Types of shielding gas
Commonly used shielding gases for laser welding mainly include N2, Ar, He, and their physical and chemical properties are different, and therefore their effects on the weld are also different.
1. Nitrogen N2
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 reduce the formation of plasma cloud and increase the effective utilization rate of laser. Nitrogen can chemically react with aluminum alloy and carbon steel at a certain temperature to produce nitrides, which will increase the brittleness of the weld, reduce the toughness, and will have a greater adverse effect on the mechanical properties of the weld joint. Therefore, 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 increase the strength of the weld joint, which will help improve the mechanical properties of the weld. Therefore, nitrogen can be used as a shielding gas when welding stainless steel.
2. Argon Ar
The ionization energy of Ar is relatively low, and the ionization degree is high under the action of laser, which is not conducive to controlling 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 and it is difficult to chemically interact with common metals. The cost of Ar is not high. In addition, the density of Ar is higher, which is beneficial to sink to the top of the weld pool, and can better protect the weld pool, so it can be used as a conventional shielding gas.
3. Helium He
He has the highest ionization energy, and the ionization degree is very low under the action of laser, which can well control the formation of plasma cloud. Laser can act on metal very well, and He has very low activity and basically does not chemically react with metal. It is a good shielding gas for welding seams, but the cost of He is too high. Generally, mass production products will not use this gas. He is generally used for scientific research or products with very high added value.
Blowing method of protective gas.
Figure1
At present, there are two main ways to blow the shielding gas: one is to blow the shielding gas on the side shaft side, as shown in Figure 1; the other is the coaxial shielding gas, as shown in Figure 2.
Figure 2
How to choose the two blowing methods is a comprehensive consideration from many aspects. Generally, it is recommended to use the side blowing protective gas method.
Principles for choosing the way of blowing protective gas
First of all, it needs to be clear that the so-called “oxidation” of the weld is only a common name. Theoretically, it refers to the chemical reaction of the weld with harmful components in the air, which leads to the deterioration of the quality of the weld. It is common that the weld metal is at a certain temperature. It reacts chemically with oxygen, nitrogen, hydrogen, etc. in the air.
To prevent the weld from being "oxidized" is to reduce or avoid contact of such harmful components with the weld metal in a high temperature state. This high temperature state is not only the molten metal in the molten pool, but from the time the weld metal is melted to the molten metal. The pool metal solidifies and its temperature drops below a certain temperature during the entire time period.
For example
For example, titanium alloy welding can quickly absorb hydrogen when the temperature is above 300°C, quickly absorb oxygen when the temperature is above 450°C, and quickly absorb nitrogen when the temperature is above 600°C, so the titanium alloy weld is solidified and the temperature is reduced to 300°C Effective protection effects are required in the following stages, otherwise it will be "oxidized".
From the above description, it is not difficult to understand that the blowing shielding gas not only needs to protect the weld pool in a timely manner, but also needs to protect the just solidified area that has been welded. Therefore, the side shaft side shown in Figure 1 is generally used. Blowing shielding gas, because this method of protection has a wider protection range than the coaxial protection method in Figure 2, especially for the area where the weld has just solidified.
Sideshaft side blowing For engineering applications, not all products can use sideshaft side blowing shielding gas. For some specific products, only coaxial shielding gas can be used, which needs to be carried out from the product structure and joint form. Targeted choices.
Selection of specific shielding gas blowing method
Straight weld
The weld shape of the product is straight, and the joint form can be butt joint, lap joint, internal corner joint or overlap welded joint. This type of product adopts the side-shaft side blowing shielding gas method shown in Figure 1. Better.
Plane closed graphic weld
The weld shape of the product is a closed pattern such as a flat circle, a flat polygon, a flat polyline, etc. The joint form can be a butt joint, a lap joint, a stacked joint, etc., this type of product is shown in Figure 2 Coaxial shielding gas method is better.
The selection of shielding gas directly affects the quality, efficiency and cost of welding production. However, due to the diversity of welding materials, in the actual welding process, the selection of welding gas is also more complicated. It is necessary to comprehensively consider the welding material, welding method, and welding position. As well as the required welding effect, a more suitable welding gas can be selected through the welding test to achieve better welding results.
