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Fusion welding processes are widely used for joining metal structures, such as pipes, ships, and cars. In general, these joining processes offer a very good compromise between reliability, safety, cost and maintenance which are important issues in the current economical context. The negative heat effects of welding, i.e. distortions and residual stresses of the welded parts, are well known and many researches in this field have already been done in the last decades in order to minimize them. On the experimental side, many sophisticated procedures have become state of the art to deal with this problem. On the computational side, the improvement of the simulation algorithms and the computing power enables the simulations of many physical phenomena occurring during the welding process. The implementation of welding simulation techniques is nevertheless not an easy task and often associated with expert knowledge which hinders their global application in an industrial environment. This paper is focused on the industrial requirements of a welding simulation software with special respect to the needs of the automotive industry. The necessary information to run a welding simulation and the expectations of a weld specialist without deep knowledge in numerical methods are investigated. These expectations are tested on an automotive welded assembly with a commercially available welding simulation software designed especially for the needs of the automotive industry. A welding experiment is done and the measured temperature distributions and distortions serve as reference to validate the simulation results. The result quality of the simulations of temperature fields and distortions is in best agreement with experimental data. The workflow is well adapted for the considered industrial requirements and the time-tosolution as well as the computational costs are acceptable, whereas the efficient calibration of the heat input model is still a point which will be further investigated in current and future research works.
During welding, residual stresses build-up created by the steep thermal gradient that occurs in the weld zone from localized heating and cooling, and phase transformations appearing in low-alloyed structural steel is inevitable. Welding of rather simple test plates do not cover the actual structural effects, which have to be considered during real component welding. However, the resulting welding-induced residual stress state is highly influenced by the structural characteristics, i.e. restraint conditions, of the welded construction. Therefore, a unique large-scale testing facility providing a specific shrinkage restraint while welding and subsequent cooling was used for the present investigations. Hereby, a six bead multi-pass gas metal arc weld of 20 mm thick structural steel S355J2 + N was welded under shrinkage restraint. The residual stresses were experimentally and numerically investigated, and compared to an analysis of plates welded under force-free support and free shrinkage conditions.
The experimentally determined and calculated residual stresses using both 2D and 3D numerical models are in a good agreement. Furthermore, the influence of a shrinkage restraint on the residual stress distribution is both experimentally and numerically shown for the present test set-up.
Welding is one of the most widely used joining processes in structural applications, like in car body production in the automotive industry. It is well-known that distortions and residual stresses occur during and after the welding process. Many procedures exist to decrease these negative heat effects of welding, but are often coupled with highly cost intensive experiments. For several decades, simulation models have been developed to understand and predict the heat effects of welding and to reduce experimental effort. In the production planning of various Original Equipment Manufacturers (OEM), some simulation tools are already well established, e.g. for crash test, forming or casting simulations. For welding, the demand is high but the implementation of welding simulation software is still not established yet. Welding is a complex process and the development of a flexible simulation tool, which produces good simulation results without expert knowledge in simulation, is not an easy task. In this paper, a welded assembly from the automotive industry has been simulated and compared to experimental data. Temperature fields and transient distortion distributions have been measured with thermocouples and with an optical 3D deformations analysis tool, respectively. The simulation has been run with a commercially available welding simulation software. The simulated temperature fields match the numerical ones perfectly. The simulated distortions are also qualitatively in best agreement with the experimental ones. Quantitatively, a difference of approximately 20 % between the simulated and the measured distortions is visible; this is acceptable considering the simplifications and assumptions of the simulation model. The global time to solution to get these results without expert knowledge in welding simulation was between 4 and 6 weeks, which is a reasonable time frame for an industrial application of welding simulation.
