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MAT SCI 298 > Lecture 3 - Laser-Material Interactions > Flashcards

Lecture 3 - Laser-Material Interactions Flashcards

1
Q

What is the laser energy determined by?

A

laser power, beam size
wavelength, frequency

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2
Q

What are the four types of laser?

A

CO2
Nd:YAG
Yb-fiber
Excimer

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3
Q

What laser is most often used for commercial machine?

A

Nd:YAG

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4
Q

What laser is the cheapest?

A

CO2

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5
Q

What laser is most often used for PBF techniques?

A

Yb-fiber

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6
Q

What is the Excimer laser used for?

A

Polymers (UV cure)

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7
Q

How does a long pulse laser (microseconds) compare to short pulse laser (femtoseconds)?

A

The shorter the pulse, the less melt zone, ejected material, and damaged surface

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8
Q

What is a short pulse laser used for?

A

machining and cutting

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9
Q

What is rb and r?

A

rb is the laser beam radius, typically smaller than the melt zone radius (r)

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10
Q

What profile is assumed for the laser?

A

Gaussian beam profile

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11
Q

What is the 1/e^2 width?

A

the width of the laser beam at 1/e^2 * maximum value

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12
Q

What is the radius of the beam W(z)?

A

W(z) = FWHM(z)/sqrt(2*ln2) ~ 1.7FWHM(z)/2

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13
Q

What is FWHM?

A

full width at half maximum

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14
Q

What is the equation for absorbance, reflectance, and transmittance?

A

A + R + T = 1

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15
Q

Why can particles reflect multiple times?

A

powder materials will reflect with each other in the powder bed

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16
Q

What is Fresnel’s Equation?

A

A = 1 - R = 1 - ((n-1)^2 + k^2)/((n+1)^2 + k^2)

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17
Q

What is n?

A

refractive index (real part) measured as n = c/v

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18
Q

What is k?

A

refractive index (imaginary part) or extinction/attenuation coefficient

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19
Q

What is the experimental equation for absorptivity?

A

A = (Energy absorbed)/(total input energy)

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20
Q

What are the three experimental methods to determine A?

A

wide-range UV-vis-IR spectrometer
direct calorimetry method
integrating sphere method

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21
Q

What is material has good absorptivity?

A

316L (stainless steel)

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22
Q

What material does not absorb well?

A

W (tungsten)

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23
Q

What factors affect absorptivity?

A

type
powder size and distribution
powder layer thickness
surface conditions of powder and roundness
green packing density of powder bed
laser power and speed
pulsed vs continuous laser

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24
Q

What is the problem with copper?

A

High thermal transmittivity, which reduces absorptivity

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25
Q

What is the usual green packing density?

A

65%

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26
Q

Is absorptivity dependence on laser power linear or nonlinear?

A

nonlinear

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27
Q

What is keyhole mode?

A

D>W/2, depth of melting is greater than half the width

28
Q

What is conduction mode?

A

D<W/2, depth of melting is less than half the width

29
Q

What is melt pool depth?

A

D, from surface to end

30
Q

What is melt pool width?

A

W, width of melt pool

31
Q

What is the Eagar model?

A

prediction of the melt pool shape based on welding models

32
Q

What is the correlation between weld width and power?

A

more power = more width

33
Q

What does the Rosenthal equation do?

A

calculate temperature within a build

34
Q

What does the Rosenthal equation assume?

A

the heat capacitance remains constant (actually is a function of the temperature)

35
Q

What is the hatch spacing?

A

distance between lines

36
Q

How much hatch spacing is preferred?

A

50% overlapping with width

37
Q

What is spatter?

A

formation of metal droplets that leave the melt pool from flow conditions in the capillary and melt pool

38
Q

How do you eliminate laser turn-around pore formation?

A

Reduce the power towards the edge of a part

39
Q

How are laser turn-around pores formed?

A

laser is exposed on part for too long of a time towards the edge

40
Q

What is laser expulsion?

A

particles are launched off the surface by a laser

41
Q

What is self-replication?

A
42
Q

What is shadowing effect?

A

scattering of particles absorb and scatter incoming laser beam, which results in less power on the particles below

43
Q

What is the denudation zone?

A

area without powder

44
Q

What happens when your laser speed is too fast?

A

pores are trapped due to depression collapse

45
Q

What is laser tracing?

A

simulation done for shadowing effect by professor’s colleagues

46
Q

When is the shadowing effect most significant?

A

When particles melt together to block the laser at a bigger scale

47
Q

What are the critical conditions for keyhole instability?

A

scan speed and power

48
Q

What happens if the power and scan speed do not align?

A

keyhole porosity or stable melting

49
Q

What can happen to a pore once formed?

A

shrinking, splitting, movement, rebound

50
Q

How is layer thickness defined?

A

The overlapping depth based on the hatch spacing

51
Q

What are the laser scan strategies?

A

all x, all y, x-y alternate, chequerboard/island (offset), chequerboard/island no offset

52
Q

What is contour vs raster fill?

A

countour is outside, raster fill is inside

53
Q

What mode is in between conduction mode and keyhole mode?

A

transition

54
Q

What is vapor depression?

A

vapor forming within the melt pool

55
Q

What is melt pool instability?

A

weird shaped melt pools (pinched off vapor depression)

56
Q

What characteristic materials can see laser cross-sections with x-rays?

A

light-z materials

57
Q

What leads to a drastic increase of absorptivity for each material?

A

a critical power number

58
Q

What is the integrating sphere method?

A

measures reflected light to calculate absorbed light

59
Q

What is the direct calorimetry method?

A

measures the temperature change of a material during laser exposure to calculate absorbed energy

60
Q

What are two characteristics of the direct calorimetry method?

A

versatile (different laser powers and scanning speeds)
commonly used in L-BPF process

61
Q

What is the wide-range UV-vis-IR spectrometer method?

A

uses variety of lasers (green, diode, Nd:YAG) to measure absorptivity

62
Q

What are two characteristics of the wide-range UV-vis-IR spectrometer method?

A

inconvenient for measuring absorptivity at elevated temperatures or different laser powers

63
Q

What is the range of the wide-range UV-vis-IR spectrometer method?

A

200-1700 nm

64
Q

What is the range of the green laser?

A

520-532 nm

65
Q

What is the range of the diode laser?

A

810-1064 nm

66
Q

What is the range of the Nd:YAG laser?

A

1064-1070 nm

67
Q

What is Drude’s theory?

A

a (complicated) method to evaluate refractive indexes (real and imaginary) for Fresnel Equation

MAT SCI 298 (11 decks)

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