Michael Rethmeier

Spectral diagnostics of a vapor-plasma plume produced during welding with a high-power ytterbium fiber laser

We have conducted spectroscopic studies of the welding plasma formed in the process of welding with an ytterbium fiber laser delivering output power of up to 20 kW. The influence of shielding gases (Ar, He) on different parts of the welding plume is investigated. The absorption coefficient of the laser radiation by the welding-plume plasma is estimated. Scattering of 532-nm probe radiation from particles of the condensed metal vapor within the caustic of a high-power fiber laser beam is measured. Based on the obtained results, conclusions are made on the influence of the plasma formation and metal vapor condensation on the radiation of the high-power fiber laser and the stability of the welding process.

Environmental energy efficiency of single wire and tandem gas metal arc welding

This paper investigates gas metal arc welding (GMAW) with respect to energy consumption and its associated environmental impacts. Different material transfer modes and power levels for single wire GMAW (SGMAW) and tandem GMAW (TGMAW) are evaluated by means of the indicator electrical deposition efficiency. Furthermore, the wall-plug efficiency of the equipment is measured in order to describe the total energy consumption from the electricity grid. The results show that the energy efficiency is highly affected by the respective process and can be significantly enhanced by a TGMAW process. The wall-plug efficiency of the equipment shows no significant dependency on the power range or the material transfer mode. Moreover, the method of life cycle assessment (LCA) is adopted in order to investigate the influences of energy efficient welding on the environmental impacts. In the comparative LCA study, the demand of electrical energy is reduced up to 24%. In consequence, the indicator values for global warming potential (100), acidification potential, eutrophication potential, and photochemical ozone creation potential are reduced up to 11%.

Experimental investigation of the laser-plume interaction during high power fiber laser welding

The effect of the well-known plasma absorption and refraction in CO2-laser metal welding plumes is incase of high power solid state laser welding negligibly small. By contrast, the diffraction effects of shorter wavelength laser radiation are considerable. According to the results of preliminary studies, the fine condensed metal particles in the welding plume can lead to essential worsening of the laser beam quality.This work is devoted to the investigation of the laser-matter interaction during up to 20 kW ytterbium fiber laser welding of thick mild steel plates. The plume attenuation of a probe 1.3u2005µm wavelength diode laser beam as well as of continuous radiation in 250-600u2005nm wavelength range was measured during welding with and without Ar shielding gas supply. The measured results allow it to calculate average size and concentration of fine condensed metal particles in different plume areas using the multi-wavelength method and the Mie scattering theory. The plume temperature, whichdetermines the condensation conditions, was measured by means of Fe I atom spectral line emission registration.The obtained results can be also of interest for remote metal treatment with high-power fiber or disc lasers.The effect of the well-known plasma absorption and refraction in CO2-laser metal welding plumes is incase of high power solid state laser welding negligibly small. By contrast, the diffraction effects of shorter wavelength laser radiation are considerable. According to the results of preliminary studies, the fine condensed metal particles in the welding plume can lead to essential worsening of the laser beam quality.This work is devoted to the investigation of the laser-matter interaction during up to 20 kW ytterbium fiber laser welding of thick mild steel plates. The plume attenuation of a probe 1.3u2005µm wavelength diode laser beam as well as of continuous radiation in 250-600u2005nm wavelength range was measured during welding with and without Ar shielding gas supply. The measured results allow it to calculate average size and concentration of fine condensed metal particles in different plume areas using the multi-wavelength method and the Mie scattering theory. The plume temperature, whichdetermines the cond...

Numerical simulation of electromagnetic melt control systems in high power laser beam welding

