HOW TO USE “GAS” CORRECTLY IN LASER WELDING?
The role of protective gas
In laser welding, shielding gas will affect the weld formation, weld quality, weld penetration and penetration width. In most cases, blowing shielding gas will have a positive effect on the weld, but it may also bring adverse effect.
1. Positive effect
1) Correctly blowing shielding gas will effectively protect the weld pool from reducing or even avoiding oxidation;
2) Correctly blowing in the shielding gas can effectively reduce the spatter generated during the welding process;
3) The correct blowing of the protective gas can promote the uniform spreading of the weld pool when it solidifies, so that the weld is formed uniformly and beautifully;
4) Correctly blowing in the protective gas can effectively reduce the shielding effect of the metal vapor plume or plasma cloud on the laser, and increase the effective utilization of the laser;
5) Properly blowing shielding gas can effectively reduce weld pores.
As long as the gas type, gas flow rate and blowing method are selected correctly, the ideal effect can be obtained.
2. Negative effect
1) Improper blowing of shielding gas may lead to poor welds:
① Selecting the wrong type of gas may cause cracks in the weld, and may also reduce the mechanical properties of the weld;
② Selecting the wrong gas injection flow rate may lead to more serious oxidation of the weld (whether the flow rate is too large or too small), and may also cause the weld pool metal to be seriously disturbed by external forces, causing the weld to collapse or form unevenly;
③ Selecting the wrong gas blowing method will cause the weld to fail to achieve the protective effect or even have no protective effect or have a negative impact on the weld formation;
2) Blowing into the shielding gas will have a certain effect on the weld penetration, especially when thin plates are welded, it will reduce the weld penetration.
3. Types of protective gases
Commonly used laser welding shielding gases are mainly N2, Ar, He, and their physical and chemical properties are different, so the effect on the weld is also different.
The ionization energy of N2 is moderate, higher than that of Ar, and lower than that of He. Under the action of laser, the ionization degree is average, which can better reduce the formation of plasma cloud, thereby increasing the effective utilization of laser. Nitrogen can chemically react with aluminum alloy and carbon steel at a certain temperature to generate nitrides, which will increase the brittleness of the weld and reduce the toughness, which will have a greater adverse effect on the mechanical properties of the weld joint, so it is not recommended to use nitrogen. Aluminum alloy and carbon steel welds are protected.
The nitride produced by the chemical reaction between nitrogen and stainless steel can improve the strength of the weld joint, which will help improve the mechanical properties of the weld, so nitrogen can be used as a protective gas when welding stainless steel.
The ionization energy of Ar is relatively low, and the degree of ionization under the action of the laser is relatively high, which is not conducive to controlling the formation of plasma clouds, and will have a certain impact on the effective utilization of the laser. However, the activity of Ar is very low, and it is difficult to chemically react with common metals. reaction, and the cost of Ar is not high. In addition, the density of Ar is large, which is conducive to sinking to the top of the weld pool, which can better protect the weld pool, so it can be used as a conventional shielding gas.
The ionization energy of He is the highest, and the ionization degree is very low under the action of the laser, which can well control the formation of the plasma cloud. It is a good weld shielding gas, but the cost of He is too high. Generally, this gas is not used in mass-produced products. He is generally used for scientific research or products with very high added value.