In laser welding, the use of protective gas can significantly affect the formation, quality, depth, and width of the weld seam. In the vast majority of cases, the proper introduction of protective gas can have beneficial effects on the weld seam. However, improper use may lead to adverse effects.

I. Positive Effects of Protective Gas in Laser Welding

  1. Proper introduction of protective gas effectively shields the weld pool, reducing oxidation and preventing it.
  2. Correct introduction of protective gas can reduce splattering during welding, acting as a shield for the focusing lens or protective mirror.
  3. Proper introduction of protective gas promotes the even spreading of the weld pool during solidification, resulting in a uniform and aesthetically pleasing weld seam.
  4. Correct introduction of protective gas can reduce the shielding effect of metal vapor plumes or plasma clouds on the laser, increasing the energy reaching the workpiece surface and enhancing laser utilization.
  5. Proper introduction of protective gas can effectively reduce weld porosity.

II. Adverse Effects of Improper Use of Protective Gas in Laser Welding As long as the gas type, flow rate, and introduction method are chosen correctly, ideal results can be achieved. However, improper use of protective gas can have negative effects on welding.

  1. Improper introduction of protective gas may lead to poor weld seam quality.
  2. Choosing the wrong gas type can result in weld cracking or decreased mechanical properties of the weld seam.
  3. Incorrect gas flow rate selection may lead to increased oxidation of the weld seam (either with excessive or insufficient flow), as well as significant interference on the weld pool metal from external forces, causing collapse or uneven formation of the weld seam.
  4. Choosing the wrong gas introduction method may result in ineffective or no shielding of the weld seam, or may have a negative impact on the weld seam formation.
  5. The introduction of protective gas can have a certain impact on the depth of the weld seam, especially in thin plate welding, where it may reduce the weld penetration.

III. Types of Protective Gases Commonly used protective gases in laser welding include N2, Ar, and He, each with different physicochemical properties and varying effects on the weld seam.

  1. Nitrogen (N2) – Suitable for Stainless Steel Welding N2 has a moderate ionization energy, higher than Ar but lower than He. Under laser action, the degree of ionization is moderate, effectively reducing plasma cloud formation and increasing laser utilization. Nitrogen can chemically react with aluminum alloys and carbon steel at certain temperatures, forming nitrides that can increase weld joint strength. However, these reactions can reduce ductility, leading to decreased mechanical properties, so it is not recommended for use with aluminum alloys and carbon steel. For stainless steel welding, nitrogen reacts to form nitrides that can enhance weld joint strength, improving mechanical properties and making it suitable for use as a protective gas.
  2. Argon (Ar) – Cost-effective and Commonly Used Ar has the lowest ionization energy, resulting in higher ionization under laser action. This can affect laser utilization but Ar is chemically inactive with common metals. Additionally, Ar’s density aids in sinking to the weld pool, providing better protection. As such, Ar is commonly used as a general-purpose shielding gas.
  3. Helium (He) – Best Performance but Expensive He has the highest ionization energy, resulting in minimal ionization under laser action, effectively controlling plasma cloud formation. He has excellent shielding properties and does not react chemically with metals. However, due to its high cost, He is typically used for scientific research or in products with high added value.

IV. Selection of Gas Introduction Method

The two main methods of introducing protective gas are axial and coaxial. Axial gas introduction (as shown in Figure 1) and coaxial gas introduction (as shown in Figure 2).

Figure 1
Figure 2

The choice of the two gas introduction methods depends on a comprehensive consideration of various factors. In general, it is recommended to use the side blow protective gas method.

Principles for selecting the gas introduction method:

  • Straight seam welding is better suited for the axial side blow method, while planar enclosed shapes are better suited for the coaxial method.

Firstly, it is important to clarify that the term “oxidation” of the weld seam is merely a colloquial term. In theory, it refers to the chemical reaction between harmful components in the air, such as oxygen, nitrogen, and hydrogen, and the weld seam, leading to a decrease in weld seam quality. The common scenario is the chemical reaction of weld metal with these elements at certain temperatures.

Preventing “oxidation” of the weld seam involves reducing or avoiding contact between these harmful components and the weld metal in a high-temperature state. This high-temperature state includes not only the melted pool metal but also the entire process from when the weld metal is melted until the melted pool metal solidifies and its temperature decreases to a certain level.

For example, in titanium alloy welding, hydrogen absorption occurs rapidly at temperatures above 300°C, oxygen absorption occurs rapidly at temperatures above 450°C, and nitrogen absorption occurs rapidly at temperatures above 600°C. Therefore, effective protection is required for the weld seam of titanium alloys during the phase after solidification and when the temperature decreases to below 300°C, otherwise it will be “oxidized.”

From the above description, it is clear that the introduced protective gas not only needs to protect the weld pool in a timely manner but also needs to protect the just-solidified area that has already been welded. Therefore, generally, the axial side blow protective gas method (as shown in Figure 1) is adopted. This method of protection is relatively broader in scope compared to the coaxial protective gas method (as shown in Figure 2), especially providing good protection for the newly solidified areas of the weld seam.

For engineering applications, not all products can use the axial side blow protective gas method. For certain specific products, only the coaxial protective gas method can be used. The choice needs to be made based on the product structure and joint form, with a targeted selection approach.

Both methods should be chosen based on comprehensive considerations. Generally, for engineering applications, axial gas introduction is recommended for its wider range of protection, especially for newly solidified areas of the weld.

V. Specific Selection Principles for Gas Introduction Methods:

  1. Straight Seam Welding For products with straight seam welding, such as butt joints, lap joints, fillet welds, or overlay welds, axial gas introduction (Figure 3) is preferable.

2. Planar Enclosed Shape Welding For products with planar circular, polygonal, or multi-segment enclosed shapes (Figure 4, 5, 6 ), coaxial gas introduction (Figure 2) is preferable.

    The choice of protective gas directly impacts the quality, efficiency, and cost of welding production. Due to the diversity of welding materials, the selection of welding gases is complex. It requires a comprehensive consideration of material, welding method, welding position, and the desired welding effect. Through welding tests, the most suitable welding gas can be selected to achieve optimal welding results.

    Similar Posts