Theoretical Analysis of Laser Cutting of Metals at 1 and 10 micrometer wavelength
Michael Heinrich Brügmann

We present a theoretical analysis of laser cutting of metals based on a model originally proposed by Niziev and an extension which includes heat conduction. Specifically, we investigate the dependence of relevant parameters, such as maximum cutting speed, shape of the cutting front et cetera, on wavelength, polarization, and laser beam properties. A special emphasis is on the comparison between results obtained for lasers around 1 μm and CO2 lasers at 10 μm wavelength, respectively. To test the model we compare the numerical solutions for a cold-work steel work-piece to experimental results presented elsewhere. We find good agreement between theoretical and experimental observations. The main differences between laser cutting with 1 and 10 μm lasers arise from the different absorptivity profiles and absorbed intensities. In most of the analysed cases the computed mean absorbed intensities as well as the absorbed intensity profiles show that the energy transfer is more efficient for laser cutting with 1 μm lasers.

Keywords: Macro-Processing; Cutting; Solid-State Laser; CO2-Laser

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Performance and efficiency of an industrial direct diode source with an extremely low BPP in laser cutting of Fe-based and reflective alloys
Barbara Previtali, Giovanni Riva, Erica Librera, Maurizio Sbetti, Mattia Vanin, Giacomo Biscaglia, Francesco Villa, Bien Chann, Bryan Lockman

The performance and efficiency of a 2kW industrial direct diode laser source with an extremely low BPP are investigated when carbon and stainless steels as well as aluminium and brass sheets are laser cut. The results confirm the industrial feasibility and robustness of the direct diode laser source as tool for laser processes. In particular in the oxidation laser cutting of iron-based alloys the low BPP together with relative larger transport fiber diameter allows quality and cutting speed equivalent to the active fiber and disk laser sources in a very large range of thickness (up to 15 mm). When higher power densities are required, because inert laser fusion cutting of structural steel is carried out or because high reflective alloys need to be cut, the low BPP and the shorter wavelength are favorable figures and produce comparable performances with the mentioned laser sources.

Keywords:Direct diode; Laser cutting; Fe-based alloys; Reflective alloys, Performance; Efficiency;

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Effect of the laser beam polarization state on the laser cut surface quality
Alexander Golyshev, Victor Shulyatyev, Anatoly Orishich

It is well-known that laser cut characteristics highly depend on the polarization state of laser beam. The works on laser cutting deal mainly with the effect of the polarization on the cutting speed, whereas the effect on the cut surface quality was almost neglected. This paper presents the experimental investigation of the effect of the laser beam polarization state on the laser cut surface roughness. The СО2 laser was used to cut steel sheets of 3, 5, 8 and 10 mm. The cut surface roughness and maximal cutting speed were measured in the cases of the circular polarization of the laser beam, and plane polarization at various angles between the polarization plane and cutting speed direction. For all thicknesses, the roughness is minimal when the cutting speed direction is perpendicular to the polarization plane. The cutting speed and cut surface quality of the samples, cut by the СО2 laser at various polarization states, were compared with the respective parameters of the samples cut by the fiber laser.

Keywords: laser cutting, CO2 laser, fiber laser, polarization, surface roughness;

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Multiple Wavelength Laser Processing Technology for Flexible Manufacturing
Joe Hillman, Yefim Sukhman, Chris Risser

Multiple wavelength laser processing technology allows laser beams of several different wavelengths to be combined to form a single, coaxial beam. This hybrid laser beam is capable of cutting composite materials, which are composed of matrix and reinforcing materials that have different optical and physical characteristics. For example, carbon fiber reinforced polymer can be cut using a combination of a 1.06 micron laser beam, which cuts thought the carbon fibers, and a 10.6 micron laser beam, which simultaneously vaporizes the polymer matrix material. This technology not only allows two or more wavelengths to be combined, but also allows for seamless switching from one laser wavelength to another. This capability enables flexible manufacturing processes for many applications.

