First, welding

When welding, the diffusion and combination between atoms are used to make the separated metal materials firmly connected and become a whole process.

原子之间的扩散与结合，通常采用加热、加压或两者并用。可以用填充材料（或不用），将金属加热到熔化状态。

Diffusion and bonding between atoms, usually by heating, pressurization, or a combination of both. The metal can be heated to a molten state with (or without) a filler.

Second, the classification of welding methods

Arc welding

Arc welding is the process of creating an arc between the electrode and the welding body, and the heat generated by the arc will be melted by the welding metal and the electrode metal, and form a permanent joint, called arc welding.

(1) Manual arc welding

Selection of welding specifications:

A, the diameter of the commonly used electrode is 3 ~ 5mm, the thicker electrode can be used for the flat welding butt, the diameter of the vertical welding electrode should not exceed 5mm, the diameter of the vertical welding electrode should not exceed 4mm, and the diameter of the welding electrode should not exceed 3.2mm for the YI layer of multi-layer welding, to ensure that the root welding is thorough.

B, the welding current is too small, the arc is unstable, it will cause defects such as incomplete welding and slag inclusion, and the production efficiency is low. If the current is too large, the weld is easy to produce defects such as edge biting and welding penetration.

C, the arc voltage is also the working voltage, its size is determined by the arc length. If the arc is long, the arc voltage is high. If the arc is short, the arc current is low. In the welding process, the arc should not be too long, otherwise there will be unstable arc combustion, increase metal splash, reduce penetration, and produce defects such as edge biting, but also due to the intrusion of oxygen and nitrogen in the air, so that the weld will produce porosity. Therefore, the short arc should be used as far as possible, and the arc length should not exceed the electrode diameter.

Manual arc welding features:

Manual arc welding can be widely used, mainly because it has: flexible process, strong applicability, and can be applied to various positions, commonly used steel grades, and workpieces of different thicknesses. Especially for irregular welds, short welds, upward welds, high altitude and narrow position welding, more flexible.

The disadvantages of manual arc welding: low production efficiency, welding quality is affected by the level of welders, and labor intensity is large.

(2) submerged arc automatic welding

The forming process of submerged arc automatic welding seam

Submerged arc automatic welding is arc welding under the flux layer. After the arc is generated between the end of the wire and the workpiece, the flux around the arc is melted when the arc is hot, the flux in the metal and the steam at high temperature will melt the molten slag away, forming a closed space, so that the arc is isolated from the outside air, the arc continues to burn in this space, the welding wire is melted down and the molten base metal is mixed into a liquid metal melt pool, the arc continues to move forward. The molten pool is also cooled and solidified to form a weld, and the slag with a lighter gravity floats on the surface of the molten pool, cools to form a slag shell, and covers the weld metal. The molten flux protects the weld metal pool.

Advantages of submerged arc automatic welding:

A. High production efficiency. Because the welding current is large, the welding wire is melted fast, the arc penetration is strong, the weld penetration is large, the arc heat concentration is high, and the welding speed is fast, so the welding productivity is 5-10 times higher than the general manual welding.

B, good weld quality. Because the automatic welding process is stable, the protection is reliable, the adverse effect of air on the weld pool is reduced, the appearance of the weld is neat and smooth, and some defects caused by manual welding due to the technical level of the welder and the replacement of the electrode are eliminated.

C, save welding and electric energy. Because of the deep penetration of submerged arc welding, it can not open the groove or open a small groove for welding, saving the welding wire and metal nuclear energy consumed by processing and filling the groove.

D, small deformation of weldment. The heat of submerged arc welding is concentrated, the welding speed is fast, and the welding heat affected zone is small, so the welding deformation is small.

E, improve the working conditions of welders. Submerged arc welding has no harmful effect of arc light, less harmful gas, and automation reduces labor intensity.

Gas shielded arc welding

1. Characteristics of gas shielded arc welding

Gas shielded arc welding is a welding method that uses gas to separate air and molten metal machinery from oxidation and nitriding of air, and the protective gas used should not have harmful effects on molten metal. Commonly used gases are argon, carbon dioxide gas and so on.

Advantages of gas shielded arc welding:

(1) It is open arc welding, arc and molten pool are clearly visible, easy to adjust welding parameters, control welding quality.

(2) Due to the compression of the protective gas on the arc column, the arc heat is concentrated, the molten pool is small, and the crystalline block is conducive to the spatial position and thin plate welding.

(3) There is no slag in the welding process, which is easy to realize mechanization and automation, and also reduces the cost, reduces the auxiliary labor, and improves the work efficiency.

(4) When argon, helium and other inert gases are used to protect welding active metals, it has good welding quality.

2. Argon arc welding

(1) Principle of argon arc welding

Argon arc welding is an arc welding method using argon as a shielding gas.

(2) Classification of argon arc welding

(1) Argon tungsten arc welding

It is a welding method using a high melting point tungsten rod as an electrode, under the protection of argon, relying on the arc heat generated between the tungsten rod and the weldment to melt the basic metal and fill the welding wire.

(2) argon arc welding

It is a welding method using a continuous wire as an electrode, under the protection of argon, relying on the arc heat generated between the wire and the weldment to melt the basic metal and fill the wire.

3. Carbon dioxide gas welding

Carbon dioxide (CO2) gas shielded welding is an advanced welding method, automatic and semi-automatic two. It has the characteristics of fast, good and province.

Compared with other welding methods, it has advantages:

(1) High production efficiency. The current density is large, the melting rate is high, the melting is very large, there is no slag, and the slag cleaning time is saved.

(2) Low cost.

(3) Good crack resistance. CO2 gas has strong oxidation at high temperature, which can reduce the content of free hydrogen in the metal melt pool and reduce the tendency of cold cracking after welding. The sensitivity of CO2 gas shielded welding to rust is small, and the cleaning requirements of the workpiece before welding are not high.

CO2 gas shielded welding is mostly used for welding of low carbon steel and low alloy steel.