Laser Beam – Material Interaction

The laser cutting process is characterized by energy thresholds due to the fact that a

temperatures - while energy is lost from the interaction zone by heat conduction to the substrate metal.

When the focused high intensity laser beam radiation strikes the surface of a metal workpiece, some radiation is reflected, and some is absorbed. The photons of the incident laser beam radiation are absorbed by the free electrons in an electron gas (a process known as inverse bremsstrahlung effect). The absorbed energy sets the electrons in forced vibration motion which can be detected as heat. If sufficient laser energy is absorbed, the thermal vibrations become so intense that the molecular bonding is stretched so far that it is no longer capable of exhibiting mechanical strength resulting in melting of the metal at the interaction zone. And with higher incident laser intensity, the stronger vibrations cause the bonding to loosen further and the material evaporates ². The flow of heat in the metal workpiece is described by Fourier’s law on heat conduction given as:

) radiation is defined as the ratio of the laser power absorbed at the surface to the incident laser power. For an opaque material such as a metal, the absorptivity A is given as^A1^R, where R is the reflectivity of the workpiece surface. Absorptivity depends on the wavelength of laser radiation, plane of polarization of the light beam, angle of incidence, material type, and temperature and phase of the material 6, 61, 62

. Absorption in metals increases going towards the visible and ultraviolet regions and decreases in going to longer infrared wavelengths. With vertical incidence (angleof incidence 0^), the beam component oriented parallel to the incidence plane (R_P) and the normal component (R_s) are absorbed equally. However, with increasing angle of incidence, the coefficient of reflectivity of the parallel polarized light (R_P) decreases while that of the perpendicularly polarized light (R_s) increases. There exists an angle of incidence known as Brewster angle where the coefficient of reflectivity of R_Pis lowest while the coefficient of reflectivity ofR_sis close to one. Consequently, the absorption coefficient of the parallel polarized light (R_P) increases with increase in angle of incidence and is highest at the Brewster angle while the absorption coefficient of the perpendicularly polarized light (R_s) decreases with increase in angle of incidence. ^{6, 62}. Absorptivity of the light beam by the metal workpiece generally increases when the material is heated to the melting temperature 6, 61, 62

. However, a decrease in absorptivity with increasing temperature is also reported for 1µm wavelength light ⁶.

Laser cutting of a metal workpiece is initiated by piercing of the workpiece with a focused incident laser beam to generate a melt surface throughout the workpiece thickness.

Initiation of laser cutting of metals suffers from the effect of surface reflectivity which

limits the amount of laser energy coupled to the workpiece. Metals with high surface reflectivity - such as aluminium - require higher power intensity for cut initiation. The more energetic photons of the shorter wavelength radiation can be absorbed by a greater number of electrons such that the reflectivity of the metal surface falls and absorptivity of the surface is increased ². It is not just the total power incident on the workpiece which creates the melt but it is primarily the power intensity at the focal point that enables melting of the workpiece at the applied cutting speed ⁶³. The critical laser beam parameter that influences the focusability of the laser beam is the beam quality. A high quality laser beam – i.e. near diffraction limited beam quality – is essential for focusing of the incident laser beam to very high power intensities necessary for melting the workpiece at the focused point at the required high cutting speed. The laser beam quality generally degrades with increase in laser output power due to nonlinear effects in the laser cavity and the traditional solid-state lasers (Nd: YAG lasers) suffer more than the gas lasers e.g. CO2

lasers. The thermal load on the active crystal of the solid-state lasers causes thermal lensing effects which greatly deteriorate the beam quality with increase in output power of the laser system. However, the improved cooling mechanisms in the fibre laser and disc laser (solid-state lasers) have enabled near diffraction limited beam quality to be realized at high output power ^{2, 7}.

The melting efficiency of a particular laser is the ratio of the absorbed laser power utilized in melting of the kerf volume to the total incident laser power. The melting efficiency increases with increase in the absorptivity. In this work the term melting efficiency is used in a similar manner as absorptivity regardless of the fact that some of the absorbed laser power is lost through conduction to the substrate material and not utilized in melting the kerf volume.

Olsen ^{64, 65} described the mechanisms of the cutting front formation and identified zones that comprise the cutting front as the following: the melt surface which propagates through the material with a velocity that depends on the energy input, thermal properties of the workpiece material and the molten material removal mechanisms; the melt film; and the melt front. After cut initiation, the laser cutting process proceeds through absorption of the incident laser beam at the melt surface. There is a minimum melt film thickness necessary for transmission of the absorbed energy from the melt surface to the melt front. The melt front velocity increases with increasing laser power intensity which enhances the penetration speed and the maximum temperature occurs below the material surface ⁶⁶. This could be due to the multiple reflections that take place inside the thick-section cut kerfs and result in increased absorption of the incident laser beam towards the bottom of the cut kerf. Duan et al. reported that the multiple reflections within the cut kerf are a function of the cutting depth and angle of inclination of the cutting front and become more significant with increase in material thickness (> 3 mm) and cutting speed ⁶⁷. The conduction power loss from the cutting front to the substrate metal increases - in an approximately linear manner - with increase in workpiece thickness. The conduction power loss decreases with increase in cutting speed.

2.2 Methods of Laser Cutting of Metal