Welding is a local rapid heating and cooling process, the welding area can not expand and contract freely due to the constraints of the surrounding workpiece body, and welding stress and deformation will occur in the weldment after cooling. Important products need to eliminate welding stress and correct welding deformation after welding.
Modern welding technology has been able to weld welds without internal and external defects and with mechanical properties equal to or even higher than the joined body. The mutual position of the welded body in space is called the welded joint, and 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 are butt, lap, T-joint (orthogonal) and angle joint.
The cross-sectional shape of the butt joint weld depends on the thickness of the welded body before welding and the groove form of the two joints. When welding thicker steel plates, grooves of various shapes are made at the edges for penetration so that electrodes or wires can be fed more easily. The groove form has a single-sided welded groove and a two-sided welded groove. When choosing the groove form, in addition to ensuring welding penetration, factors such as convenient welding, less filler metal, small welding deformation and low groove processing costs should also be considered.
When two steel plates with different thicknesses are docked, in order to avoid serious stress concentration caused by rapid changes in cross-section, the thicker plate edge is often gradually thinned to achieve equal thickness at the two joint edges. The static and fatigue strength of butt joints is higher than other joints. For joints operating under alternating or shock loads or in cryogenic high-pressure vessels, butt joint welding is often preferred.
The pre-welding preparation of lap joints is simple, the assembly is convenient, and the welding deformation and residual stress are small, so they are often used in site installation joints and unimportant structures. In general, lap joints are not suitable for operation under alternating loads, corrosive media, high or low temperatures, etc.
The use of T-joints and corner joints is usually due to structural needs. The working characteristics of the unpenetrated 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 frontal fillet weld, and the surface shape of the weld will cause different degrees of stress concentration; The force situation of the fillet weld weld through is similar to that of the butt joint.
The angle joint has a low bearing capacity, generally not used alone, only when welded through, or when there are fillet welds inside and outside, it is improved, and it is mostly used at the corners of closed structures.
Welded products are lighter than rivets, castings and forgings, which can reduce their own weight and save energy for means of transportation. The sealing of welding is good, suitable for the manufacture of various types of containers. The development of joint processing technology, so that welding combined with forging, casting, can be made into large, economical and reasonable casting and welding structure and forging and welding structure, economic benefits are very high. The use of welding process can effectively use materials, welding structure can use different performance materials in different parts, give full play to the characteristics of various materials, to achieve economy, high quality. Welding has become an indispensable and increasingly important processing method in modern industry.
In modern metal processing, welding developed later than casting and forging processes, but the development speed is 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. 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 the welding quality and safety and reliability, such as improving the existing arc, plasma arc, electron beam, laser and other welding energy; The use of electronic technology and control technology to 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 welding machine to achieve program control and digital control; Development of special welding machines that automate all processes from preparation process, welding to quality control; In the automatic welding production line, the promotion and expansion of CNC welding manipulators and welding robots can improve the level of welding production and improve the health and safety conditions of welding.
