Lasers Flashcards
What does laser stand for?
Light Amplification by Stimulated Emission Radiation
What are the 4 main properties of Lasers
- Collimation - One direction
- Monochromaticity - Single wave/ one colour
- Coherence - Light waves all in phase
- High Energy Density - Lots of photons produced
What is the wavelength of a C02 laser?
10.6 microns
(x 10^-6)
What is the equation for frequency?
Frequency = speed of light / wavelength
Describe how electrons gain and release energy.
Electrons gain energy by moving away from the nucleus.
Electrons release energy by moving closer to the nucleus.
Describe stimulated emission.
one photon joins, 2 are released. moving from E2 to E1 energy states.
How many levels are required to achieve population inversion?
3 levels - E0 and E1
4 levels - E1 and E2
What are the different categories of laser?
- Gas lasers
- Solid state lasers
- Semi conductor diode lasers
Give examples of Gas Lasers, their pros and cons, and their light range.
C02 lasers, Infrared light range:
- high power
- common technology
- low absorption by metals
- cant be transmitted by fibre optic.
Excimer laser, UV light range:
- for fine detailed work.
- Good for polymer
- Multiple wavelengths
- uses corrosive gases, h & s implications.
What are the types of solid state lasers? give pros, cons.
Fibre Laser:
- Cheaper, faster and smaller than Nd:Yag
- Modern replacement of Nd:Yag
- Low processing speed.
-Beam size limited by wavelength.
Nd:Yag:
- Better for pulsing
- High peak power possible
- Low average power
What are the characteristics of a diode laser?
- cheap
- stackable
- small
- collimating optics required
- age related red shift
what affect does material have on choosing a laser?
The wavelength required.
What functions does the laser nozzle have?
- Houses the focussing optics
- protects the focusing optics
- directs the assist gas
What are the functions of assist gas? And give examples of assist gases.
- protects melt pool and workpiece from atmospheric contamination oxidation
- Cools substrate and blows away spatter
- Different gases for different materials.
- Protects focus optics from spatter
- oxygen, argon, nitrogen
What are the main hazards of lasers, and how do you mitigate these hazards?
-Interaction with the eye causing blindness
-Fumes
- Electrification
- Damage to skin
- Enclosed beam
-Extraction fan - Emergency shut off
- PPE
Describe Fusion Cutting
- Material is melted by the laser and forced out of the bottom of the hole by the assist gas.
- Less energy required than reactive fusion cutting.
Describe Reactive Fusion cutting
- Assist gas is oxygen, this reacts and creates another heat source.
- Much faster cutting than regular fusion cutting. Rougher cuts.
What is the equation for power required for laser cutting?
nP = w * t * V * p ( Cp * T + Lf + m’ Lv)
P = Incident Power
n = coupling coefficient
w = kerf width
t = material thickness
V = cutting speed
p = density
m’ = fraction of melt vaporised
Cp = specific heat capacity
T = temp change
Lf = latent heat of fusion
Lv = Latent heat of vaporisation
What are the main material defects associated with laser cutting?
Dross
- Solid metal stuck to the top or
bottom of material being cut. - Speed range to produce no dross, keep speed high in corners to prevent dross.
Striations
- Lines down the cross section of the cut
- Get larger as they go down the material.
Spatter
- From drilling or first part of cut
- Material ejected from top and land on material
- Ejected by extraction system or assist gas (to prevent)
Focal length
- Lack of focus (too high or too low)
- Wider kerf (cut width) than wanted.
- Thick materials can have a thin cut at top, wide cut at bottom.
Describe the 4 main laser drilling techniques.
Trepanning:
laser moved around circumference of final hole
Helical trepanning:
drill complex hole, several passes to work through material
Single shot:
large power pulse, one shot through material
○ Thin materials,
○ Fast
○ Not that accurate
Percussion:
pulse laser gradually getting further through material
laser stays in same place as opposed to moving in trepanning
Most common for thick materials.
Most precise.
What is the equation for drilling velocity?
V = F0 / p (Lf + Lv + Cp x T)
F0 = absorbed power density
p = density
Lf = latent heat of fusion
Lv = latent heat of vaporisation
Cp = specific heat capacity
T = temp difference between initial temp and vaporisation temp
What are the different types of laser welding?
Conduction limited welding:
- Melting only - Shallow weld pool.
Conduction limited transmission welding:
- Used for to bond polymers - Would not
Keyhole:
- Deeper - Melting and Vaporising - Filler material - Stronger weld - Quality not as good as Conduction.
What are the benefits and disadvantages of Conduction Welding?
Benefits:
- Very Stable
- Very high quality weld
- No joint prep so no loss of material
- Tolerant of joint mismatch
Issues:
- Slow
- High surface reflectivity
What are the benefits and issues with keyhole welding?
Benefits:
- High speed
- deep welds
- Low heat input
Issues:
- Filler material
- Defects
- Spatter
What is the equation for frequency of emission from given energy transition?
𝑣21 = 𝐸2−𝐸1/ℎ
What is the equation for fluence?
Fp = Ep / A
Fp = Fluence
Ep = pulse energy
A = area
Whats the equation for power density of a beam?
F = P/A
F = Power density
P = Incident power
A = Area
What is the equation for power related to energy?
P = E/t
P = power
E = energy
t = material thickness