Welding is a key technology in the area of industrial production due to its flexibility and efficiency. However, new materials and welding techniques necessitate permanent research activities in order to keep up with the demands. A detailed knowledge about the process itself and the heat effects of welding, e.g., temperatures, distortions, and stresses, is the basis for a target-oriented optimization instead of a trial-and-error approach. Numerical welding simulation is a powerful tool to meet these demands. Complementary to an experimental investigation, it enables the analysis of the specimen during the welding process, commonly known as computational welding mechanics (CWM). Whereas simulation is nowadays a common tool in different development processes, the modeling of welding still remains difficult because of the multiple physical effects taking place. One of the most important problems for the user is the lack of knowledge about the material properties as input data for the simulation. Furthermore, any scattering of the data causes uncertainties that can have major effects on the calculations. The objective of this paper is to give an overview about the experimental determination and analysis of the material properties needed as input data for a welding simulation. The measurement techniques and the occurring deviations of the results are discussed. Additionally, the collected data for three representative alloys (dual-phase steel, austenitic steel, precipitation-hardenable aluminum alloy) are analyzed. Finally, the temperature-dependent thermophysical and thermomechanical material properties for these three alloys are given in a ready-to-use format for a numerical welding simulation.
The results of numerical welding simulations strongly depend on its temperature field. In the present paper, the temperature field of a pulsed gas metal arc weld of structural steel S355J2+N (ASTM A572 Gr. 50) with a thickness of 5 mm is experimentally and numerically investigated. In the case of temperature field validation, volumetric Gauss and double-ellipsoid Goldak heat sources are applied. Additionally, different heat source configurations, including adaptations of thermal conductivity, are analyzed regarding their influence on the calculation of welding-induced distortion.
The investigations clarify the influence of heat source configurations on the calculated results, thus, contribute to an improved prediction of welding-induced distortion.
Kornfeinung im Schweißgut kann die mechanischen Eigenschaften der Schweißnaht und die Schweißeignung des Grundwerkstoffs deutlich verbessern. Eine Möglichkeit korngefeintes Schweißgut zu erreichen, ist das Versetzen des Schmelzbades mit kornfeinenden Mitteln. In dieser Studie wird gezeigt wie Titan- und Borzusätze Korngröße und -struktur von WIG-Schweißnähten der Al-Legierung 5083 (Al Mg4,5Mn0,7) beeinflussen. Dazu wurden in einem Gießprozess stäbchenförmige Einlagen hergestellt, die aus Grundwerkstoff und definierten Zusätzen der Kornfeinungslegierung Al Ti5B1 bestanden. Sie wurden als Ersatz für einen Schweißzusatzwerkstoff in einer Nut im Grundwerkstoff untergebracht und im WIG-Verfahren überschweißt. Durch die Steigerung des Titan- und Borgehalts im Schweißgut konnte dessen mittlere Korngröße deutlich verringert werden. Außerdem wurde eine Änderung der Kornstruktur beobachtet. Die Ergebnisse können als Grundlage genutzt werden, um die empfohlene chemische Zusammensetzung von Schweißzusätzen für Lichtbogenschweißen von Aluminium anzupassen.
Die Kornfeinung beim Schweißen ist mit einem Übergang von groben, stängeligen Körnern zu kleineren, globulitischen Körnern im Schweißgut verbunden. Die Feinung der Korngröße und Form führt sowohl zu einer wesentlichen Verbesserung der Schweißeignung als auch zu verbesserten mechanischen Eigenschaften. Heute gebräuchliche Schweißzusatzwerkstoffe enthalten zum Teil bereits geringe Mengen kornfeinender Zusätze, deren Wirksamkeit jedoch von einer Reihe weiterer Randbedingungen abhängt. Durch Zugabe definierter Mengen an Keimbildnern (AlTi5B1) in das Schmelzbad wurde die Abhängigkeit der Korn-größe und –form vom Titan/Bor-Gehalt untersucht. Weiterhin wurde der zur vollständigen Kornfeinung notwendige Mindestgehalt an Keimbildner für unterschiedliche Legierungen und unter unterschiedlichen Schweißbedingungen, sowohl beim WIG-Schweißen als auch beim Laserstrahlschweißen bestimmt. Die Ergebnisse zeigen, dass sich die Korngröße im Schweißgut bis auf ein Minimum von ca. 20µm verringern lässt und zugleich der Anteil der globulitischen Körner zunimmt. Der Mindestgehalt an Keimbildner für vollständige Korn-feinung hängt stark von Schweißgeschwindigkeit und Legierungszusammensetzung ab. Der Einfluss der Kornfeinung auf die Festigkeit, Duktilität sowie die Heißrissempfindlichkeit der Schweißverbindungen wird an ausgewählten Bespielen aufgezeigt.
