The role of shielding gas
In laser welding, shielding gas affects weld formation, weld quality, weld depth and weld width. In most cases, blowing in shielding gas will have a positive effect on the weld, but it can also have a negative effect.
Positive effects
- Correctly blown shielding gas will effectively protect the weld pool from oxidation;
- Correctly blown in shielding gas will effectively reduce the spatter generated during the welding process;
- The correct blowing into the shielding gas can promote the solidification of the weld melt pool evenly spread, making the weld shape uniform and beautiful;
- The correct blowing of shielding gas can effectively reduce the shielding effect of the metal vapor plume or plasma cloud on the laser, increasing the effective utilization of the laser;
- Correctly blowing in shielding gas can effectively reduce the weld seam porosity. As long as the gas type, gas flow rate and blowing method are chosen correctly, the ideal effect can be obtained.
However, incorrect use of shielding gas can also have adverse effects on welding.
The adverse effects are as follows:
- Incorrectly blown-in shielding gas may result in a poor weld seam:
- Selection of the wrong type of gas may lead to cracking of the weld and may also lead to a reduction in the mechanical properties of the weld;
- the wrong choice of gas blow-in flow rate may lead to more severe oxidation of the weld (either the flow rate is too high or too low) and may also lead to severe disturbance of the weld pool metal by external forces resulting in collapse of the weld or uneven forming;
- the wrong choice of gas blowing in can result in the weld not achieving a protective effect or even basically no protective effect or a negative impact on the weld forming;
- blowing into the shielding gas will have a certain impact on the weld depth, especially in thin plate welding, will reduce the weld depth.
Types of shielding gas
Commonly used laser welding shielding gases are mainly N2, Ar, He, their physical and chemical properties are different, and therefore the effect on the weld seam also varies.
1. Nitrogen N2
The ionization energy of N2 is moderate, higher than that of Ar and lower than that of He. The degree of ionization under the action of the laser is average, which can better reduce the formation of plasma clouds and thus increase the effective utilization of the laser. Nitrogen can chemically react with aluminum alloy and carbon steel at a certain temperature to produce nitride, which will increase the brittleness of the weld and reduce the toughness, and will have a greater adverse effect on the mechanical properties of the welded joint, so it is not recommended to use nitrogen to protect the aluminum alloy and carbon steel weld. The chemical reaction between nitrogen and stainless steel produces nitrides that can improve the strength of the welded joint and will help improve the mechanical properties of the weld, so nitrogen can be used as a shielding gas when welding stainless steel.
2. Argon Ar
Ar's ionization energy is relatively low, the degree of ionization under the action of the laser is high, is not conducive to control the formation of the plasma cloud, will have a certain impact on the effective utilization of the laser, but Ar activity is very low, it is difficult to chemical reaction with common metals, and Ar cost is not high; in addition, the density of Ar is greater, conducive to sink to the top of the weld pool, can better protect the weld pool, so can be used as a conventional shielding gas.
3. Helium He
The ionization energy of He is the highest, and the ionization degree is very low under the action of laser, so it can control the formation of plasma cloud well, and the laser can work well on the metal. However, the cost of He is so high that it is not used for mass-produced products. He is generally used for scientific research or products with very high added value. The way the shielding gas is blown in.
There are two main types of shielding gas blowing in: one is side shaft side blowing shielding gas and the other is coaxial shielding gas.
In general, it is recommended to use the side-blowing protective gas method.
Protective gas blowing method selection principle
First of all, it should be clear that the so-called "oxidation" of the weld is only a common name, theoretically refers to the chemical reaction between the weld and the harmful components in the air, resulting in the deterioration of the quality of the weld, commonly the weld metal at a certain temperature and the chemical reaction of oxygen, nitrogen, hydrogen, etc. in the air.
To prevent the weld from being "oxidized" is to reduce or avoid contact between such harmful components and the weld metal at high temperature, which is not only the molten pool metal, but the whole process from when the weld metal is melted until the pool metal solidifies and its temperature is reduced to below a certain temperature.
For example, titanium alloy welding, when the temperature above 300 ℃ can quickly absorb hydrogen, 450 ℃ above the rapid absorption of oxygen, 600 ℃ above the rapid absorption of nitrogen, so titanium alloy welds in the solidification and temperature down to 300 ℃ below this stage are subject to effective protection effect, otherwise it will be "oxidation".
From the above description, it is easy to understand that the protective gas blown into not only the timely protection of the weld pool, but also the need to have been welded just solidified area for protection, so the general use of side axis side blowing protective gas, because this way of protection compared to the coaxial protection of a broader range of protection, especially for the weld just solidified area has better protection.
For engineering applications, not all products can be protected by side-blown shielding gas, and for some specific products, only coaxial shielding gas can be used, which requires targeted selection from the product structure and joint form.
The choice of specific protective gas blowing method
1. Straight line welding seam
The shape of the weld seam is straight, and the form of the joint is butt joint, lap joint, corner joint or stacked weld head.
2. Plane closed pattern welding seam
The shape of the weld seam is a closed pattern such as a plane circumferential shape, a plane multilateral shape, a plane multi-segment line shape, etc. The form of the joint is butt joint, lap joint, stacked weld head, etc. This type of product is preferable to the coaxial shielding gas method.
The selection of shielding gas directly affects the quality, efficiency and cost of welding production, but due to the diversity of welding materials, the selection of welding gas is also complicated in the actual welding process, which requires comprehensive consideration of welding materials, welding methods, welding positions, and the required welding results, and through welding tests to select a more suitable welding gas and achieve better welding results.
