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What are the classification of welding and why

Back to list Source: Junye Browse: Date of issue:2021-01-19 14:57【B M S
There are more than 40 kinds of metal welding methods. According to the state of the metal in the welding process, the welding methods can be divided into three categories: fusion welding, pressure welding and brazing.
 
Fusion welding
 
It is a method to heat the workpiece interface to the melting state in the welding process and complete the welding without pressure. During fusion welding, the heat source rapidly heats and melts the interface between the two workpieces 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 a whole.
 
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. When nitrogen and steam in the atmosphere enter the weld pool, some defects such as porosity, slag inclusion and cracks will be formed in the weld during the subsequent cooling process, which will deteriorate the quality and performance of the weld.
 
In order to improve the welding quality, people have developed a variety of protection methods. For example, gas shielded arc welding is to use argon, carbon dioxide and other gases to isolate the atmosphere, so as to protect the arc and molten pool rate during welding; another example is steel welding, adding titanium iron powder with high affinity for oxygen in the electrode coating for deoxidation, which can protect the beneficial elements such as manganese and silicon from oxidation and enter the molten pool, and obtain high-quality weld after cooling.
 
Pressure welding
 
It is under the condition of pressure, so that two workpieces in the solid state to achieve atomic bonding, 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 two workpieces, the temperature rises due to the high resistance. When heated to the plastic state, the connection becomes one under the action of axial pressure.
 
The common feature of all kinds of pressure welding methods is to apply pressure in the welding process without adding filler material. Most pressure welding methods, such as diffusion welding, high frequency welding and cold pressure welding, have no melting process, so there is no such problems as burning loss of beneficial alloy elements and penetration of harmful elements into the weld as fusion welding, so as to simplify the welding process and improve the safety and health conditions of welding. 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 smaller. For many materials that are difficult to be welded by fusion welding, pressure welding can be used to form high-quality joints with the same strength as the base metal.
 
Brazing
 
It is a method of using metal material with lower melting point than the workpiece as solder, heating the workpiece and solder to a temperature higher than or lower than the melting point of solder, wetting the workpiece with liquid solder, filling the interface gap and realizing mutual diffusion between atoms and workpiece, so as to realize welding.
 
The joint formed during welding to connect two connected bodies is called weld. During welding, both sides of the weld will be affected by welding heat, and the microstructure and properties will change. This area is called heat affected zone. During welding, due to the difference of workpiece material, welding material and welding current, overheating, embrittlement, hardening or softening may occur in the weld and heat affected zone after welding, which also makes the weldment performance decline and worsens the weldability. Therefore, it is necessary to adjust the welding conditions. Preheating the interface of weldment before welding, heat preservation 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. The welding area can not expand and contract freely due to the restraint of the surrounding parts. After cooling, welding stress and deformation will occur in the weldment. Important products need to eliminate welding stress and correct welding deformation after welding.
 
Modern welding technology has been able to weld the weld without internal and external defects, whose mechanical properties are equal to or even higher than the connected body. The mutual position of the welded body in space is called welded joint. The strength of the joint is not only affected by the quality of the weld, but also related to its geometry, size, stress and working conditions. The basic forms of joints are butt joint, lap joint, T-joint (forward joint) and corner joint.
 
The cross section shape of butt joint depends on the thickness of the welded body before welding and the groove form of the two edges. When welding thick steel plate, in order to weld through, the groove of various shapes is made at the edge, so that the welding rod or welding wire can be easily put in. There are two types of groove: one side welding groove and two sides welding groove. When selecting the groove form, in addition to ensuring the penetration, the factors such as convenient welding, less filler metal, small welding deformation and low groove processing cost should also be considered.
 
When two steel plates with different thickness butt joint, in order to avoid the serious stress concentration caused by the sharp change of cross section, the thicker plate edge is often gradually thinned to achieve the same thickness at the two joint edges. The static strength and fatigue strength of butt joint are higher than other joints. Butt joint welding is often preferred for joints working under alternating and impact loads or in low temperature and high pressure vessels.