Welding, also known as welding, is a manufacturing process and technology for joining metals or other thermoplastic materials such as plastics by heating, high temperature, or high pressure.
There are many energy sources for modern welding, including gas flames, electric arcs, lasers, electron beams, friction and ultrasonic waves. In addition to being used in factories, welding can also be carried out in a variety of environments, such as the field, underwater and space. No matter where it is, welding may bring danger to the operator, so appropriate protective measures must be taken when welding. The possible harm to the human body caused by welding includes burns, electric shock, visual impairment, inhalation of toxic gases, excessive ultraviolet radiation, etc.
Introduction to welding
Welding: usually refers to the welding of metals. It is a forming method in which two separate objects are combined into one body by heating or pressurizing, or both.
Classification: According to the heating degree and process characteristics of the welding process, welding methods can be divided into three categories.
(1) Fusion welding. The welding part of the workpiece is locally heated to a molten state to form a molten pool (usually adding filler metal), and the weld is formed after cooling and crystallization, and the welded workpiece is combined into an inseparable whole. Common fusion welding methods include gas welding, arc welding, electroslag welding, plasma arc welding, electron beam welding, laser welding and so on.
(2) Pressure welding. In the welding process, whether it is heated or not, a pressurized welding method is required. Common pressure welding includes resistance welding, friction welding, cold pressure welding, diffusion welding, explosive welding and so on.
(3) Brazing. After melting with a solder (filler metal) whose melting point is lower than that of the metal to be welded, the joint gap is filled and interdiffused with the metal to be welded to achieve connection. During the brazing process, the welded workpiece does not melt and generally has no plastic deformation.
Characteristics of welding production:
(1) Save metal materials and light structure.
(2) Combining small and large to small, manufacturing heavy and complex machine parts, simplifying casting, forging and cutting processes, and obtaining the best technical and economic effects.
(3) The welded joint has good mechanical properties and airtightness.
(4) The bimetal structure can be manufactured, so that the performance of the material can be fully utilized.
Application: Welding technology is widely used in machine manufacturing, shipbuilding industry, construction engineering, power equipment production, aviation and aerospace industry, etc.
Insufficiency: Welding technology also has some shortcomings, such as the welding structure cannot be disassembled, which brings inconvenience to maintenance; welding stress and deformation will exist in the welding structure; the structure and performance of the welded joint are often uneven, and welding defects will occur.
1. Electrode arc welding:
Principle——The arc welding method of welding with manual welding rod. The stable burning arc established between the welding rod and the weldment is used to melt the welding rod and the weldment to obtain a firm welded joint. It belongs to gas-slag joint protection.
Main features-flexible operation; low assembly requirements to be welded; wide range of weldable metal materials; low welding productivity; strong dependence on weld quality (depending on the welder’s operating skills and on-site performance).
Application-widely used in shipbuilding, boilers and pressure vessels, machinery manufacturing, building structures, chemical equipment and other manufacturing and maintenance industries. Suitable for welding of various metal materials, various thicknesses, and various structural shapes (in the above-mentioned industries).
2. Submerged arc welding (automatic welding):
Principle-The arc burns under the flux layer. The heat generated by the arc burning between the welding wire and the weldment is used to melt the welding wire, flux and the base material (weldwork) to form a weld. Belongs to slag protection.
Main features-high welding productivity; good weld quality; low welding cost; good labor conditions; difficult to weld in space; high quality requirements for weldment assembly; not suitable for welding thin plates (when welding current is less than 100A, arc stability Bad) and short welds.
Application-widely used in shipbuilding, boilers, bridges, hoisting machinery and metallurgical machinery manufacturing. Submerged arc welding can be used for all weldments whose welds can be kept in a horizontal position or whose inclination angle is not large. The thickness of the board must be greater than 5 mm (to prevent burn through). Welding carbon structural steel, low-alloy structural steel, stainless steel, heat-resistant steel, composite steel, etc.
3. Carbon dioxide gas shielded welding (automatic or semi-automatic welding):
Principle: A molten electrode arc welding method using carbon dioxide as a shielding gas. It is gas protection.
Main features-high welding productivity; low welding cost; small welding distortion (centralized arc heating); high welding quality; simple operation; high spatter rate; difficult to weld with AC power; poor wind resistance; no welding of colored materials that are easy to oxidize metal.
Application-Mainly for welding low carbon steel and low alloy steel. Suitable for various thicknesses. It is widely used in automobile manufacturing, locomotive and vehicle manufacturing, chemical machinery, agricultural machinery, mining machinery and other sectors.
4. MIG/MAG welding (Melting extremely inert gas/active gas shielded welding):
Principle of MIG welding-an arc welding method using inert gas as shielding gas and welding wire as melting electrode.
The shielding gas is usually argon or helium or their mixture. MIG uses inert gas, and MAG adds a small amount of active gas, such as oxygen, carbon dioxide, etc., to the inert gas.
Main features-good welding quality; high welding productivity; no deoxidation and dehydrogenation reaction (easy to form welding defects, especially strict requirements for surface cleaning of welding materials); poor wind resistance; complex welding equipment.
Application-Almost all metal materials can be welded, mainly used for welding non-ferrous metals and their alloys, stainless steel and some alloy steels (too expensive). The thinnest thickness is about 1 mm, and the maximum thickness is basically unlimited.
5. TIG welding (tungsten inert gas shielded welding)
Principle——Under the protection of inert gas, the base metal and filler wire are melted by the arc heat generated between the tungsten electrode and the weldment (or without filler wire) to form a welding seam. The electrode does not melt during welding.
Main features-strong adaptability (stable arc, no spatter); low welding productivity (poor current carrying capacity of the tungsten electrode (anti-melting and evaporation of the tungsten electrode, anti-welding tungsten)); high production cost.
Application——Almost all metal materials can be welded. It is commonly used for welding stainless steel, high temperature alloys, aluminum, magnesium, titanium and their alloys, refractory active metals (zirconium, tantalum, molybdenum, niobium, etc.) and different metals. Welding thickness is generally less than 6 mm weldment, or the bottom welding of thick parts. The use of small-angle grooves (narrow groove technology) can realize automatic TIG welding of narrow gaps with a thickness of 90mm or more.
6. Plasma arc welding
Principle-With the help of the restraining effect of the water-cooled nozzle on the arc, a high-energy-density plasma arc welding method is obtained.
Main features (compared with argon arc welding)——⑴Concentrated energy and high temperature. For most metals, a small hole effect can be obtained within a certain thickness range, and a weld with full penetration and uniform formation on the reverse side can be obtained. ⑵The arc stiffness is good, the plasma arc is basically cylindrical, and the arc length change has a relatively small effect on the heating area and current density on the weldment. Therefore, the change of the arc length of plasma arc welding has no obvious influence on the weld formation. ⑶The welding speed is faster than argon arc welding. ⑷Able to weld thinner and thinner processed parts. ⑷The equipment is complicated and the cost is high.