manufacturing LASER PROCESSING Flashcards
(96 cards)
1
Q
is laser manufacturing conventional or non-conventional?
A
non-conventional
2
Q
what does laser stand for?
A
Light
Amplification by
Stimulated
Emission of
Radiation
3
Q
where did the idea of lasers come from?
A
Einstein - paper on photoelectricity
4
Q
when was first industrial laser constructed?
A
1960s
5
Q
is laser light coherent or non coherent
A
The light produced by a laser is coherent
6
Q
What is coherent light?
A
Coherent light is made up of waves all of the same wavelength and in phase.
it is collimated (parallel and non-diverging)
and monochromatic (same wavelength => same colour)
7
Q
what property of laser light makes it good for subtractive manufacturing, as opposed to visible light
A
laser light can be directed to a very narrow region with a very high energy density so can burn materials, hence its use in subtracting manufacturing.
Visible light scatters everywhere and so doesnโt burn material like lasers.
8
Q
what range of the EM spectrum is laser light a part of
A
from Infrared to UV light, depending on wavelength used.
[Radio -> micro -> IR -> visible -> UV -> Xray -> Gamma]
9
Q
2 useful properties of lasers
A
- Spatial coherence
(Allows a laser to be focussed to a tight spot at a high energy density. Also allows laser beam to stay narrow over long distances (collimation). Enables applications such as laser cutting) - High temporal coherence
*(allows very narrow spectrum; ie are monochromatic, as they are non-divergent. Can be used to produce pulses of light - as short as a fs.)
10
Q
Examples of laser consumer product applications
A
- laser printers
- laser pointers
- laser light shows
- temperature guns for checking for hot spots in fire
fighting situations - monitoring materials in process of
heating and cooling
11
Q
Examples of laser medical applications
A
- laser skin procedures bloodless surgery
- laser healing
- surgical treatment
- kidney stone treatment
- eye treatment
- dentistry
12
Q
examples of laser industrial applications
A
- cutting
- welding
- material heat treatment
- marking
13
Q
examples of laser research applications
A
- spectroscopy
- laser ablation
- laser annealing
- laser scattering
- laser interferometry
14
Q
what is wave-particle duality
A
the dual nature that EM radiation is said to have. Itโs behaviour is sometimes a wave, but sometimes a stream of particles (photons)
15
Q
how is the wavelength and energy of a photon related?
A
as wavelength increases, energy decreases
16
Q
2 energy states of atoms
A
ground state (low energy)
excited state (high energy)
17
Q
what theory is the generation of laser theory based on?
A
Boltzmannโs Law of Thermo
18
Q
What is Boltzmannโs law of thermo?
A
There will always be more atoms in the ground state than the excited state for a system to be in thermal equilibrium.
19
Q
How are lasers generated?
(3 required interactions to produce a high energy laser beam)
A
- Population Inversion
- Stimulated Emission
- Amplification
20
Q
What is Population Inversion?
A
where more atoms are in the excited (transitory) state (higher energy) than the ground state (lower energy). ie not in thermal equilibrium.
21
Q
How is population inversion achieved?
A
through Pumping
= the act of transferring energy from an external source into the laser medium, usually thought a form of light or electric current
22
Q
What is Stimulated Emission?
A
Where excited atoms emit photons and produce a sudden burst of coherent radiation.
Atoms travel back from excited to the ground state when being stimulated with a small pulse of laser light. A photon with frequency equal to the energy difference between the excited and ground states, strikes an excited atom causing it to emit a second photon, which is coherent to the first. This creates a rapid chain of discharged atoms which produces a sudden burst of coherent radiation.
23
Q
What is amplification?
A
The amplification of radiation, due to an increase in the amplitude, that causes a large number of coherent photons. The coherence causes the light to shine in an extremely bright and straight beam.
This is achieved by a system of mirrors at either end of a glass amplifier, which cause the photons to travel back and fourth. This stimulates more atoms to the ground state causing the emission of even more photons.
24
Q
What is significant about the characteristics of mirrors used in amplification?
