Macro Processing: System Technology (LiM 2015)

Laser fume particles of ultra fast processing: New demand on extraction and filtration technology
Stefan Jakschik, Stefan Blei

Laser treatment is increasingly used in material patterning. Avoiding thermal impact is for some of them crucial to achieve high quality products. This requirement let to recent increased utilization of ultra-fast laser processes. The following article investigates laser safety requirements of ultra-fast processes with respect to particle generation, shape and filtration. Especially, particles of ultra-fast processes are significant smaller and have a non-hemispherical shape, which asks for specialized solutions to achieve a long filter life time.

Keywords: Ablation, Ultra Fast Lasers, Process Control, filtration


System Technology for High Speed Laser Welding
Peter Hoffmann, Bernd Pögel, Roland Dierken

Improved laser sources in terms of beam parameter product initiated the idea of remote welding with a laser scanner. Various concepts for combining a standard robot with a scanner have been realized during the last years. Key problems have been the missing accuracy of the robot ́s guiding behavior, the interlinking of the control units of the base machine and the scanner and last but not least the generation of programs for welding-on-the-fly. Therefore, applications were limited to less complicated joint geometries such as overlap joints on more or less flat assemblies. ERLAS was the first company to decide using laser remote welding for three dimensional processing and butt seam welding in mass production. A new type of machine was born in 2012. Special features of this machine are hybrid kinematics for movement of both, the 3D-scanner by Cartesian Axes and the work piece by rotating and tilting axes. Based on these ideas ERLAS decided to go forward again and place a novel laser welding cell on market.

Keywords: Macro, Processing, System Technology, Joining


Latest trends in high power disk laser technology
Volker Rominger, Marco Holzer, Matthias Koitzsch, Tracey Ryba

Diode pumped solid state lasers have become an important tool for many industrial materials processing applications. They combine ease of operation with efficiency, robustness and low cost. This paper will give insight in latest progress in disk laser technology ranging from kW-class CW-Lasers over fundamental mode beam quality to frequency converted lasers, as well as advances in new applications realized with disk lasers. As of today, the disk principle has not reached any fundamental limits regarding output power per disk or beam quality, and offers numerous advantages over other high power resonator concepts, especially over monolithic architectures. Recently a TruDisk 6001 with 6 kW from a single disk completed the TRUMPF portfolio. The disk laser enables high beam quality at high average power and at high peak power at the same time. The power from a single disk was scaled from 1 kW around the year 2000 up to more than 10 kW today. Coupling of two disks in a common resonator results in even higher power of 20 kW. Consisting of two 16 kW units laser beam can be applied scalable up to 32 kW through one single twin-fiber.
Recently was demonstrated more than 4 kW of average power from a single disk close to fundamental mode beam quality (M² = 1.38). The extremely low saturated gain makes the disk laser ideal for internal frequency conversion. We show >1 kW average power and >6 kW peak power in multi ms pulsed regime from an internally frequency doubled disk laser emitting at 515 nm (green). Also external frequency conversion can be done efficiently with ns pulses. More than 500 W of average UV power was demonstrated. Besides improvements in efficiency, latest state-of-the-art disk lasers come up with intelligent energy saving modes, wake up and sleep timer functions that allow energy savings also within idle times. The conclusion will show a quick overview of what type of applications can be performed by these lasers including cutting, welding, remote welding, LMD, hardening and hybrid welding.

Keywords: Macro Processing; System technology; Disk laser; High Power, Brillance, Energy saving mode, frequency conversion


Creation of system for studying of optical materials and coatings for their damage threshold, mastering of measuring methodic.
Elena Krehova, Alexander Ignatov, Vasily Kurakin, Anatoliy Pozdnyakov, Yuri Kalinin, Boris Krayev, Andrey Skrynnik, Michael Filipov, Vladimir Serebryakov

This article describes the system for testing the damage threshold of laser phosphate glass active elements that should be resistant to impulse radiation with duration of 5 ns at wavelength 1054 nm and stand single exposure not less than 30 J/cm2. Measurements are conducted by direct focused laser radiation effect on bulk or surface of sample with recording the laser radiation parameters (energy, area of the exposure spot, distribution of radiation density in the exposure spot).

