Solid-State and СО2 Laser Welding Zink-Coated Steel Sheets and Unconditional Blanks in Argon and its Mixtures
In the case of high power CO2 laser welding (up to 12kW) Helium can be replaced by 100% Argon keeping the welding speed and bead properties unchanged and reducing considerably the manufacturing costs.
In the case of YAG lasers, the use of fast dynamic Argon jets makes possible to weld critical materials, as well as well-known critical welding configurations like zinc coated steel stacks.
Laser Welding: state of the art
The “keyhole mode welding” or “deep penetration laser welding” occurs when the power density of the focalized laser beam is higher than 1MW/cm2. The laser energy is absorbed by the metal surface and the metal is vaporized. The recoil pressure due to this strong vaporization generates a narrow capillary within the melted pool called keyhole. This keyhole is filled with a mixture of dense and hot metal vapors and plasma absorbing partially the incident laser beam. In this way the laser energy is deposited deeply into the material. The laser welding process results from the displacement of this keyhole into the melted pool. The keyhole behavior is described with two different approaches: the “static cylindrical description” [1,2,3] and the “dynamical description” . In the static description the keyhole is a vertical rigid cylinder moving towards the welding direction. The keyhole is stable and stationary and its equilibrium is described as a simple balance between opening forces coming from the bottom of the cylinder and the closure forces due to the melted metal. In the dynamical description the keyhole does not have a defined shape; this shape is generated continuously by the succession of local beam absorption/ vaporization/ reflection cycle on many and small parts of the keyhole wall surface.
In the case of 1μm lasers, the shielding gas is principally used to protect the weld from atmospheric pollution. Nevertheless the role of the shielding gas for high power high brightness lasers (like Yb:YAG 10kW lasers or more) is controversy. Some authors prefer to use a gas protection to help laser/metal coupling [7,8], while others [9} affirm that blowing out the plume exiting from the keyhole with a fan is sufficient to obtain a good weld with correct penetration at the right welding speed.
CO2 laser welding: He and LASALMIX
Ar/He mixture shielding gas
Due to its high ionization-potential (24.5eV), Helium is considered as the reference gas for CO2 laser welding process [10,11]. Other gases such as argon or nitrogen can be used, but their low ionization potential limits their use to low laser power and/or power density. The following table summarizes the values of ionization potentials of the main welding gases (table 1):
Helium has the highest ionization potential (24.5eV). It is very difficult to ionize this atom. Helium is an inert gas which does not affect the metallurgy of the weld seam.
The ionization potential of argon is lower than that of helium (15.7eV). Argon is used for CO2 laser powers up to 3kW. As helium, argon is an inert gas which does not affect the weld seam. The reduced cost of Argon versus Helium makes Argon a good candidate to replace Helium.
The ionization potential of atomic nitrogen (15.5eV) is closed to the Argon one, however the shielding gas plasma formation process in N2 (N ≡ N) is more complex. The ionization of nitrogen atom occurs only after the break of the Di-Nitrogen molecule. The energy of the triple bond N ≡ N is around 9eV12. The direct ionization of the nitrogen molecule N2 is possible but the resulting ion species (N2+...) are not stable. Thus, it is possible to use nitrogen up to 5kW without losing the laser/metal coupling. Nitrogen reacts with several metallic species, for this reason Nitrogen is not largely used as laser shielding gas.
LASALMIX Ar/He mixtures can be used as shielding gas. The key point to determine the maximum amount of Argon acceptable in the shielding gas is the laser power density available above the metallic plasma plumes Figure 2. Higher is the power density above the plasma plume, higher is the risk of ionization breakdown. For short focal length the power density decreases with the increase of Z faster that in the case of long focal length.
Remembering that the height of the metallic plasma plume is evaluated around 5mm and 10mm , we can related the power density above the plasma plume with the focal length of the focalization mirror, as described in the table 2.
From an operational point of view is easiest to talk about focal length instead of power density above the plasma plume; for this reason the results are summarized as follow:
For 12kW CO2 laser, natural beam of 25mm the suggested gas mixtures are:
40% Ar–60% He with a focal length of 250mm,
65% Ar–35% He with a focal length of 200mm,
75% Ar–25% He with a focal length of 150mm.
