Laser Material Interaction

Question 1

What are the three primary ways light interacts with matter?

Answer 1

Reflection, absorption, and transmission.

Question 2

Why is reflectivity important in laser-material interactions for metals?

Answer 2

Reflectivity determines how much laser energy is absorbed for processing; high reflectivity reduces efficiency.

Question 3

How does the absorption coefficient of materials vary with laser wavelength?

Answer 3

Absorption decreases with longer wavelengths (e.g., metals absorb better at 1 µm than 10.6 µm).

Question 4

Which laser type is more suitable for cutting metals: CO₂ or fiber lasers? Why?

Answer 4

Fiber lasers (1 µm wavelength) are better for metals due to higher absorption compared to CO₂ lasers (10.6 µm).

Question 5

How does temperature affect the absorptivity of materials during laser processing?

Answer 5

Absorptivity increases with temperature, especially near melting/vaporization points.

Question 6

What happens to absorptivity when a metal reaches its melting point?

Answer 6

Absorptivity approaches 100% as the metal melts.

Question 7

What are the key physical effects of laser-material interaction as intensity increases?

Answer 7

Heating, melting, vaporization, plasma formation, and ablation.

Question 8

What is the role of plasma formation in laser-material interaction?

Answer 8

Plasma enhances energy absorption via multiple reflections and stabilizes the keyhole in deep penetration processes.

Question 9

How is interaction time (τ) defined in laser processing?

Answer 9

Interaction time (τ) = beam diameter (dₛ) / beam velocity (v).

Question 10

What is the difference between irradiance and fluence in laser processing?

Answer 10

Irradiance is power per unit area (W/cm²); fluence is energy per unit area (J/cm²).

Question 11

What are the typical power levels and beam sizes for laser cutting of metals?

Answer 11

Power: 1–20 kW; beam size: 0.1–0.6 mm for cutting, 0.05–0.1 mm for micromachining.

Question 12

What components are included in a laser cutting head?

Answer 12

Focusing optics, nozzle, gas supply, and protective window.

Question 13

Why is shielding gas used in laser cutting?

Answer 13

To blow away molten material, prevent oxidation (with inert gases), or enhance cutting (with reactive gases like O₂).

Question 14

What are the common defects observed in a laser-cut kerf?

Answer 14

Dross, striations, HAZ, top-edge rounding, and kerf width variations.

Question 15

How does the heat-affected zone (HAZ) in laser cutting compare to plasma cutting?

Answer 15

HAZ in laser cutting is much smaller (microns) compared to plasma cutting.

Question 16

What standards are used to define laser cut quality?

Answer 16

ISO 9013 for cut quality, including kerf width, perpendicularity, and surface roughness.

Question 17

What are the three fundamental methods of laser cutting?

Answer 17

Fusion cutting, vaporization cutting, and cold ablation.

Question 18

How does melt-and-blow cutting differ from vaporization cutting?

Answer 18

Melt-and-blow uses gas to eject molten material; vaporization relies on boiling material without gas assistance.

Question 19

What gases are typically used in melt-and-blow cutting?

Answer 19

Inert gases like N₂, Ar, or He.

Question 20

What is the purpose of reactive fusion cutting with oxygen?

Answer 20

Oxygen reacts exothermically with metals, adding energy to increase cutting speed or thickness capacity.

Question 21

How does oxidation enthalpy contribute to reactive fusion cutting?

Answer 21

Oxidation releases heat (e.g., 260 kJ/mol for Fe), reducing the required laser energy.

Question 22

What causes striations in reactive fusion cutting?

Answer 22

Striations result from periodic ignition and extinguishing of the oxidation reaction along the cut path.

Question 23

How does the lumped heat capacity model apply to melt-and-blow cutting?

Answer 23

The model balances laser energy with material removal energy (melting + vaporization).

Question 24

What is the relationship between laser power, cutting speed, and material thickness in melt-and-blow cutting?

Answer 24

Higher power allows faster cutting or thicker materials; speed ∝ power / (thickness × kerf width).

Question 25

Why is oxygen not typically used for cutting stainless steel or titanium?

Answer 25

Oxygen causes chromium loss in stainless steel and excessive heat in titanium, leading to poor cut quality.

Question 26

What are the advantages of vaporization cutting for polymers?

