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Laser welding is mainly aimed at the welding of thin-walled materials and precision parts and can realize spot welding, butt welding, stitch welding, sealing welding, etc., With a high aspect ratio, small weld width, small heat-affected zone, small deformation, and fast welding speed. The welding seam is flat and beautiful, no need for simple treatment after welding. The welding seam is of high quality, has no pores, can be precisely controlled, the focusing spot is small, the positioning accuracy is high, and it is easy to realize automation. So how to use the shielding gas correctly during 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 effects.
- Correctly blowing shielding gas will effectively protect the weld pool from reducing or even avoiding oxidation;
- Correctly blowing the shielding gas can effectively reduce the spatter generated during the welding process;
- Correctly blowing in the protective gas can promote the uniform spreading of the weld pool when it solidifies, making the weld shape uniform and beautiful;
- 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;
- Correct blowing of shielding gas can effectively reduce weld porosity.
As long as the gas type, gas flow rate, and blowing method are selected correctly, the ideal effect can be obtained. However, improper use of shielding gas can also have adverse effects on welding.
● The improper blowing of shielding gas may result in poor welds;
● Selecting the wrong type of gas may cause cracks in the weld, and may also cause the mechanical properties of the weld to be reduced;
● Choosing the wrong gas blowing flow rate may cause 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;
● Choosing the wrong gas blowing method will cause the weld to fail to achieve or even have no protective effect or have a negative impact on the weld formation;
● Blowing in the shielding gas will have a certain influence on the weld penetration, especially when welding thin plates, it will reduce the weld penetration.
Type of Protective Gas
The commonly used shielding gases for laser welding are mainly N2, Ar, and He and their physical and chemical properties are different, so their effects on the weld are also different.
The ionization energy of N2 is moderate, higher than that of Ar, and lower than that of He. Under the action of the 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. 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 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 are 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 high, and the degree of ionization under the action of the laser is very low, 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.
Shielding Gas Blowing Method
At present, there are two main ways to blow the protective gas: one is to blow the protective gas on the paraxial side, as shown in Figure 3; the other one is the coaxial protective gas, as shown in Figure 4.
How to choose the two blowing methods is a comprehensive consideration? Generally, it is recommended to use the side blowing protective gas method.
Selection Principle of Protective Gas Blowing Method
First of all, it should be clear that the so-called “oxidation” of the weld is only a common name. In theory, it means that the weld is chemically reacted with harmful components in the air, which leads to the deterioration of the weld. It is often the case that the weld metal is exposed to the same temperature at a certain temperature. Oxygen, nitrogen, hydrogen, etc. in the air react chemically.
Preventing the weld from being “oxidized” is to reduce or prevent such harmful components from coming into contact with the weld metal at high temperatures, not just the molten pool metal, but from the time the weld metal is melted until the pool metal solidifies and its temperature drops below a certain temperature over the course of the period.
Selection of Specific Protective Gas Blowing Methods
As shown in Figure 5, the shape of the welding seam of the product is a straight line, and the joint form is a butt joint, a lap joint, an internal corner seam joint, or a lap welded joint. It is better to blow protective gas on the shaft side.
Flat Closed Shape Welds
As shown in Figure 6, the shape of the welding seam of the product is a closed shape such as a plain circle, a plane polygon, and a plane multi-segment line. It is better to use the coaxial shielding gas method shown in Figure 4.
The selection of shielding gas directly affects the quality, efficiency, and cost of welding production. However, due to the diversity of welding materials, the selection of welding gas is also relatively complicated in the actual welding process.
It is necessary to comprehensively consider welding materials, welding methods, and welding positions. As well as the required welding effect, only through the welding test can a more suitable welding gas be selected to achieve better welding results.