Diagnosis and Modelling of Nonlinear Dynamics in Laser Cutting, Welding and Drilling
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Diagnosis and Modelling of Nonlinear Dynamics in Laser Cutting, Welding and Drilling
W. Schulz Fraunhofer Institut f¨ur Lasertechnik Steinbachstrasse 15, 52074 Aachen, Germany
ABSTRACT
Laser cutting and welding are well established industrial applications. To maintain productivity and to guarantee product quality the industry tries to introduce monitoring and control systems. The long term goal is the autonomous laser machine. Signal assessment is advancing by monitoring and simulation of the dynamical processes. Applying the advanced results about diagnosis and modelling broadens the potentials to cope with productivity and quality features in drilling, trepanning and fine cutting. As result, in cutting two mechanisms for the formation of adherent dross are revealed theoretically, identified by the monitoring system and can be avoided by modulation of the laser beam power. In welding, the dynamic model predicts the formation of pores sets in or is suspended depending on modulation frequency for the laser power. In drilling the mechanisms governing the maximum depth of the drilled hole – still showing efficient melt removal – are identified experimentally and can be related to the processing parameters theoretically
INTRODUCTION
Laser processing covers the leading percentage (86%) of the present west European laser market for industrial applications, where metal sheet cutting (44.8%) dominates metal sheet welding (27.7%) and marking (13.5%) as well as the remaining not yet widespread applications such as drilling, soldering etc. Scientific research and development are related to monitoring, quality assurance and adaptive control in precision machining. Fundamental research activities are focussed on the dynamical features of the process – such as ripple formation [1] and adherent dross [2, 3] – in contour cutting [4], processing with elevated speed, precision machining of microstructures [5, 6] and exploiting the potentials of diode pumped solid state and fiber lasers [7]. The fundamental physical processes – also present in caving, welding and drilling – are related to the movement of free boundaries separated by the melt flow establishing the dynamical shape of the cutting front. Fundamental results are present for the physics of thin film flow [8, 9, 10], wetting [11, 12] and flow separation [13, 14], as well as evaporation and condensation [15]. Actual investigations deal with the development and application of mathematical methods for the analysis of partial differential equations (asymptotic methods [16, 17, 18], integral (variational) [19, 20] and spectral methods [21]) and Free Boundary Problems [20], which yield the formulation of reduced (asymptotically exact) models [16, 17, 22, 23, 24]. Recently developed numerical methods for tracking of free boundaries (Level Set method [25, 26],
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adaptive ”sparse grids” [27]) are applied for numerical simulation [28]. The comparison of approximate models with advanced numerical simulations and experimental identification [29, 4] by diagnostic m
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