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Schweißzugeigenspannungen in Strahlschweißnähten auf Höhe der lokalen Werkstoffstreckgrenze können die Bauteileigenschaften sowie deren Lebensdauer beeinflussen. Die Nutzung der Schweißquelle für eine nachträgliche Behandlung der geschweißten Bauteile bietet eine kostengünstige und flexible Methode zur Reduzierung dieser Spannungen. Dabei wird durch einen oszillierenden defokussierten Elektronen- bzw. Laserstrahl der Werkstoff beidseitig der Schweißnaht auf mehrere hundert Grad Celsius erwärmt, um dort zusätzliche Zugspannungsbereiche hervorzurufen. Die so erzeugten Längszugspannungszonen führen zu einer mechanischen Entlastung der Schweißnaht. Experimentelle Messungen zeigen, dass die Längseigenspannungen in der Schweißnaht mit diesem Verfahren um bis zu 70% reduziert werden können. Hierbei wurde neben der experimentellen Untersuchung dieses Verfahrens an Linearnähten an 5 mm dicken Blechen des Werkstoffs S355J2+N auch eine umfassende Analyse der Einflüsse der Verfahrensparameter auf die Spannungsreduktion mittels FEMSimulation durchgeführt. Anhand der Ergebnisse wird der Entlastungsmechanismus diskutiert. ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Residual tensile welding stresses in beam welds at the level of the local yield strength of the material may influence the properties of components as well as their service lives. The utilisation of the welding source for the subsequent treatment of the welded components offers a cost-favourable and flexible method of reducing these stresses. In this respect, an oscillating defocused electron or laser beam serves to heat the material on both sides of the weld up to several hundred degrees Celsius in order to induce additional tensile stress regions there. The longitudinal tensile stress zones produced in this way lead to the mechanical relief of the weld. Experimental measurements show that the longitudinal residual stresses in the weld can be reduced by up to 70% with this process. In this case, not only was the experimental investigation into this process conducted on linear welds on 5 mm thick sheets made of the S355J2+N material but a comprehensive analysis of the influences of the process parameters on the stress reduction was also made by means of FEM simulation. The relief mechanism is discussed on the basis of the results.
Schweißzugeigenspannungen in Strahlschweißnähten auf Höhe der lokalen Werkstoffstreckgrenze können die Bauteileigenschaften sowie deren Lebensdauer negativ beeinflussen. Die Nutzung der Schweißquelle für eine nachträgliche Behandlung der geschweißten Bauteile bietet eine kostengünstige und flexible Methode zur Reduzierung dieser Spannungen. Dabei wird durch einen oszillierenden defokussierten Elektronen- bzw. Laserstrahl, der Werkstoff beidseitig der Schweißnaht auf mehrere hundert Grad Celsius erwärmt, um dort zusätzliche Zugspannungsbereiche hervorzurufen. Die so erzeugten Längszugspannungszonen führen zu einer mechanischen Entlastung der Schweißnaht. Experimentelle Messungen der Längseigenspannungen an linearen Elektronen- und Laserstrahlschweißnähten im ersten Teil des Beitrags zeigten, dass die Spannungen mit diesem Verfahren um bis zu 70% reduziert werden können. In weiteren experimentellen Untersuchungen wird dieses Verfahren für kompliziertere Schweißnahtgeometrien getestet. An axialen sowie radialen Rundnähten an ferritischen Werkstoffen konnten ebenfalls hohe Spannungsreduktionen von über 50% erzielt werden. Zudem wurde dieses Verfahren mit einer für das Schweißen üblichen Laserstrahlschweißoptik ohne Scan-Funktion geprüft. Hierbei zeigte das Wärmebehandeln in zwei Arbeitsschritten noch größere Spannungsreduktionen im Vergleich zur Anwendung einer Laserstrahlscanneroptik zur quasisimultanen Wärmebehandlung auf beiden Seiten der Schweißnaht. -------------------------------------------------------------------------------------------------------------------------------------------
Residual tensile welding stresses in beam welds at the level of the local yield strength of the material may influence the properties of components as well as their service lives. The utilisation of the welding source for the subsequent treatment of the welded components offers a cost-favourable and flexible method of reducing these stresses. In this respect, an oscillating defocused electron or laser beam serves to heat the material on both sides of the weld up to several hundred degrees Celsius in order to induce additional tensile stress regions there. The longitudinal tensile stress zones produced in this way lead to the mechanical relief of the weld. Experimental measurements of the longitudinal residual stresses on linear electron and laser beam welds in the first part of the article showed that the stresses can be reduced by up to 70% with this process. In further experimental investigations, this process is tested for more complicated weld geometries. It was also possible to achieve great reductions in the stresses (over 50%) on both axial and radial circular welds on ferritic materials. Moreover, this process was tested with laser beam welding optics which are customary for welding and do not perform a scanning function. In this respect, the heat treatment in two work steps showed even greater reductions in the stresses in comparison with the application of laser beam scanner optics for quasi-simultaneous heat treatment on both sides of the weld.
The results of an in-situ plume-laser interaction measurement during welding of mild steel with a 5 kW ytterbium fiber laser are reported. A measurement of the attenuation of probe laser beam passing through the plume has allowed to estimate the plume characteristics like the size of the extinction area and the spatial distribution of the extinction coefficient. The power loss of the fiber laser radiation propagating through the whole plume length was calculated. Together with a measured temporal characteristics of extinction the result indicates a significant decreasing of the laser radiation stability, which can lead to the formation of the macroscopic welding defects.
