Industrial Lasers for Welding

1. Industrial Lasers for Welding Ing. M. Muhshin Aziz Khan

2. Laser is essentially an optical amplifier that • Generates and • Amplifies Stimulated Emission Facts About Laser:Laser Basics Light Amplification by Stimulated Emission of Radiation • Laser Components Lasing Medium:Provides appropriate transition and Determines the wavelength (it must be in a metastable state) • Pump:Providesenergy necessary for populationinversion • Optical Cavity:Provides opportunity for amplification and Produces a directional beam (with defined length and transparency) • Properties of Laser • Coherent (synchronized phase of light) • Collimated(parallel nature of the beam) • Monochromatic (single wavelength) • High intensity (~1014W/m2)

3. Facts About Laser:Laser History

4. Energy Highly excited State • For material with Two-Level system • Absorption and stimulated processes neutralize one another. • The material becomes transparent. Two-Level 2° Excited State E3 1° Excited State Laser Transition Population Inversion Excitation E2 Ground State Ground State E1 - + Facts About Laser:Laser Material: Energy Levels of Atomic or Molecular System Laser operation takes place via transitions between differentenergy levels of an atomic or molecular system Population inversion is impossible

5. Highly excited State Highly excited State Fast Decay Fast Decay Three-Level Four-Level Metastable State Higher Metastable State Laser Transition Population Inversion Excitation Excitation Laser Transition Population Inversion Lower Metastable State Natural Depopulation Ground State Ground State Facts About Laser:Laser Material: Energy Levels of Atomic or Molecular System

6. Amplification by stimulated emissionincreases the Intensity according to • I(z) = I(0) exp-(gz) • Absorption by atoms in Level 1decreases the Intensity according to • I(z) = I(0) exp-(σz) According to BoltzmannRatio Where E2 > E1 Net effect of passage of light through the material Facts About Laser:Optical Pumping: Population Inversion • Moreatoms or molecules are in a higherenergy state • Nonequilibriumdistribution of atoms among the variousenergy level of atomic system Is populationinversion by thermal excitation possible!!??!! • Process producing populationinversion is called Pumping • Energy needed for populationinversion is supplied by opticalexcitation with lightsource • Flash lamps (Pulsed laser), Arc lamp (CW Laser), Semiconductor Diode • Excitation by electron collisions and resonanttransfer of energy (Gaseous) Is population inversion a necesarycondition for laser operation!!??! Light with intensity I(z) passing through a laser medium with densities of atoms N1 and N2 in higher and lower energy levels

7. Stability Diagramfor laser resonatorsconsisting of two mirrors Condition for stability of aresonator Facts About Laser:Optical Cavity: Mirror Configuration • Resonatorcavity is formed by placing mirrors at the ends of the active medium The mirrors are perpendicular to the axis along which the laser light travels • Acts as Positive feedback system • Provides amplification and directionalityto a laser beam via oscillation • The resonantcavity generally is much longer than it’s width Mirror configurations are judged on two criteria • Stability Light rays bouncingback and forth between mirrors will be re-entrant. • Filling of the active medium by light Spatial profile defined by the light rays fills all the volume of the active medium

8. Possible Mirror Configurations Facts About Laser:Optical Resonator: Mirror Configurations Plane paraller and Confocal Mirror Configurations • Both mirrorconfigurations have marginal or delicatestability • Plane parallel mirrors have good filling whereas confocal mirrors offers poor filling of active medium • For plane parallel mirrors, allignment is really crucial. However, for confocal mirrors even if the configuration is not exactly perfect, the light rays will still be reentrant. Long-radius mirror configuration is most often used in moderncomercial lasers • It falls within a region of goodstability • Beam spatial profilefills active medium reasonablywell

9. Laser gain and losses Laser turn-on and gain saturation • Gaindecreases as output power increases • Saturation Facts About Laser:Optical Resonator: Gain • Optical loss in resonant cavity • r1 and r2 : mirror loss/coupling loss Due to non-unity reflectivity on the mirrors Loss is Independent of cavity length • exp(-2αL) ~ 1-2αL : distributed loss/internal loss • Due to absorption/scattering in • the cavity material • Loss is proportional to cavity • length Oscilation Condition: Gain ≥ Losses

