Laser Drilling

Question 1

What is laser drilling good at

Answer 1

Fast Accurate, suitable for almost any material, not restricted to circular holes.

Question 2

What is laser drilling?

Answer 2

• Process of melting, vaporising or
  ablating material to create a hole within a workpiece
• All cutting operations (unless they start from the edge of a piece of material)
  must start by drilling a hole
• Control of process is extremely important if a high quality hole is to be produced (especially for micro drilling)
• Possible to achieve hole diameters as
  small as a few μm.

Question 3

What are the other alternatives to laser drilling?

Answer 3

• Mechanical drilling
• Electric discharge machining (EDM)
• Electro chemical machining (ECM)
• Ion and electron beam machining

Question 4

What are the advantages of laser drilling over other techniques?

Answer 4

• Holes can be drilled very quickly
• Burr and spatter can be controlled (to some extent)
• Any material that will absorb the radiation can be drilled
  (regardless of hardness)
• The diameter and the shape of a hole can be controlled by a
  technique known as trepanning
• Lasers can pierce material at almost any angle
• Holes of very small diameter can be achieved

Question 5

What are processing challenges of laser drilling?

Answer 5

• The need to increase the speed for faster production
• Reduction or control of taper
• Reduction or elimination of the re-solidified layer on the hole wall (HAZ – Heat Affected Zone)
• Precise cross section shape – square corners, star shapes, etc.
• Elimination of spatter
• Repeatability
• High aspect ratio holes (depth to width)
• Drilling through coated material

Question 6

What are the 4 main steps of laser drilling?

Answer 6

1. Melting and removing the molten materials with a gas jet
2. Removal by exothermic reaction – use of oxygen assist gas
3. Vaporization
4. Ablation by photo-chemical or ultrafast interaction.

Question 7

What are the 4 main types of laser drilling?

Answer 7

Single shot drilling, percussion, trepanning or helical trepanning

Question 8

What is single shot drilling?

Answer 8

• A single burst of laser energy to vaporise or ablate material
• Only really suitable for thin films
• Very fast drilling (suitable for on-the-fly production lines)
• Poor reproducibility
• Diameter of holes typically < 500 μm, but of that magnitude
• Applications in drilling holes in thin foils and food packaging
• Can also be used for roughening surfaces (blind
  holes)

Question 9

What is percussion drilling?

Answer 9

• A series of pulses with specified energies and durations are directed onto the same spot of the material.
• Suitable for thicker materials (of the order 10 mm)
• Quite fast processing time
• High aspect ratio possible
• Good reproducibility
• Diameter of holes typically between 100 μm and 1 mm.
• Applications in drilling holes in turbine blades, filter systems

Question 10

What is the material removal process for percussion and single shot drilling?

Answer 10

1. Absorption and heating
   * Drilling normally relies on vaporisation of material
   * First light heats up surface to create keyhole
   * Keyhole leads to sudden increase in absorptivity (multiple reflections) Multiple mechanisms for material to eject
   * Generated vapour escapes through top of hole
   * Liquid melt ejection - Vaporisation induced recoil pressure & Boiling
   * Melt driven up walls to escape as spray
   * When pulse finishes melt can fall back as spatter around hole
   * Generally more melt = poorer quality hole
   * Plasma (ionised gas) can be formed and absorb some light.

Question 11

What is waveguiding?

Answer 11

• The phenomena of multiple reflections in the hole cavity is commonly associated with the drilling of thicker materials.
• Once a crater is formed the laser beam pulses are incident on the hole
  walls. This leads to multiple reflections.

Question 12

What can waveguiding cause in holes?

Answer 12

Deeper cavities
Thinner recast layer
More cylindrical shape

Question 13

What are the 2 types of liquid melt ejection alongsize vaporisation?

Answer 13

Vaporisation induces recoil pressure and nucleation boiling

Question 14

What is vaporisation induced recoil pressure?

Answer 14

The reaction force on the liquid surface which pushes the melt layer out of the beam path.
This is referred to as vaporization-induced recoil pressure. Melt extraction from deeper holes (not holes in thin foils) is achieved with this mechanism.

Question 15

What is nucleation boiling?

