Classification of welding

Metal welding can be divided into three categories according to the characteristics of its process: fusion welding, pressure welding and brazing.

During the welding process, if the atmosphere is in direct contact with the high-temperature molten pool, the oxygen in the atmosphere will oxidize the metal and various alloy elements. Nitrogen, water vapor, etc. in the atmosphere enter the molten pool, and will also form defects such as pores, slag inclusions, and cracks in the weld during the subsequent cooling process, deteriorating the quality and performance of the weld.

In order to improve the quality of welding, people have developed various protection methods. For example, gas shielded arc welding uses argon, carbon dioxide and other gases to isolate the atmosphere to protect the arc and molten pool rate during welding; another example is when steel welding, ferrotitanium powder with a high affinity for oxygen is added to the electrode coating for deoxidation. It can protect beneficial elements such as manganese and silicon in the welding rod from oxidation and entering the molten pool, and obtain high-quality welds after cooling.

The common feature of various pressure welding methods is that pressure is applied 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. Therefore, there are no problems like burning of beneficial alloy elements and intrusion of harmful elements into the weld like fusion welding, thus simplifying the welding process. It also improves welding safety and hygiene 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 smaller. Many materials that are difficult to weld with fusion welding can often be welded with pressure welding to form high-quality joints with the same strength as the base metal.

The seam formed during welding and connecting two connected bodies is called a weld. During welding, both sides of the weld will be affected by the welding heat, resulting in structural and performance changes. This area is called the heat-affected zone. When welding, the workpiece material, welding material, welding current, etc. are different. Deterioration of weldability requires adjustment of welding conditions. Preheating of 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. The welding area 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. After welding, important products need to eliminate welding stress and correct welding deformation.

Modern welding technology can already produce welds with no internal or external defects and mechanical properties equal to or even higher than those of the connected objects. The mutual positions of the welded bodies in space are called welded joints. In addition to being affected by the quality of the weld, the strength of the joint is also related to its geometry, size, stress and working conditions. The basic forms of joints include butt joints, lap 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 joining edges. When welding thicker steel plates, various shapes of grooves are made at the joint edge for welding penetration, so that the welding rod or wire can be fed more easily. The groove forms include single-sided welding grooves and double-sided welding grooves. When selecting the groove form, in addition to ensuring weld penetration, factors such as ease of welding, small amount of filler metal, small welding deformation and low groove processing costs should also be considered.

When two steel plates with different thicknesses are butted together, in order to avoid severe stress concentration caused by sharp changes in the cross-section