1. Laser welding gas
In laser welding, a shielding gas is used to improve welding results and prevent sediment build-up on laser tools. Shielding gas can be divided into three categories: auxiliary gas (MDE gas), shielding gas and jet gas.
Auxiliary gas, particularly useful in yttrium aluminum garnet laser welding, helps reduce the absorption of the laser beam in the metal vapor plasma. The shielding gas, in turn, expels air from the welding area to avoid any reaction with the air components.
Jet gas is used in welding processes that produce excessive amounts of spatter and steam. The air curtain directs the gas from the air curtain to the machining head at a 90 degree angle through a nozzle, protecting the machining head from splash and mist during welding. The air curtain has no effect on the molten metal or shielding gas.
2. What is the role of shielding gas?
The laser produces a beam of energy necessary for the welding process. This energy is directed to the joint position of the workpiece through a combination of a steering mirror, laser optical cable and focusing device.
To ensure precise guidance of the focused laser beam, the workpiece must be positioned and clamped correctly. The focusing optical element is then moved along the seam position, directing the laser beam towards the workpiece.
The high power density of the laser beam at the focus point causes the material to melt and a small portion to vaporize. The pressure of the flowing metallic vapor is so strong that it forms a small hole known as a “keyhole”. This keyhole penetrates the material several millimeters deep.
When the focusing lens is moved above the workpiece, the keyhole also moves under the focusing lens. This allows the molten metal to flow together, resulting in the molten material solidifying into a tight weld.
However, many metals can react with components present in the air in a molten state, leading to a reduction in welding quality. The shielding gas expels these air components, positively affecting the characteristics of the weld.
3. Shielding gas
Inert gas is commonly used in metal laser welding due to its properties of not reacting, or rarely reacting, with the matrix material. Some recommended shielding gases include nitrogen (N2), argon (AR), and helium (He).
It is important to note that industrial gases often contain small amounts of impurities. The purity of the gas is indicated through a digital system, where the first number represents the number of nines in the percentage value and the second number represents the last digit of the percentage value. For example, He 4.6 indicates a helium purity of 99.996% (by volume).
The useful life of a gas cylinder can be easily calculated. Welding gas is stored in gas cylinders, with a typical gas storage cylinder containing 50 liters of gas at a pressure of 200 bar.
- T: Service life
- V: Volume of the gas cylinder
- Q: Inflation pressure
- Q: Unit gas consumption
Example:
V = 50l,p = 200bar,Q =40l/h → T = 50l • 200bar/40l/h = 250 h
Nitrogen (N2)
Nitrogen is a colorless and odorless inert gas suitable for welding chrome-nickel steel. However, it is not recommended for use with zirconium alloys and titanium materials, as although it is inactive, it can form compounds with these materials.
It is important to note that when welding steel with nitrogen, the presence of nitrogen can slightly reduce rust resistance by dissolving the chromium and nickel in the steel.
Recommendation
The following table provides an overview of recommended shielding gases.
| Air | He | N 2 | Observation | |
| Aluminum and aluminum alloy | – | + | – | Smooth, shiny welds can be formed with hydrogen or hydrogen mixture.
Using hydrogen will cause pores in the material Hydrogen-containing gas will cause pores in the material Very high quality welds can be formed when welding with carbon dioxide (CO2) or a hydrogen/carbon dioxide mixture. However, the smoothness and brightness of these welds are slightly poor. |
| Nickel chrome steel | + | – | + | It is recommended to use argon in devices prone to gas overflow because it is heavier than nitrogen. To prevent corrosion, argon must be used because nitrogen reacts with the chromium and nickel in the material. |
| Titanium and titanium alloys | + | – | + | Titanium reacts strongly with air components. As long as the solder temperature after cooling is still 200 ℃, it is necessary to completely cover the solution pool with argon (for example, a glove box can be used) |
| chrome alloy | + | – | – | – |
| Copper | – | – | – | It is generally not necessary to use shielding gas when welding copper. |
Suggestions for raw material shielding gas: “+” = yes, “-” = no
Note: When welding on narrow devices, self-shielding occurs as the metal vapor expels oxygen from the surrounding environment. In this case, the use of shielding gas is not necessary.
4. Shielding gas inlet
The ways to insert shielding gas into the processing position are:
- Pass through the nozzles
- Fixing device through bench
The following parameters should be optimally configured for use:
- Type of gas, pure gas or mixed gas
- Angle of incidence
- Incident range
- air flow
- Nozzle geometry.
The amount of shielding gas input must be adjusted based on the type of laser (continuous or pulsed), the welding speed and the weld. TRUMPF supplies some standard nozzles, which will be described in more detail in the subsequent information.
Linear gas supply
The linear nozzle is an advanced version of the composite tube, where each tube is assembled individually.
The linear nozzle has the following advantages:
- Better welding quality.
