A study of the heat transfer and fluid flow phenomena in laser drilling.

For absorbed laser intensities of around 1010--10 12 W/m2, both melting and vaporization of the target workpiece occurs. Vaporization of the melt causes a pressure rise in the melt, a recoil pressure due to the momentum of the vaporizing molecules. Radial variations in the laser intensity lead to differing amounts of vaporization over the extent of the laser beam. Due to the differences in vaporization, pressure gradients are established in the melt, and these pressure gradients act to drive the melt away from the centerline and towards the edge of the beam where melt ejection occurs. Therefore, hole drilling is a result of both material vaporization and melt expulsion. Understanding the fundamental heat transfer and fluid flow phenomena in this drilling process is the key to linking the input laser characteristics with the resulting drilled hole. The focus of this work is the development and application of semi-analytic and numerical analysis techniques to the laser drilling process accounting for recoil-pressure-driven melt expulsion as a means of material removal. Stagnation flow, integral, and Fourier analysis techniques are applied to both pulsed and continuous wave laser operation with a variety of laser powers, beam radii, pulse lengths, pulse frequencies, and pulse shapes. Drilling trends are presented both in a qualitative and quantitative manner as functions of the operating characteristics of the laser. One of the significant advances of these analyses over previous research efforts is the treatment of the melt flow due to recoil pressure. The results of the analyses for several different materials show that only a small amount of the material is vaporized. Typically less than 20% is vaporized and the rest is removed via melt expulsion, removal mechanics that result in a more energy-efficient drilling process. It was determined that operating the laser in pulsed mode results in higher pressure gradients, faster drilling speeds, and thinner melt layers when compared with cw drilling for the same average power. When compared with experimental data from the literature, reasonably good agreement is observed.PhDApplied SciencesMechanical engineeringPlasma physicsPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/132302/2/9963739.pd