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What Are The Main Parameters of Laser Deep-penetration Welding (3)

Aug 03, 2020

Protective gas

       In the laser welding process, inert gas is often used to protect the molten pool. When some materials are welded, the surface oxidation may not be considered, but the protection may not be considered. However, for most applications, helium, argon, nitrogen and other gases are often used as protection to make the workpiece in Avoid oxidation during welding.

  Helium is not easy to ionize (higher ionization energy), which allows the laser to pass smoothly, and the beam energy reaches the surface of the workpiece without hindrance. This is the most effective shielding gas used in laser welding, but it is more expensive.

   Argon is cheaper and denser, so the protection effect is better. However, it is susceptible to high-temperature metal plasma ionization, which shields part of the beam from being directed to the workpiece, reduces the effective laser power for welding, and also damages the welding speed and penetration. The surface of the weldment protected by argon is smoother than when protected by helium.

   Nitrogen is the cheapest shielding gas, but it is not suitable for welding certain types of stainless steel, mainly due to metallurgical problems, such as sometimes producing pores in the overlap area.

   The second function of using shielding gas is to protect the focusing lens from metal vapor contamination and splashing of liquid droplets. Especially in high-power laser welding, because the ejected material is very powerful, it is more necessary to protect the lens at this time.

   The third role of shielding gas is to dissipate the plasma shielding produced by high-power laser welding. The metal vapor absorbs the energy of the laser beam and ionizes into a plasma cloud, and the shielding gas around the metal vapor is also ionized by heating. If there is too much plasma, the laser beam energy is consumed by the plasma to some extent. Plasma exists on the surface of the workpiece as a second source of energy, which makes the penetration depth shallow and the surface of the weld pool wider. Increase the electron recombination rate by increasing the three-body collision of electrons with neutral atoms to reduce the electron density in the plasma. The lighter the neutral atom, the higher the collision frequency and the higher the recombination rate. On the other hand, only the shielding gas with high ionization energy will not increase the electron density due to the ionization of the gas itself.

Material

Helium

Argon

Nitrogen

Aluminum

Magnesium

Iron

Atomic (molecule) mass

4

40

28

27

24

56

Ionization energy (eV)

24.46

15.68

14.5

5.96

7.61

7.83

 Table 1. Atomic (molecule) mass and ionization energy of several gases and metals

       It can be seen from the table that the plasma size is related to the different shielding gas used, helium is the smallest, followed by nitrogen, and the largest when using argon. The larger the plasma size, the shallower the penetration depth. The degree of ionization and gas density make the difference in plasma size.

      Helium has the least ionization and density. It can quickly drive out the rising metal vapor generated from the molten metal pool. Therefore, the use of helium as a shielding gas can suppress the plasma to the greatest extent, thereby increasing the penetration depth and increasing the welding speed; because of its light weight, it can escape, and it is not easy to cause pores. Of course, from the actual welding effect, the effect of argon protection is not bad. The influence of plasma cloud on penetration is most obvious in the low welding speed zone. When the welding speed increases, its influence will be weakened.

  The shielding gas is ejected through the nozzle opening with a certain pressure to reach the surface of the workpiece. The hydrodynamic shape of the nozzle and the diameter of the outlet are very important. It must be large enough to drive the sprayed shielding gas to cover the welding surface, but in order to effectively protect the lens and prevent metal vapor contamination or metal splash damage to the lens, the size of the nozzle must also be limited. The flow rate should also be controlled, otherwise the laminar flow of the shielding gas will become turbulent, and the atmosphere will be drawn into the molten pool and eventually form pores.

  In order to improve the protection effect, additional side blowing can also be used, that is, the shielding gas is directly injected into the deep penetration welding hole at a certain angle through a smaller diameter nozzle. The shielding gas not only suppresses the plasma cloud on the surface of the workpiece, but also exerts an influence on the formation of plasma and small holes in the hole, and the penetration depth is further increased, and a weld with an ideal depth and width is obtained. However, this method requires precise control of the size and direction of the air flow, otherwise it is easy to produce turbulence and destroy the molten pool, which makes the welding process difficult to stabilize.