Section 3.3 Laser energy absorption by target material

The physical processes in laser material interaction is important for understanding the capabilities and limitations of laser machining processes. When laser beam strikes on the target material, part of the energy is reflected and part of it is absorbed, the absorbed energy heats up the target materials. This absorption and reflection has a Resonant Feature due to the microsturctures of materials. When laser beam acts on the material, laser energy is first absorbed by free electrons. The absorbed energy then propagates through the electron subsystem, then transferred to the lattice. In this way laser energy is transferred to the ambient target material, as illustrated by figure 3.6. We say this process has a resonant feature because materials show different absorptions to lasers with different wavelengths, this dependence of absorption on wavelength is decided by the microstructure and electromagnetic properties of the material. For example, Copper has an absorptivity of 2% for 10.6 micron CO2 lasers, but has much higher absorptivity for UV lasers(about 60%).

Figure 3.6 Laser energy absorption by target material

Given the laser pulse duration, one can estimate the Depth of Heat Penetration, which is the distance that heat can be transfered to during the laser pulse.

D=sqrt(4*alfa*dT)

where D is the depth of heat penetration, alfa is the diffusivity of materials, dT is the pulse duration.

Conversely, one can estimate the minimum pulse duration needed to penetrate a certain depth. Then we have:

dT=D^2/(4*alfa)

Example: Thermal diffusivity for electrolytic copper is 1.14 cm^2/s, find the thermal penetration depth for dT=50 nanosecond, 50 msec and 50 microseconds. Also calculate the pulse duration needed to penetrate a plate with thickness 1mm, 1cm.

Solve: For dT=50nanoseconds, D=sqrt(4*1.14*50E-9)=4.775 microns, for dT=50 msec, D=sqrt(4*1.14*50E-3)=4.775 mm, for dT=50 microseconds, D=sqrt(4*1.14*50E-6)=0.15 mm.

The pulse duration to penetrate 1mm is 2.2 msec, and for 1cm, it becomes 0.2193 s! 100 time bigger as that for 1mm thickness.

This gives you some idea of why laser processes can have very thin affected zones for short pulses and why thick plate cutting, welding and drilling is difficult.

Thermal conductivity, density, heat capacity and thermal diffusivity are the important parameters that influnce heat conduction into the target material. Since laser machining involves strong phase changes, the property variations with temperature should be considered. Other properties are melting temperature, vaporization temperature, melting latent heat and vaporization latent heat. For details of material properties, please refer to Chapter 5.

A simple relation for surface absorption of laser energy is:

A = 1 - R - T

where A is the surface absorptivity, R is reflection, T is transmission. For opaque material, T=0, then A=1-R.

This relation should be revised for 2D or 3D conditions when the direction of incident beam relative to the target material is important. Then all energy balances should be considered in the normal direction of the surface. See Leve 3, section 3.1 for details.

Under the action of laser irradiation, the surface quickly rises up to melting temperature. This melting range expands through heat conduction. Maximum melting without vaporization is desired for welding, but this happens in a very narrow range of laser intensity and pulse durations. If the laser intensity is too high, surface starts vaporizing before a significant melting depth of molten material is formed. In laser machining, however, vaporization is prefered. higher intensities than that used in welding is acted on the surface of target material, the surface quickly reaches vaporization temperature, both melting and vaporization exist, materials are removed through direct vaporization ablation or through hydrodynamic ablation. Hydrodynamic ablation here means that material is removed as liquids. For laser pulse durations longer than microseconds, hydrodynamic ablation is the dominate component over pure vaporization ablation. The most common regime for laser drilling and cutting is 100 usec to 10 msec, this time scale allows the surface to heat to the vaporization temerature and remain there for some time. When vaporization occurs, it generates a pressure than acted on the molten material, this pressure is call recoil pressure. Also there exists strong temperature gradients in the molten material, usually the center is hotter than the outer because of the profile of the laser intensity. The recoil pressure and temperature gradient drives the motlen material out as liquids. The combination of vaporization and hydrodynamic ablation drill or cut target material in depth.

When the laser intensity is even higher, the vaporized material will be ionized and plasma is formed. This makes the situation much more complex. If the plasma density is low, laser energy can still be transmitted to the material without obvious absorption, if the plasma density is high enough, absorption by the plasma should be considered. This absorption is related to the electron density in the plasma. When the plasma density reaches certain value, high absorption and reflection happens. Then laser energy cannot be effectively transmitted to the target material, thus laser and material interaction is decoupled. When the plasma expands, dissipates and rarefies, then laser energy can reach the surface again, another cycle of plasma generation, decoupling and dissipation happens. In this process, shock wave is generated. So the highest laser intensity is optimum for laser machining. Laser machining works in the range below strong plasma generation. For your special interest, laser shock waves can be used for material processing, it is called Laser Shock Processing which is used to improve surface hardness and stress distribution.

Laser energy transmission in the target material is governed by the Lambert law:

I(z)=I0*exp(-a*z)

Where z is the distance from the surface, a is the absorption coefficient, I0 is the intensity at surface. For metals a is about 100000 /cm.

Combining the results in section 3.2 and 3.3, we have the following formula for laser energy transmitted to the material at depth z:

I(x,y,z,t)=A*I0(t)exp(-a*z)SP(x,y)

Where A is the coefficient considering surface reflection and plasma absortion, I0(t) is the temporal distribution of laser intensity, a is the absorption coefficient, SP is the spatial distribution of the laser intensity.

For a Gaussian beam with beam radius r and for a mterial with A=1-R, R is the reflectivity, we have:

I(x,y,z,t)=(1-R)I0(t)exp(-a*z)exp(-(x^2+y^2)/r^2)

In above we have discussed photothermal degradation of materials, which is the thermal degradation of materials caused by laser thermal effects. For excimer UV laser ablation of some materials, such as polymers, the mechanism of Photochemical Degradation is suggested. It was observed that excimer laser ablation of polymers can happen with little to no debris or edge damage, it sometimes called Cold Cutting.The photon energy of excimer laser is 4.9eV, which is similar to the bond energy of many organic materials. It was suggested that this photon energy break the bond of organic materials directly, the photon energy is almost totally used for bond breaking, the long chains of organic materials is decomposed into shoter shorter units, side effects is greatly reduced. How great if this can happen to any materials! But this is limited to organic materials presently. Laser sources that can produce photon energy comparable to the bonding activation energy are required to realize this dream.