Development of welding process

There are more than 40 metal welding methods, which are mainly divided into three categories: welding, pressure welding and brazing. Fusion welding is a method in which the workpiece interface 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 joints of the two workpieces to be welded to form a molten pool. The molten pool moves forward with the heat source, and after cooling, forms a continuous weld to join the two workpieces into one body. During the fusion welding process, if the atmosphere is in direct contact with the high temperature molten pool, oxygen in the atmosphere oxidizes the metal and various alloying elements. Nitrogen, water vapor, etc. in the atmosphere enter the molten pool, and defects such as pores, slag inclusions, cracks, etc. are formed in the weld during the subsequent cooling process, which deteriorates the quality and performance of the weld.

In order to improve the quality of welding, various protection methods have been developed. For example, gas-shielded arc welding is to insulate the atmosphere with a gas such as argon or carbon dioxide to protect the arc and the molten pool rate during welding. For example, when the steel is welded, the ferrotitanium powder with a large affinity for oxygen is added to the electrode coating for deoxidation. It can protect the beneficial elements of the electrode, such as manganese and silicon, from oxidation and enter the molten pool, and obtain high-quality welds after cooling.

Pressure welding is to achieve atomic bonding in the solid state under pressure conditions, also known as solid state welding. The commonly used pressure welding process is resistance butt welding. When the current passes through the connection end of the two workpieces, the temperature rises due to the large resistance, and when heated to the plastic state, the connection is integrated under the axial pressure.

A common feature of various pressure bonding methods is the application of pressure during the welding process without the addition of filler material. Most pressure welding methods, such as diffusion welding, high-frequency welding, cold-welding, etc., do not have a melting process, so there is no problem of burning of beneficial alloying elements like welding and intrusion of harmful elements into the weld, thereby simplifying the welding process. Improved safety and hygiene conditions for welding. At the same time, since the heating temperature is lower than the fusion welding and the heating time is short, the heat affected zone is small. Many materials that are difficult to weld by fusion welding can often be welded to a high quality joint of the same strength as the base material.

Brazing is to use a metal material with a lower melting point than the workpiece as a brazing material to heat the workpiece and the brazing material to a temperature higher than the melting point of the brazing material and lower than the melting point of the workpiece. The workpiece is wetted by the liquid brazing material, filling the interface gap and realizing with the workpiece. The method of mutual diffusion between atoms to achieve welding.

The seam formed by welding when joining two connected bodies is called a weld bead. Both sides of the weld are subjected to welding heat during welding, and changes in structure and properties occur. This area is called the heat affected zone. Due to different welding materials and welding currents of the workpiece during welding, overheating, embrittlement, hardening or softening may occur in the weld and heat affected zone after welding, and the weldment performance may be degraded and the weldability may be deteriorated. This requires adjustment of the welding conditions, pre-heating of the weldment interface before welding, welding heat preservation and post-weld heat treatment can improve the welding quality of the weldment.

In addition, the welding is a local rapid heating and cooling process. The welding zone cannot be freely expanded and contracted due to the restraint of the surrounding workpiece body, and the welding stress and deformation are generated in the weldment after cooling. Important products need to eliminate welding stress after welding and correct welding deformation.

Modern welding techniques have been able to weld welds with no internal or external defects and mechanical properties equal to or even higher than the welded joints. The mutual position of the welded body in space is called the welded joint, and the strength of the joint is affected by the quality of the weld, and also related to its geometry, size, stress and working conditions. The basic forms of the joints are 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 joints. When welding a thick steel plate, grooves of various shapes are opened at the joint edges for penetration to facilitate feeding of the welding rod or the welding wire. The groove form has a single-sided welded groove and a double-sided welded groove. When selecting the groove form, in addition to ensuring the penetration, the welding should be considered, the amount of filler metal is small, the welding deformation is small, and the processing cost of the groove is low. When two steel plates with different thicknesses are butt jointed, in order to avoid severe stress concentration caused by sharp changes in the cross section, the thicker plate edges are often thinned to the same thickness at the two joint edges. The static strength and fatigue strength of the butt joint are higher than other joints. Welding of butt joints is often preferred over joints that operate under alternating, impact, or low temperature, high pressure vessels. The pre-weld preparation of the lap joint is simple, the assembly is convenient, the welding deformation and the residual stress are small, so it is often used in the installation of joints on the construction site and unimportant structures. In general, lap joints are not suitable for operation under alternating loads, corrosive media, high temperatures or low temperatures.

The use of T-joints and corner joints is usually due to structural requirements. The work characteristics of the fillet welds that are not welded on the T-joint are similar to the fillet welds 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 shape of the weld surface will cause different degrees of stress concentration; the weld of the fillet weld is similar to that of the butt joint.

The angle joints have low load carrying capacity and are generally not used alone. They are only improved when they are welded through, or when there are fillet welds inside and outside. They are mostly used at the corners of closed structures.

Welded products are lighter in weight than rivets, castings and forgings, saving weight and saving energy for transportation. The welding has good sealing properties and is suitable for manufacturing various types of containers. The development of joint processing technology, combining welding with forging and casting, can be made into a large, economically reasonable cast-welded structure and forged welded structure with high economic benefits. The welding process can effectively utilize the materials, and the welded structure can adopt different performance materials in different parts, and fully exert the special features of various materials to achieve economic and high quality. Welding has become an indispensable and increasingly important processing method in modern industry.
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