Prof. Franz-Josef Kahlen Profile

Prof. Franz-Josef Kahlen

Associate Professor at Univ of Cape Town

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Proceedings Article | 7 February 2000 Paper

Plasma instability and optimum utilization of laser energy in laser materials processing

Franz-Josef Kahlen, Hermann Emminger, Srikanth Sankaranarayanan, Aravinda Kar

Proceedings Volume 3888, (2000) https://doi.org/10.1117/12.377016

KEYWORDS: Plasma, Absorption, Laser energy, Laser processing, Liquids, Metals, Continuous wave operation, Energy transfer, Aluminum, Pulsed laser operation

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A plume consisting of vapor and ionized particles of the workpiece is usually formed during various types of laser materials processing. The process parameters such as the laser power, spot diameter, scanning speed, material properties and shielding gas affect the properties of this plume. A one- dimensional model is presented to understand the effects of the vapor-plasma plume in continuous wave (cw) laser materials processing. A model for pulsed laser materials processing is also discussed. These models are used to analyze the transmission of the laser beam through the plume and the deposition of energy on the melt pool at the substrate surface. An experimental technique described as the pinhole experiment is devised for pulsed laser operations to measure the partitioning of laser energy between the plume and workpiece and to identify the process parameter regime for efficient energy transfer from the laser beam to the workpiece. The attenuation coefficient of the vapor-plasma plume was measured during cw CO₂ laser-assisted metal deposition conditions by directing a CO₂ probe laser beam horizontally through the plume and determining the ratio of irradiance of the beam after and before the plume. Assuming an isotropic attenuation coefficient through the plume, the energy partitioning between the plume and workpiece was determined.

Proceedings Article | 2 September 1997 Paper

One-step microlithography

Franz-Josef Kahlen, Srikanth Sankaranarayanan, Aravinda Kar

Proceedings Volume 3225, (1997) https://doi.org/10.1117/12.284548

KEYWORDS: Optical lithography, High power lasers, Carbon monoxide, Gas lasers, Metals, Helium

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Subject of this investigation is a one-step rapid machining process to create miniaturized 3D parts, using the original sample material. An experimental setup where metal powder is fed to the laser beam-material interaction region has been built. The powder is melted and forms planar, 2D geometries as the substrate is moved under the laser beam in XY- direction. After completing the geometry in the plane, the substrate is displaced in Z-direction, and a new layer of material is placed on top of the just completed deposit. By continuous repetition of this process, 3D parts wee created. In particular, the impact of the focal spot size of the high power laser beam on the smallest achievable structures was investigated. At a translation speed of 51 mm/s a minimum material thickness of 590 micrometers was achieved. Also, it was shown that a small Z-displacement has a negligible influence on the continuity of the material deposition over this power range. A high power CO₂ laser was used as energy source, the material powder under investigation was stainless steel SS304L. Helium was used as shield gas at a flow rate of 15 1/min. The incident CO₂ laser beam power was varied between 300 W and 400 W, with the laser beam intensity distribute in a donut mode. The laser beam was focused to a focal diameter of 600 (Mu) m.

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