We can conclude that the mixture 70% He-30%Ar is suitable for the largest number of configuration, nevertheless a specific evaluation of the mixture required is strongly suggested.
CO2 laser welding with 100% Argon shielding gas – LASAL JET NOZZLE
Nevertheless Helium or Helium based mixtures like LASALMIX are generally used for CO2 laser welding. We can go further and replace the Helium shielding gas with 100% Argon fast gas jet.
The basic principle is to stop the propagation of the induced gas plasma with a dynamic Argon jet crossing the axis of the laser beam just above the metallic plasma plume. Its action is to blow out the ionised gas particles away from the focused laser beam path Figure 3. This fast gas jet reduces the interaction time between the gas particles and the laser beam and the avalanche process cannot occur.
We develop a device, called LASAL jet Nozzle, made with a fast dynamic jet nozzle and a dedicated positioning system, Figure 4. It can be used to weld with CO2 laser up to 12kW. The position of the nozzle can be adjusted in the three main directions XYZ in a range of 5mm and the positioning system allows to set up the nozzle in a few seconds. This device is adapted to work with all kind of industrial focusing heads with a focal length between 200mm and 300mm. The nozzle has two orifices, the first one dedicated to the blowing, and the second to protect the weld from atmospheric pollution. The solution is Air Liquide patented.
The LASAL jet nozzle Figure 5 allows welding with 100% Argon without generating gas plasma in many industrial welding configurations as tailored blank, flat panels, pipe lines, etc.
In Figure 6 we present the comparison between a weld with Helium shielding gas, (a) and with 100% Argon dynamic gas jet (b). The beads have no defects or porosity, the shape is similar in the two cases.
CO2 laser welding: 100% Argon for tailored blanks applications
We test the solution on tailored blank application. The working parameters are:
8kW CO2 laser sources, focal 200mm
LASAL jet nozzle with 30l/min of Argon,
3 configurations zinc coated plates (1.02mm/ 1.76mm), (2.47mm/2.47mm), (1.02mm/1.02mm.)
The welds are shown in Figure 7 and in Figure 8.
Visual analysis of the surface (concavities, holes...), macrographies, X-ray control and Erichsen/Olsen tests Figure 9 show the good quality of the welds. (For the Erichsen/Olsen test we consider 75% deformation limit before cracking). The beads have no defects, good mechanical properties and they are less expensive than the ones obtained with Helium shielding gas.
We observe also a reduction of the amount of fumes deposited around the weld. Figure 10. We suppose that the dynamical gas jet avoid fume depositions on the plates. Fumes do not disappear; they are simply blown away from the surface of the plate.
YAG laser welding: Argon for keyhole stabilization
As described in the initial paragraph, the keyhole is unstable [14,15,16]; its shape is determined by the dynamical equilibrium between the vaporization pressure and the closure forces due to the movement of the melted metal inside and around the keyhole itself [17,18]. It has been shown that the interaction between the metallic vapors and the keyhole rear wall was responsible of several fluid instabilities like spattering, liquid collapses or waves travelling on the melted pool surface. The use of 100% Argon dynamical gas jet,  oriented inside the keyhole can stabilize it, reducing several welding defects . In fact the gas jet increases significantly the pressure into the keyhole; the resulting opening is large enough to avoid unwanted closing phenomenon.
The positioning, the dimensions of the argon jet and the flow rate are critical parameters. The gas jet has to be laminar, fast, with an exit section as close as possible to the width of the weld bead and oriented only inside the keyhole; the better results are obtained with a gas jet directed toward the rear wall of the keyhole Figure 11.
If the flow rate is too high, the liquid metal is pushed back too far, the liquid solidify prematurely, and humps are generated. If the flow rate is too low, there is no significant keyhole opening and we are close from the standard laser welding configuration.