Answer 15

Causes violent boiling in the irradiated zone during drilling.
= the material below the surface is super-heated which gives rise to a rapid liquid-vapour phase change.
In turn, this results in small vapour bubbles developing in the liquid zone which expand rapidly, causing a thermal explosion due to the high pressure.
Following such an explosion, the neighbouring liquid is ejected.

Question 16

What is the depth to diameter ratio for a shallow, intermediate, deep and through hole?

Answer 16

shallow 2:1, intermediate ~4:1, deep >4:1and through hole&raquo_space;4:1

Question 17

What happens in material property wise in regard to each of the 4 hole formations?

Answer 17

Shallow 1 PULSE
* focussed spot diameter
* pulse parameters
* materials properties
Entry hole diameter set
Intermediate 3/4 PULSE
* depth of focus
* materials properties
* Stagnant gas jet
Deep 12 PULSE
* wave-guiding effect
* direct beam ejection
* material properties (i.e.
viscosity)
* Stagnant gas jet
Through hole 100 PULSE
* wave-guiding effect
* gas dynamics
* material properties

Question 18

What is Trepanning drilling?

Answer 18

Effectively cutting a hole by performing a relative motion between the laser beam and the workpiece
* Beam focused into very small
diameters
* Very good hole quality and
repeatability
* Time consuming process

Question 19

What are the 3 main options of trepanning drilling?

Answer 19

Move the workpiece, move the laser, move the beam

Question 20

What is helical trepanning drilling?

Answer 20

Full penetration is not achieved with the first pulses; layers
of material are removed to enhance accuracy
* More deviation from circular geometry can be achieved than with trepanning
* Load on the opposite wall of the hole is minimised
* Recast layers are reduced or avoided

Question 21

Most of our examples have been about laser drilling of metals. What
would be different about machining:
– Ceramics?
– Thermosetting plastics?
– Bone?

Answer 21

Ceramics are brittle and have high melting points making them susceptible to thermal shock from drilling
Thermosetting plastics are more likely to burn or degrade under high laser heat.
Bone has a lower melting point than metals but requires a much higher laser power which would cause vaporisation of organic components and ablation of inorganic components.

Question 22

What are 6 laser drilling defects?

Answer 22

Hole tapering; spatter; recast layer; microcracking; dross and barrelling.

Question 23

What is hole tapering in drilling?

Answer 23

• Hole tapering in laser percussion drilling is due to erosion caused by the
  expulsion of molten and vaporized material from the hole.
• Varying the pulse energy, pulse width, number of pulses, as well as the
  design of the optical system can control the degree of taper.
• Hole tapering is affected by:
  – The focal position;
  – If the material thickness is more than the depth of focus
  then parallelism is difficult to achieve.
  – Thermal diffusivity and gas pressure.

Question 24

What is spatter in drilling?

Answer 24

• Spatter results when any molten or vaporized material is not completely expelled from the drill site and
  resolidifies on the material surface. This is unavoidable without surface protection.

Question 25

What is the recast laser drilling defects?

Answer 25

• Recast layer results when molten or vaporized material is not completely expelled from hole and resolidifies on the wall of the hole.
  Recast layer and microcracking can be avoided by selecting:
  1. Laser power densities that effectively expel the molten material from inside the hole .
  2. Pulse widths short enough and pulse frequencies low enough to minimize heating of the
  surrounding parent material.
  3. Pre-heating
  4. The use of diffractive optical elements to shape the laser beam profile.

Question 26

What is microcracking in material drilling defects?

Answer 26

• Microcracking occurs in the hole walls and is due to thermally induced stresses.

Question 27

What is dross from drilling?

Answer 27

• Dross is resolidified material from the meltpool that is not totally ejected from underside of the hole.
• Dross is less of a problem than spatter as it is usually smaller and more loosely attached.
• But dross can become a problem when:
  – Unsuitable gasses are used that raise the heat.
  – The laser beam is not centrally aligned to the nozzle.
  – Inappropriate gas pressure is used – higher the better.

Question 28

What is plasma formation?

Answer 28

• Dross is resolidified material from the meltpool that is not totally ejected from underside of the hole.
• Dross is less of a problem than spatter as it is usually smaller and more loosely attached.
• But dross can become a problem when:
  – Unsuitable gasses are used that raise the heat.
  – The laser beam is not centrally aligned to the nozzle.
  – Inappropriate gas pressure is used – higher the better.