Laser Welding Gas
In laser welding, shielding gas is utilized to enhance the welding process and prevent deposits on the laser tool.
Depending on the application, it can be divided into.
- Auxiliary gas (MDE gas)
- Shielding gas
- Jet gas
Why Do I Need Shielding Gas?
Laser continuous welding uses a high-energy laser beam as a heat source to irradiate the surface of the workpiece to melt and join the workpiece, resulting in excellent welding results.
In the high power laser welding process, the laser irradiates the material’s surface to melt the workpiece. Still, the high temperature is accompanied by the vaporization of the metal to form a metal vapor plasma.
The formed metal vapor plasma absorbs, refracts, and reflects the laser light, which weakens the energy that reaches the workpiece’s surface and affects the melt pool’s stability. Therefore, it is necessary to blow a shielding gas with high ionization energy to suppress the plasma generation.
At the same time, the shielding gas isolates the air from the welding process and keeps the molten pool free from oxidation. It also reduces welding spatter and makes the weld surface uniform and smooth.
Function of The Shielding Gas
In laser welding, the shielding gas affects the weld formation, quality, penetration, and width.
In most cases, blowing the shielding gas will positively affect the weld, but it may also have an undesirable effect.
Positive effects
- The correct use of shielding gas can protect the weld pool and reduce or even avoid oxidation.
- The correct shielding gas can effectively reduce spatter during welding.
- The correct shielding gas can distribute the weld pool evenly during solidification and give the weld seam a uniform and beautiful shape.
- The right shielding gas can effectively reduce the shielding effect of metal vapor fume or plasma cloud on the laser and improve the effective utilization of the laser.
- The correct shielding gas can effectively reduce the weld seam porosity.
The desired results can be achieved with the correct choice of gas type, flow rate, and blowing method.
However, incorrect use of shielding gas may have a counter effect.
Negative effects
- Incorrect use of shielding gas may lead to deterioration of the weld seam.
- Choosing the wrong type of gas may lead to cracks in the weld and may also lead to deterioration of the mechanical properties of the weld.
- Incorrect selection of blowing gas flow rate may lead to more severe oxidation of the weld (whether the flow rate is too high or too low) or severe disturbance of the weld pool metal by external forces, resulting in collapsed or unevenly formed welds.
- Choosing the wrong type of gas blowing can result in a weld that fails to achieve protection, or even has essentially no protection, or adversely affects the weld formation.
- Blowing shielding gas can affect the permeability of the weld, especially when welding thin plates.
Types of Shielding Gas
No, He and Ar are the three shielding gases most frequently employed in laser welding.
Since their physical and chemical characteristics fluctuate, so does the effect on the weld seam.
N2
Nz has moderate ionization energy, higher than AR and lower than He. Under the action of the laser, the ionization degree of Nz is average, which can reduce the formation of a plasma cloud and improve the effective utilization of the laser.
A specific temperature can cause nitrogen to react with carbon steel and aluminum alloy to create nitride. This will improve the brittleness of the weld, reduce the toughness, and significantly negatively affect the welded joint’s mechanical properties.
Therefore, nitrogen is not advised to preserve the weld seam of aluminum alloy and carbon steel.
The nitride formed due to the chemical interaction between nitrogen and stainless steel can enhance the weld’s mechanical characteristics and strength. So it can be used as a shielding gas when welding stainless steel.
Argon gas
Because of AR’s comparatively low ionization energy and the relatively high ionization degree caused by the laser, controlled plasma clouds cannot develop.
This will have some impact on the effective use of laser light. However, the low activity of argon makes it difficult to react with common metals, and the cost of argon is not high.
In addition, the high density of argon facilitates sinking above the welding pool, which can better protect the environment. Therefore, it can be used as a conventional shielding gas.
Helium
Helium has the highest ionization energy and is very low ionized by the laser, which allows good control of the plasma cloud formation.
The laser can act well on the metal, while It has very low activity and does not react with the metal, so it is a good shielding gas for welds.
However, the cost of He is so high that it is not used for general mass-produced products but scientific research or products with high added value.
The Way of Blowing Shielding Gas
At present, there are two main ways of blowing shielding gas: one is side-axis side-blowing shielding gas, as shown in Figure 1; the other is coaxial shielding gas, as shown in Figure 2.