The availability of laser sources with a power of 20 kW upwards prepared the ground for laser beam welding of up to 20 mm thick metal parts. Challenges are the prevention of gravity-driven melt drop-out and the control of the dynamics mainly due to the Marangoni flow.Coupled numerical turbulent fluid flow, thermal and electromagnetic simulations and experimental validation with aluminum AlMg3 and stainless steel AISI 304 were done for alternating and steady magnetic fields perpendicular to the process direction. The first can prevent melt sagging in full-penetration welding by Lorentz forces in the melt induced by an AC magnet located below the weld specimen counteracting gravitational forces. The latter controls the Marangoni flow by Lorentz braking forces in the melt by the so-called Hartmann effect.The simulations show that the drop-out of aluminum and stainless steel can be avoided for 20 mm thick full-penetration welds with moderate magnetic flux densities of 70 mT and 95 mT at oscillation frequencies of 450 Hz and 3 kHz, respectively. The experiments are in good agreement but show somewhat larger values for steel, whose weakly ferromagnetic properties are a possible reason. The investigations with steady magnetic fields reveal the possibility to mitigate the dynamics significantly beginning with around 500 mT at laser penetration depths of approximately 20 mm.The availability of laser sources with a power of 20 kW upwards prepared the ground for laser beam welding of up to 20 mm thick metal parts. Challenges are the prevention of gravity-driven melt drop-out and the control of the dynamics mainly due to the Marangoni flow.Coupled numerical turbulent fluid flow, thermal and electromagnetic simulations and experimental validation with aluminum AlMg3 and stainless steel AISI 304 were done for alternating and steady magnetic fields perpendicular to the process direction. The first can prevent melt sagging in full-penetration welding by Lorentz forces in the melt induced by an AC magnet located below the weld specimen counteracting gravitational forces. The latter controls the Marangoni flow by Lorentz braking forces in the melt by the so-called Hartmann effect.The simulations show that the drop-out of aluminum and stainless steel can be avoided for 20 mm thick full-penetration welds with moderate magnetic flux densities of 70 mT and 95 mT at oscillation frequencie...

Design of neural network arc sensor for gap width detection in automated narrow gap GMAW

An approach to develop an arc sensor for gap width estimation during automated NG-GMAW with a weaving electrode motion is introduced by combining arc sensor readings with optical measurements of the groove shape to allow precise analyses of the process. The two test specimen welded for this study were designed to feature a variable groove geometry in order to maximize efficiency of the conducted experimental efforts, resulting in 1696 individual weaving cycle records with associated arc sensor measurements, process parameters and groove shape information. Gap width was varied from 18 mm to 25 mm and wire feed rates in the range of 9 m/min to 13 m/min were used in the course of this study. Artificial neural networks were applied as a modelling tool to derive an arc sensor for estimation of gap width suitable for online process control that can adapt to changes in process parameters as well as changes in the weaving motion of the electrode. Wire feed rate, weaving current, sidewall dwell currents and angles were defined as inputs to calculate the gap width. The evaluation of the proposed arc sensor model shows very good estimation capabilities for parameters sufficiently covered during the experiments.

Hot cracking in high power laser beam welding of thick high strength structural steels under restraint conditions

High power laser beam welding represents a cost-effective alternative for fast joining of thick components. However, the reliability of the welded structures can be severely affected by solidification crack formation, caused by constructional restraints. Experimental investigations using 15u2005mm thick plates of the fine grained structural steel S690, welded with a 20 kW fiber laser under different restraint conditions revealed a relationship between hot cracking susceptibility and restraint intensity. Systematic laser welding experiments were conducted in the IRC (Instrumented Restraint Cracking)-test facility, under both free shrinkage as well as defined restraint intensities. In order to assess the stress-strain condition of the weld during solidification which is crucial in the hot crack formation process but also difficult, if not impossible to acquire experimentally, a two-dimensional thermo-mechanical finite element model of the IRC-Test was developed. The results constitute a good approach to explain and understand the detrimental effects of high restraints on hot cracking phenomena during laser beam welding.High power laser beam welding represents a cost-effective alternative for fast joining of thick components. However, the reliability of the welded structures can be severely affected by solidification crack formation, caused by constructional restraints. Experimental investigations using 15u2005mm thick plates of the fine grained structural steel S690, welded with a 20 kW fiber laser under different restraint conditions revealed a relationship between hot cracking susceptibility and restraint intensity. Systematic laser welding experiments were conducted in the IRC (Instrumented Restraint Cracking)-test facility, under both free shrinkage as well as defined restraint intensities. In order to assess the stress-strain condition of the weld during solidification which is crucial in the hot crack formation process but also difficult, if not impossible to acquire experimentally, a two-dimensional thermo-mechanical finite element model of the IRC-Test was developed. The results constitute a good approach to explain...