10. Output versus input power for an optically pump power Slope efficiency (or differential efficiency) The slope of the curve obtained by plotting the laser output versus the pump power. Facts About Laser:Laser Efficiency

11. Facts About Laser:Laser Quality and Its Effect Effects of Beam Quality Beam Quality • A measure of Lasers’ capability to be • propagatedwith low divergenceand • focused to a small spot by a lensormirror • Beam Quality is measured by M2 or BPP (Beam Product Parameter, mm.mrad) • Ratio of divergence of actual beam to a theoretical diffraction limited beam with samewaist diameter • M2= 1; Ideal Gaussian Beam, perfectly diffraction limited • Value of M2 tends to increase with increasing laser power • Smaller focus at constant aperture and focal length • Longer working distance at constant aperture and spot diameter • Smaller aperture (‘slim optics’) at constant focal diameter and working distance A higher power density by a smaller spot size with the same optics, or The samepower density at lowerlaser power

12. Facts About Laser:Primary Adjustable Parameters and Their Effects Change in Pulse Duration Primary Controllable Parameters • Laser Beam Energy Output Characteristics (i) Voltage (ii) Pulse Duration • Laser Focus Characteristic (iii) Laser Beam Diameter Increased pulse duration results in deeper and wider melting Change in Voltage Change in Voltage and Pulse Duration Increased voltage results in deeper physical penetration with less melting due to physical pressure Simultanousincreasein voltage and pulse duration results in deeper melting Change in Beam Diameter Increased beam diameter results inshallow soft penetration and wide, but soft melting

13. Facts about lasers for welding Laser Characteristics, Quality and Application • Typical commercial lasers for welding • CO2 Laser • Nd3+:YAG Lasers • Lamp-pumped • LD-pumped • DiskLaser • Diode Laser • Fiber Laser CO2 Laser: M2 values [CW]

14. Facts about lasers for Welding:YAG Laser Laser Characteristics, Quality and Application YAG Laser: M2 values [CW & PW]

15. Facts about lasers for welding:Disk Laser Laser Characteristics, Quality and Application Recent Development(Mann 2004; and Morris 2004): • Commercially available disk laser system: 1 and 4 kW class • Beam delivery with 150and 200µm diameter fiber • Even a 1 kW class laser is able to produce • a deepkeyhole-type weld bead • extremely narrow width in stainless steel and aluminum alloy

16. Facts about lasers for welding:Diode Laser Laser Characteristics, Quality and Application Recent Development(Hayashi 2004; and Zediker 2001): • Commercially available Diode laser system: Direct and/or fiber-coupled modes • Found suitable for welding of • plastics and • thin sheets of aluminum or steel at high speed • Fiber-delivered laser is used for brazing Zn- coated steel using robot.

17. Facts about lasers for welding:Fiber Laser Laser Characteristics, Quality and Application Recent Development(Thomy et.al. 2004; and Ueda 2001): • Fiber lasers of 10kW or more are commercially available • Fiber lasers of 100kW and more are scheduled • Fiber laser at 6.9kW is able to provide deeply penetrated weld at highspeed • Fiber laser is able to replace high quality (slab) CO2 laser for remote or scanning welding

18. Facts about lasers for welding Comparison of different laser systems CorrelationofBeamQualitytoLaserPower (Katayama 2001; O’Neil et. al. 2004; Shiner 2004; Lossen 2003): • Overlaid with condition regimes • Beam quality of a laser worsens with an increase in power • LD-pumped YAG, thin disk, CO2 and fiber lasers can provide high-quality beams • The development of higher power CO2 or YAG lasers is fairly static and, hence Main focuson development: i. high-power diode, ii. LD-pumpedYAG, iii. disk and/or iv. fiber lasers

19. CO2 Laser Facts about lasers for welding Wavelengths of some important laser sources for materials processing Expanded portion of the electromagnetic spectrum showing the wavelengths at which several important lasers operate

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