There are more than 40 kinds of metal welding methods, which are mainly divided into three categories: fusion welding, pressure welding and brazing.

Fusion welding is a method in which the interface 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 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.

In order to improve the welding quality, various protection methods have been developed. 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; another example is to add iron titanium powder with a high affinity for oxygen to the electrode coating to deoxidize when welding steel. It can protect the beneficial elements such as manganese and silicon in the electrode from oxidation and enter the molten pool, and obtain high-quality welds after cooling.

Pressure welding is to make two workpieces realize interatomic bonding in a solid state under pressure, also known as 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 during the welding process without adding filler materials. 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 seam 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.

In addition, welding is a local rapid heating and cooling process. The welding zone cannot expand and contract freely due to the constraints of the surrounding workpiece body. After cooling, welding stress and deformation will occur in the weldment. Important products need to eliminate welding stress and correct welding distortion after welding.

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

The cross-sectional shape of the butt joint weld is determined by the thickness of the welded body before welding and the groove form of the two joints. When welding thick steel plates, grooves of various shapes are made at the joint for penetration, so that the welding rod or wire can be fed in more easily. The groove forms include one-sided welding groove and two-side welding groove. When selecting the groove form, in addition to ensuring the penetration of the weld, consideration should be given to factors such as convenient welding, small amount of filler metal, small welding deformation and low processing cost of the groove.

When two steel plates with different thicknesses are butted, in order to avoid severe stress concentration caused by sharp changes in cross section, the thicker plate edges are often thinned gradually to reach the same thickness at the two joint edges. The static strength and fatigue strength of butt joints are higher than other joints. For connections that work under alternating or impact loads or in low-temperature and high-pressure vessels, butt joint welding is often preferred.

The pre-welding preparation of lap joints is simple, easy to assemble, and the welding deformation and residual stress are small, so it is often used in the installation of joints and unimportant structures on the site. Generally speaking, lap joints are not suitable for working under conditions such as alternating loads, corrosive media, and high or low temperatures.

The use of T-joints and angle joints is usually due to structural needs. The working characteristics of the incomplete fillet weld on the T-joint are similar to the fillet weld of the lap joint. When the weld is perpendicular to the direction of the external force, it becomes a front fillet weld. At this time, the surface shape of the weld will cause different degrees of stress concentration; the force of the welded fillet is similar to that of butt joints.

Corner joints have low load-carrying capacity and are generally not used alone. They can only be improved when the weld is penetrated, or when there are fillet welds inside and outside. They are mostly used at the corners of closed structures.

Welding products are lighter than riveted parts, castings and forgings, which can reduce their own weight and save energy for transportation vehicles. The welding has good sealing performance and is suitable for manufacturing various types of containers. Develop joint processing technology to combine welding with forging and casting, which can produce large, economical and reasonable cast-welded structures and forging-welded structures with high economic benefits. The use of welding technology can effectively use materials, and the welding structure can use materials with different properties in different parts, and give full play to the advantages of various materials to achieve economy and high quality. Welding has become an indispensable and increasingly important processing method in modern industry.

In modern metal processing, welding has developed later than casting and forging processes, but the development speed is very fast. The weight of welded structures accounts for about 45% of steel output, and the proportion of aluminum and aluminum alloy welded structures is also increasing.

In the future welding process, on the one hand, it is necessary to develop new welding methods, welding equipment and welding materials to further improve welding quality and safety and reliability, such as improving existing welding energy sources such as arc, plasma arc, electron beam, and laser; using electronic technology And control technology, improve the process performance of the arc, and develop a reliable and lightweight arc tracking method.

On the other hand, it is necessary to improve the level of welding mechanization and automation, such as the realization of program control and digital control of welding machines; the development of special welding machines that automate all processes from preparation procedures, welding to quality monitoring; and the promotion and expansion of CNC welding on automatic welding production lines Manipulators and welding robots can increase the level of welding production and improve the health and safety conditions of welding.

(Plastic) Welding The method of fusing two or more surfaces of a thermoplastic product into a whole by using heat and pressure or other methods.