Laser cutting technology

LASER FUNDAMENTALS

Very versatile technology in which a very small spot can be obtained.

Smaller the spot, higher the power density.

The laser beam heats up the material; the electromagnetic waves at low frequency (visible range) can interact just with electrons.

Laser beam = heat source

The interaction depends on the frequency of the beam but also on the structure of the material itself (different absorption).

Laser beam can interact only with the electrons in the material because the heavier nuclei are not able to follow the high laser frequency.

INTERNAL ABSORPTION

How deep the beam can go inside the material?

The incident beam is very focussed.

• A part will be reflected: I_r
• A part will be transmitted inside the material: I_t

If the material is very transparent, the beam could even go outside (we don't want that, we want heat to be dissipated).

The dissipation follows an exponential law; BEER-LAMBERT LAW.

I_t(z) = I_t(0) e^-2αz = AI_t e^-2αz

If we calculate I_t at depth ≈ s => I_t(z=s) = I_t(0) e^-2αs = I_t(0) e^{-2 × 0,135} I_t(0)

I_t(z=2s) = I_t(0) e^-2αs∕2 = I_t(0) e^-2 = 0,018 AI_t(0)

At a depth equal to s ≈ already 86% of the power has been absorbed.

At a depth of 2s ≈ only 1.8% of power is remained, almost 99% has been absorbed.

• Cutting
• Welding
• Heat-Treatments
• Drilling
• Marking
• Milling
• Cleaning
• Measurements

LIGHT AS ELECTROMAGNETIC FIELD

An electromagnetic field is made by an electric field perpendicular to a magnetic field.

E_y(z,t) = E_y0⋅sin(ωt -^2π/_λz + ϕ)

ω = angular velocity rad/s ω = 2πf

ϕ = phase [rad]

λ = wavelength [m]

f⋅λ = V

velocity of propagation of the laser beam inside a given medium [m/s]

If the medium is vacuum : V = c = 3⋅10⁸ m/s

SPECTRUM OF ELECTROMAGNETIC WAVES

Our interest is from UV to near infrared (10 _μm, industrial laser)

Light can be considered:

• electromagnetic wave
• beam made of particles having given energy (QUANTUM)

PROPERTIES OF LASER RADIATIONS

1. MONOCHROMATICITY (single wavelength)
2. COLLIMATION (nearly parallel rays)
3. BEAM COHERENCE (stationary interference)
4. ELEVATED BRIGHTNESS (or radiance)
5. SMALL BEAM DIAMETER (low divergence)
6. DIFFERENT TRANSVERSE ELECTROMAGNETIC MODES (TEM)
7. DIFFERENT TEMPORAL MODES

1- MONOCHROMATICITY (same wavelength)

Just one wavelength, just one color that will focus on a very little spot.

Since there is a lens, there is a little divergence θ. In any location of the beam:

θ · d₀ = K · λ = constant

θ = divergence [mrad] K = depends on the beam shape

2- COLLIMATION (directionality)

Small divergence = propagates hundreds of Km without expanding much

θ · d₀ = K · λ

Monochromatic laser can be collimated very well.

P_Pk = max_0<t<τHP(t)   PEAK POWER

PH = √τH &integral;τH0P(t) dt   AVERAGE PULSE POWER

Q = √τH0P(t) dt = τH·PH   TOTAL PULSE ENERGY

P_av = f₀·Q = f₀·τ_H·P_H   AVERAGE POWER

• Free running: source naturally pulsed (like light already pulsed)

τ_H = 10^-8 s, short but not so short time

No losses in efficiency

• Q-switched: rotating mechanical devices inside the source

τ_H = 10^-9 s

then we loose some power

• Mode locking: pulsed by optical interference technique inside the source

τ_H = 10^-14 s

No losses

ADVANTAGES OF PULSED LASER

Solid state laser: λ = 1064 nm

M² = 1.7 decent quality

With the same total energy expenditure, the average power is much larger in pulsed lasers. 12 KW vs 12 GW, 100 ns vs 100 fs

P_H = 12 gi…kW low track …k … loads and heat, the electromagnetic waves directly

Ablation

Ablation can be obtained by pulsing the laser (high intensity required).

Inverting the pulse...

• COLD ABLATION
• HOT ABLATION
• MELT EXPULSION

Cold Ablation

Ultra short pulses: interaction time shorter than electron relaxation time (1 ps for metals), no time for the microstructure to relax when the pulse is finished, so no time for heat conduction. It breaks atomic bonds without heating the material, the heat is not felt even by the neighbor —> no HAZ.

If τ