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- Laser beam welding (60)
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Organisationseinheit der BAM
- 9 Komponentensicherheit (143)
- 9.3 Schweißtechnische Fertigungsverfahren (140)
- 5 Werkstofftechnik (2)
- 5.2 Metallische Hochtemperaturwerkstoffe (2)
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- 7 Bauwerkssicherheit (1)
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Full penetration 15 kW Yb fibre laser butt welding of thick AlMg3 (AW 5754) plates was performed in PA position. A contactless inductive electromagnetic weld pool support system was used to prevent gravity dropout of the melt. The welding speed needed to achieve 20 mm penetration was ~0·5 m min-1. An ac power supply of ~244 W at 460 Hz was necessary to completely suppress gravity dropout of the melt and eliminate sagging of the weld pool root side surface. The oscillating magnetic field can suppress the Marangoni convection in the lower part of the weld pool. The system was also successfully used in the full penetration welding of 30 mm thick AlMg3 plates.
Spot welding is one of the most important technologies for joining sheet metal. While there are lot of approaches to non-destructive testing, quality assurance still mainly relies on welding parameter monitoring and destructive testing, leading to significant failure rates. In this paper an approach to spot weld testing using flash thermography is presented. The main focus of attention is on the identification of two typical error classes: stick welds and welds at the splash limit. Besides investigating the principal feasibility of thermography for zinc plated samples the results of a series test of spot welds joining 1 mm thick TRIP steel are shown. Based upon these results a statistical criterion is developed which allows a reliable classification of the named error classes.
The objective of this paper is to demonstrate a new simulation technique which allows fast and automatic generation of temperature fields as input for subsequent thermomechanical welding simulation. The basic idea is to decompose the process model into an empirical part based on neural networks and a phenomenological part that describes the physical phenomena. The strength of this composite modelling approach is the automatic calibration of mathematical models against experimental data without the need for manual interference by an experienced user. As an example for typical applications in laser beam and GMA-laser hybrid welding, it is shown that even 3D heat conduction models of a low complexity can approximate measured temperature fields with a sufficient accuracy. In general, any derivation of model fitting parameters from the real process adds uncertainties to the simulation independent of the complexity of the underlying phenomenological model. The modelling technique presented hybridises empirical and phenomenological models. It reduces the model uncertainties by exploiting additional information which keeps normally hidden in the data measured when the model calibration is performed against few experimental data sets. In contrast, here the optimal model parameter set corresponding to a given process parameter is computed by means of an empirical submodel based on relatively large set of experimental data. The approach allows making a contribution to an efficient compensation of modelling inaccuracies and lack of knowledge about thermophysical material properties or boundary conditions. Two illustrating examples are provided.
Bei hochfesten austenitischen Stählen können unter den Produktionsbedingungen in der Rohkarosseriefertigung schweißbedingte Risse beim Widerstandspunktschweißen entstehen. Der Einfluss solcher Risse auf die dynamische Festigkeit ist derzeit nicht hinreichend bekannt. Im Rahmen dieser Arbeit wurden daher Schweißrisse verschiedener Lage und Ausbildung hinsichtlich ihrer Wirkung auf die Festigkeit und das Bruchverhalten von Widerstandspunktschweißverbindungen im Hochgeschwindigkeitsscherzug untersucht. Dabei konnte durch die Analyse der Kraft-Weg-Verläufe, der Scherzugfestigkeit und des Energieaufnahmevermögens nachgewiesen werden, dass Oberflächenrisse im Übergangsbereich und im Randbereich des Elektrodeneindrucks bei den hier vorliegenden Bedingungen keinen negativen Einfluss auf die dynamische Festigkeit haben. ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
In the case of high-strength austenitic steels, welding-induced cracks may arise during resistance spot welding in the rough conditions of body shell fabrication. The influence of such cracks on the dynamic strength is not sufficiently well-known at present. Within the framework of this paper, welding cracks in various positions and with various formations were therefore investigated with regard to their effect on the strength and fracture behaviour of resistance-spot-welded joints subjected to high-speed shear tension. In this respect, the analysis of the force/displacement courses, the tensile shear strength and the energy absorption capacity served to prove that surface cracks in the interfacial region and in the peripheral region of the electrode indentation do not have any negative influence on the dynamic strength for the tested circumstances.
