Modeling and measurement of processes with liquid -vapor interface created by high power density lasers.

This research investigates the underlying physics of laser-material interactions, focusing on the formation of keyholes, and further creates computational models of laser drilling and keyhole welding. The liquid-vapor (L/V) interface processes are common in many laser processings. In this study, all important interface phenomena, such as free surface evolution, evaporation, kinetic Knudsen layer, homogeneous boiling and multiple reflections, are considered and applied to laser drilling and keyhole welding models. First, a laser drilling model is presented. An emphasis has been placed on the implementation of the evaporation and homogeneous boiling processes near the critical point. Due to the presence of a thin liquid layer, a scheme to model fluid flow is developed, incorporating both thermo-capillary effect and recoil pressure, keeping energy and mass balances complete. All process variables are modeled as being transient. Effects of laser intensity on material removal mode, liquid layer thickness, surface temperature, surface recession speed and effective beam profile due to multiple reflections are presented and discussed. This model also provides very interesting conclusions regarding the process, such as the narrowing of the keyhole and dominance of evaporative mass removal at high laser intensities. Second, a laser keyhole welding model is presented. For sharp L/V interface, the level set approach is used to incorporate the L/V interface boundary conditions into the Navier-Stokes equation. Both thermo-capillary force and recoil pressure, which are the major driving forces for the melt flow, are incorporated in the formulation. The capillary effect is considered as well. For melting and solidification processes at the solid-liquid (S/L) interface, the mixture continuum model has been employed. Therefore, this model has accounted for all the important factors in the laser welding process. All computations are performed in three-dimensions, and the effects of keyhole formation on the liquid melt pool are investigated. This model shows very interesting results, such as intrinsic instability of keyholes, role of recoil pressure and effect of beam scanning. For verification purposes, visualization experiments have been performed to measure melt pool geometry and surface velocity. The theoretical predictions show a reasonable agreement with the experimental observations.Ph.D.Applied SciencesMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/125678/2/3016883.pd