Lecture 3 - Laser-Material Interactions Flashcards
What is the laser energy determined by?
laser power, beam size
wavelength, frequency
What are the four types of laser?
CO2
Nd:YAG
Yb-fiber
Excimer
What laser is most often used for commercial machine?
Nd:YAG
What laser is the cheapest?
CO2
What laser is most often used for PBF techniques?
Yb-fiber
What is the Excimer laser used for?
Polymers (UV cure)
How does a long pulse laser (microseconds) compare to short pulse laser (femtoseconds)?
The shorter the pulse, the less melt zone, ejected material, and damaged surface
What is a short pulse laser used for?
machining and cutting
What is rb and r?
rb is the laser beam radius, typically smaller than the melt zone radius (r)
What profile is assumed for the laser?
Gaussian beam profile
What is the 1/e^2 width?
the width of the laser beam at 1/e^2 * maximum value
What is the radius of the beam W(z)?
W(z) = FWHM(z)/sqrt(2*ln2) ~ 1.7FWHM(z)/2
What is FWHM?
full width at half maximum
What is the equation for absorbance, reflectance, and transmittance?
A + R + T = 1
Why can particles reflect multiple times?
powder materials will reflect with each other in the powder bed
What is Fresnel’s Equation?
A = 1 - R = 1 - ((n-1)^2 + k^2)/((n+1)^2 + k^2)
What is n?
refractive index (real part) measured as n = c/v
What is k?
refractive index (imaginary part) or extinction/attenuation coefficient
What is the experimental equation for absorptivity?
A = (Energy absorbed)/(total input energy)
What are the three experimental methods to determine A?
wide-range UV-vis-IR spectrometer
direct calorimetry method
integrating sphere method
What is material has good absorptivity?
316L (stainless steel)
What material does not absorb well?
W (tungsten)
What factors affect absorptivity?
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
What is the problem with copper?
High thermal transmittivity, which reduces absorptivity
What is the usual green packing density?
65%
Is absorptivity dependence on laser power linear or nonlinear?
nonlinear
What is keyhole mode?
D>W/2, depth of melting is greater than half the width
What is conduction mode?
D<W/2, depth of melting is less than half the width
What is melt pool depth?
D, from surface to end
What is melt pool width?
W, width of melt pool
What is the Eagar model?
prediction of the melt pool shape based on welding models
What is the correlation between weld width and power?
more power = more width
What does the Rosenthal equation do?
calculate temperature within a build
What does the Rosenthal equation assume?
the heat capacitance remains constant (actually is a function of the temperature)
What is the hatch spacing?
distance between lines
How much hatch spacing is preferred?
50% overlapping with width
What is spatter?
formation of metal droplets that leave the melt pool from flow conditions in the capillary and melt pool
How do you eliminate laser turn-around pore formation?
Reduce the power towards the edge of a part
How are laser turn-around pores formed?
laser is exposed on part for too long of a time towards the edge
What is laser expulsion?
particles are launched off the surface by a laser
What is self-replication?
What is shadowing effect?
scattering of particles absorb and scatter incoming laser beam, which results in less power on the particles below
What is the denudation zone?
area without powder
What happens when your laser speed is too fast?
pores are trapped due to depression collapse
What is laser tracing?
simulation done for shadowing effect by professor’s colleagues
When is the shadowing effect most significant?
When particles melt together to block the laser at a bigger scale
What are the critical conditions for keyhole instability?
scan speed and power
What happens if the power and scan speed do not align?
keyhole porosity or stable melting
What can happen to a pore once formed?
shrinking, splitting, movement, rebound
How is layer thickness defined?
The overlapping depth based on the hatch spacing
What are the laser scan strategies?
all x, all y, x-y alternate, chequerboard/island (offset), chequerboard/island no offset
What is contour vs raster fill?
countour is outside, raster fill is inside
What mode is in between conduction mode and keyhole mode?
transition
What is vapor depression?
vapor forming within the melt pool
What is melt pool instability?
weird shaped melt pools (pinched off vapor depression)
What characteristic materials can see laser cross-sections with x-rays?
light-z materials
What leads to a drastic increase of absorptivity for each material?
a critical power number
What is the integrating sphere method?
measures reflected light to calculate absorbed light
What is the direct calorimetry method?
measures the temperature change of a material during laser exposure to calculate absorbed energy
What are two characteristics of the direct calorimetry method?
versatile (different laser powers and scanning speeds)
commonly used in L-BPF process
What is the wide-range UV-vis-IR spectrometer method?
uses variety of lasers (green, diode, Nd:YAG) to measure absorptivity
What are two characteristics of the wide-range UV-vis-IR spectrometer method?
inconvenient for measuring absorptivity at elevated temperatures or different laser powers
What is the range of the wide-range UV-vis-IR spectrometer method?
200-1700 nm
What is the range of the green laser?
520-532 nm
What is the range of the diode laser?
810-1064 nm
What is the range of the Nd:YAG laser?
1064-1070 nm
What is Drude’s theory?
a (complicated) method to evaluate refractive indexes (real and imaginary) for Fresnel Equation