We have developed a nozzle and a positioning system dedicated to the keyhole stabilization process, called LASAL Keyhole Stabilization nozzle Figure 12. The position of the nozzle can be adjusted in the three main directions XYZ in a range of 5mm and the positioning system allows to set up the nozzle in few seconds. The nozzle presents 3 gas jets. The central one is oriented inside the keyhole, while the two surrounding gas jets have a double action: they stabilize the position of the rear wall of the keyhole and protect the weld against atmospheric pollution and oxidation. The solution is Air Liquide patented.
For a Nd:YAG laser having a spot diameter of 600μm and keyhole section around 600μm-1mm, the effect of the dynamical flow allows to open the keyhole till 3mm instead of 1mm, as it is shown in Figure 13.
YAG laser welding: Argon for zinc coated steel overlap joints welding
Laser welding of zinc coated plates in overlap configuration with zero gap presents some difficulties due to the zinc vaporisation at the plate interface. Because zinc has a low vaporization temperature, (906°C), the zinc vapors induce very strong pressure forces on the keyhole walls at the interface between the two plates. Those forces are so strong that they destabilize the keyhole, generating melted pool ejection and intense spattering, resulting in lack of welding or holes into the welding bead. The actual method to laser weld zinc coated plates in overlapped configuration is to generate an artificial gap between the plates with pressure fingers or lateral wheels pressure or embossment technique to let the zinc vapors escape through this gap.
An alternative solution is to open the keyhole significantly with a controlled argon jet and let the zinc vapors exit directly through the keyhole. Then, the zinc vapors come out without interacting with the molten metal, as shown in Figure 14 without generating defects or spatters.
Dedicated experiments on 2 overlap zinc coated configurations are described in the following. The operational parameters are:
4kW Nd: YAG laser, focal lenght 200mm, working fiber 600μm, top hat beam shape.
LASAL stabilization nozzle with 15l/min of Argon
GI=Galvanized coating composed by 99.5% Zn and 0.5% Al. Zinc thickness around 10μm.
EZ=Electro-Galvanized coating. Electrolytic 100% Zn deposition. Zinc thickness around 7.5μm.
Configuration : 0.7mm/0.7mm with zero gap.
Welding speed 5 m/min
The results are valid both for GI and EZ plates. The weld of the overlap configuration without assisted gas Figure 15 (a) presents several defects, like lack of metal, spatters, and an unacceptable superficial aspect. On the other the weld performed with 16l/min of Argon at the same welding speed has no defects.
The macros of the section of the two welds are illustrated in Figure 16. The defect in figure (a) is generated by the explosion of molten metal during the vaporisation and expansion of the zinc layers. The macro on the right has no defects. It is also to be mentioned that this device works also very well with a gap.
We note also a difference concerning the fume deposition on the plates. Comparing the images of the welding process with no shielding gas and with the LASAL stabilization nozzle at 15l/min of Argon, we note that the atmospheric pollution is more important without the LASAL keyhole stabilization nozzle. Figure 17.
Argon gas with specific dynamical properties is a good solution to replace other shielding gas in laser welding. The proposed solutions are suitable for high power CO2 laser up to 12kW, for high power YAG lasers, solid states and for all kind of applications and materials. A more detailed discussion is dedicated to two more important automotive applications: CO2 laser welding of tailored blanks and YAG laser welding of overlapped zinc coated plates. The argon gas solutions and the nozzles are AIR LIQUIDE patented.
Tailored blanks welding with CO2 laser: we can replace Helium shielding gas with 100% Argon using the new LASAL Jet nozzle. The weld quality is the same as the traditional Helium shielding gas solution. The action of the fast argon jet is to remove the ionized atoms from the laser path, in order to avoid ionization plasma breakdown. The LASAL jet nozzle works with a gas flow of 30l/min of Argon, allowing a reasonable cost saving.
Zinc coated plates in overlap configuration with zero gap can be welded with a YAG laser without any pressure device or embossment technique. The LASAL jet nozzle produces a dynamic argon gas jet oriented inside the keyhole. The gas jet increases the keyhole opening and the metal vapors escapes from it, eliminating molten metal explosion. The Argon flow rate is around 15l/min.
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