Laser drilling techniques and applications

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Laser drilling and its applications

• Laser material processing

Hamed Tasalloti Kashani
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Introduction

• Laser drilling
• A hole-making technique that utilises a focused
  laser beam at power densities sufficient to melt
  and vaporize the material

• Main features
• Non-contact and therefore wear-free drilling
• High flexibility
• Possibility for automatization
• High speed
• Small achievable diameters (less than 100µm)

• Usability
• Can be used for a wide range of materials
  including
• Steels and different metallic alloys
• High-strength materials, ceramics,
  semiconductors, carbon compounds, composites,
  diamond, or plastics

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Introduction

• Applications
• electronics, computers, communication products
• automotive parts
• aerospace industry
• drilling high quantity of closely spaced holes
  required for effusion cooling in turbine engine
  components
• holes in surgical tooling
• inkjet nozzles etc.

• Challenges
• Spatter Elimination
• Precise cross-section shape
• Repeatability
• holes in surgical tooling
• Reduction or control of taper

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Different drilling techniques
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Schematic of drilling techniques
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Single-pulse drilling

• The hole is drilled with the radiation of a
  single laser pulse
• NdYAG laser systems are generally used as the
  beam source
• Single pulse durations in the microsecond to
  millisecond
• Intensities I gt1MW/cm2
• Hole depths of up to 2mm is possible to be
  drilled
• Hole diameters of about 10500µm are achievable.
• Above 100 holes per second can be produced
• Is the most productive methods and is used when a
  large number of holes have to be drilled.
• Most of the material is expelled out from the
  hole as melt.

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Schematic of single-pulse laser drilling
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Influence of process parameters

• The hole diameter and the attainable drilling
  depths are highly dependent on beam quality
• The dependence on beam quality is greater at
  lower pulse energies
• With other identical parameters, higher beam
  quality results in greater drilling depths and
  smaller hole diameters
• Higher beam quality, results smaller variations
  in geometry from hole to hole.

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Application examples
Surgical needle
Micro-holes in fuel filter for the automotive
industry
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Percussion Drilling

• Is a pulsed process, in which material is removed
  with consecutive pulses of laser radiation.
• This enables a higher aspect ratio to be achieved
  than with single-pulse drilling (approx. 501
  compared with 201).
• Hole diameters typically can range from 100 µm
  to 1 mm,
• drilling depth of 20mm can be achieved.
• The pulse duration, intensity distribution, and
  temporal pulse shape have a significant influence
  on the quality of the drilled hole

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Percussion Drilling

• It is often difficult to produce repeatable holes
  with laser percussion drilling
• The absorption of laser radiation at a closure by
  solidified melt can result in local expansion of
  the drill hole (lateral convection) and thus a
  reduced reproducibility of the drill-hole
  geometry.

Variation of geometry for holes drilled under the
same conditions.( NdYAG laser, The workpiece
material used was EN3 mild steel)
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Influence of process gases

• Depending on the type of material involved,
  different process gases are used
• Oxygen precipitates chemical reactions that
  increases temperature during the drilling process
  and cause oxidization of the material on the
  drill-hole wall
• Nitrogen can be used to nitrate the material
• argon is used to prevent unwanted chemical
  reactions during the process
• with oxygen, the occurring spatter is just 1020
  as densely as with compressed air, nitrogen, or
  argon
• The exothermal reaction of oxygen with the molten
  material over heats the melt and cause it to be
  expelled in greater quantities in the form of
  droplets, and to turbulently solidify.
• When the process gas is argon, the melt has a
  higher cooling rate and solidifies in multiple
  layers.

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Applications

• Tool and die making
• Medical engineering
• Automotive engineering
• Aviation
• Cooling holes in the turbine blades
• Holes in filters
• Nozzles for ink-jet printers
• Feed through holes for solar cells
• etc.

Micro-filter 2000 drill holes. Material
aluminium, 15µm drilling depth
Transparent metal 6.5105 drill holes with
diameters between 10 and 30µm spaced 60µm apart
a) print heads for ink-jet printer, material
Vacrel. b) print-head nozzle drilled by KrF
laser, 248 nm
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Trepanning

• Trepanning is a combined cutting and drilling
  process
• Typically performed using a pulsed laser
• By percussion drilling a through hole is first
  pierced and then the through hole is widened to
  its final diameter in a circular cutting motion
• The duration of each pulse ranges from ns to the
  ms range
• depending on the material, the thickness of the
  workpiece, and the demanded quality of the drill
  hole
• Typical drill-hole diameters are 0.151 mm
• The melt is expelled through the exit area of
  the joint or drill hole, with a process gas
  flowing coaxially to the laser beam