- The structure is more compact, resulting in less interference in the contour.
- It can be used even if the nozzle is far from the workpiece.
prerequisite:
- CW Laser
- Objective lens focal lengths f = 150 mm, f = 200 mm, f = 250 mm and F = 300 mm.
Welding Application:
The linear nozzle is suitable for linear welding:
- Butt weld.
- Weld bead.
Linear gas supply with side MDE nozzle
Using this nozzle, the shielding gas can be directed in a straight line and the influence of metal vapor can be reduced through the use of a side MDE nozzle.
prerequisite:
- CW laser
- Objective lens focal lengths f = 150 mm, f = 200 mm, f = 250 mm and F = 300 mm.
Welding Application:
The linear nozzle is suitable for linear welding:
- Butt weld.
- Weld bead.
Gas supply from bubbling nozzle
The conical nozzle has a radius regulator, which guarantees laminar flow and uniform distribution of the shielding gas.
Bubbling nozzles can be used when the following preconditions are met:
- CW laser
- Pulsed laser.
- Objective lens focal lengths f = 150 mm, f = 200 mm, f = 250 mm and F = 300 mm.
Welding Application:
The bubbling nozzle can provide large area laminar gas supply when the beam power and welding speed are low. On the other hand, conical nozzles offer even distribution of the shielding gas, especially in difficult-to-reach areas.
It is recommended to maintain a distance of 8 – 12 mm and an angle of 30° – 50° from the workpiece, depending on the application.
Other methods
In situations where it is necessary to guarantee complete and uniform coverage of the material with shielding gas, the use of a glove box is recommended. The glove box completely surrounds the work area and prevents the protective gas from overflowing.
Since the glove box is completely filled with shielding gas, there is no need for a separate shielding gas nozzle.
5. Shielding gas nozzle layout
There are two different welding processes for laser welding:
- Thermal conductivity welding
- Deep Penetration Welding
In thermal conductivity welding, only the surface of the material melts, resulting in a weld that is only a few tenths of a millimeter deep. This welding process is mainly used with pulsed Nd:YAG lasers.
In contrast, deep penetration welding creates deep, narrow welds. This process is carried out using an Nd:YAG laser in continuous wave operating mode.
pulsed laser
To obtain the best results when welding with a pulsed laser, the welding wire (if used) is normally inserted slowly. The direction of the shielding gas inlet can be freely chosen.
CW laser
To obtain optimal results when welding with a continuous wave laser, it is necessary to insert the shielding gas forward and slow down the insertion of the welding wire (if used).
Edge welding
The shielding gas inlet nozzle must be arranged to produce a smooth and uniform flow of air. When welding along the edge, a vortex can be created, bringing oxygen from the surrounding environment into the welding area.
If the oxygen content exceeds 0.5%, the material may react with oxygen. To prevent air flow vortexing along the edges during welding, damper plates can be installed.
6. Shielding gas measurement
Accurately measuring shielding gas is crucial to achieving optimal welding results. Ideally, a smooth and uniform laminar airflow should be present above the processing point.
If the incoming amount of shielding gas is too low, it may not provide adequate protection, allowing moisture from the gas or air to enter the weld. On the other hand, if too much shielding gas is used, it can create vortices that bring air into the welding area.
The color of the weld can provide information about the amount of shielding gas used during welding. If the weld appears gray, this suggests that no shielding gas was used. If the weld appears yellow, the shielding gas measurement needs to be optimized.
If the shielding gas measurement is optimized, a high gloss weld will be produced.
A shielding gas nozzle with radius regulator can ensure a uniform flow of shielding gas. The same result can be achieved using steel wool on the nozzle.
Transverse air curtain:
Wind curtains are useful in welding applications that generate a significant amount of spatter and steam. The air curtain must be adjusted so that the flow of the air curtain does not interfere with the shielding gas.
Suggestion:
A simple test can determine whether the jet gas has been optimally adjusted. Place a piece of paper above the workpiece and adjust the air pressure of the jet so that the paper is not pushed down or pulled by the jet.
7. Role of shielding gas
Different shielding gases can produce different results, affecting the shape of the weld and creating a smoother, more polished weld surface. The choice of shielding gas can also impact the formation of pores and spatter in the weld, in addition to making it difficult to couple the laser beam.
| Air | He | N 2 | No shielding gas | |
| Welding way
b = width T = depth |
 |
 |
 |
 |
| Welding surface | ++ | + | + | – |
| Splashes | + | + | 0 | – |
| Stomach | ++ | + | + | – |
| Laser beam coupling | – | – | – | + |
| Cost | – | – | 0 | Any less |
To optimize the effect of the shielding gas, it is necessary to briefly open the shielding gas before and after welding. After the shielding gas opens, there is a period of time before the gas reaches the workpiece. The casting still cooled after welding also requires a brief coating with shielding gas.
