How to use shielding gas correctly in laser welding
 
The role of shielding gas used in laser welding

Laser means that in laser welding, shielding gas will affect weld formation, weld quality, weld penetration depth and penetration width. In most cases, blowing shielding gas will have a positive effect on the weld, but it may also Will bring unfavorable effects.

Positive effects

1) Effectively protect the weld pool to reduce or even avoid oxidation;

2) It can effectively reduce the spatter generated in the welding process;

3) It can promote the uniform spread of the weld pool during solidification, making the weld shape uniform and beautiful;

4) It can effectively reduce the shielding effect of the metal vapor plume or plasma cloud on the laser, and increase the effective utilization rate of the laser;

5) It can effectively reduce the welding seam porosity.

As long as the gas type, gas flow rate, and blowing method are selected correctly, the desired effect can be obtained. However, incorrect use of shielding gas will also have an adverse effect on welding.

1) Incorrect blowing of shielding gas may cause deterioration of the weld:

2) Choosing the wrong gas type may cause cracks in the weld, and may also reduce the mechanical properties of the weld;

3) Choosing the wrong gas flow rate may cause more serious oxidation of the weld (whether the flow is too large or too small), and may also cause the weld pool metal to be seriously interfered by external forces, causing the weld to collapse or form unevenly;

4) Choosing the wrong gas blowing method will cause the welding seam to fail to achieve the protection effect or even basically no protection effect or have a negative influence on the welding seam formation;

5) Blowing in shielding gas will have a certain impact on the weld penetration, especially when welding thin plates, it will reduce the weld penetration.

Types of protective gas

Commonly used shielding gases for laser welding mainly include N2, Ar, and He, and their physical and chemical properties are different, and therefore their effects on welds 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 moderate, 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 have a greater adverse effect on the mechanical properties of the weld joint. Therefore, it is not recommended to use nitrogen for 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. The 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 shielding gas
There are currently two main types:
One is to blow the shielding gas on the side shaft side,
One is the coaxial shielding gas,

How to choose the two blowing methods is a comprehensive consideration from many aspects. Generally, it is recommended to use side blowing protective gas.

First of all, it needs to be clear that the so-called "oxidation" of the weld is just a common name. In theory, it refers to the chemical reaction between the weld and the harmful components in the air, which causes the quality of the weld to deteriorate. It is usually the weld metal at a certain temperature. It reacts chemically with oxygen, nitrogen and hydrogen 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 pool metal, but from the time the weld metal is melted to the melt The pool metal solidifies and its temperature drops below a certain temperature during the entire time period.

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".

It is not difficult to understand from the above description that the blown 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 blowing shielding gas is generally used, because Compared with the coaxial protection method, this way of protection has a wider range of protection, 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.

The selection of shielding gas directly affects the quality, efficiency and cost of welding production, but 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 welding test to achieve better welding results.