Strategien zur Erreichung eines konstanten Volumenaufbaus bei der additiven Fertigung mittels Laser-Pulver-Auftragschweißen / Strategies to achieve constant build-up with laser metal deposition

Der Einsatz von Hochleistungswerkstoffen verlangt nach einer hohen Endformnahe der zu fertigenden Bauteile, um den Aufwand und somit die Kosten fur Materialeinsatz und Nachbearbeitung moglichst gering zu halten. Der additive Einsatz in Form des Laser-Pulver-Auftragschweisens bietet hierfur durch den gezielten Materialauftrag ein hohes Potential. Herausforderungen bestehen in Bereichen der Vorhersagbarkeit und der Reproduzierbarkeit des Materialauftrages, sowie der Fertigungszeit. Unterschiedliche Einflusse bei der Schichterzeugung fuhren dabei zu Abweichungen von der Soll-Geometrie. Die vorliegenden Untersuchungen behandeln den Einfluss von Spurgeometrie, Spuruberlappung, Verfahrweg und Aufbaureihenfolge auf die entstehende Bauteilform. Die Teilung einer Lage in Rand- und Kernbereich ermoglicht einen konturangepassten Verfahrweg und eine Erhohung der Endformnahe innerhalb einer Ebene. Die Verwendung unterschiedlicher Spurgrosen bei der Bauteilerzeugung verdeutlicht die Moglichkeiten einer hohen Auftragsrate bei gleichzeitig hoher Formgenauigkeit. Bereits kleine Unterschiede beim Materialauftrag zwischen Kern- und Randbereichen, Start- und Endpunkten sowie in Bereichen des Richtungswechsels fuhren aufgrund von Fehlerfortpflanzung nachmehreren Lagen zu Abweichungen in der Aufbaurichtung. Kompensierungen mittels angepasster Baustrategien werden aufgezeigt und diskutiert. Die Nickelbasislegierung Inconel 718, die Titanlegierung Ti-6Al-4V sowie der austenitische Stahl 316L sind Bestandteil der vorliegenden Untersuchungen. Die gewonnenen Erkenntnisse verdeutlichen das Potenzial einer angepassten Aufbaustrategie zur reproduzierbaren Erzeugung von Bauteilen am Beispiel unterschiedlicher Korpergeometrien.

Weld seam formation and mechanical properties of girth welds performed with laser-GMA-hybrid process on pipes of grade X65

With the recent introduction of multi-kilowatt fiber lasers combining high beam quality with an impressive energy efficiency, it was possible to broaden the spectrum of laser beam and laser-hybrid welding applications widely. Application of these lasers for welding thick-walled structures in different branches of industry (e.g. power generation, shipbuilding, wind power, pipeline construction) is interesting, because the fiber lasers offer sufficient penetration depth to allow economically efficient welding of thick sheet metal, i.e. with a reduced number of welding passes and with a lower amount of filler material. A girth laser-hybrid process using a 20 kW fiber laser and a gas metal arc welding (GMAW) process was examined at BAM, Federal Institute for Materials Research and Testing, Berlin. The aim of this research was to obtain a stable and crack free girth process and to demonstrate the possibility of its application in pipeline construction. The experiments were carried out on 16u2005mm thick pipe rings with 914u2005mm pipe diameter of X65. As it could be shown in previous publications the rings can be welded using a girth hybrid process that is divided into two steps each in 5G position downhill. This paper will focus on imperfections of the weld seams in the different welding positions and on the methods for their avoiding. Influences of forming gas and scanner optic parameters on the appearance of the weld root were analyzed. A serie of welding experiments with preheating was performed. Mechanical properties and weld metal microstructure for the pipe segment welds were examined for different preheat temperatures.With the recent introduction of multi-kilowatt fiber lasers combining high beam quality with an impressive energy efficiency, it was possible to broaden the spectrum of laser beam and laser-hybrid welding applications widely. Application of these lasers for welding thick-walled structures in different branches of industry (e.g. power generation, shipbuilding, wind power, pipeline construction) is interesting, because the fiber lasers offer sufficient penetration depth to allow economically efficient welding of thick sheet metal, i.e. with a reduced number of welding passes and with a lower amount of filler material. A girth laser-hybrid process using a 20 kW fiber laser and a gas metal arc welding (GMAW) process was examined at BAM, Federal Institute for Materials Research and Testing, Berlin. The aim of this research was to obtain a stable and crack free girth process and to demonstrate the possibility of its application in pipeline construction. The experiments were carried out on 16u2005mm thick pipe rings...