Manganese alloyed stainless steels represent a cost-effective alternative to conventional CrNi- stainless steels due to strong fluctuations of the market prices for nickel seen during the last years. In CrMnNi steels, nickel is partially replaced by lower-cost manganese and small amounts of nitrogen for stabilization of the austenitic phase. This also brings benefits regarding the mechanical properties, as it results in an increased material strength. Laser beam welding of such materials was investigated for direct comparison with standard CrNi steels. Main emphasis was laid on finding adequate process parameters to achieve a stable welding process and obtain a good weld quality. Two different laser sources, a 4.4 kW Nd:YAG and a 5 kW CO2 laser, were used to weld 1.5 mm stainless steel sheets in continuous wave mode. A high-Mn austenitic (1.4376) and a lean duplex (1.4162) steel, as well as the standard austenitic (1.4301) and duplex (1.4362) grades were selected as test materials. Both butt and lap joint configurations were studied. Experiments were carried out systematically, varying the welding speed, laser power and focal point position in order to determine adequate process windows. The influence of the shielding gas type and flow rate on the process stability and the weld quality were investigated. The effects of weld edge preparation on the weld appearance and quality levels attained were also examined. The obtained welded joints were subjected to radiographic tests for detection of internal imperfections. Also a metallurgical characterization of the samples regarding the resulting phase composition or balance and hardness depending on the welding process parameters was conducted. Furthermore, tensile and potentiodynamic tests were performed to evaluate the mechanical and corrosion properties, respectively. The results provide an insight into the advantages and limitations of the laser beam welding process for joining high-manganese alloyed stainless steels. Conditions for the production of defect-free and corrosion-resistant welds having good mechanical properties could be determined.
Dickwandige Rohre aus Stahl sind die Basis einer Vielzahl von Komponenten im Maschinen- und Anlagenbau, zum Beispiel bei Hydraulikkomponenten, in der Kraftwerkstechnik oder der Petroindustrie. Insbesondere, wenn hohe Lage- und Formtoleranzen gefordert werden, ist das klassische Herstellungsverfahren ein Zerspanen aus dem Vollmaterial. Dies ist jedoch zeit- und materialintensiv. Als Alternative bieten sich die Laserstrahl-MSGHybridverfahren mit modernen Hochleistungslasern an. Mit diesen Verfahren ist es derzeit möglich, Bauteile bis zu etwa 15 mm Wanddicke verzugsarm und einlagig zu verschweißen. In ersten Experimenten wurden bei Schweißungen an Rohren jedoch in Bezug auf die Schweißrichtung längs und vertikal orientierte Erstarrungsrisse, so genannte Mittelrippendefekte, festgestellt. Im Rahmen eines Forschungsprojekts konnte die Ursache der Rissentstehung ermittelt und Maßnahmen zur Vermeidung gefunden werden. Dabei ließen sich die technischen Randbedingungen einhalten. Der Artikel gibt einen Überblick über das Forschungsprojekt und die wesentlichen Ergebnisse. ---------------------------------------------------------------------------------------------------------------------------------------
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 rib defects, 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.
With additive manufacturing in the powder bed, the component size is limited by the installation space. Joint welding of additively manufactured parts offers a possibility to remove this size limitation. However, due to the specific stress and microstructure state in the additively built material, it is unclear to what extent existing evaluation rules of joint welding are also suitable for welds on additive components. This is investigated using laser beam welding of additively manufactured pipe joints. The welds are evaluated by means of visual inspection, metallographic examinations as well as computed tomography. The types of defects found are comparable to conventional components. This is an indicator that existing evaluation regulations also map the possible defects occurring for weld seams on additive components.
A mathematical model for physical processes in fusion welding has been developed. It is based on the equivalent heat source concept and consists of two parts: thermo-hydrodynamics of the weld pool and heat conduction in the weldment outside the pool. In thermo-hydrodynamic problem, temperature – dependent material properties, keyhole shape, thermo-capillary and natural convection, phase transformations and other physical phenomena are taken into consideration.
Solution of the thermo-hydrodynamic problem by the finite element method is demonstrated with keyhole laser beam welding of a 15 mm thick steel plate. Thermo-capillary convection is primarily responsible for the intricate convex-concave melt pool shape and pool enlargement near the plate surfaces. The calculated and experimental molten pool dimensions are in close agreement.
Solidification cracking phenomena taking place under controlled tensile weldability (CTW) test conditions have already been investigated both experimentally and numerically via FEA in order to get a better understanding of the mechanisms of hot crack formation during laser beam welding of austenitic steel grades. This paper develops a threedimensional finite element model employing the contact element technique to simulate the formation and propagation of solidification cracks during laser full penetration welding of fully austenitic stainless steel 1.4376. During the experimental procedure, the resulting strain and displacement directed to the laser beam in the close vicinity of the weld pool was measured at the surface of the workpiece using a digital image correlation (DIC) technique with an external diode laser as an illuminating source. Local strain fields, global loads and crack lengths predicted by the model are in good agreement with those observed in experiments.