Lasers

Lasers

Summary
1. Basics
  1. Absorption
    1. Spontaneous Emission
      1. Stimulated Emission
        Population Inversion
2. components
  1. Gain/Laser medium
  2. Pump Source
  3. Optical Resonator
3. Absorption mechanisms
  1. Fresnal
  2. Ablation
  3. Inverse Bremstrahlung
4. Laser Types
  1. Excimer
    1. 248nm
  2. Diode
    1. 808-1100nm
  3. Nd:YAG
    1. 1064nm
  4. Fibre
    1. 1070-1080nm
  5. CO2
    1. 10600nm
Laser Welding
1. Conduction Welding
  1. Process
    1. Laser absorped at surface
      1. Heat transfer: conduction and melt pool convection
  2. Advantages
    1. Stable
    2. High Quality
    3. No material loss
    4. Large spot size
  3. Disadvantages
    1. Surface Reflectivity
    2. Slow process
    3. Depth limited by heat transfer
  4. Lasers
    1. CO2 (10600nm)
    2. Diode (808-1100nm)
2. Conduction Limited Transmission Welding
  1. Near IR to weld transparent polymers
  2. Interface or bottom of workpiece IR absorbing
  3. heat energy absorbed enough to form weld
3. Deep Penetration (Keyhole Welding)
  1. Process
    1. High laser power density
      1. fast vapourisation
    2. vapour causes pressure depression in melt
      1. keyhole forms (directly below)
    3. Energy absorption
      1. different to conduction
      2. In Plasma: Inverse Bremstrahlung (point like)
      3. At walls: Fresnal (line like)
  2. Advantages
    1. High aspect ratio
    2. Deep penetration
    3. High weld speed
    4. Low heat input
      1. less distortion
  3. Disadvantages
    1. Joint fit-up
    2. Defects
      1. Spatter
      2. Cracking
      3. Porosity
4. Assist Gas
  1. Cools and removes plasma
  2. Protects from atmosphere and oxidation
  3. Protects optics from vapour
Laser Cladding
1. Process
  1. heating
    1. Metling
      1. dynamic melt pool
        Rapid solidification
  2. material fed into laser path
    1. fused to substrate
2. Methods
  1. Preplaced Powder
    1. highly complex geometries
      1. very slow
  2. Wire fed
    1. more heat energy
      1. larger melt pool
        straight line only
        high deposition rate
  3. Blown powder
    1. Most versatile
    2. Pore free tracks
    3. Feeding
      1. Off-axis
      2. Coaxial
        dis/continuous
3. Output variables/defects
  1. Microstructure
  2. Geometric tolerence
  3. Oxidisation
  4. Porosity
  5. Surface roughness
  6. Dilution
  7. Spallation
4. Lasers
  1. C02 (10600nm)
    1. Poor absorption
  2. Nd:YAG (1060nm)
    1. Good absorption
  3. Diode (808-1100nm)
    1. good absorbtion
    2. High surface coupling
  4. Fibre (1070-1080nm)
    1. Good absorption
    2. Best for pre-placed
Laser Drilling
1. Lasers
  1. Nd:YAG (1064nm)
  2. Excimer (248nm)
  3. Fibre (1070-1080nm)
2. Stages
  1. 1. Surface heating without phase change
  2. 2. surface melting
  3. 3. Vapourisation
  4. 4. Melt ejection
3. Defects
  1. Hole tapering
  2. Spatter
  3. Dross
  4. Recast layer
  5. Micro-cracking
4. Types
  1. Single Pulse
    1. large no. holes
    2. Nd:YAG (1064nm)
    3. shallow, less than 1mm
  2. Percussion
    1. higher aspect ratio
    2. inconsistent quality
    3. 20mm depth, 1mm dia.
  3. Trepanning
    1. combined drilling and cutting with pulses
    2. Freeform and contours
  4. Helical
    1. vapourization dominated
    2. very precise
    3. good microstructure
5. Pulse Duration
  1. Long
    1. Large plasma plume
    2. Large HAZ
    3. Shockwaves
    4. Microcracking
    5. Recast layer
  2. Short
    1. Smaller plasma plume
    2. smaller HAZ
Laser Cutting
1. Process
  1. heating
    1. melting/exothermic
      1. vapourization/ejection
2. Types (relative energy)
  1. Vapourization (40)
    1. Focussed beam: keyhole
      1. deepens fast
        vapour blows out melt
    2. Thin and nonconductive metals
  2. Fusion Cutting (20)
    1. 'melt and blow'
      1. melts material and blown out by gas jet
    2. No boiling-low HAZ
  3. Reactive Fusion Cutting(10)
    1. 'melt burn and blow'
      1. reactive gas used also
        less heat input
  4. Controlled Fracture Cutting (1)
    1. Brittle materials sensitive to thermal fracture
    2. localzed heating
      1. local expansion:stress build up
        rapid cooling
        Crack: stress raiser
  5. Scribing (1)
    1. Grooves/lines of holes in brittle material
      1. weaken sturcture allowing mechanical break
  6. Cold Cutting (100)
    1. Ultra short UV pulses cut without melting
      1. breaks molecular bonds
    2. no melt or charr or boiling
3. Spot size and mode
  1. smaller spot size
    1. less power density
      1. higher absoprtion
        lower kerf width
  2. High brightness beam
    1. deeper cut
      1. narrower kerf
        material removal issue
4. Gas jets
  1. Too high velocity
    1. side burning
    2. cooling and less efficient
  2. O2
    1. reactive fusion cutting
  3. Argon
    1. Aerospace
  4. Nitrogen
    1. thin sheets
    2. avoid oxygen scales on steel
  5. compressed air
    1. Low cost
    2. increased oxidation/dross

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