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Advanced Manufacturing > Lasers > Flashcards

Lasers Flashcards

1
Q

What does laser stand for?

A

Light Amplification by Stimulated Emission Radiation

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2
Q

What are the 4 main properties of Lasers

A
  • Collimation - One direction
  • Monochromaticity - Single wave/ one colour
  • Coherence - Light waves all in phase
  • High Energy Density - Lots of photons produced
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3
Q

What is the wavelength of a C02 laser?

A

10.6 microns

(x 10^-6)

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4
Q

What is the equation for frequency?

A

Frequency = speed of light / wavelength

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5
Q

Describe how electrons gain and release energy.

A

Electrons gain energy by moving away from the nucleus.

Electrons release energy by moving closer to the nucleus.

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6
Q

Describe stimulated emission.

A

one photon joins, 2 are released. moving from E2 to E1 energy states.

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7
Q

How many levels are required to achieve population inversion?

A

3 levels - E0 and E1

4 levels - E1 and E2

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8
Q

What are the different categories of laser?

A
  • Gas lasers
  • Solid state lasers
  • Semi conductor diode lasers
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9
Q

Give examples of Gas Lasers, their pros and cons, and their light range.

A

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.
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10
Q

What are the types of solid state lasers? give pros, cons.

A

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
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11
Q

What are the characteristics of a diode laser?

A
  • cheap
  • stackable
  • small
  • collimating optics required
  • age related red shift
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12
Q

what affect does material have on choosing a laser?

A

The wavelength required.

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13
Q

What functions does the laser nozzle have?

A
  • Houses the focussing optics
  • protects the focusing optics
  • directs the assist gas
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14
Q

What are the functions of assist gas? And give examples of assist gases.

A
  • 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
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15
Q

What are the main hazards of lasers, and how do you mitigate these hazards?

A

-Interaction with the eye causing blindness
-Fumes
- Electrification
- Damage to skin

  • Enclosed beam
    -Extraction fan
  • Emergency shut off
  • PPE
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16
Q

Describe Fusion Cutting

A
  • 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.
17
Q

Describe Reactive Fusion cutting

A
  • Assist gas is oxygen, this reacts and creates another heat source.
  • Much faster cutting than regular fusion cutting. Rougher cuts.
18
Q

What is the equation for power required for laser cutting?

A

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

19
Q

What are the main material defects associated with laser cutting?

A

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.
20
Q

Describe the 4 main laser drilling techniques.

A

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.

21
Q

What is the equation for drilling velocity?

A

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

22
Q

What are the different types of laser welding?

A

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.
23
Q

What are the benefits and disadvantages of Conduction Welding?

A

Benefits:
- Very Stable
- Very high quality weld
- No joint prep so no loss of material
- Tolerant of joint mismatch

Issues:
- Slow
- High surface reflectivity

24
Q

What are the benefits and issues with keyhole welding?

A

Benefits:
- High speed
- deep welds
- Low heat input

Issues:
- Filler material
- Defects
- Spatter

25
Q

What is the equation for frequency of emission from given energy transition?

A

𝑣21 = 𝐸2−𝐸1/ℎ

26
Q

What is the equation for fluence?

A

Fp = Ep / A

Fp = Fluence

Ep = pulse energy

A = area

27
Q

Whats the equation for power density of a beam?

A

F = P/A

F = Power density

P = Incident power

A = Area

28
Q

What is the equation for power related to energy?

A

P = E/t

P = power
E = energy
t = material thickness

Advanced Manufacturing (2 decks)

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