Keywords: damage threshold, pulse radiation, spot of exposure, active element


Laser joining of glass and metal
Thomas Schmidt, Benjamin Kipker, Ronny Bauer, Daniel Eilenberger, Sabine Sändig

Joining of glass and metal is in the most cases realized by ultrasonic welding, soldering or gluing. Another possibility is the direct bonding of glass and metal by glazing. This technology exists since more than 100 years. Most prominent examples are the light bulb or the television tube. At the moment, this technique is developed for additional applications in combination with the new tool laser. Actual examples are solar collector tubes or X-ray tubes. Here the most important requirement is the vacuum tightness for several 10 years. In the industry this process is realized with burners in semiautomatic processes. This paper shows a new processing method for the combination of glass and metal with CO2-laser radiation. Fundamental material properties of the bonding partners, physical and chemical correlations and the following process steps will be discussed. The presentation of the system techniques and the process cycle gives an overview of the level of automation and the aspects for production, like process time and laser power. The research in joining glass and metal regarding to stability, mechanism of bonding and the internal stress will allow for the evaluation of this joining method. An outlook for further steps in the development of this procedure will complete this article.

Keywords: joining; surface functionalization; process control; system technology


Identification of process phenomena in DMLS by optical in-process monitoring.
Robert Domröse, Thomas Grünberger

Additive manufacturing processes of metals were investigated by an optical in-process monitoring setup. Direct Metal Laser Sintering (DMLS) is an AM-process in which parts are built from metal powders, which is fused layer by layer, through exposure with an infrared laser. The basic layout of the monitoring setup is photodiode-based sensing of process light in combination with software-based data evaluation of the photodiode signals. Enhancing a monitoring or measuring system for the purpose of automatic quality inspection requires sufficient knowledge about the correlation between involved sets of variables. In this context, variables were partitioned into three categories: input variables (meaning the process control e.g. laser power, scan speed etc.), material properties and photodiode signals including their signal characteristics. Correlations between monitoring signals and process conditions were found, based on a comparison of nominal process (baseline population) and faulty process (defect population). Faulty process was induced by intentional variation of the input parameters (provoked errors, e.g. variation of laser power), as well as investigation of known and undesired process effects (e.g. overheat effects, interaction of laser beam and process smoke). Characteristics in the corresponding photodiode signals were identified. Algorithms to auto-recognize these signal characteristics were created and tested successfully . Examples are given that show the successful auto-detection of selected process effects.

Keywords: Additive manufacturing; Process Monitoring and Control


New Approaches for seam tracked laser beam brazing and welding
Daniel Reitemeyer, Stefan Liebl, Florian Albert

Laser brazing of steel and laser welding of aluminum and steel parts with filler wire are state of the art in mass production processes, e.g. in the automotive industry. As the standard supplier for wire guided brazing/welding optics Scansonic frequently receives inquires about customized beam shaping. Within this contribution beam shaping approaches aiming on process optimization by optimized energy distributions are presented. Therefore, technical approaches, resulting intensity distributions at the work piece and processing results are discussed. One example shown here is beam shaping by the use of a diffractive optical element (DOE) inside of the brazing and welding optics Scansonic ALO3. The DOE shapes the collimated laser beam for a resulting characteristic intensity distributionin the focal plane. The application addressed in this contribution is the Bifocal Hybrid Laser Welding (BHLW), an innovative technique for welding aluminum alloys with high surface quality and reduced hot cracking susceptibility of the weld seam. The DOE allows for bringing the BHLW process with laser powers up to P = 8 kW from research to industrial application.


Statistical distributions of the protection time of passive laser safety barriers – Normal distribution or is there a better description?
Florian Peter Lugauer, Florian Moosbauer, Michael Friedrich Zäh

Recent developments in laser material processing technology, like the rapid increase of maximum output power, the remarkable improvement of the beam quality, and the availability of deflection optics, have put a focus on laser safety technology. Moreover, the development of new application fields, such as processing of CFRP, has led to a higher demand for laser technology, and an increased need for suitable safety technology. The protective exposure limit (PEL) according to IEC 60825-4 is currently calculated based on the assumption that protection times are normally distributed. This approach is criticised for generating too low protection times and for not reflecting the reality. Thus, the question arises whether there is a better way to describe the distribution of protection times for a more accurate calculation of the PEL. The aim of this work was to take the first step to answer this question by executing protection time tests, using steel plates and a fibre laser, and to determine the statistical distribution. In the following, random based protection time calculations were executed by the aid of the resulting distributions. This normative procedure was opposed to an alternative method, and the results were compared. It could be demonstrated, that both approaches lead to similar results for the examined case.

