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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.
New brilliant high-power lasers such as disc laser and fibre laser open up new fields of applications for laser-hybrid welding, e.g. power generation, shipbuilding and pipeline construction. For the use of laser-hybrid welding in pipe laying, insensibility towards tolerances and the possibility of orbital welding are very important factors in addition to the weld seam quality.
Within the scope of a basic research project, first promising results for laser-hybrid welding of thick materials could be demonstrated. As base material, the typical pipeline steel API 5L X65 was selected. With the help of a 20 kW fibre laser in combination with an arc-welding process it was possible to produce high-quality welds in plates of up to 20 mm thickness in a single pass and of up to 32 mm in three to five passes, both welded in position PA. Various joint preparations were examined to weld 20 mm thick plates in one pass. Besides different welding positions, gap bridgeability and misalignment were studied. Results for microhardness and Charpy toughness are presented to proof the applicability of laser-hybrid welding for joining in pipe laying.
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.