Fast temperature field generation for welding simulation and reduction of experimental effort
(2011)
The quality of welding processes is governed by the occurring induced distortions yielding an increase in production costs due to necessary reworking. Especially for more complex specimens, it is difficult to evaluate the optimal configuration of welding sequences in order to minimize the distortion. Even experienced welding operators can solve this task only by trial and error which is time and cost consuming. In modern engineering the application of welding simulation is already known to be able to analyse the heat effects of welding virtually. However, the welding process is governed by complex physical interactions. Thus, recent weld thermal models are based on many simplifications. The state of the art is to apply numerical methods in order to solve the transient heat conduction equation. Therefore, it is not possible to use the real process parameters as input for the mathematical model. The model parameters which allow calculating a temperature field that is in best agreement with the experiments cannot be defined directly but inversely by multiple simulations runs. In case of numerical simulation software based on finite discretization schemes this approach is very time consuming and requires expert users. The weld thermal model contains an initial weakness which has to be adapted by finding an optimal set of model parameters. This process of calibration is often done against few experiments. The range of model validity is limited. An extension can be obtained by performing a calibration against multiple experiments. The focus of the paper is to show a combined modelling technique which provides an efficient solution of the inverse heat conduction problem mentioned above. On the one hand the inverse problem is solved by application of fast weld thermal models which are closed form solutions of the heat conduction equation. In addition, a global optimization algorithm allows an automated calibration of the weld thermal model. This technique is able to provide a temperature field automatically that fits the experimental one with high accuracy within minutes on ordinary office computers. This fast paradigm permits confirming the application of welding simulation in an industrial environment as automotive industry. On the other hand, the initial model weakness is compensated by calibrating the model against multiple experiments. The unknown relationship between model and process parameters is approximated by a neural network. The validity of the model is increased successively and enables to decrease experimental effort, For a test case, it is shown that this approach yields accurate temperature fields within very short amount of time for unknown process parameters as input data to the model contributing to the requirement to construct a substitute system of the real welding process.
Für die Abgrenzung der Anwendbarkeit verschiedener Methoden der numerischen Schweißsimulation sowie für die Vereinheitlichung der Voraussetzungen und der durchzuführenden Schritte bei der Simulation sind normative Regelwerke für den Anwender erforderlich. Da es derzeit noch keine normähnlichen Dokumente auf diesem Gebiet gibt, wurde vom DIN der Arbeitsausschuss NA 092-00-29-A "Schweißsimulation" gegründet, welcher sich mit der Erarbeitung der entsprechenden Dokumente befasst. Dieses Gremium wurde gemeinsam mit dem Gemeinschaftsausschuss FA 12 "Anwendungsnahe Schweißsimulation" der Forschungsvereinigung des DVS initiiert.
Das Ziel der Normungsarbeit im Bereich der numerischen Schweißsimulation besteht in der Erarbeitung einer DIN SPEC, die die Vorgehensweisen bei der Simulation verschiedener Schweißphänomene und -verfahren langfristig standardisieren soll. In einem durch den genannten Arbeitsausschuss bereits vorbereiteten Entwurf der DIN SPEC 32534-1 wurde die Struktur des Hauptdokuments festgelegt. Im Hauptdokument sind die Anwendungsbereiche und die wesentlichen Begriffe der Schweißsimulation spezifiziert. Des Weiteren wurde die allgemein gültige Simulationsstruktur erarbeitet, welche als Empfehlung für den Auftraggeber und den Auftragnehmer bei der Formulierung und Abwicklung eines Dienstleistungsauftrages sowie für den Neueinstieg in die Schweißsimulation dienen soll.
