A mathematical model of ablation in the keyhole and droplet formation in the plume in deep penetration laser welding

Abstract

CW CO2, CO and Nd:YAG lasers are well established as power sources for the welding of metals. The mechanisms for the coupling of energy from the beam to the weld specimen are important; one such mechanism is Fresnel absorption of laser light in the surface of the metal and another is inverse bremsstrahlung absorption in the plasma of the plume above the surface of the work piece, and in the keyhole produced by the process. For the inverse bremsstrahlung mechanism to work a plasma must be present. It probably forms as a result of ablation of gas from the walls of the keyhole that is then heated up under the action of the laser beam, forming a partially ionised plasma. This emits radiation and so its temperature drops. The radiative losses can be calculated but the process can be complicated by the line spectrum of radiation that is in general emitted. The radiative losses from argon plasmas have been studied in detail and if metallic plasmas have comparable radiative losses, their temperatures would remain high enough for the Saha equation to predict partially ionised plasmas under conditions of local thermodynamic equilibrium. For partially ionised metallic plasmas in electric arcs data exist that suggest a much higher level of radiation losses. This could lower the temperature of the plasma to a value at which ionisation in a state of local thermodynamic equilibrium could be too low to be of significance even for CW CO2 laser wavelengths. Levels of ionisation are lower for CW CO laser wavelengths and probably negligible for the Nd:YAG laser. Plasmas are nevertheless thought to occur when Nd:YAG lasers are used for laser welding. This suggests that some non-equilibrium mechanism is present in laser welding leading to plasma generation. Such a mechanism arises from spray formation at the walls of the keyhole when the temperature is at or slightly above the boiling point of the metal under laser action. Spray ejected from the keyhole walls is quickly vaporised in the beam path. Droplets of small radii experience high levels of surface tension that increase the internal pressure and hence their boiling point can be high enough to generate plasma. The lifetimes of droplets outside the path of the laser beam could be much larger than in the presence of the beam, resulting in condensation in a suitable region of the plume. Their presence could be detected by scattering of a low-power transversely directed laser beam; they have been observed by Matsunawa [1]. The formation of bubbles in the keyhole wall that gives rise to the spray is studied here. Droplet generation is considered under conditions of local thermodynamic equilibrium by kinetic rate processes in the environment of the keyhole; so too is their ablation to produce plasma. Condensation in the plume outside the path of the laser beam is analysed so a possible mechanism is described for plasma formation under the conditions of laser keyhole welding.CW CO2, CO and Nd:YAG lasers are well established as power sources for the welding of metals. The mechanisms for the coupling of energy from the beam to the weld specimen are important; one such mechanism is Fresnel absorption of laser light in the surface of the metal and another is inverse bremsstrahlung absorption in the plasma of the plume above the surface of the work piece, and in the keyhole produced by the process. For the inverse bremsstrahlung mechanism to work a plasma must be present. It probably forms as a result of ablation of gas from the walls of the keyhole that is then heated up under the action of the laser beam, forming a partially ionised plasma. This emits radiation and so its temperature drops. The radiative losses can be calculated but the process can be complicated by the line spectrum of radiation that is in general emitted. The radiative losses from argon plasmas have been studied in detail and if metallic plasmas have comparable radiative losses, their temperatures would remain...