What Is Fusion Welding?

The so-called fusion welding means that the welding joint is melted under the action of high temperature during the welding process. Because the workpieces to be welded are closely attached, under the action of temperature field and gravity, without pressure, the molten melt of the two workpieces will be mixed. After the temperature is lowered, the melting part is coagulated, and the two workpieces are firmly welded together to complete the welding method. ^[1-2]

Fusion welding , also known as fusion welding , is one of the most common

1. During the welding process, a large amount of gas is filled in the welding area.

When welding with an acid electrode, the main gas components are CO, H2, H2O; when welding with an alkaline electrode, the main gas components are CO, CO2; when submerged arc welding, the main gas components are CO, H2.

The gas in the welding area mainly comes from the following aspects: First, in order to protect the welding area from the invasion of air, artificially add a layer of protective gas in the welding area, such as the gas generator (starch, wood flour, marble in the coating) Etc.) The gas produced by thermal decomposition, the shielding gas (CO2 gas, Ar gas) used in gas shielded welding, etc .; the second is the gas that is released when welding with a wet electrode or flux, the air that is inadequately protected and penetrates, the welding wire and Impurities (oil stains, rust, paint, etc.) on the surface of the base material are generated by heating, and gases generated by high temperature evaporation of metals and slag.

2.The effect and influence of nitrogen, hydrogen and oxygen on the weld metal

The nitrogen is mainly from the air around the welding area. In hand arc welding, the surfacing metal contains about 0.025% nitrogen. Nitrogen is an element that increases the strength of the weld metal, reduces plasticity and toughness, and is one of the main reasons for the occurrence of pores in the weld.

Tritium hydrogen mainly comes from the skin of the electrode, the moisture in the flux, the organic matter in the skin, the dirt (rust, oil) on the surface of the welding piece and the wire, and the moisture in the air. Various welding methods increase the hydrogen content of the weld, but the degree of hydrogen increase is different: the hydrogen content of the weld obtained by hand welding with cellulose covered electrodes is 70 times higher than the base metal; only low-hydrogen type electrodes are used When welding, the hydrogen content of the weld is relatively low; when using CO2 gas shielded welding, the hydrogen content is the lowest.

Hydrogen severely reduces the plasticity of the weld metal, promotes porosity and time-lapse cracks in the weld joint, and also forms white spots on the fracture surface of the tensile specimen.

(3) Oxygen mainly comes from the oxides in the air, skin and flux, and the oxides on the surface of the welding material. With the increase of the oxygen content in the weld, its strength, hardness and plasticity will obviously decrease, it can also cause hot brittleness, cold brittleness and aging hardening of the metal, and it is also the main reason for the formation of pores (CO pores) in the weld. One.

In short, nitrogen, hydrogen, and oxygen entering the weld metal are all harmful elements.

3.Protect the welding area

Alloying is the transition of the required alloying elements into the weld metal (or surfacing metal) through the welding material.

The phenomenon of uneven distribution of each constituent element in the alloy during crystallization is called segregation. During the primary crystallization process of the welding molten pool, due to the fast cooling rate, the chemical components in the solidified weld metal have no time to diffuse, resulting in uneven distribution and segregation.

There are three types of segregation in the weld:

Welding

Microsegregation

When the molten pool is crystallized once, the first crystallized center metal is the purest, the later crystallized part contains other alloying elements and impurities slightly higher, and the last crystallized part, that is, the outer end and leading edge of the crystal, contains the other alloying elements and impurities the highest. The uneven distribution of chemical composition inside a columnar grain and between grains is called microsegregation.

Regional segregation

During the primary crystallization of the molten pool, due to the continuous growth and progress of columnar crystals, impurities will be "hurried" towards the center of the molten pool, so that the impurity content in the center of the molten pool is more than other parts. This phenomenon is called regional segregation. The shape of the weld section has a great influence on the distribution of regional segregation. For narrow and deep welds, the boundary of each columnar crystal is at the center of the weld, so there are more impurities in the center of the weld. Such a weld seam is prone to thermal cracking at its center. For wide and shallow welds, impurities accumulate in the upper part of the weld, which has a high resistance to thermal cracking.

layered segregation

During the crystallization of the molten pool, the latent heat of crystallization should be continuously released. When the latent heat of crystallization reaches a certain value, the crystallization of the molten pool will temporarily stop. In the future, with the heat dissipation of the molten pool, the crystallization restarts, forming periodic crystals. With the periodic fluctuation of the impurity concentration in the liquid metal at the leading edge of the crystal, periodic segregation is called layered segregation. Layered segregation concentrates some harmful elements, so defects often appear in layered segregation. Stomata caused by layered segregation. ^[1]