Keywords: macro processing; laser cutting; composites; multiple wavelength; multiwave

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Remote laser cutting of composites with a fibre guided thin-disk nanosecond high power laser
Sven Bluemel, Veit Angrick, Stefan Bastick, Peter Jaeschke, Oliver Suttmann, Ludger Overmeyer

Carbon fibre reinforced plastics (CFRP) are of high interest for lightweight construction within many industrial sectors. The automotive industry shows already an increasing demand for CFRP parts, but the implementation of CFRP parts is limited by the lack of automatable, reliable and cost efficient processes. In the field of laser cutting of CFRP materials, there are also different scientific approaches such as the use of nanosecond lasers with high average power. At this
point, the German research project HolQueSt 3D starts dealing with 3-dimensional high power laser processing of lightweight CFRP structures in enhancing quality and quantity. Within this paper, the authors will describe the first results achieved with a newly developed fibre-guided high power nanosecond laser. The laser emits at a wavelength of λ = 1030 nm with a pulse duration of tp = 30 ns. The laser has an average power of Pavg = 1.5 kW with a maximum pulse energy of Ep,max = 80 mJ. The laser beam was deflected by a 3D programmable focusing optic with a focal length of lf = 255 mm. CFRP based on an epoxy resin with two different
reinforcements was used for the investigations. The gained results were analysed concerning the achieved heat affected zone and the optical quality of the cutting edges. The average HAZ width b could be minimized to a value below b <40 μm for both materials. Furthermore, the maximum effective cutting velocity for selected laser parameters was determined. The maximum effective cutting speed achieved within this investigation is veff = 1.8 m/min. The results of the investigation revealed the potential of the fibre-guided nanosecond laser for industrial applications. This is in particular the case for 3D applications due to the possibility of robot based remote processes.

Keywords: cutting, composites, nanosecond laser

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F-Theta at Jenoptik - a holistic approach
Tim Baldsiefen

The continuing development in laser technology and laser applications drives the development of suitable optical systems and components. E.g. both trends to higher laser power as well as shorter laser pulses pose different new challenges for the design, manufacturing, and testing of F-Theta lenses. This contribution will present how Jenoptik Optical Systems GmbH addresses these challenges, employing experiences as a manufacturer for customized optical systems for both laser as well as high-NA UV applications.

system technology, physical simulations, optical system design, high power laser applications

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Laser or Plasma Cutting – Is there a Choice?
Volker Krink, Thomas Dr. Rümenapp, Michael Dr. Schnick, Nicole Dönicke

There are various methods that can be used for cutting metals. Depending on the type of material as well as economic and quality-related aspects, each technology has its own strengths and thus its justification. On the one hand, there is the versatile laser cutting; on the other hand there is the constantly improved plasma cutting technology. Thus, new areas of application for cutting metallic work-pieces with plasma could be opened up over the past years and process variants have been developed which cover even non-conductive or interrupted materials in a wide performance range and a wide range of material thicknesses. Today, some plasma cutting systems achieve in many areas cut qualities which are equal or almost equal to laser cutting. Due to the significantly lower investment costs compared to laser, the costs per cutting metre for plasma cutting are considerably lower. The achieved performance increase of the plasma cutting units with regard to thick mild steel sheets is also increasingly competing with gas cutting methods. But which cutting method is the right one? This contribution shows the strengths and weaknesses, which every user should consider. It does not always have to be a laser, because laser cutting also has its limits, or there are areas of application where other methods, like
plasma cutting, can be more economic and/or achieve high quality.

Keywords: Cutting; Plasma; Laser; Fiber

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Laser Micro-Cutting of Thick Tungsten Sheets
Ramunas Šniaukas, Gediminas Raciukaitis

In recent years, laser-based technologies become important or even dominant in industrial applications such as welding or cutting. Further possibilities and innovations are still in progress concerning the area of laser micro processing. Laser technologies enables to manufacture materials with micrometer accuracy, however, there are obstacles to reaching desirable speeds when cutting thick materials (>200 μm). Different cutting strategies were investigated to achieve the high cutting speed and quality at the same time in a 300 μm thick tungsten with picosecond lasers. A parametric study of laser cutting was performed taking into account trade-offs between precision and cutting speed. Experimental results proved that picosecond lasers are suitable to process quite thick tungsten sheets with micrometer accuracy. Furthermore, improvements in overall performance were achieved by optimizing the beam guiding approach over the full size of the component to cut.