A
one is fully silvered so no photons can escape the glass. (High Reflector)
the other is partly silvered to allow some photons to escape as laser light, while some is still reflected causing amplification. (Output Coupler)
25
how can the profile of a beam be described?
it usually follows Gaussian distribution but can be changed with device choice
26
when is the laser at its sharpest point on a surface?
when the spot on surface is at its focal distance from the lens
27
DoF def
zf = Depth of Focus = the region within which the structure/texturing is "sufficiently sharp" or functionality is within the required tolerance ranges
28
when would you choose to use less energy density than at max DoF?
when full vaporisation isn't desired eg in laser polishing
29
at what point is the diameter of the laser beam at a minimum
at zfmax
ie maximum DoF (its at its narrowest point)
30
what is the Scan Field and how can it be achieved?
The area the laser can process and can be achieved using mirrors.
31
What is the beam spot size?
aka diameter of beam spot = dF
32
what is the beam mode?
A fixed value from supplier of beam, describing its quality
33
5 phenomena that occur when EM radiation is subject to a surface
Reflection
Refraction
Absorption
Scatter
Transmission
34
Why is absorption the most important phenomenon to evaluate?
Need to know how much *heat* is absorbed by the material, as well as the light.
35
5 effects of absorption
heating
melting
vaporisation
plasma formation
ablation
36
How do temperature and wavelength greatly influence absorptivity?
1. absorptivity **increases** at **shorter** wavelengths for common metals
2. absorptivity **increases** as temperature **increases**
37
relationship between temperature and the reflectivity and absorptivity of materials
a strongly reflective material at low temp may become a strongly absorptive material at high temps
38
4 material processing techniques that are based on absorption
- laser drilling
- laser cutting
- laser welding
- 3D laser machining
39
what happens to different material types when laser beam is irradiated on their surface?
metals: excitation of free electrons
insulators: vibrations
semiconductors: both
40
how can a laser cause phase transformations of a material?
if the laser intensity is high enough, it can result in phase transformations of the material, eg melting or vaporisation.
Depth of melting increases with increasing pulse time until it reaches its max, which occurs when the surface reaches boiling point.
Further increase in laser power density/pulse duration causes the evaporative material removal from the surface. Once vaporisation is initiated, continued laser irradiation will cause the liquid-vapour interface to move inside the material.
41
what limits the amount way which depth of melting can increase?
limited by the max achievable surface temp. Once the surface reaches boiling point, maximum depth is reached, meaning no more material can be melted. Further power causes material to vaporise.
42
which processes rely on laser vaporisation of material?
- laser drilling
- laser cutting
43
What is plasma?
highly ionised vapour caused by interactions between the laser beam and the vapour
44
At what point does plasma form?
At laser power densities (irradiation intensities) above *Ip*
45
What is plasma coupling?
When laser irradiation > *Ip*, plasma forms near the evaporating surface and remains confined to this region. Whole laser energy is confined within this plasma and transferred to material, and can significantly increase absorptivity in highly reflective materials.
46
What is Plasma Shielding?
When laser irradiation >> *Ip*.
The plasma interacts with laser irradiation causing the rapid expansion of the plasma. Plasma becomes decoupled from the surface causing the transfer of energy to the material to cease. Laser irradiation is absorbed in the plasma.
causes laser to stop operating
47
What is ablation?
Material removal process used for micro machining (cutting/drilling)
48
how to manage thermal damage to material during ablation?
pulse time must be shorter than that of thermal relaxation for minimal thermal damage to the material
49
what processes use ablation?
- micro machining
- cutting
- drilling
- precision ablation of tissues such as human corneal tissue
50
3 material-dependent time constants
Te = electron cooling time
Ti = lattice heating time
TL = laser pulse duration
51
laser interaction regimes
- fs: TL < Te < Ti (athermal/photonic process โด no surface damage during removal of material)
- ps: Te < TL < Ti
- ns: Te < Ti < TL (thermal process used in laser milling to remove material from surface)
52
7 laser properties
Intensity
Wavelength
Spatial Coherence
Temporal Coherence
Angle of Incident
Polarisaion
Illumination Time
53
5 material properties
absorptivity
thermal conductivity
density
specific heat
surface condition
54
types of additive laser-based manufacturing processes
SL, LENS, SLS, SLM, LOM
55
types of subtractive laser-based manufacturing processes
drilling
cutting
milling
polishing
texturing
56
components of a laser
**X and Y galvanometric scan heads** (move in XY optical axes)
**Energy source** (laser beam)
**Lens** to focus beam
**Z-module** to keep beam in focal position (Z axis of optical system)
**mechanical stage** (moves in XYZ mech axes)
**workpiece** can rotate around XYZ in ABC directions
57
how many degrees of freedom in laser equipment?