Den Schwerpunkt stellt die Vorstellung der neuen DIN SPEC mit der Erläuterung der wesentlichen Simulationsschritte dar. Im Weiteren wird auf die Klassifizierung der Unterdokumente in Abhängigkeit des Schweißverfahrens und des angestrebten Simulationsergebnisses eingegangen. Schließlich wird ein Ausblick auf die weiteren Themenfelder des Arbeitsausschusses NA 092-00-29-A "Schweißsimulation" sowie auf die internationalen Aktivitäten auf diesem Gebiet gegeben.
The present investigations cover different relevant influences on the numerical calculation of welding-induced distortion. Therefore, a single-layer pulsed gas metal arc (GMA) weld of structural Steel S355J2+N with a thickness Öf5 mm is experimentally and numerically investigated. The influences of mesh density, tack-welds, and continuous cooling transformation (CCT) diagrams on welding-induced distortion are studied. The quality and quantity of These effects are clarified based on the used experimental and numerical set up. The occurring differences between the investigated cases achieve significant values. Consequently, prediction of welding-induced distortion can be improved considering the present investigations.
For differentiating the applicability of various numerical welding simulation methods as well as unifying the prerequisites and the steps to be taken in simulation, normative codes are required for the user. Since any standard-like documents are currently still lacking in this field, the DIN German Institute for Standardization in partnership with the Research Association of DVS German Welding Society have set up a standards committee dealing with the preparation of respective documents.
This article focuses on the presentation of the new DIN Spec 32534-1 explaining the major simulation steps and specifying the application fields and the key terms of welding simulation. In addition, a generally valid simulation structure has been established which is intended to serve as a recommendation for customers and suppliers in formulating and handling a service order as well as for persons who start doing welding simulation for the first time. It additionally gives an overview of the other subject areas dealt with in the standards committee as well as of the international activities in this field.
This paper presents volume heat sources and the corresponding functional analytical Solutions for the transient temperature field. The considered energy distributions are normal, exponential and parabolic. The method follows the common approach in Computational Welding Mechanics (CWM) to account for the physics of the welding process and the resulting temperature field by phenomenological models for heat conduction. Therefore, the used heat source models are apparent heat sources that incorporate the real heat input as well as the fluid flow in the weld pool and the latent heat connected with phase transformations. The heat source models provide welding characteristics like thermal cycle and Fusion line in the cross section within short computational time. Consequently, inverse techniques on basis of optimisation algorithms enable the adaptation of the models to the experimental data efficiently. Furthermore, the direct evaluation of the energy distribution for the experimental fusion line in the cross section is demonstrated which enhances the numerical optimisation by reducing the number of unknown model Parameters and providing a reasonable initial guess within the model parameter space. The proposed temperature field models are validated with real laser beam welding experiments.
The paper presents bounded volume heat sources and the corresponding functional-analytical expressions for the temperature field. The power density distributions considered here are normal, exponential and parabolic. The sources model real heat sources like the welding arc, laser beam, electron beam, etc., the convection in the weld pool as well as the latent heat due to fusion and solidification. The parameters of the heat source models are unknown a priori and have to be evaluated by solving an inverse heat conduction problem. The functional-analytical technique for calculating 3D temperature fields in butt welding is developed. The proposed technique makes it possible to reduce considerably the total time for data input and solution. It is demonstrated with an example of laser beam welding of steel plates.
