Modeling for Self-Optimization in Laser Cutting

Modeling for Self-Optimization in Laser Cutting

Torsten Hermanns (RWTH Aachen University, Germany), Ulrich Thombansen (RWTH Aachen University, Germany), Markus Nießen (Fraunhofer Institute for Laser Technology, Germany), Ulrich Jansen (RWTH Aachen University, Germany) and Wolfgang Schulz (Fraunhofer Institute for Laser Technology, Germany)
DOI: 10.4018/978-1-4666-8823-0.ch019
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Abstract

Laser based cutting processes comprise a significant part of industrial manufacturing. To ensure a high level of cutting quality the process parameters have to be kept at the right values and within defined tolerances. In this context, the position of the laser beam focus is one of the most important parameters. However, due to thermal heating of optical elements the focal position shows a transient behavior. The handling of this focus shift is aggravated by the fact that the focal position is not directly detectable during processing. In this chapter an introduction in the topic of laser cutting is given. Subsequently, a closed mathematical formulation of laser fusion cutting is presented before a reduced model is developed that allows the determination of a focus shift by an experimentally accessible surrogate criterion. The theoretical predictions are finally validated by experimental results.
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Introduction

Laser based processes depict a significant part in today’s industrial manufacturing. Caused by continuous enhancements of existing laser based processes as well as the new developments in the field of laser processing a future increase of the importance of laser based industrial manufacturing can already be foreseen. Currently laser cutting is the leading application in the field of laser processing from the processing of flat metal sheets for construction over cutting of pre-formed parts in white goods industry to trimming of hardened 3D-parts in automotive industry. All these variants require a reliable performance concerning the resulting product quality which is typically determined by the roughness of the cut faces – the ripples – and the occurrence of solidified melt adherent at the lower side of the work piece – the dross. However, several phenomena of the cutting process are yet not completely understood so that a satisfactory description of the correlation between the process adjustments and the resulting product quality is prevented. Thus, the common procedure to ensure the requested cut quality is to establish setting parameters for the cutting machine so that the requested quality is always reached and to collect the latter in so called technology tables. However, to find these setting parameters time consuming empirical experimental testing is required, where test series comprising several thousand experimental tests are not unusual. Furthermore, influences like unforeseeable changings of the machine status, typically caused by disturbances of the setting parameters, cannot be covered. These aspects show the necessity for further modeling of the cutting process. On the one hand a reliable description of the resulting quality in dependence of the setting parameters reduces the time-consuming and uneconomical experimental testing. On the other hand a comprehensive process model is the basis of the development of a process control in order to react on changes of the machine status during processing. In this context, the focal position of the laser beam, i.e. the position of the beam waist relative to the work piece, is one of the most critical setting parameters in a real world scenario. While the focal position is not directly detectable during processing caused by the inaccessibility of the process zone it has a major impact on the resulting cutting quality. In this context, a challenging problem is the occurrence of disturbances or degradations of the optical elements of the cutting machine and a subsequent shift of the focal position during processing. Thus, a focal shift during the cutting process leads to uncontrollable variations of the resulting cutting quality which cannot be compensated by conventional process control. The common approach to handle the focus shift is to cut a thin metal sheet with different adjustments for the focal position and to observe the resulting cut widths. As the energy distribution widens with increasing distance from the beam focus such a cut results in a sample with an increased kerf width. This so called comb-cut allows the correlation of the focal position to the resulting kerf width by manually testing with a gauge. However, the determined reference position based on the comb-cut approach only describes the condition of the machine at the moment when the comb-cut is being produced. Variations during the process cannot be covered so that the status of the machine is known only after each job by reproducing a further comb-cut. In this chapter a reduced process model for laser fusion cutting is presented which offers a possibility of tracking the focal position during processing. In detail, a surrogate criterion for the focal position is defined which is measurable during the process: The first molten point at the upper side of the work piece in feed direction – the melting point. Subsequent reduction of the underlying physical equations and comparison with experimental explorations lead to the result that the correlation between the melting point and the focal position can be described by a two-dimensional one-phase model, i.e. a model that only considers the solid state of the work piece (one-phase model) and neglects the heat conduction in the direction of the laser beam axis (two-dimensional).

Key Terms in this Chapter

Process Parameter: Parameter that evolves during the process, e.g. surface temperature of the melt film or the roughness of the cut faces.

Setting Parameter: Independent parameter of the process, e.g. laser power or cutting speed.

Beam Focus: The beam waist, i.e. the Location of the smallest extent of the beam caustic.

Sagittal Plane: Plane in the middle of the cut kerf that is spanned up by the laser beam axis and the feed direction.

Surrogate Criterion: Detectable process parameter that is measured as surrogate for a not accessible process parameter which can be connected with the measured surrogate by a process model.

Focus Shift: Shift of the focal position during processing typically caused by disturbances or degradations of optical elements.

Melting Point: First molten point in feed direction within the sagittal plane.

Self-Optimizing Cutting Machine: Cutting machine that is able to detect its current process status and to react adequately in order to achieve one (or several) target(s), e.g. stabilization of the focal position or reduction of ripple formation at the cut surfaces.

Beam Caustic: The envelope of the intensity distribution which encloses the region where the laser beam radiation is concentrated

Reduced Model: Model that especially not describes the overall complexity of a problem in order to identify fundamental properties and mechanisms.

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