How should "gas" be used correctly during laser welding? Is side blowing better, or is coaxial gas flow preferable?
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2021-04-24
In laser welding, the shielding gas significantly influences weld bead formation, weld quality, penetration depth, and weld width. In most cases, introducing a shielding gas has a positive effect on the weld; however, it can also potentially lead to adverse outcomes.
The role of protective gas
1. Positive Impact
1. Properly introducing protective gas effectively shields the weld pool, reducing or even preventing oxidation.
2. Properly introducing the shielding gas can effectively reduce spatter generated during the welding process.
3. Properly introducing the shielding gas can promote uniform spreading of the weld pool as it solidifies, resulting in a consistent and aesthetically pleasing weld bead.
4. Properly introducing protective gas can effectively reduce the shielding effect of metal vapor plumes or plasma clouds on the laser, thereby increasing the laser's overall efficiency.
5. Properly introducing the shielding gas can effectively reduce porosity in the weld.
As long as the gas type, gas flow rate, and injection method are chosen correctly, you can definitely achieve the desired results.
However, improper use of shielding gas can also negatively affect the welding process.
2 Adverse Effects
1. Incorrectly introducing protective gas can lead to poor weld quality:
2. Choosing the wrong type of gas can lead to cracking in the weld, as well as a reduction in the weld's mechanical properties.
3. Selecting the wrong gas flow rate can lead to more severe weld oxidation (whether the flow is too high or too low), and may also cause the weld pool metal to be significantly disturbed by external forces, resulting in weld collapse or uneven weld formation.
4. Choosing the wrong gas injection method can result in inadequate weld protection—sometimes leading to virtually no protection at all—or even negatively impact the weld bead formation.
5. Introducing protective gases can significantly affect weld penetration, especially in thin-plate welding, where it tends to reduce the weld depth.
Types of shielding gases
The commonly used laser welding shielding gases include N₂, Ar, and He, each with distinct physicochemical properties—and consequently, they produce varying effects on the weld seam.
1 Nitrogen gas N2
N2 has a moderate ionization energy—higher than that of Ar but lower than He—resulting in a typical degree of ionization under laser irradiation. This property helps minimize the formation of plasma clouds, thereby enhancing the effective utilization of the laser beam. At certain temperatures, nitrogen can chemically react with aluminum alloys and carbon steel, forming nitrides that increase weld seam brittleness and reduce toughness. Such reactions can significantly impair the mechanical performance of the welded joint, making it highly undesirable to use nitrogen as a shielding gas for welding aluminum alloy and carbon steel seams.
The nitrides formed when nitrogen chemically reacts with stainless steel can enhance the strength of weld joints, thereby improving the mechanical properties of the weld. Therefore, nitrogen gas can be used as a shielding gas during stainless steel welding.
2 Argon gas Ar
Argon has the lowest ionization energy, making it highly susceptible to ionization under laser irradiation. This, however, makes it challenging to control the formation of the plasma cloud, potentially impacting the laser's overall efficiency. On the other hand, argon is extremely inert, rarely reacting chemically with common metals. Additionally, argon is cost-effective and inexpensive. Beyond these advantages, argon’s relatively high density allows it to settle naturally above the weld pool, providing superior protection for the molten metal. For these reasons, argon remains a practical and widely used shielding gas in welding applications.
3 Helium He
He has the highest ionization energy and exhibits very low ionization under laser irradiation, allowing for precise control over plasma cloud formation. The laser can effectively interact with metals. Follow us on WeChat Official Account: Wei Welder. Additionally, He is highly inert—practically unreactive with metals—making it an excellent shielding gas for welds. However, due to its high cost, He is typically not used in large-scale production of commercial products. Instead, it’s mainly employed in scientific research or for applications where the product carries exceptionally high added value.
The method of introducing protective gas
Currently, there are two main methods for introducing protective gas: one is side-blowing protective gas with an off-axis arrangement, as shown in Figure 1, and the other is co-axial protective gas, as illustrated in Figure 2.
Figure 1: Off-Axis Side-Blown Protective Gas
Figure 2: Coaxial Shielding Gas
Choosing between the two blowing methods depends on a variety of factors, but in most cases, it’s recommended to use the side-blowing method for shielding gas protection.
Principles for Selecting the Protective Gas Inlet Method
First, it’s important to clarify that the so-called "oxidation" of welds is merely a common term. Theoretically, it refers to chemical reactions between the weld and harmful components in the air, which can degrade weld quality. A typical example is when weld metals react chemically with oxygen, nitrogen, hydrogen, and other elements in the air at elevated temperatures.
Preventing weld seams from "oxidizing" means minimizing or entirely avoiding contact between these harmful elements and the high-temperature weld metal—where "high temperature" refers not just to the molten pool but encompasses the entire period, from when the weld metal first melts until the molten pool solidifies and its temperature drops below a certain threshold.
For example
For example, when welding titanium alloys, they rapidly absorb hydrogen at temperatures above 300°C, oxygen above 450°C, and nitrogen above 600°C. Therefore, the weld seam of titanium alloys must be effectively protected during the cooling phase after solidification—specifically, once the temperature drops below 300°C—otherwise, it will "oxidize."
As is evident from the description above, the protective gas not only needs to shield the weld pool at the right time but also protect the freshly solidified areas that have already been welded. That’s why the side-axis gas shielding method shown in Figure 1 is typically used—this approach offers a broader coverage area compared to the coaxial protection method depicted in Figure 2, providing particularly effective protection for the regions where the weld has just begun to solidify.
For engineering applications, not all products can utilize the side-blowing method with protective gas; for certain specific products, only coaxial protection gas is suitable. The choice should be carefully tailored based on the product’s structural design and connector type.
The specific selection of protective gas injection method
1. Straight weld seam
As shown in Figure 3, the weld seam of the product is linear in shape, and the joint types can include butt joints, lap joints, internal corner welds, or overlap welds—whichever is appropriate. For this type of product, it is recommended to use the side-axis lateral gas-shielded welding method illustrated in Figure 1.
Figure 3: Linear Weld Seam
2-D Closed-Shape Weld Joints
As shown in Figure 4, the weld seam of the product can take on closed shapes such as planar circular, planar polygonal, or planar polyline configurations. The joint types can include butt joints, lap joints, and fillet welds—any of these are suitable. For this type of product, it is recommended to use the coaxial shielding gas method illustrated in Figure 2.
Figure 4: Planar Closed-Shape Weld Joints
The selection of shielding gas directly impacts the quality, efficiency, and cost of welding production. However, due to the wide variety of materials being welded, choosing the right welding gas in practice can be quite complex. It requires a comprehensive consideration of factors such as the welding material, welding method, welding position, and the desired welding outcome. Ultimately, only through welding tests can the most suitable gas be identified, ensuring optimal welding results.
Source: Welding & Cutting Alliance
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