"Keywords: laser safety; protection of employees; health and safety; IEC 60835-4"


Development and integration of an adaptive focus position control system for a new high-performance laser remote welding head
Georg Cerwenka, PD Dr.-Ing. habil. Jörg Wollnack, Prof. Dr.-Ing. Claus Emmelmann

The Institute of Laser and System Technologies (iLAS) of the Hamburg University of Technology (TUHH) investigated an intelligent vision guided laser remote scanner (LRS) system not only for thin sheet or structure applications, but also for thick sheet welding tasks with brilliant high power lasers up to 30 𝑘𝑤 optical laser power. Two different wavelength acting; 532 𝑛𝑚 for measuring and analyzing processes, 1070 𝑛𝑚 for welding and other laser processes. Mechanical, thermal and optical effects mainly influence the manufacturing results after position, shape and machining path measuring processes. Especially the high laser power induced interaction between the laser beam and optical components will change the optical guidance properties in the course of time. Changing the refraction index 𝑛 is one of
the important effects for the thermo-optical reaction; focal shift takes place. The conclusion is now, that the machining position of an infrared high power laser spot does not fit anymore to the measured and analyzed position of the green pointing laser spot. The paper describes a first basic simulation model of the optics from this new LRS system with influencing components and important parameters. Furthermore the thermal influences have been simulated and the first results are presented. These results demonstrate the importance to develop within the next steps a combined practical and fast correction algorithm design supported by real sensor signals and implemented in the laser remote scanner controller unit.

Keywords: laser remote technology, scanner technology, laser remote scanner, laser remote welding, high power laser, focus shift,
thermal lense, system technology, process monitoring and control, simulation, joining, welding, brazing


Sub-100µs latency feedback control of laser machining using FPGA-powered inline coherent imaging
Ethan Jenkins, Cole Van Vlack, Paul J. L. Webster, James M. Fraser

Accurate real-time measurement of laser penetration depth for feedback control is desirable for a variety of laser material processing applications in both micro- and macro-processing fields. Inline coherent imaging (ICI), a recently-developed interferometric technique, provides this capability with micron-scale precision whereby images can be captured at rates up to 312 kHz[1]. However, ICI image analysis is computationally intensive, and its usefulness in high-speed feedback control is
limited by the >1ms latency that is introduced by PC architecture. To address this issue, we have implemented ICI analysis for real-time applications on a dedicated Field-Programmable Gate Array (FPGA). FPGAs consist of configurable digital logic that offers superior parallel processing capabilities with exceptionally deterministic performance. Using this FPGA technology, we have managed to reduce the latency of a complete depth measurement cycle (including both acquisition and analysis) to 54±5 μs, more than an order of magnitude improvement over traditional processing methods.
We will present the FPGA ICI system's performance as a sensor for an autofocus feedback loop, wherein the ICI depth signal is used to control the height of a mechanical motion stage to maintain machining beam focus at the surface of samples with variable morphology. The response time of this system (41 ms) is dominated by the motion of the stage and not by the latency of the feedback signal. Since industrial kW-class laser powers can be modulated at much faster rates, active control of laser welding depth fully exploits the low-latency of our FPGA imaging system. We will also present the results of this preliminary FPGA-based welding control.


Reinventing thermal laser power measurements
Michele Zahner, Etienne Schwyter, Susanne Dröscher, Thomas Helbling

A novel design of a thermal laser power detector is presented, which allows minimizing the sensor dimensions, namely thickness and passive device area. The detector is intended to monitor medium laser power between 5 and 50 W. As its key feature, the sensor exhibits a rise time of just 200 ms, which is 5 times faster than conventional disc sensors. Such a low rise time is achieved by an axial thermopile arrangement combined with a minimized thermal mass. In order to fully exploit the speed advantage of the new design, a specific optical absorber coating has been developed. The coating has a broadband absorption characteristic and damage threshold of 1.5 kW/cm2.

Keywords: Power monitoring; Thermopile; Absorption coating; Diagnostics and Control