9 in total (3 optical, 3 mech, 3 rotational)
but don't need all 9 at once as this is expensive.
Only need rot axes for 3D or curved surfaces
58
Comparison between mechanical and optical axes accuracy
mech = 1-2๐m accuracy
optical = ~10๐m
mech more accurate but is much slower than movement of galvanometers. Use mech stage to position laser in accurate position. Galvos can move beam at a very fast rate (up to 10m/s) so can be used to increase productivity
59
what components of laser are needed for cutting operations?
Laser
Collimator
mech XYZ stage
60
what components of laser are needed for AM operations?
Laser
Adjustable mirrors
X-Y scan head (galvos)
Powder bed/UV sensitive resins
Z stage
61
what components of laser are needed for milling/marking/welding operations?
Laser
Adjustable mirrors
Z-module
X-Y scan head (galvos)
62
general process of laser drilling
a high intensity, stationary beam is focussed onto the surface at power densities; feeding workpiece surface with laser pulses with sufficient to heat, melt and eject the material in liquid and vapour form
63
Advantages of laser drilling
- non-contact, reproducible and precise
- can drill holes in difficult-to-machine materials (ceramics/composites) without any tool wear
- can achieve drilling rates of 100holes/s
64
4 Laser Drilling methods
**Single pulse**: can only achieve very small hole depths (useful for thin workpiece thicknesses)
**Percussion**: series of pulses hitting the workpiece, with an increased depth of hole
**Trepanning**: pilot hole created then diameter and depth increased with pulses in rotating motion
**Helical drilling**: beam travels round the periphery, increasing the depth layer by layer (most accurate and best qual; longest processing time)
65
What is single pulse laser drilling?
- typically used for drilling narrow holes (<1mm) through thin plates (<1mm)
- high pulse energies are supplied with a single pulse with high enough energy levels to vaporise material in single pulse
- high speed production
- poor tolerances compared to trepanning
66
What is percussion?
- typically used for drilling narrow holes (<1.3mm) through thicker metals (<25mm)
- series of short pulses separated by long time periods directed at the same spot
_+ves:_
- high speed
- very cost effective in applications where lots of holes are required
_-ves:_
- resolidified material remains at the hole wall (recast layer)
- tapering - decrease in hole diameter with depth
67
what is trepanning?
- Typically wider holes (<3mm) in thick plates (<10mm).
- Produced by drilling a series of overlapping holes around a circumference of a circle. (More overlap increases edge quality)
- Either the workpiece or the focusing optic is moved.
+ves:
Compared to percussion drilling, reduced taper and better tolerance can be achieved.
68
what is helical laser drilling?
- Relatively new technique
- Breaks up the process into multiple ablation steps.
- Laser goes round in a circle removing material (similar to trepanning).
- The difference between the two is that helical laser drilling does not involve the creation of an initial pilot hole for the desired hole to be created.
- Repeats this multiple times before the hole goes all the way through the material.
+ves:
- The recast layer is greatly reduced or completely avoided.
- The process is efficient when the helical diameter is close to the diameter of the laser.
-ves:
- Costs more than percussion drilling.
69
what is laser microdrilling used for?
to create 3D cavities on freeform surfaces to improve retention of the cutting fluid, thereby reducing friction
two side drilling is used for deeper drilling
70
How does laser cutting work?
a highly intense laser beam is focussed on the workpiece to heat and melt/vaporise it; creating a cut
The molten material is expelled by a pressurised assist gas jet.
Either the workpiece or laser beam moves.
Best suited to high-volume manufacturing processes.
71
4 PROS of laser cutting
1. ease of automation (CNC)
2. fine and precise
3. high cutting speed
4. good cut quality (min HAZ and thermal stresses)
72
2 Disadvantages of laser cutting
1. high energy consumption
2. hardening along the edges
73
what is laser milling?
a machining process which involves the removal of material to a specified depth, creating a variety of features (grooves/slots/profiles)
74
how laser milling works
single laser beams scan material systematically creating a series of overlapping craters and grooves
vaporisation is the primary removal mechanism
ultrashort pulses can create features below the laser spot size. Each laser pulse creates a crater on the substrate. The beam is moved while pulses repeat.
