Ultimate Guide to Laser Welding Shielding Gases

Laser welding and arc welding differ in how they heat and melt metals. Laser welding does not corrode with high-temperature ionized gas, but directly heats the solid, resulting in minimal oxidation.

Protective Gas for Laser Welding

During laser welding, inert gases are commonly used to protect the molten pool. While for some materials, surface oxidation can be disregarded, in most applications, gases such as argon and nitrogen are often used to protect the workpiece from oxidation during the welding process.

For flat and enclosed welds, a coaxial gas protection method is typically employed, as shown in Figure 2-36.

Figure 2-36 Coaxial Gas Shielding Arrangements
Figure 2-36 Coaxial Gas Shielding Arrangements

Helium, due to its high ionization energy, allows the laser to pass smoothly, enabling the beam’s energy to reach the workpiece surface unhindered. Helium is the most effective protective gas for laser welding but is relatively expensive. Argon, while less expensive, has a higher density, providing better protection.

However, it is prone to ionization by high-temperature metal plasma, leading to partial beam shielding and a reduction in effective laser power, impacting welding speed and penetration. Surfaces protected with argon tend to be smoother compared to those protected with helium.

Nitrogen, being the most economical, is not suitable for welding certain types of stainless steel due to metallurgical issues such as absorption, which can sometimes lead to porosity in the overlap area.

The second function of using protective gas is to shield the focusing lens from metal vapor contamination and splattering of liquid droplets. This becomes particularly crucial during high-power laser welding when the ejection becomes very forceful, making the protective lens essential.

The third function of the protective gas is to effectively shield the plasma generated by high-power laser welding. Metal vapor absorbs the laser beam, ionizing into a plasma cloud, and the surrounding protective gas also ionizes due to heating. If there is an excessive plasma presence, the laser beam is somewhat consumed by the plasma.

The plasma, as a second energy source on the work surface, reduces penetration depth and widens the weld pool surface. Increasing the collision rate of electrons, ions, and neutral atoms helps reduce electron density in the plasma. Lighter neutral atoms lead to higher collision frequencies and composite rates. Additionally, only protective gases with high ionization energy can prevent an increase in electron density due to the gas’s own ionization.

The size of the plasma cloud varies with the type of protective gas used, with helium being the smallest, followed by nitrogen, and the largest when using argon. The larger the plasma size, the shallower the penetration depth. This difference is primarily due to variations in the ionization level of gas molecules and the differing densities of the protective gases, leading to differences in metal vapor diffusion.

Helium has the lowest ionization and density, allowing it to quickly disperse the rising metal vapor from the molten pool. Therefore, using helium as a protective gas can effectively suppress the plasma, thereby increasing penetration depth and improving welding speed. Its lightweight nature allows it to escape easily, minimizing the formation of pores.

However, in practical welding, the protective effect of using argon is still satisfactory. The impact of the plasma on penetration depth is most noticeable at low welding speeds. As the welding speed increases, its influence diminishes.

The protective gas is ejected onto the workpiece surface through a nozzle at a certain pressure. The fluid dynamics shape of the nozzle and the size of its outlet diameter are crucial.

It must be large enough to drive the ejected protective gas to cover the welding surface, yet limited in size to effectively protect the lens, preventing metal vapor contamination or damage from metal spattering. The flow rate also needs to be controlled; otherwise, laminar flow of the protective gas can turn into turbulent flow, leading to air entrainment in the molten pool and ultimately forming pores.

Blowing Method

To enhance the protective effect, an additional lateral blowing method can be used, where a smaller diameter nozzle directs the protective gas at a certain angle directly into the small hole of deep penetration welding.

This not only suppresses the plasma cloud on the workpiece surface but also affects the plasma inside the hole and the formation of the small hole, further increasing penetration depth and achieving an ideal depth-to-width ratio for the weld.

However, this method requires precise control of airflow volume and direction; otherwise, turbulent flow can disrupt the weld pool, making the welding process unstable. Most straight-line welds employ the lateral blowing method, as shown in Figure 2-37.

Figure 2-37 Lateral Blowing Method

For thicker materials, use a negative defocus. Be sure to increase the welding speed appropriately. Within the scope of allowable costs, use a mixture of helium and argon as the shielding gas. While pure helium is optimal, it is also more costly. For applications with stringent process requirements, it is best not to use argon alone, as its low ionization energy can easily generate a plasma cloud.

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top