steel plates with different thicknesses are butted, in order to avoid serious stress concentration caused by the sharp change of cross section, the thicker plate edges are often gradually thinned to reach the same thickness at the two butted edges. The static strength and fatigue strength of butt joint are higher than those of other joints. Butt welding is usually the first choice for connections working under alternating and impact loads or in low temperature and high pressure vessels.

Lap joint is easy to prepare before welding, easy to assemble, with small welding deformation and residual stress, and is often used for on-site installation of joints and unimportant structures. Generally speaking, lap joints are not suitable for working under alternating load, corrosive medium, high temperature or low temperature.

T-joints and corner joints are usually used for structural needs. The working characteristics of incomplete fillet weld on T-joint are similar to those of lap joint. When the weld is perpendicular to the direction of external force, it becomes a frontal fillet weld, and the surface shape of the weld will cause stress concentration to varying degrees. The stress of penetration fillet weld is similar to that of butt joint.

The bearing capacity of fillet joint is low, so it is generally not used alone. It can only be improved by full welding, or when there are fillet welds inside and outside, and it is mostly used at the corner of closed structure.

Welded products are lighter than riveted parts and cast forgings, which can reduce their own weight and save energy for transportation tools. The welding has good sealing performance and is suitable for manufacturing various containers. With the development of combined machining technology, welding, forging and casting can be combined to manufacture large-scale, economical and reasonable cast-welded structures and forged-welded structures, which has high economic benefits. The welding process can effectively use materials, and the welding structure can use materials with different properties in different parts, giving full play to the advantages of various materials and achieving economic and high quality. Welding has become an indispensable and increasingly important processing method in modern industry.