Keywords: Ablation; Micro-cutting; Picosecond laser;

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The influences of pulse overlap on cut quality during fiber laser cutting of electrodes for Lithium-ion batteries
Tobias Reincke, Stefan Kreling, Klaus Dilger

Concerning e-mobility and the research of highly developed battery technologies within the automotive sector the quality improvement and cost reduction of Lithium-ion batteries is an important challenge. Therefore the Battery LabFactory Braunschweig (BLB) examines the entire production chain with the objective of improving the energy density, quality and reliability of these traction batteries. Within this development programme the cutting of the electrodes represents a significant challenge for the production of Lithium-ion batteries. Compared to die cutting the approach of contactless laser cutting offers a higher flexibility and reduced tool costs. Furthermore the decreased edge quality due to the tool wear can be prevented by using laser cutting. Referring to laser cutting of thin metal and composite sheets by using a fiber laser a particular challenge is the improvement of the cut quality as well as the reduction of the heat affected zone leading to a degradation of properties.
This comparative study investigates different process parameters such as pulse frequency or cutting speed and focuses on the influence of pulse overlap or respectively yielded energy to minimize the heat affected zone and thus improve the quality of the cut. The influences of the pulse overlap on the delamination width which is defined as the delamination of the coated layer of the metal substrate at the laser cut kerf are examined.

Keywords: Laser cutting; Lithium-ion battery; Fiber laser; Pulse overlap

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Laser welding simulation of microfluidic devices
Arnaud Francois, Anne Henrotin, Jose A. Ramos

In this study, a numerical approach is presented for the simulation of the transparent laser welding process of thermoplastic polymers. In particular, the numerical tool is used to develop the welding process of a microfluidic device. The studied microfluidic device consists of two thermoplastic sheets, one of them has been previously micro-machined by a specific UV femtosecond laser setup. The assembly of the microfluidic device is then obtained by scanning a laser beam over its entire surface. The weld quality of laser welded thermoplastics is strongly influenced by the amount of laser energy that is converted into heat, which also depends on the optical and thermal properties of the materials. In practice, finding suitable processes parameters for new products is often a difficult task given that temperatures at the interface need to reach the melting point without exceeding the degradation temperature of the polymer. Furthermore, the presence of micro channels in microfluidic devices modifies the heat absorption and heat transfer resulting in inhomogeneous temperature distributions at the weld interface. The numerical approach followed here gives access to values that are difficult to measure experimentally, in particular the temperatures in the melted zone. The development of the process was carried out in steps of increasing complexity, from the study of the assembly of two plates without any micromachining to the analysis of the microfluidic device. Experimental welding tests are also presented and were carried out to validate the simulation observations and are also presented.

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Temporally and spatially resolved measurement of the cut front geometry while cutting with a solid-state laser
Oliver Bocksrocker, Tim Hesse, Peter Berger, Meiko Boley, Thomas Graf

Solid-state lasers at the wavelength of about 1 μm have gained a rapidly increasing importance in the last few years for cutting metal sheets. As known from numerous investigations, the spatial distribution of the absorbed laser power is a key factor for the efficiency and the quality of the cutting process. Therefore, the exact cut front geometry is one major input parameter in order to investigate the influence of the absorption process. In the present paper, measurements of the cut front geometry while cutting mild steel will be presented. A novel monitoring system, a so called quotient goniometer, enables recording temporally and spatially resolved measurements
of the cut front geometry. The quotient goniometer exploits the angular- and polarization-dependent thermal emission of hot surfaces in order to measure the angle of the cutting front. The study shows that the cut front has a characteristic, straight profile while cutting with standard parameters. In addition the dynamic behavior of the molten material can be visualized with the quotient goniometer. It will be shown, that there is a high fluctuation of the cut front profile near the bottom of the cut front, while the shape in the upper half of the cut front typically remains constant over time. The measurements are compared to longitudinal sections to investigate the cut front geometry in the cutting direction.

