Interaction of laser beam radiation and material

In laser materials processing, the required energy in the form of heat for the welding process is supplied by the radiation energy of the laser beam. Using optical elements, the laser beam is focused as a small spot on the material surface, the energy is absorbed by the material and converted into heat. The temperature on the surface can be determined based on the balance of released energy and absorbed energy, which defines the resulting thermal mechanisms. Thus, the effect of the laser beam radiation is determined by the absorbed energy, the interaction time with the workpiece, and the geometrical and material properties of the workpiece. The power density E on the workpiece surface is calculated with the beam power P and the laser beam radius rb as:

𝐸 = ^𝑃

𝜋𝑟_𝑏². (2.7)

Power density can be used to determine the principal physical mechanism of the interaction with the material, see Figure 2.12. At power densities lower than 10³ W/cm² to 10⁴ W/cm², radial heat conduction will dominate. In this range, significant heating of the workpiece takes place while the temperature remains below the melting point of the material. When increasing the power density to the order of 10⁵ W/cm², by raising the laser power or reducing the radius of the laser beam, the melting point in the zone of interaction is reached, which leads to the generation of a melt pool. This state satisfies the conditions for heat conduction welding. When the power density is between 10⁵ W/cm² and 10⁶ W/cm², the vaporisation temperature is reached and a cavity, called a keyhole is generated in the workpiece. Heat conduction ensures that there is sufficient melting of the walls of the keyhole to enable a weld to be made. Intensifying the power density further to around 10⁷ W/cm²to 10⁸ W/cm² increases the vaporisation rate in the keyhole. This can lead to high pressure in the interaction zone which, consequently, rapidly expels liquid melt and increases the generation of plasma and metal vapour. Both the plasma and metal vapour can be ionised, which absorbs the laser beam energy to a great extent and impairs energy transfer into the workpiece. Figure 2.13 gives an overview of the power densities and interaction times applicable for the most important laser processes (Kroos, 1993), (Steen, 2003), (Ion, 2005), (Olsen, 2009).

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Figure 2.12: Laser material interaction at increasing power density and principal physical principles of material interaction, based on Kroos (1993).

Figure 2.13: Range of laser processes mapped against power density and interaction time for metallic materials, based on Steen (2003), Ion (2005) and Hügel (2009).

When the laser beam radiation delivers sufficient energy onto the surface of the workpiece to first start melting and then vaporising the material, the recoil force of vaporisation from the liquid surface expels the melt and forms a cavity. Once this cavity is deep enough the laser beam radiation is reflected from the cavity wall. This causes a sudden increase of the vaporisation process which leads to further deepening of the cavity until a stationary keyhole is formed. The actual welding process is performed by moving the keyhole relative to the workpiece. Liquid material is generated at the front side of the keyhole, flows around it, solidifies behind the keyhole and produces a deep and narrow weld with

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a high aspect or depth to width ratio, Figure 2.14, (Beyer, 1995), (Ion, 2005), (Poprawe, 2005), (Hügel, 2009).

Figure 2.14: Principal physical mechanisms in the keyhole, based on Kaplan (1994).

The most important physical processes for the absorption of energy in the keyhole and for maintaining the equilibrium between opening and closing forces are (Kroos,1993), (Kaplan, 1994) (Beck, 1996):

− Fresnel absorption, which describes the angular dependent absorption of laser beam radiation on the wall of the keyhole. It is guided through the keyhole to the base of the keyhole or, in the case of full penetration reflected out at the bottom side of the keyhole.

− Absorption by multiple reflections on the keyhole wall, which increases the absorption rate, especially in the formation phase of the keyhole.

− Plasma absorption by inverse Bremsstrahlung, which is the transfer of energy from photons to electrons. The incident radiation into the keyhole is partly absorbed by the plasma in and above the keyhole. The absorbed energy is conducted into the melt pool.

− Recoil pressure created by the vaporisation process, which accelerates vapour particles desorbed from the condensed phase to the keyhole wall and thus keeps the keyhole open.

− Surface tension pressure, which is dependent on the material properties and tries to close the keyhole.

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− Hydrodynamic pressure, which is proportional to the density of the melt, the depth of the keyhole and gravity. It acts to close the keyhole.

− Dynamic pressure of material flow around the keyhole, which creates a higher pressure at the front of the keyhole and a lower pressure at the back.

By determining the single forces of the pressure balance, it can be calculated that recoil pressure and pressure from surface tension are of equal value. The hydrostatic pressure is negligible and the dynamic pressure of the material flow around the keyhole gains significance only at very high welding speeds.