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.
Design of experiments for laser metal deposition in maintenance, repair and overhaul applications
(2013)
Modern and expensive parts lead to an increasing demand for maintenance, repair and overhaul (MRO) technologies. Instead of part replacement, MRO technologies are economically advantageous throughout the life cycle. Laser metal deposition as modern MRO technology can be used to repair cracks or protect damaged surfaces with a hard facing layer. It is necessary to adjust weld bead profile to the specific task. For this purpose, Design of Experiment (DoE) has a high potential to decrease experimental effort. In this paper, a full factorial design is used to determine the effect of process parameters on the geometric dimensions of the weld bead. The paper is of interest to engineers working with laser metal deposition as well as DoE methods.
The aim of the present work was to investigate the possibilities of hybrid laser arc welding regarding reliable production of longitudinal welds of high strength pipe steels X80 and X120 and to evaluate achievable mechanical properties of laser hybrid welds. The study focused on weld toughness examination in low temperature range up to –60°C. Suitable filler materials were identified in the context of this task. It could be shown that metal cored electrodes guaranteed sufficient Charpy impact toughness at low temperature for both investigated materials. Modern arc welding technologies such as modified pulsed spray arc were used to promote deeper penetration of the filler material into the narrow laser welding gap. Edge preparation with a 14 mm deep root face was considered as optimum, because no penetration of the filler material could be detected beyond this depth limit, and therefore, any metallurgical influences on the weld metal properties through the welding wire could be excluded.
Laserstrahl-MSG-Hybridschweißen dicker Bleche aus Rohrleitungsstahl mit Faserlasern hoher Leistung
(2010)
In diesem Beitrag werden die Ergebnisse aus einem gemeinschaftlich durchgeführten Forschungsprojekt zur Schweißeignung unlegierter Stähle mit Laserstrahl- und Lichtbogenschweißverfahren vorgestellt. Es wurden Schweißversuche in Ein- sowie Mehrlagentechnik mit Blechdicken von 16, 20 und 28 mm durchgeführt sowie die Prozessgrenzen für die Spaltüberbrückbarkeit, den Kantenversatz und das Schweißen in Zwangspositionen untersucht. Für die Schweißversuche kamen Faserlaser mit Leistungen von 8, 20 und 30 kW in Verbindung mit einem MAG-Schweißprozess als Laserstrahl-MSG-Hybridschweißen zum Einsatz. Mit dem 30-kW-Faserlaser war es möglich, 28 mm dicke Bleche einlagig zu schweißen, jedoch besteht weiterer Forschungsbedarf, um Aussagen über die erreichbare Qualität treffen zu können. Bei 20 kW Laserstrahlleistung wurde zum einlagigen Schweißen von Blechen mit einer Dicke von 20 mm eine entsprechende Nahtvorbereitung benötigt, 16 mm dicke Bleche konnten dagegen ohne Vorbereitung der Naht im Stumpfstoß geschweißt werden. In der Mehrlagentechnik wurden mit dem 8-kW-Faserlaser 16 mm dicke Bleche vorbereitet mit einer Y-Naht geschweißt. Hierbei wurde die Wurzellage hybrid geschweißt, die Füll- und Decklagen mit einem reinen MAG-Prozess.
