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What is laser vaporisation?


When laser material processing of metals, increasing the intensity of the laser beam to values close to 1x106 W/cm2 leads to boiling and the formation of a column of vapour with a diameter of approximately the size of the focussed laser beam - a 'keyhole', surrounded by a tube of molten material. The metallic vapour in the keyhole partially absorbs the laser light which is subjected to many reflections off the keyhole walls. However, the main laser beam absorption mechanism in the keyhole walls is governed by Fresnel absorption. As a consequence, the effect of the laser wavelength on absorptivity becomes less important once the keyhole is formed. The Knudsen layer, positioned between the vapour and the liquid phases and only a few mean free paths thick, is believed to regulate the vaporisation rate and, very probably, the formation mechanisms for the particles that make up the vapour. Figure 1 shows schematically the components of a laser 'keyhole'. Conduction and convection in the liquid surrounding the keyhole transfer heat to the solid material increasing the molten volume.

Fig.1. Schematic representation of laser beam/keyhole coupling
Fig.1. Schematic representation of laser beam/keyhole coupling

When keyhole welding (the domain of high power CO2 and solid state lasers), relative motion of the beam and the workpiece drives the keyhole in the welding direction, leaving a fused zone behind it. The column of vapour in the keyhole is of great importance because its pressure keeps the keyhole open, preventing collapse due to surface tension and gravity forces and allowing the beam to penetrate deeply into the material being welded.

At power densities of 9 x 105 W/cm2 (when CO2 laser welding metals) and higher, some of the electrons in the metallic vapour become ionised by inverse Bremsstrahlung. These free electrons can also absorb energy directly from the incoming laser beam, resulting in higher temperatures, more ionisation and increased absorption. Therefore, a plasma of partially ionised metallic vapour, which can absorb laser light and emit radiation, is formed when CO2 laser welding. As long as the plasma is controlled, the plasma will stay inside the keyhole and contribute to the process. However, if the plasma is not properly controlled, the plasma vapour column exits the keyhole, forming a 'cloud' over its opening. This plasma 'cloud' absorbs and defocuses the laser light, decreasing the energy entering the keyhole. The defocusing of the laser beam increases the width of the weld and dramatically decreases penetration. Various techniques, mainly using jets of gas such as helium, with a high ionisation potential (24eV), can be used to control the plasma effects above the workpiece and improve coupling into the keyhole.

At power densities ~106W/cm2 (when Nd:YAG laser welding metals) and higher, an energetic plume of evaporated particles is ejected from the keyhole. Since inverse Bremsstrahlung is directly proportional to the square of the wavelength, the shorter the wavelength the less ionisation. As a result the ionisation of the vapour is very small when processing with Nd:YAG lasers and the temperature of the vapour plume (<3,000K) is not as high as when CO2 laser welding (~10,000K). In the case of Nd:YAG laser welding, the mechanism by which metallic atoms evaporate from the surface or keyhole walls and grow by coalescence and/or form particles or clusters, is not well understood. However, the metallic particles in the vapour can interact with the laser beam via Mie scattering, absorbing and defocusing the beam, and reducing the weld penetration and increasing the top weld width.

For drilling, energy densities above 107 W/cm2 are usually obtained with pulsed lasers, using high-power in the peak of the pulse. Then, effective material removal can be initiated by pressure gradients between the interaction zone and the surrounding atmosphere, resulting in a drilling action. Liquid and vapour from the hole are expelled upwards from the beam focus region by the rapidly expanding vapour, thereby revealing a new surface to be drilled further. For laser drilling, the laser must reach this high peak power in short pulses, so that minimum heat is conducted sideways from the hole into the parent material. Furthermore, shorter wavelengths are also beneficial for drilling small holes.

As the power density increases further ~1x108 W/cm2 , material removal becomes completely vaporisation dominated (also called ablation, ) and the heat diffusion during the pulse is more limited. Both these factors contribute to less re-cast layer and heat affected zone (HAZ) width. At these power densities, laser wavelength becomes important and the frequency multiplied solid state laser, can be used to good effect, as the modified wavelength results in a reduced focus spot size thereby further increasing power density.

If a series of laser drilled holes are connected together, for example by moving the workpiece, then depending on the speed, a cut can be produced. This is the dominant mechanism for laser cutting using pulsed solid state lasers, which are applied extensively for the production of fine, two and three-dimensional parts, in many materials.

Using the ultra-short pulse length Femtosecond (~10-15 seconds) lasers, which can produce power densities as high as 1015 W/cm2 , holes can be produced in a range of materials that show no apparent re-cast at all, no heat affected zone and no microcracking. This is believed to be due to the fact that since typical thermal diffusion times are of the order of several picoseconds, the femtosecond pulse energy from the ultra short pulse laser is deposited within the skin depth of the material before any significant thermal diffusion into the lattice. The energy forms a plasma that explodes and vaporises material directly without forming a liquid phase. However, the average power available from these ultra fast lasers is low (1-10W) and component throughput is slow compared to that available from a Q-switched Nd:YAG laser, for example, with two orders of magnitude higher average power.

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