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What are the limitations of Continuous Wave (CW) CO2 and Nd:YAG lasers?

   
  • Laser power : the output power of a laser determines its performance in terms of depth of penetration or welding speed, for instance. Currently, CO2 lasers are commercially available up to 15kW in power, lamp and diode-pumped Nd:YAG lasers up to 6kW, Yb-fibre lasers up to 20kW (or more), Yb:YAG disc lasers up to 8kW and direct-diode lasers up to 6kW.
  • Laser efficiency : the efficiency with which laser light can be generated is low, in particular, when compared with other welding processes. Typical wallplug efficiencies for a number of laser types are given below:
  • - CO2 laser : 10-12%
    - Lamp-pumped Nd:YAG laser : 3-5%
    - Diode-pumped Nd:YAG laser : 8-10%
    - Yb-fibre laser : 25-30%
    - Yb:YAG thin-disc laser: 25-30%

    - Direct-diode laser : 30-50%

  • Capital costs : the capital cost of a laser is high, particularly when compared with equipment required for conventional arc welding processes. When purchasing a laser system, the cost of a chiller unit, gas supplies, beam delivery systems and beam manipulation, for instance, should also be considered. The running costs of a laser system can therefore be high, if not used to its maximum capacity.
  • Operators : trained staff are needed for setting up the laser, workpiece and processing conditions, although the process itself is fully automated.
  • Maintenance : the different laser technologies require skilled servicing at regular intervals, generally carried out by the manufacturer. Cleaning and realignment of mirrors in CO2 laser systems (typically every 500 hours) and changing lamps/diodes for some solid-state lasers (typically every 1000/10000 hours, respectively), however, can be done in-house by trained staff. General maintenance is not required for the Yb-fibre lasers.
  • Health and Safety : all material processing lasers are Class IV and as such, the radiation produced should be regarded as potentially dangerous. The regulations for use of Class IV laser sources can be found in the standard EN 60825-1 : 1994 (IEC 60825-1 : 1993) (Safety of Laser Products - Equipment classification, requirements and user's guide).
  • Materials : materials that can be welded using conventional arc welding processes, can generally be laser welded. The high speeds achieved with the laser process, however, may for some materials increase the tendency for solidification cracking, or for porosity to form in the weld because of the high cooling rates. Care should be taken when welding materials with a high surface reflectivity to ensure absorption of the laser light into the material.
  • Joint fit-up and material preparation : laser welding in the keyhole mode requires a good fit-up in terms of joint gap and joint alignment (with values dependent on the material thickness). The addition of filler wire enlarges the fit-up tolerance of the process. To achieve a good laser weld quality, material edge preparation is recommended immediately prior to welding to eliminate oil, dust, scale, rust, etc.

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