October 11, 2005
Evaluating the gas equipment you need to produce the shielding mixture you want for optimal laser hybrid welding is critical. Different shielding gases yield different results and should be considered carefully for your specific application.
|Laser welding is the basis of hybrid laser-arc welding.|
First flat-sheet bed cutting expanded job shop capabilities. Then multiaxis processing came along. Today, even as the dark side squeezes hard on job shop profits, new opportunities for differentiation continue to arise.
Enter hybrid laser-arc welding, a significant force for propelling profits into the next millennium and the latest in a technology trilogy shaping metal fabrication.
Hybrid laser-arc welding introduces a secondary energy source to the weld pool area (see Figure 1). It combines typical laser welding benefits—high travel speeds, limited heat-affected zone (HAZ), narrow weld joint, and good bead appearance—with those of gas metal arc welding (GMAW): process energy efficiency, gap-bridging, slow cooling rates, and energy coupling efficiency.
Hybrid laser-arc welding couples the laser beam with a secondary energy source. Source: M. Douglass, Ph.D., Visteon, "Recent Developments in Laser Welding Power Train Applications," in proceedings from ALAC 2004—Advanced Laser Applications Conference & Exposition, sponsored by the National Center for Manufacturing Sciences, Ann Arbor, Mich., Sept. 20-22, 2004, Vol. 2, p. 59.
Shielding gas components are critical to obtaining the best bead profile, plasma formation, and coupling efficiency from hybrid laser-arc welding. Evaluating the gas equipment required to produce the shielding mixture completes the saga.
Before you understand how different shielding gas combinations affect hybrid laser-arc welding, you should understand some basics about the process.
The GMAW wire can be introduced before or after the laser beam. Laser beam attenuation (scattering and absorption) caused by vapor particles evacuating the keyhole or weld area reduces the amount of beam energy coupled to the base material.1Laser beam scattering and absorption reduce the travel speed and depth of the weldment.2The larger the particles, the greater the attenuation effect.
To select the appropriate shielding gas combination, you must understand the basic gas properties of ionization potential, thermal conductivity, density, and reactivity. Both the laser beam and arc process provide enough energy to ionize (electron-donate) the outermost electron of the shielding gas molecule, thereby creating the plasma of flowing electrons.
Figure 2shows the ionization potential of various gases. Gases with low ionization potential, such as argon, offer good GMAW arc initiation and stability. However, this must be balanced with the negative effects of plasma formation, keyhole vapor particle size, and the attenuation effect on the incident laser beam.
Likewise on the positive side, helium shielding gas produces the smallest average vapor particle size. On the other hand, 100 percent helium tends to reduce GMAW depth of penetration and arc stability. Again, balance is necessary—in this case, you should balance the amount of laser-induced plasma formation and the resulting effect on the GMAW arc transfer.
Adding 30 percent to 50 percent argon to helium can provide the low plasma formation attributed to helium and the good GMAW arc initiation and control associated with argon.
Shielding Gas Ionization Potential Source: AWS C5.10-C5.10M: 2003, Recommended Practices for Shielding Gases for Welding and Cutting (Miami: American Welding Society), p. 3.
Shielding Gas Thermal Conductivity Source: Isidoro Martinez, Professor of Thermodynamics, Department of Thermodynamics, Ciudad Universitaria, Madrid.
This mixture may be acceptable for thin materials requiring minimal penetration. However, thicker materials may benefit from a 3 percent to 10 percent addition of carbon dioxide and/or oxygen. Once the arc is established, the plasma gas begins to distribute the heat circumferentially toward the workpiece. Gases with a low thermal conductivity, such as carbon dioxide, exhibit a narrow GMAW arc with a high inner core temperature that produces a deep "V" penetration profile.
Figure 3illustrates, in ascending order, the thermal conductivity of various gases. High-thermal-conductivity gases distribute heat away from the core at a greater rate, thereby producing a wider but shallower penetration profile. Depending on joint configuration, adding 1 percent to 5 percent oxygen to the helium-argon blend will stabilize the GMAW arc and improve weld puddle fluidity. This provides better tie-in at the toe of the weld and minimizes weld joint stresses.
The greater the density of a mixture, the more quickly vapor particles can be evacuated from the keyhole to minimize laser beam attenuation. More energy is coupled to the workpiece, enabling higher travel speeds and productivity. Also, high-density argon and carbon dioxide components dwell longer about the cooling weld. This facilitates higher travel speeds while protecting against atmospheric contamination. The added heat of GMAW also slows the cooling rate, thereby allowing trapped gases to escape and preventing porosity formation. Gas mixture flow rates are lower than helium, so this reduces shielding gas consumption.
Minor additions of 1 percent to 5 percent hydrogen can be added to the helium-argon gas mixture to improve puddle fluidity. Hydrogen's reducing, electron-donating characteristic allows it to react with the cooling weld puddle to minimize the amount of oxide formation. This creates a cleaner weld without reducing toughness.
Now that you've learned about the general effects of various shielding gas mixtures, it's time to select the necessary equipment to deliver a precise shielding gas mixture.
An infinite-adjustment gas blending system helps provide optimal shielding gas for hybrid laser-arc welding.
For manufacturers, planning a growth strategy is a must for keeping shielding gas costs competitive. Asset flexibility also is important.
High-pressure cylinder clusters for monthly consumptions up to 28,000 cubic feet comprise the typical helium supply mode. High-pressure tube trailers are optimal, but logistically, tube trailers can create storage space problems because they require an area with two so-called receiving bays for loading and unloading. Blending 40 percent to 50 percent argon can reduce the volume of helium consumed per month, extending the usefulness of helium in high-pressure clusters.
Blending systems generally offer infinite adjustment from 0 percent to 100 percent argon, helium, and other components for process optimization (see Figure 4). Argon can be delivered in liquid cans for monthly consumptions up to 40,000 cubic feet. However, you must address liquid can vent and residual losses. Pressure differential switchover technology does not address the 230-, 350- and 500-pound-per-square-inch (PSI) liquid cylinders' pressure-building and economizer dynamics adequately to reduce these losses.
A recent development in intelligent switchover technology can help resolve these concerns. An intelligent switchover uses microprocessor controls to analyze the output of precise pressure transducers to assign cylinder bank priority in a logical manner to reduce vent and residual losses. Balanced stem seat technology switchovers can provide switchover response flexibility by eliminating static and decaying inlet pressure effects. Other options include microbulk or bulk vessels.
Hydrogen and oxygen in 1 percent to 5 percent concentrations can be delivered via individual or clusters of high-pressure cylinders. Simple pressure differential switchovers can supply gas to the mixer continuously. The only other consideration is the use of a remote alarm and intrinsically safe electrical circuits. Oxygen may have other uses within the facility, which would enable using existing on-site assets.
Victory is at hand as the job shop seeking differentiation can reach a key goal—process profitability—with the latest advances in lasers. Hybrid laser-arc welding with attention to shielding gas selection and supply mode can provide the productivity customers demand.
Lead photo courtesy of TRUMPF Inc., Farmington, Conn.
1. A. Matsunawa and T. Ohnawa, "Beam-Plume Interaction in Laser Materials Processing," Trans. JWRI 21, 1 (1991).
2. J. Greses, P.A. Hilton, C.Y. Barlow, and W.M. Steen, "Plume Attenuation Under High Power Nd:Yttrium-Aluminum-Garnet Laser Welding," Journal of Laser Applications, Vol. 16, No. 1 (2004).
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