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The project is concentrated on two aspects of material treatment by laser: laser drilling and laser welding.

Laser drilling

Hole drilling by laser is a thermal process by which holes are created by focusing a beam of relatively low energy density into a high-density beam impinging onto a workpiece. This results in a spot where energy is sufficiently high to vaporize most of the material in the path of the beam. Advantages of laser drilling include
  • no tool wear or breakage
  • ability to accurately locate the holes
  • possibility to drill very small holes
  • low operating costs
On the other hand, disadvantages include
  • high equipment costs,
  • possible tapering in hole shape,
  • impurities being deposited on the workpiece,
  • restrictions in the depth of the holes.

The project involves a collaboration between the team of Prof. J.F. Tu (MAE-NCSU) and his students (mainly, A. Paleocrassas) and Prof. P. Gremaud (MA-NCSU) and his students (J. Collins and R. Rodriquez). The present goal is to find optimal firing schedules of low power drilling systems so as to maximize the attainable depth of the holes to be drilled. The study is supported by theoretical and experimental approaches. Laser ablation problems for ultra short pulses (fs, ps or ns) has been the focus of intense research. However, studies in the micro-second range, which is of interest here and leads to much larger removal rates, are few and far between.

As part of the project, a mathematical model of the phenomenon under study has been proposed. The model includes basically four ingredients. First, thermal reactions in the workpiece are taken into account. Second, the presence of a Knudsen layer at the melt surface is considered. Under the conditions of interest, this layer has sub-micron thickness and is just here approximated as a surface of a discontinuities across which temperature, density and pressure jump. Third, the behavior of the metal vapor is tracked by solving the Euler equations of gas dynamics. Fourth and finally, the influence of ambient air is also taken into account and the equations of gas dynamics are assumed to hold there as well. Mathematically, the problem involves two free boundaries (solid-vapor and vapor air) and corresponds to a coupling between parabolic (heat equation) and hyperbolic (gas dynamics) problems.

A discretization of the equations has been proposed, implemented and is in the process of being calibrated. The gas dynamics equations are discretized using a globally second-order centered scheme and a ``multi-fluid" approach is taken. Namely, the vapor-air interface is simply implemented by considering an appropriate local modification in the numerical fluxes. The numerical resolution is complicated by the presence of very short duration spikes in the laser power during ignition (up to 400% of the steady state power level). While the current code is one-dimensional, an axi-symmetric generalization of the approach is also under development.

Experimentally, different schedules have already been tested in Prof. Tu's lab, see picture above. However, post-processing is very time consuming and involves delicate etching techniques to reveal the holes after drilling. The combination of experimental and numerical techniques will allow for a much more efficient approach to optimal firing schedules.

Finally, a simple thermal code predicting the shape of the melt pool and its position related to the laser beam for laser welding problems has also been implemented. Publications are in preparation.