75
how is material removal controlled in laser milling?
by controlling the amount of overlap using the *laser frequency* and *scanning speed*
76
what is long pulse laser interaction in milling?
ms and ns
pulse length > 100ps
mainly a thermal process
causes thermal damage => debris and HAZ
77
what is ultra-short laser interaction in milling?
ps and fs
pulse length < 10ps
down to fs
mainly a photonic process
no thermal damage
78
types of milling material removal mechanisms
- one way hatch
- cross hatch
- random angle hatch
- hatch and border
- profile offsets
- special strategies
79
laser milling flat surfaces
cone shape is created in beam by focussing it in a spot. the cone creates the side walls of the feature.
For flat, relatively big surfaces, the best results are achieved using random angle hatching as it creates the most overlap.
80
laser milling vertical surfaces
more complex than for flat surfaces
as the angle of incident affects material removal rate, as it increases;
- depth decreases
- width and curvature increase
80
laser milling vertical surfaces
more complex than for flat surfaces
as the angle of incident affects material removal rate, as it increases;
- depth decreases
- width and curvature increase
(control using a lens to control the angle of the beam)
81
3 criteria of surface integrity in laser milling
- surface roughness (controlled by scanning speed and track displacement)
- microstructure (HAZ)
- microhardness (can increase in the processed area)
82
how to reduce surface roughness in laser milling
change scanning layer by 90ยบ layer-to-layer to reduce roughness. It's influenced by the overlap of craters
83
What is LST?
Laser Surface texturing
used to impart functionalities onto the surface by using a textured design. Can be aesthetic (appearance) or functional (improve tribological properties; medical/friction)
84
how SLT is used to improve tribological properties
- use a decreased cutting force in machining
- create dimples and channels in the form of micro and nano-structures
85
comparison of LST with mechanical micro milling to get textured surfaces
LST can achieve very fine structures as the beam is order of 5-10๐m, whereas mech micromilling can't go finer than 0.1mm
LST can induce much more thermal damage than micromilling.
86
how is LST used for medical applications?
for superhydrophobic (antibacterial) and super hydrophilic (surgery) surfaces
antibacterial: laser texturing reduces the impact of bacteria on the surface
surgical: laser texturing can reduce sticking between the soft tissue and the surgical tool
87
why is LST used?
to create textured surfaces that can have
- antibacterial properties (for use in biomedical/food industries)
- anti-ice properties (for use in aerospace applications at high temperatures to stop jamming of mechanical parts)
88
LST process chain for shark skin (to reduce drag)
1. bio-mimetic modelling
2. CAM solutions
3. filling simulation
4. laser milling
5. IM replication
89
What is laser polishing?
a post processing technique that reduces surface roughness of AM parts (as they typically have a very high surface roughness). It is a flexible, contactless method that can be fully automated without the need for dedicated equipment.
90
what materials can laser polishing be used on?
- diamond
- glass
- silica
- metals (incl steel nickel/titanium alloys and sometimes aluminium alloys)
91
why are aluminium alloys difficult to polish
as they have a relatively high thermal conductivity
92
3 process mechanisms of laser polishing
1. large area ablation
2. localised ablation
3. re-melting at macro (depth > 20-200๐m) or micro (depth > 0.5-5๐m) polishing regimes
93
main characteristics of polishing by re-melting
- high level of automation
- short machining times (esp compared to manual polishing)
- no pollutive impact from grinding/polishing wastes/chemicals
- polishing of grained and microstructured surfaces without damaging the structures
- the generation of user-definable and localised surface roughness
- no change of the form of the workpieces. Deviations of the form aren't corrected but an already perfect form isn't damaged.
- small micro roughness as the surface results from the liquid phase
94
limitations of AM parts
- high surface roughness (typically Ra= 5-15๐m)
- stair-step effects
- balling
- adverse residual stresses
- low dimensional accuracy
- time-consuming post processing
95
difference between laser polishing and laser milling
similar operations but polishing only *melts* the material, whereas milling *removes* the material.
