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498431
Lasers
Description
Mind Map on Lasers, created by smith_legend on 21/01/2014.
Mind Map by
smith_legend
, updated more than 1 year ago
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Created by
smith_legend
over 10 years ago
505
2
0
Resource summary
Lasers
Summary
Basics
Absorption
Spontaneous Emission
Stimulated Emission
Population Inversion
components
Gain/Laser medium
Pump Source
Optical Resonator
Absorption mechanisms
Fresnal
Ablation
Inverse Bremstrahlung
Laser Types
Excimer
248nm
Diode
808-1100nm
Nd:YAG
1064nm
Fibre
1070-1080nm
CO2
10600nm
Laser Welding
Conduction Welding
Process
Laser absorped at surface
Heat transfer: conduction and melt pool convection
Advantages
Stable
High Quality
No material loss
Large spot size
Disadvantages
Surface Reflectivity
Slow process
Depth limited by heat transfer
Lasers
CO2 (10600nm)
Diode (808-1100nm)
Conduction Limited Transmission Welding
Near IR to weld transparent polymers
Interface or bottom of workpiece IR absorbing
heat energy absorbed enough to form weld
Deep Penetration (Keyhole Welding)
Process
High laser power density
fast vapourisation
vapour causes pressure depression in melt
keyhole forms (directly below)
Energy absorption
different to conduction
In Plasma: Inverse Bremstrahlung (point like)
At walls: Fresnal (line like)
Advantages
High aspect ratio
Deep penetration
High weld speed
Low heat input
less distortion
Disadvantages
Joint fit-up
Defects
Spatter
Cracking
Porosity
Assist Gas
Cools and removes plasma
Protects from atmosphere and oxidation
Protects optics from vapour
Laser Cladding
Process
heating
Metling
dynamic melt pool
Rapid solidification
material fed into laser path
fused to substrate
Methods
Preplaced Powder
highly complex geometries
very slow
Wire fed
more heat energy
larger melt pool
straight line only
high deposition rate
Blown powder
Most versatile
Pore free tracks
Feeding
Off-axis
Coaxial
dis/continuous
Output variables/defects
Microstructure
Geometric tolerence
Oxidisation
Porosity
Surface roughness
Dilution
Spallation
Lasers
C02 (10600nm)
Poor absorption
Nd:YAG (1060nm)
Good absorption
Diode (808-1100nm)
good absorbtion
High surface coupling
Fibre (1070-1080nm)
Good absorption
Best for pre-placed
Laser Drilling
Lasers
Nd:YAG (1064nm)
Excimer (248nm)
Fibre (1070-1080nm)
Stages
1. Surface heating without phase change
2. surface melting
3. Vapourisation
4. Melt ejection
Defects
Hole tapering
Spatter
Dross
Recast layer
Micro-cracking
Types
Single Pulse
large no. holes
Nd:YAG (1064nm)
shallow, less than 1mm
Percussion
higher aspect ratio
inconsistent quality
20mm depth, 1mm dia.
Trepanning
combined drilling and cutting with pulses
Freeform and contours
Helical
vapourization dominated
very precise
good microstructure
Pulse Duration
Long
Large plasma plume
Large HAZ
Shockwaves
Microcracking
Recast layer
Short
Smaller plasma plume
smaller HAZ
Laser Cutting
Process
heating
melting/exothermic
vapourization/ejection
Types (relative energy)
Vapourization (40)
Focussed beam: keyhole
deepens fast
vapour blows out melt
Thin and nonconductive metals
Fusion Cutting (20)
'melt and blow'
melts material and blown out by gas jet
No boiling-low HAZ
Reactive Fusion Cutting(10)
'melt burn and blow'
reactive gas used also
less heat input
Controlled Fracture Cutting (1)
Brittle materials sensitive to thermal fracture
localzed heating
local expansion:stress build up
rapid cooling
Crack: stress raiser
Scribing (1)
Grooves/lines of holes in brittle material
weaken sturcture allowing mechanical break
Cold Cutting (100)
Ultra short UV pulses cut without melting
breaks molecular bonds
no melt or charr or boiling
Spot size and mode
smaller spot size
less power density
higher absoprtion
lower kerf width
High brightness beam
deeper cut
narrower kerf
material removal issue
Gas jets
Too high velocity
side burning
cooling and less efficient
O2
reactive fusion cutting
Argon
Aerospace
Nitrogen
thin sheets
avoid oxygen scales on steel
compressed air
Low cost
increased oxidation/dross
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