Keywords: laser cutting; cut front geometry; thermal radiation; polarization; process monitoring

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Optical cutting tear detection system for industrial fiber laser based cutting machines
Benedikt Adelmann, Benedikt Neumeier, Max Schleier, Eugen Wilmann, Ralf Hellmann

In this study, we demonstrate an optical cutting tear detection and evaluation system affixed to a 4 kW fiber laser cutting machine. The sensor is mounted between the cutting head and collimator and collects the thermal radiation from the process zone. The process radiation is detected by a stacked silicon and InGaAs photodiode combination and digitalized with 20 kHz sample rate. The acquired signal for two exemplarily chosen cuts in 2 mm stainless steel which one of them include cutting tears are shown wherein the piercing, the waiting time and laser switch on/off are clearly be resolved. These signals are high pass filtered and the fluctuation range is calculated. In the resulting signal, a cutting tear is indicated by the fluctuation range of the Si diode exceeding the fluctuation range of the InGaAs diode multiplied with
a correction factor. With this characteristic signature, 193 cuts including 83 cutting tears were analyzed revealing 96.4 % detection rate (alpha error 3.6 %) and 0 % beta errors. The easy integration in existing cutting systems, the direction independent signal and the high detection rate highlight the systems potential for cutting tear detection in industrial cutting machines.

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Modeling of Transport Phenomena in Metal Fusion Cutting Using High Power Laser
Karim Kheloufi, El Hachemi Amara

In the present study, a three-dimensional transient numerical model was developed to study the temperature field and cutting kerf shape during laser fusion cutting. The finite volume model has been constructed, based on the Navier–Stokes equations and energy conservation equation for the description of momentum and heat transport phenomena, and the Volume of Fluid (VOF) method for free surface tracking. The Fresnel absorption model is used to handle the absorption of the incident wave by the surface of the liquid metal and the enthalpy-porosity technique is employed to account for the latent heat during melting and solidification of the material. To model the Physical phenomena occurring at the liquid film/gas interface, including momentum/heat transfer, a new approach is proposed which consists of treating friction force, pressure force applied by the gas jet and the heat absorbed by the cutting front surface as source terms incorporated into the governing equations. All these physics are coupled and solved simultaneously in Fluent CFD®. The main objective of using a transient phase change model in the current case is to simulate the dynamics and geometry of a growing laser-cutting generated kerf until it becomes fully developed. The model is used to investigate the effect of some process parameters on temperature fields and the formed kerf geometry.

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Dynamic beam shaping for laser fusion cutting
Cindy Goppold, Thomas Pinder, Patrick Herwig, Achim Mahrle, Andreas Wetzig, Eckhard Beyer

Sheet metal with thicknesses above 8 mm has a distinct cutting performance. The optical configuration, composed of fiber diameter, collimation and focal length, allows for many opportunities to influence the stationary beam geometry. Previous analysis points out the limits of this method in the thick section area. The achieved cut quality of a stationary beam shape is restricted by the laser power. Most operators would like to improve the cutting results without changing their available laser sources, even for thicker materials. To overcome some limits, new approaches are desired. Within the present study an experimental investigation of fiber laser fusion cutting of thick section stainless steel was performed, by means of dynamical beam oscillation. The aim is to compare stationary and dynamical beam shaping,
through evaluation of the cut quality, the kerf geometry, and the applied process parameters. The investigation emphasizes promising procedural possibilities for improvements of cutting performance in fiber laser cutting of thick stainless steel.

Keywords: laser fusion cutting; beam shaping; oscillation; dynamic; cut kerf

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Productive Laser Processing of CFRP
Volkher Onuseit, Torben Prieß, Christian Freitag, Margit Wiedenmann, Birgit Faisst, Roswitha Giedl-Wagner, Thomas Rettich, Rudolf Weber, Peter Middendorf, Thomas Graf

