A computational study of laser-produced evaporation and plasma plume dynamics

In laser materials processing, localized heating, melting and evaporation caused by focused laser radiation forms a vapor on the materials surface. The plume is generally an unstable entity, fluctuating according to its own dynamics. The beam is refracted and absorbed as it traverses the plume, thus modifying its power density on the surface of the condensed phases. This modifies material evaporation and optical properties of the plume.; Numerical models of the dynamics of laser-induced evaporation and plasma plume formation are presented. The material model includes conduction heat transfer as well as melting and evaporation phase changes. The phase changes are modeled with kinetic and energy balance equations and the equations are solved in one dimension using a central finite difference scheme. Transformations in the state of the vapor as it evolves across the Knudsen layer are modeled assuming strong evaporation with relatively small backpressure.; The quasi-one-dimensional plasma plume is modeled using compressible Navier-Stokes equations in a quasi-one dimensional domain that expands to match the focusing laser beam propagation path. The governing equations are solved using the MacCormack explicit scheme. In addition, ionization, laser beam absorption and radial conduction heat losses are considered. Considering the active response to material surface from plume vapor, we calculated surface temperature and reflected laser power at the each time step. The transient behavior of established plumes is investigated using this simulation. It is found that, at surface power densities commonly associated with welding, the plume is not stable and convects away in the flow or evaporant. At low surface power densities such as might occur on the tilted front wall of a keyhole, the vapor flow is slower and the simulation predicts that a stable plume might be established.; The laser-produced plasma plume simulation is completed using the axisymmetric, high-temperature gas dynamic model including the laser radiation power absorption, refraction, reflection, and thermionic emission. The physical properties and velocity profiles are verified using the published experimental and numerical results. The parametric studies provided the effect of plasma plume fluctuations on the laser power density, evaporation rate on the material surface. It is found that the beam absorption, reflection and defocusing effects through the plume are essential to obtain appropriate mathematical simulation results. The mechanisms of the steady plasma plume involve relatively low velocities, low pressures, and thermionic emission near the tilted material surface. It is also found that the helium is more efficient in reducing the beam refraction and absorption effect for common laser materials processing.