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Trepanning
Schematic of the trepanning process
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Process gas pressure

• During trepanning, the thickness of the melt
  depends on the process gas pressure
• The melt formed during trepanning closes the
  holes again at low process gas up to10 bar
• In order to blow out the melt from the bottom,
  the process gas pressure has to be larger than 14
  bar.
• The higher pressure is required for the deeper
  trepanning kerf, to remove the melt from the hole
• with argon as process gas, a pressure higher
  than16 bar is necessary to maintain a sufficient
  melt ejection rate

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Applications

• power-generation and turbine engineering
• The automotive industry
• and tool-making

(a) Vent holes in a mould for disks (b)
longitudinal section of a vent hole
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Helical drilling

• Pulsed laser radiation is used in the nano- to
  picoseconds range
• Only a very small volume of material is removed
  by each pulse
• Relative movement takes place between the
  workpiece and the laser beam.
• The spin of the laser beam on the helical path
• The rotation of the laser beam in itself (proper
  rotation)
• Proper rotation
• To compensate for a non-circular beam profile
• Makes it possible to produce small drill holes
  with a diameter in the range of the beam
  cross-section
• Full perforation only can obtain after multiple
  revolutions
• Changing the geometry of the drill hole can be
  achieved by changing the angle of incidence and
  the workpiece and the point of impact

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Helical drilling
Diagram of helical drilling (a). Branding marks
at low laser output on the surface of the
workpiece illustrates the rotating motion (b)
Types of drill holes that can be produced by
helical drilling
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Main characteristics

• Helical drilling provides the greatest precision
  of all currently known laser drilling techniques
• The roundness of the drill hole
• The sharpness of the edges at the entrance and
  exit of the drill hole
• Decreased surface roughness
• Thinner recast layer of solidified melt
• The wall of the drill hole is more homogeneous

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Helical Drilling Optic
Principles of laser beam rotation
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New drilling approaches

• Underwater laser drilling
• In recent years, micro-channels drilling in water
  with femtosecond laser have been investigated
  extensively
• It has great potential in many applications
  especially for brittle materials
• The method could improve the efficient of debris
  ejection.
• water carries away the debris generated in the
  drilling process, thus reducing the pileup around
  the hole.
• Reduces heat accumulation at the edges of the
  hole
• The cooling effect of water can reduce the size
  of the heat-affected zone

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New drilling approaches
Configuration of the underwater laser drilling
Interaction between laser, water, and glass
substrate
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New drilling approaches

• Anti-spatter composite coating(ASCC)
• The method is based on the application of a
  specifically developed anti spatter composite
  coating (ASCC) on the workpiece surface before
  laser percussion drilling.
• This coating contains a mixture of ceramic filler
  particles embedded in a silicone elastomer matrix
• ASCC effectively prevents the deposition of
  spatter for all the tested assist gases (O2, air,
  N2 and Ar)

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New drilling approaches
Optical micrographs of laser drilled array holes
(2 mm hole pitch) in uncoated(left) and coated
(right) Nimonic 263 alloy using (a) O2, (b) air,
(c) N2 and (d) Ar assist gases. Fibre-optic
delivered 400 W NdYAG .Percussion drilling
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Summary
Laser drilling technique Typical usage Pulse duration Description
Single-pulse drilling Large number of holes with diameters 1mm and depths 3mm In the range of 100µs to 20ms The hole is drilled with the radiation of a single laser pulse. NdYAG laser systems are generally used as the beam source for single-pulse drilling
Percussion drilling Holes with diameters 1mm and depths up to 20 mm From fs to ms is a pulsed process, in which material is removed with consecutive pulses of laser radiation and enables a higher aspect ratio to be achieved than with single-pulse drilling. The machined holes differ in quality regarding recast layer, aspect ratio, conicity, or cracks in the recast layer and the base material
Trepanning Typical drill-hole diameters are 0.151 mm In the range of µs to ms Is a combined drilling and cutting process typically applied with pulsed laser radiation. Free-form holes with different shapes and contours on the hole entrance and the hole exit is possible
Helical drilling Positive and negative taper with minimum diameters of 40 µm In the nanosecond range Laser radiation is rotated relative to the workpiece. Drilling process is dominated by vaporization. This helps to avoid the formation of a large melt pool at the hole bottom. The helical drilled holes are very precise and exhibit a good microstructure quality
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