Fusion welding is widely used in the automotive industry to join metal structures. It is well-known that distortions and residual stresses occur during and after the welding process. Many procedures exist to decrease these negative heat effects of welding, but are often coupled with highly cost intensive experiments. The implementation of a welding Simulation tool to reduce this very expensive experimental procedures is therefore of high interest. Despite the fact that the automotive industry is a key sector for Simulation procedures, welding Simulation Software is nevertheless not yet widely implemented. This is mainly due to the complexity of the Simulation tools requiring expert users and the resulting high time to Solution.
In this study, a new fast thermo-mechanical Simulation of a complex and large laser beam welded automotive sheet metal assembly with several non linear welds is simulated. Assumptions and simplifications of the complex physical welding phenomenon, which are made to keep the computational cost and the complexity of the Simulation in an industrial frame, are discussed. The calibration time of the phenomenological heat source model has been optimized with a very fast analytical thermal model and the resulting simulated temperature fields match perfectly with the measured ones. Additionally, the user experience and the time-to-solution are kept within a reasonable time frame for arr industrial environment. All Simulation results are validated with experimental results.
This article studies the three dimensional transient weld pool dynamics and the influence of joint preparation angle on welding of low carbon structural steel plates using the ForceArc® process. ForceArc is a new gas metal arc welding technology which allows adequate fusion and penetration with a smaller V groove angle. This enhances welding efficiency significantly because of reduction of layers and low material consumption.
The deformation of the weld bead is calculated with an accurate coupling of the heat transfer with fluid flow through continuity, momentum and the energy equations combined with the effect of droplet impingement, gravity, electromagnetic force, buoyancy, drag forces and surface tension force (Marangoni effect). Four different angles of V groove are employed with the same welding parameters and their influence on the weld pool behavior and weld bead geometry is calculated and analyzed, to allow subsequent calculations of residual stress and distortion of the workpiece.
Such a simulation is an effective way to study welding processes because the influence of all the welding parameters can be analyzed separately with respect to thermal cycle, weld bead formation, and the microstructure of the weld. Good agreement is shown between the predicted and experimentally determined weld bead dimensions. It was found that with a larger groove angle, the penetration depth increases. Furthermore, a higher wire feeding rate is needed to fill the larger groove. The model presented can be used for further analyses of GMAW processes as well as input data for the numerical calculation of welding induced residual stresses and distortions using Computational Weld Mechanics CWM.
Welding simulation is a powerful, cost-efficient tool to predict welding induced distortion. Nevertheless, effects on calculation result quality are often unknown, thus, sensitivity analyses should be performed to evaluate the influences of certain parameters on distortion development.
In the present paper, a single-layer gas metal arc (GMA) weld of 5 mm thick structural steel S355J2+N is experimentally and numerically investigated. Subsequent to welding, the numerical modeling begins with a mesh analysis based on modal analyses. Hereby, the influence of different coarsening methods and element edge length (EEL) in welding direction on the deformation behavior or the stiffness of the discrete geometry is the focus of the analysis. Secondly, phase transformations in structural steels such as S355J2+N are decisive for final product properties. The sensitivity of welding-induced distortion is examined regarding different continuous cooling transformation (CCT) diagrams for S355J2+N.
The present investigations deal with different relevant influences on numerical calculation of welding-induced distortion. The quality and quantity of these effects are clarified based on the experimental and numerical set-up employed. Consequently, prediction of welding-induced distortion is possible and potential for pre-production optimization is present.
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.
Mit Ausgabedatum März 2011 ist die Spezifikation DIN SPEC 32534-1 erschienen. Die Spezifikation wurde im DIN/DVS-Gemeinschaftsausschuss NA 092-00-29 AA „Schweißsimulation (DVS AG I 2.1)" des Normenausschusses Schweißtechnik (NAS) im DIN in Zusammenarbeit mit dem Ausschuss für Technik (AfT) im DVS - Deutscher Verband für Schweißen und verwandte Verfahren e. V. nach dem Vornormverfahren erarbeitet.