Laser welding equipment uses high-energy pulse laser focusing to melt the target metal for welding between workpieces.
The effect of this new welding technology is very good, but if it is not operated properly, it will also cause hot cracks or cold cracks. So how does the laser welding machine produce welding cracks?
The unbalanced rapid heating and rapid cooling of laser welding makes the whole joint in a complex stress state, which constitutes the mechanical factor of joint cracking;
Laser welding is the synthesis of a series of unbalanced processes. In the process of rapid metallurgical solidification, there will inevitably be uneven component distribution and hardened structure with low crack resistance, which constitutes the metallurgical factors promoting crack initiation.
There are three kinds of cracks produced in the welding process of laser welding machine: product crack, liquefaction crack and reheat like crack.
The crystal crack is related to the non-uniformity of the composition in the macro region of the weld.
The central area of the weld is the place where the liquid phase crystallization is the latest. The columnar crystals on both sides of the weld meet here.
At the same time, a large number of impurities with low melting point also accumulate here to form centerline segregation, which reduces the bonding strength at this place.
Under certain mechanical conditions, cracks occur at this position.
After cracks occur, we need to find and solve them in time. So how to reduce the occurrence of welding cracks in laser welding machine?
There are two main measures:
Changing parameters of laser welding machine
The main causes of thermal cracks during pulse laser welding are: during solidification after laser welding, the weld passes through the temperature zone with low plasticity, which is the brittle temperature zone.
The crystallization of weld metal is carried out under the action of tensile stress.
This tensile stress is formed because the welded metal is difficult to shrink.
The existence of tensile stress causes the elastic-plastic deformation of the weld.
If the weld metal of welding equipment is in the brittle temperature zone and the plastic deformation exceeds the plasticity of the metal, cracks will be formed.
Reasonably improve the alloy system of the material according to the nature of the crack
Causes of cold crack: during the cooling process of pulse laser welding, martensite structure is formed near the seam, and the stress state of welding stress and microstructure stress is also formed.
The metal with martensitic structure has high strength and low plasticity, and there is residual stress in the cooling process of welding metal.
The stress state of welded joints of ferritic or austenitic quenched steel is characterized by the formation of longitudinal compressive stress in the near weld area, which is caused by martensitic transformation in this area during welding.
These kinds of stresses will produce cold cracks.
Causes of thermal crack:
The generation of thermal crack during laser welding depends on the chemical composition of weld metal and near weld area, the conditions and characteristics of crystallization process, the expansion degree of chemical and physical micro heterogeneity, the size of deformation and the growth rate of deformation.
At present, the mechanism of crystal crack formation is described as follows:
In the solidification process of plate bending machine, the weld of laser welding machine passes through the temperature zone with low plasticity, which is the brittle temperature zone.
The crystallization of weld metal is carried out under the action of tensile stress. This tensile stress is formed because the welded metal is difficult to shrink.
The existence of tensile stress causes the elastic-plastic deformation of weld. If the weld metal of welding equipment is in the brittle temperature zone, the plastic deformation exceeds the plasticity of the metal, and a crack is formed.