Laser processing of carbon fiber reinforced plastic (CFRP) is a very promising method to solve a lot of the challenges for large-volume production of lightweight constructions especially in automotive and airplane industries. Laser processes are very promising for tasks like cutting of dry fibers, trim cutting of parts after the curing process, or drilling of holes for riveting. The challenge for these processes is to reach both, the productivity and quality which is needed for large-
volume production. In this paper processing with different laser sources including cw-lasers with high average power up to 6 kW and ps-lasers with average power from 30 W up to 1 kW will be compared in terms of productivity and quality for different applications. The main issue reducing the quality of laser-processed CFRP parts is the heat affected zone which results from heat conduction into the material. In the present paper, the influence of the heat affected zone on the mechanical strength of CFRP components will be discussed. It will be shown that damage-free laser processing of CFRP is possible when using high intensities above 108 W/cm² and avoiding any kind of heat accumulation in the processing zone. To reach this intensity it is favorable to use short and ultra-short pulsed lasers with pulse duration of
several ns down to ps. However, the productivity of the process is dominated by the average laser power available which is actually below 150 W for commercial systems. Therefore it is necessary to increase the efficiency by using advanced process strategies.

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Innovative distance control for laser cutting based on inline low coherence interferometry
Robert Schmitt, Guilherme Mallmann, Timo Kosanke

Laser based cutting processes are present today in a large number of industrial applications, processi ng different materials from micro to macro-ranges with high flexibility and automation levels. Modern systems use different sensor units to monitor and consequently enable a robustness enhancement through a controlled process. In the case of a distance control, e.g. an integrated capacitive sensor monitors the distance between laser cutting head and workpiece, with the aim of maintaining the workpiece within the system’s allowed parameter window. The application of this technique is however just possible on conductive materials, not covering several carbon fiber reinforced plastic (CFRP), plastic or glass workpieces. Additionally the measurement spots of capacitive sensors are very large, in the range of some millimeters, limiting its usage next to workpiece discontinuities. Within this paper, the integration of a low coherence interferometer in the optical path of a laser cutting head is presented for inline distance measurements. This solution presents a promising alternative to the state of the art, enabling a material and surface independent monitoring / control. Furthermore the dimensions of the measurement spot in the range of micrometers (same range of the laser spot) also enables the monitoring / control next to workpiece discontinuities. The achieved results validate its usage for inline distance measurement in different parameter scenarios.

Keywords: laser cutting; process monitoring; process control; inline metrology; low coherence interferometry

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Areas of application for TEA CO2-Laser induced shock waves
Stefan Veenaas, Frank Vollertsen

Increasing batch size and shorter lifecycle in micro manufacturing is a challenge for production processes. Conventional processes have limits regarding the technical feasibility for producing micro parts, due to so-called size effects. Therefore, a new flexible production approach using TEA-CO2-laser induced shock waves for manufacturing micro parts is presented in this paper. For thin sheet in the range of 15 μm to 100 μm materials there are some technical challenges using a conventional mechanical process. The right tool alignment, for forming, punching and blanking processes, between punch and die is an increasing challenge when the process dimensions are decreasing. Additionally, in a punching process the clearance between punch and die is such a critical parameter. The size of a suitable cutting clearance correlates with the sheet thickness, e.g. for a fine blanking process 0.5 % of the material thickness, which results for 20 μm in clearance dimension of just 0.1 μm. Therefore, it is shown how a laser shock process in micro range can replace conventional forming, punching and blanking processes. For thermal joining processes, there are restrictions due to the principle of joining, especially for the use in clean rooms or regarding the creation of intermetallic phases for hybrid joints. Therefore,
laser induced shock waves can be used to join different sheets by a plastic forming process. This technology enables the joining of different sheet materials with thicknesses between 20 μm and 300 μm. The manufacturing of these joints is an incremental process where several laser induced shock waves are needed to form an undercut, which presents the joint itself. The joining of aluminum/steel and aluminum/copper joints is shown in this paper. For different thicknesses ranging from 15 μm to 100 μm of Al99.5-sheet perforations were successfully generated. The punching geometry is also not limited to circular holes. Rectangular perforations were proved to be applicable as well. Methods and approaches to use a laser as a flexible tool for forming, cutting and joining processes for the micro range are presented in this work.

Keywords: laser shock forming, mechanical joining, laser shock cutting