Distortion optimisation of beam-welded industrial parts by means of numerical welding simulation
(2010)
Analytical and numerical methods are used to estimate the temperature field due to the heat effects of welding. Numerical techniques are more adapted for industrial complex applications where analytical solutions do not exist yet. However, computational time is much lower with analytical models and a combination of both methods is investigated. Therefore, the two approaches are introduced and confronted in this paper. The finite-element software Ansys has been used for numerical simulations and Scilab for analytical simulations. In order to get a similar result quality, both methods have to be analysed and compared with respect to boundary conditions. These configurations are presented in this paper. Before starting any analysis, the analytical and numerical models have to be comparable. For the numerical simulation, every in- or output is given in discrete form and, for the analytical simulation, in continuous form. Thus, an analysis of the energy input distribution in both models is compulsory to ensure that the same amount of energy is applied. After this first study, a comparison of the analytical and numerical temperature field simulation is done from a fix point source in an infinite volume in steady state to a moving point source in a finite dimension in a transient state. A good agreement between the analytical and the numerical simulation results is found. However, some techniques, like a consideration of an image heat source for the analytical model or the selection of boundary conditions for the numerical model, need to be taken into consideration when the degree of complexity of the study (finite dimension or cooling time) increases. The limit of the comparison is reached when the geometry becomes too complex and when the effect of variable thermal properties with temperature cannot be neglected.
Zugeigenspannungen, wie sie beim Schweißprozess durch inhomogene Temperaturverteilungen und Schrumpfungen hervorgerufen werden, können die Lebensdauer geschweißter Verbindungen signifikant herabsetzen. Eine neue und außerordentlich attraktive Methode, um Druckeigenspannungen bereits während des Schweißens gezielt einzustellen, gelingt mit sogenannten LTT (Low Transformation Temperature)-Legierungen. LTT-Legierungen weisen eine martensitische Phasenumwandlung bei relativ niedrigen Temperaturen auf, wobei die damit verbundene Volumenexpansion zu einer Reduktion der Schrumpfeigenspannungen bzw. Erzeugung von Druckeigenspannungen führt. Zum direkten Nachweis der Phasenumwandlungen und der damit verbundenen resultierenden Schweißeigenspannungen wurden erstmals In-situ-Schweißexperimente unter Nutzung hoch energetischer, polychromatischer Synchrotronstrahlung (Weißstrahl) realisiert, um die Umwandlungskinetik während eines realen Schweißprozesses und die daraus resultierenden Schweißeigenspannungen zu analysieren. Es wird gezeigt, dass mit LTT-Legierungen signifikante Druckeigenspannungen in der Schweißnaht erreicht werden.
The results of a collaborative research project on laser beam weldability of carbon steels of high sheet thickness are presented. That includes single and multiple pass welding of 16 mm and 20 mm thick plates, as well as the investigation of acceptable tolerances i.e. gap bridgeability and edge misalignment. For the welding experiments fibre lasers with 8 kW, 20 kW laser power and different GMAW-techniques were used in various applications. With the 20 kW fibre laser 16 mm plates could be welded with a single pass, 20 mm required a seam preparation or alternatively preheating of the material. For multi pass welding with 8 kW laser power a joint preparation with a single V-butt joint with a broad root face (Y-groove) was applied. The root pass was always welded with a hybrid process, the filler passes with a hybrid process as well as a GMAW process which produced the best results.
For the designing of safety-relevant components particularly by means of finite element calculations, great significance is attached to the determination of local material parameters in the welding region. Conventional test methods can merely provide global parameters which, however, are frequently not sufficient precisely for the calculation of resistance-spot-welded components since these are characterised by the existence of joining zones with tight local limitations. In contrast, modern test methods such as optical strain field measurement permit the determination of the characteristic material data for concrete locations and times. In order to establish the potential of optical strain field measurements for the characterisation of the local material behaviour of spotwelded joints, tensile shear tests as well as tensile tests were carried out on resistance-spot-welded high-strength steels. The results show the high potential of this procedure not only for the quantification of the local strain behaviour in the joining region but also for the determination of local characteristic material data.
