Classification of Welding

Metal welding can be divided into three categories based on the characteristics of its process: fusion welding, pressure welding, and brazing
During the welding process, if the atmosphere comes into direct contact with a high-temperature molten pool, the oxygen in the atmosphere will oxidize the metal and various alloying elements. Nitrogen and water vapor from the atmosphere entering the molten pool can also form defects such as pores, slag inclusions, and cracks in the weld seam during subsequent cooling, deteriorating the quality and performance of the weld seam.
In order to improve welding quality, various protection methods have been developed. For example, gas shielded arc welding uses gases such as argon and carbon dioxide to isolate the atmosphere and protect the arc and molten pool rate during welding; For example, when welding steel, adding titanium iron powder with high affinity for oxygen to the electrode coating for deoxidation can protect beneficial elements such as manganese and silicon in the electrode from oxidation and entering the molten pool. After cooling, high-quality welds can be obtained.
The common feature of various pressure welding methods is to apply pressure during the welding process without adding filler material. Most pressure welding methods, such as diffusion welding, high-frequency welding, cold pressure welding, etc., do not have a melting process, so there is no problem of beneficial alloy element burnout and harmful element invasion into the weld seam like fusion welding, which simplifies the welding process and improves welding safety and hygiene conditions. Meanwhile, due to the lower heating temperature and shorter heating time compared to fusion welding, the heat affected zone is smaller. Many materials that are difficult to melt weld can often be pressure welded into high-quality joints with the same strength as the base material.
The joint formed during welding that connects two connected bodies is called a weld seam. The two sides of the weld seam are subjected to welding heat during welding, resulting in changes in microstructure and properties. This area is called the heat affected zone. Due to differences in workpiece material, welding current, and other factors during welding. To deteriorate weldability, it is necessary to adjust the welding conditions. Preheating at the interface of the weldment before welding, insulation 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, and the welding area cannot expand and contract freely due to the constraints of the surrounding workpiece body. After cooling, welding stress and deformation occur in the welded part. Important products need to eliminate welding stress and correct welding deformation after welding.
Modern welding technology can produce welds without internal or external defects, with mechanical properties equal to or even higher than those of the connected body. The mutual position of the welded body in space is called a welded joint, and the strength at the joint is not only affected by the quality of the weld seam, but also by its geometric shape, size, stress situation, and working conditions. The basic forms of joints include butt joint, lap joint, T-joint (positive joint), and corner joint.
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 joint edges. When welding thicker steel plates, various shapes of grooves are cut at the joint to facilitate the insertion of welding rods or wires. The groove forms include single-sided welding groove and double-sided welding groove. When choosing the groove form, in addition to ensuring full penetration, factors such as easy welding, less filler metal, small welding deformation, and low groove processing costs should also be considered.
When two steel plates with different thicknesses are joined together, in order to avoid severe stress concentration caused by sharp changes in cross-section, the thicker plate edge is often gradually thinned to achieve equal thickness at the two joint edges. The static strength and fatigue strength of the docking joint are higher than those of other joints. Welding of butt joints is often preferred for connections working under alternating, impact loads, or in low-temperature and high-pressure vessels.
The pre welding preparation work for lap joints is simple, easy to assemble, and the welding deformation and residual stress are small. Therefore, it is often used in the installation of joints and unimportant structures on construction sites. Generally speaking, lap joints are not suitable for working under conditions such as alternating loads, corrosive media, high or low temperatures.
The use of T-joints and corner joints is usually due to structural requirements. The working characteristics of fillet welds on T-joints that are not fully penetrated are similar to those of fillet welds on lap joints. When the weld seam is perpendicular to the direction of external force, it becomes a positive angle weld seam, and the surface shape of the weld seam will cause varying degrees of stress concentration; The stress situation of the welded fillet weld is similar to that of the butt joint.
Corner joints have low load-bearing capacity and are generally not used separately. They are only improved when welded through or when there are corner welds both inside and outside. They are mostly used at the corners of enclosed structures.
Welded products are lighter in weight 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. Developing joint processing technology, combining welding with forging and casting, can produce large and economically reasonable cast welded and forged welded structures with high economic benefits. The use of welding technology can effectively utilize materials, and welding structures can use materials with different properties in different parts, fully utilizing the strengths of various materials to achieve economy and high quality. Welding has become an indispensable and increasingly important processing technique in modern industry.
In modern metal processing, welding developed later than casting and forging processes, but its development speed was very fast. The weight of welded structures accounts for about 45% of steel production, and the proportion of aluminum and aluminum alloy welded structures is also increasing.
The future welding technology requires the development of new welding methods, welding equipment, and welding materials to further improve welding quality and safety reliability, such as improving existing welding energy sources such as arc, plasma arc, electron beam, laser, etc; By utilizing electronic and control technologies, we aim to improve the process performance of electric arcs and develop reliable and lightweight arc tracking methods.
On the other hand, we need to improve the level of welding mechanization and automation, such as implementing program control and digital control for welding machines; Develop a specialized welding machine that automates the entire process from preparation, welding, to quality monitoring; Promoting and expanding CNC welding robotic arms and welding robots on automatic welding production lines can improve the level of welding production and enhance welding hygiene and safety conditions.