Protective Gas Blowing Mode Selection Principles
First, it should be clear that the “oxidation” of the weld is just a common name. Theoretically, it refers to the chemical reaction of the weld with harmful components in the air, which leads to the deterioration of the weld quality.
The common cause is that the weld metal reacts with oxygen, nitrogen, and hydrogen in the air at a certain temperature. To prevent the weld from being “oxidized,” reduce or avoid such harmful components at high temperatures with the weld metal.
This high-temperature state refers not only to the molten pool metal but also to the entire process, from melting the weld metal to solidifying the weld metal and the temperature dropping below a certain temperature.
For example, titanium alloy welds can rapidly absorb hydrogen at temperatures above 300°C, oxygen above 450°C, and nitrogen above 600°C.
Therefore, titanium alloy welds must be effectively protected after solidification and in the phase below 300°C, or they will be “oxidized.”
From the above description, it is easy to understand that the purge shielding gas needs to protect not only the weld pool in time but also the newly solidified weld area.
Therefore, the side shaft blowing shielding gas shown in Figure 1 is generally used. Compared to the coaxial shielding in Figure 2, this method provides a wider range of protection, especially for the newly solidified area of the weld.
Not all products can be protected by side-axis side-blowing shielding gas for engineering applications. For some special products, only coaxial shielding gas can be used, and specific choices must be made regarding product structure and joint form.
The Choice of a Specific Blowing Method for Shielding Gas
Straight line welding seam
As shown in Figure 3, the shape of the weld seam of this product is linear, and the form of the joint can be a butt, lap, internal corner joint, or lap weld.
For this product, it is better to use the side shaft blowing protective gas, as shown in Figure 1.
Planar closed graphical welds
As shown in Figure 4, the shape of the weld seam for this product is a closed figure such as a planar circle, planar polygon, planar multi-segment straight shape, etc. The form of the joint can be a butt, lap, overlap joint, etc.
Therefore, the coaxial shielding gas method shown in Figure 2 is preferred for this type of product.
The shielding gas selection directly impacts welding output’s effectiveness, cost, and quality.
The choice of gas is more difficult during the welding process because of the variety of materials.
The welding material, the welding method, the welding position, and the desired welding result must be considered.
Only through welding tests can a more suitable welding gas be selected to achieve better welding results.
The Effect of Shielding Gas on The Weld Seam Morphology
In addition to selecting a suitable shielding gas according to the welding material, it is also necessary to study the effect of the shielding gas blowing’s angle, direction, and flow rate on the weld seam morphology.
The effect of shielding gas with different blowing angles on the weld seam was studied based on the same welding conditions.
Through experimental tests, the trend of the effect on weld seam morphology is the same for different flow rates under the same control of other variables.
However, the greater the flow rate, the more pronounced the effect on weld penetration and the lesser the effect on the weld surface. The effect on the weld surface and the lower weld width.
Therefore, when the shielding gas flow rate was 5 L/min and other variables were controlled, only the blowing angle was varied to study the blowing angle.
The test results are shown in Figure 5, and the weld morphology cross-sectional metallographic images are shown in Figure 6.
The experimental results show that the weld penetration rises and falls when the blowing angle increases.
At 0° or beyond 45°, the penetration rate decreases rapidly.
The weld penetration reaches a maximum at a blowing angle of 30°.
The attenuation of the laser determines the weld seam width by the plasma and the effect of the gas flow on the weld pool.
When the blowing angle is 0°, the width of the melt is minimal.
As the blowing angle increases, the melt width increases.
When the angle is greater than 45°, the width of the weld does not change much.
Analysis of The Results
The influence of the shielding gas on the weld morphology is mainly determined by controlling the size of the plasma to determine the power density of the laser reaching the surface of the workpiece.
By observing the metallographic picture of the weld cross-section, it can be seen that at 0° or 75°, the weld morphology tends to the heat conduction welding mode. In comparison, at 30° and 45°, the deep penetration welding morphology is obvious.
In summary, under the same welding process parameters, it is recommended that the shielding gas being blown at an angle of 30° to obtain greater penetration.
A blowing angle of 45° is recommended if the surface width is larger.
A blowing angle of 0° or 75° is recommended if the lower melt width is larger.
