1. Types of protective gas
The commonly used shielding gases for laser welding are N2, Ar and He. Their physical and chemical properties are different, so the effect on the weld is also different.
1) N2
The ionization energy of N2 is moderate, higher than that of AR and lower than that of he. The ionization degree of N2 is general under the action of laser, which can reduce the formation of plasma cloud and increase the effective utilization of laser. Nitrogen can react with aluminum alloy and carbon steel at a certain temperature to produce nitride, which will improve the weld brittleness, reduce the toughness and have a great adverse effect on the mechanical properties of the weld joint. Therefore, it is not recommended to use nitrogen to protect the weld of aluminum alloy and carbon steel.
The nitride produced by the chemical reaction between nitrogen and stainless steel can improve the strength of the weld joint, which is conducive to the improvement of the mechanical properties of the weld, so nitrogen can be used as protective gas when welding stainless steel.
2) Ar
The ionization energy of AR is relatively low, and the ionization degree is relatively high under the action of laser, which is not conducive to the control of the formation of plasma cloud, and will have a certain impact on the effective utilization of laser. However, the activity of AR is very low, it is difficult to react with common metals, and the cost of AR is not high. In addition, the density of AR is large, which is conducive to sink above the weld pool, It can better protect the weld pool, so it can be used as conventional shielding gas.
3) He
The ionization energy of he is the highest, and the ionization degree is very low under the action of laser, which can control the formation of plasma cloud well. Laser can act on metal well, and he activity is very low, which basically does not react with metal, so he is a good weld protection gas. However, the cost of he is too high, so it is not used in general mass production products, He is generally used in scientific research or high value-added products.
2. Blowing mode of shielding gas
At present, there are mainly two ways to blow the protective gas: one is to blow the protective gas on the side of the side shaft, as shown in Figure 1; The other is coaxial shielding gas, as shown in Figure 2.
Figure1 Side shaft side blowing protective gas
Figure2 Coaxial shielding gas
How to choose the two ways of blowing is a comprehensive consideration in many aspects. In general, it is recommended to use the way of side blowing protective gas.
Selection principle of the blow in mode of the protective gas
First of all, it needs to be clear that the so-called "oxidation" of weld is just a common name. Theoretically, it refers to the chemical reaction between the weld and the harmful components in the air, which leads to the deterioration of the weld quality. The common reason is that the weld metal reacts with oxygen, nitrogen, hydrogen in the air at a certain temperature.
To prevent the weld from being "oxidized" is to reduce or avoid the contact of such harmful components with the weld metal at high temperature, which is not only the molten pool metal, but also the whole process from the time when the weld metal is melted to the time when the weld metal solidifies and its temperature drops below a certain temperature.
For example, when the temperature of titanium alloy welding is above 300 ℃, it can absorb hydrogen rapidly, when the temperature is above 450 ℃, it can absorb oxygen rapidly, and when the temperature is above 600 ℃, it can absorb nitrogen rapidly. Therefore, titanium alloy welding should be protected effectively after solidification and when the temperature is below 300 ℃, otherwise it will be "oxidized".
It is not difficult to understand from the above description that the blown shielding gas not only needs to protect the weld pool in time, but also needs to protect the just solidified area that has been welded. Therefore, the side shaft side blowing shielding gas shown in Fig. 1 is generally used, because the protection range of this method is wider than that of the coaxial protection method in Fig. 2, Especially, it has better protection for the area where the weld has just solidified.
For engineering application, not all products can adopt the way of side shaft side blowing protective gas. For some specific products, only coaxial protective gas can be used. Specific selection should be made from product structure and joint form.