Answer 26

Polymers vaporize easily at low temperatures, requiring no assist gas and enabling clean cuts.

Question 27

What is the key difference between vaporization cutting and fusion cutting?

Answer 27

Vaporization cutting avoids molten material, relying solely on boiling, while fusion cutting uses melt ejection.

Question 28

What parameters influence the practical performance of laser cutting?

Answer 28

Beam properties (power, spot size), transport properties (speed, focal position), gas type, and material properties.

Question 29

How does focal position affect laser cutting quality?

Answer 29

Focal position controls power density; subsurface focusing improves penetration for thick materials.

Question 30

What is the role of stand-off distance (SOD) in laser cutting?

Answer 30

SOD ensures optimal gas flow and protects the nozzle from spatter (typically 0.5–1.0 mm).

Question 31

How does wavelength affect laser cutting performance?

Answer 31

Shorter wavelengths (e.g., 1 µm) increase absorption and enable smaller spot sizes.

Question 32

Why are fiber lasers preferred over CO₂ lasers for metal cutting?

Answer 32

Fiber lasers offer higher beam quality (M² ≈ 1) and better absorption for metals at high powers.

Question 33

How does surface roughness affect laser cutting efficiency?

Answer 33

Rough surfaces absorb more due to multiple reflections but may reduce final product quality.

Question 34

What are the main types of contemporary laser cutting machines?

Answer 34

2D sheet cutters, tube cutters, 3D robotic systems, and remote scanner-based systems.

Question 35

What is the advantage of robotic systems in laser cutting?

Answer 35

Robots enable complex 6-axis motion for cutting curved or tubular parts.

Question 36

How does remote laser cutting differ from proximity cutting?

Answer 36

Remote cutting uses scanners for high-speed beam movement without gas assist; proximity cutting uses nozzles.

Question 37

What is the function of an f-theta lens in remote cutting?

Answer 37

F-theta lenses focus deviated beams onto a flat field, enabling linear scanning.

Question 38

How does laser cutting compare to punching in terms of cost for small production runs?

Answer 38

Laser cutting has no tooling costs, making it cheaper for small batches; punching is economical for large volumes.

Question 39

What are the advantages of laser micromachining over conventional methods?

Answer 39

No tool wear, high precision, small feature sizes, and applicability to diverse materials.

Question 40

What is the significance of pulse duration in laser micromachining?

Answer 40

Pulse duration determines thermal effects: shorter pulses (fs/ps) reduce HAZ and melt formation.

Question 41

How does cold ablation differ from thermal ablation?

Answer 41

Cold ablation (fs pulses) removes material without melting; thermal ablation (ns pulses) generates melt/vapor.

Question 42

What is the two-temperature model in ultrashort pulse laser interactions?

Answer 42

Electrons absorb energy first (Tₑ); if pulses are shorter than lattice heating time (τᵢ), heat doesn’t propagate.

Question 43

Why are femtosecond pulses preferred for high-quality micromachining?

Answer 43

Femtosecond pulses minimize HAZ and dross, enabling high-precision features (e.g., 200 µm holes).

Question 44

What are the typical applications of laser micromachining?

Answer 44

Fuel injector drilling, stent manufacturing, surface texturing, and microfluidic device fabrication.

Question 45

How does pulse duration affect the quality of laser-drilled holes?

Answer 45

Shorter pulses (fs/ps) produce cleaner holes; longer pulses (ns) cause melt deposition and rough edges.

Question 46

What parameters determine the spot size in laser micromachining?

Answer 46

Spot size depends on M², wavelength (λ), focal length (f), and collimated beam diameter (d_c).

Question 47

How can harmonic generation be used to reduce spot size?

Answer 47

Harmonic generation (e.g., 1064 nm → 532 nm) reduces λ, enabling smaller spots but with power loss.

Question 48

What are the common drilling strategies in laser micromachining?

Answer 48

Single-pulse, percussion, trepanning, and helical trepanning (for deeper or larger holes).

Question 49

What is the drawback of using shorter focal lengths for smaller spot sizes?

Answer 49

Shorter focal lengths reduce spot size but also Rayleigh range, making focus positioning critical.

Question 50

How does the Rayleigh range affect laser micromachining precision?

Answer 50

Smaller Rayleigh range (shorter focal lengths) reduces tolerance for workpiece height variations.