The usage of continuous cooling transformation (CCT) diagrams in numerical welding simulations is state of the art. Nevertheless, specifications provide limits in chemical composition of materials which result in different CCT behavior and CCT diagrams, respectively. Therefore, it is necessary to analyze the influence of variations in CCT diagrams on the developing residual stresses. In the present paper, four CCT diagrams and their effect on numerical calculation of residual stresses are investigated for the widely used structural steel S355J2 + N welded by the gas metal arc welding (GMAW) process. Rather than performing an arbitrary adjustment of CCT behavior, four justifiable data sets were used as input to the numerical calculation: data available in the Sysweld database, experimental data acquired through Gleeble dilatometry tests, and TTT/CCT predictions calculated from the JMatPro and Edison Welding Institute (EWI) Virtual Joining Portal software. The performed numerical analyses resulted in noticeable deviations in residual stresses considering the different CCT diagrams. Furthermore, possibilities to improve the prediction of distortions and residual stress based on CCT behavior are discussed.
High welding residual stresses can cause service life reducing consequences. Even though many processes have been developed to reduce these stresses, they are only applicable for wider welds and simple component geometries or are cost-intensive, respectively. The presented method uses a defocused beam after welding for heating the material regions on both sides of the weld. In this way, the welding stresses are decreased without contacting the surfaces using the available equipment. Different process parameters could be used depending on the component geometry and the laser power. The mechanism and the influence of the process parameters were investigated by FEM-simulation and experiments on S355J2+N steel and showed a stress reduction of about 73%.
The main characteristic feature of deep penetration laser beam welding is a large temperature difference between the plasma cavity (keyhole) in the centre of the weld pool and the melting/solidification front. Large temperature gradients in the weld pool result in a very intensive thermocapillary (Marangoni) convection. The weld pool surface width becomes very large and unstable. However, an externally applied oscillating magnetic field can stabilize the surface of the melt (the Garnier-Moreau effect, 1983). In the present work this technology was used to stabilize the surface of the weld pool in partial penetration 4.4 kW Nd:YAG laser beam welding of AW-5754 alloyin PA position. The AC magnet was mounted on the laser welding head. The oscillating magnet field was oriented perpendicular to the welding direction. It was found that the AC magnet field can drastically reduce the surface roughness of welds. The analysis of the x-ray images shows a rastic reduction of porosity content in the welds. This effect can be explained as a result of electromagnetic ectification of the melt.
Thick-walled pipes made of steel are the basis of a large number of components in mechanical and installation engineering, e.g. for hydraulic components, in power station technology or in the petroleum industry. Particularly when strict positional and shape tolerances are demanded, the classical manufacturing process is chip-producing machining from the solid material. However, this is time-intensive and material-intensive. The laser/GMA hybrid processes with modern high-power lasers are a suitable alternative. With these processes, it is currently possible to weld components with a wall thickness up to approx. 15 mm with little distortion in one pass. However, in initial experiments, solidification cracks with longitudinal and vertical orientations in relation to the welding direction, so-called central line cracks, were established in welds on pipes. Within the framework of a research project, it was possible to determine the cause of the cracking and to find measures in order to avoid it. The technical boundary conditions could be complied with in this respect. The article gives an overview of the research project and the essential results.
Simulation of an inductive weld pool support for deep penetration laser beam welding of metal parts
(2012)
three-dimensional laminar steady state numerical model was used to investigate the influence of an altemating current (ac) magnetic field during single pass high power laser beam keyhole welding of 20 mm thick aluminum. The three-dimensional heat transfer, fluid dynamics and electromagnetic field equations were solved with the commercial finite element package COMSOL Multiphysics. Dominant physical effects of the process were taken into account: Thermo-capillary (Marangoni) convection at the upper and lower weld pool surfaces, natural convection due to the gravity influence and the latent heat of solid-liquid phase transition. Simulations were conducted for several magnetic field strengths and it was found that the gravity drop-out associated with welding of thick plates due to the hydrostatie pressure can be prevented by the application of an ac magnetic field below the weld specimen of around 70 mT (rms) at an oscillation frequency of 450 Hz. The inductive support System allows for single-pass laser beam welding of thick aluminum plates. The flow pattem in the molten zone and the temperature distributions are significantly changed by the application of the electromagnetic forces in the weld pool.