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A method of solving the thermo-fluid dynamics problem is presented, enabling the prediction of the shape and dimensions of the weld pool during keyhole laser welding of thick plates. It is shown that the rear weld pool boundary can be satisfactorily approximated by a set of superellipses (Lamé curves). The presence of a convex rear weld pool boundary in the mid-plane has been observed experimentally and reproduced numerically. It was shown that in this zone the concentration of liquating impurities increases and the local solidification temperature decreases, contributing to the susceptibility to hot cracking.
It is proposed to model the experimentally observed weld pool boundary with superellipses (Lamé curves) and to find the unknown parameters of the curves using optimization methods. It has been shown experimentally that during laser welding of austenitic stainless steel with a thickness of 2 mm at a speed of 20 mm/s, the rear weld pool part has a shape close to triangular which can be accurately approximated by a superellipse. Analytical dependences of the trajectory and growth rate of the crystal and its cross-sectional area on the geometry of the rear weld pool part are obtained.
МОДЕЛИРОВАНИЕ ГИДРОДИНАМИЧЕСКИХ И ТЕПЛОВЫХ ПРОЦЕССОВ ПРИ ЛАЗЕРНОЙ СВАРКЕ СО СКВОЗНЫМ ПРОПЛАВЛЕНИЕМ
(2020)
Разработана модель физических процессов при сварке плавлением на основе концепции эквивалентных источников теплоты. Модель включает в себя две части: термогидродинамику сварочной ванны и теплопроводность свариваемого тела вне ванны. В задаче термогидродинамики учитываются температурные зависимости свойств материала, форма парогазового канала, термокапиллярная и естественная конвекция, фазовые превращения и другие физические явления.Приведено решение задачи термогидродинамики методом конечных элементов на примере сварки стальной пластины толщиной 15 мм со сквозным проплавлением лазерным лучом (по технологии "замочная скважина"). Показано, что термокапиллярная конвекция жидкого металла является основной причиной сложной выпукло-вогнутой формы границы ванны с увеличенными размерами в приповерхностных областях. Получено удовлетворительное совпадение расчетных и экспериментальных размеров сварочной ванны.
Приведено аналитическое решение задачи теплопроводности после прекращения действия подвижного источника теплоты для различных комбинаций источника и свариваемого тела. Показано, что после выключения источника возможно дополнительное плавление основного металла за счет перегретого жидкого металла сварочной ванны. Например, при лазерной сварке со сквозным проплавлением стальной пластины толщиной 2 мм длина кратера шва может быт на 19% больше установившейся длины сварочной ванны. Установлено, что центр кратера, в котором заканчивается затвердевание жидкого металла, смещен в сторону хвостовой части кратера относительно оси теплового источника в момент прекращения его действия. Скорость и направление кристаллизации металла сварочной ванны и кратера различны.
Ein wesentliches Ziel der deutschen Industriepolitik besteht darin, Rahmenbedingungen zu schaffen, welche die Positionierung nachhaltiger Produkte und Prozesse innerhalb globaler Wertschöpfungsketten ermöglichen und ökoeffiziente Fertigungsprozesse langfristig sichern. In diesem Beitrag werden aktuelle Forschungsarbeiten zur Bewertung der Umweltwirkungen der schweißtechnischen Fertigungskette anhand ausgewählter Schmelzschweißverfahren unter Berücksichtigung vor- sowie nachgelagerter Fertigungsschritte dargestellt. Eine weit verbreitete und standardisierte Methode zur Abschätzung der Umweltwirkungen eines Produkts oder Prozesses ist die Ökobilanzierung. Hierbei stellen die Sachbilanzierung, das heißt die Ermittlung sämtlicher relevanter Energie- und Ressourcenverbräuche
während der schweißtechnischen Fertigung, aufgrund des hohen Dokumentationsaufwands, sowie die Ableitung assoziierter Wirkkategorien das größte Hindernis für eine Etablierung und Akzeptanz der Ökobilanzierung in der Praxis dar. Gleichwohl wird seitens Unternehmen, welche als Zulieferer von OEM’s (engl.: original equipment manufacturer) agieren, die Dokumentation des fertigungsspezifischen CO2-Verbrauches gefordert.
Ein weiterer Aspekt der Arbeiten beinhaltet Methoden zur automatisierten Erfassung von schweißtechnischen Produktionsdaten sowie deren Nachverfolgbarkeit. Anhand unterschiedlicher Schweißverfahren werden die aus den Produktionsdaten abgeleiteten Energie- und Ressourcenverbräuche automatisiert in die relevanten Umweltwirkungen überführt. Die analysierten Schweißprozesse umfassen dabei ein breites für kleine und mittlere Unternehmen (KMU) relevantes Spektrum. Durch die softwareseitige Bereitstellung der aufgestellten Umweltprofile ist der Anwender in der Lage, Schweißprozesse unter ökologischen Aspekten zu bewerten und die effizienteste Variante zu identifizieren.
Wire arc additive manufacturing enables the production of near-net shape large-volume metallic components leveraging an established industrial base of welding and cladding technology and adapting it for layer-wise material deposition. However, the complex relationship between the process parameters and resulting mechanical properties of the components still remains challenging. In case of high-strength Al-Mg-Si aluminum alloys, no commercial filler wires are yet available due the high susceptibility of solidification cracking as well as the necessary efforts to obtain acceptable mechanical properties. To address this need, we evaluated a novel filler wire based on AlMg0.7Si doped with a Ti5B1 master alloy to foster fine equiaxed grains within the deposited metal. The correlation between the process parameters and component quality was examined by analyzing the size and distribution of pores as well as the grain morphology. Furthermore, we evaluated the influence of different post-weld heat treatment strategies to achieve mechanical properties corresponding to the reference wrought material. We demonstrated that fine equiaxed grains in the weld metal reduced the susceptibility of solidification cracking significantly. The novel AlMg0.7Si-TiB (S Al 6063-TiB) filler wire facilitated wire arc additive manufacturing of high-strength aluminum components with mechanical properties that were almost as superior as the corresponding wrought base material.
High-strength steels are increasingly applied in modern steel constructions to meet today’s lightweight requirements. Welding of these steels demands a profound knowledge of the interactions between the welding process, cooling conditions, heat input, and the resulting metallurgical occurrences in the weld and its vicinity. Additionally, welding stresses may be detrimental for the safety and performance of high-strength steel component welds during fabrication and service, especially due to the high yield ratio. For a development of strategies to adjust welding heat control, all these effects should be considered, to reach a complete exploitation of the high-strength steel potential. In recent researches at BAM, multilayer GMAW experiments were performed with high-strength steels, in which cooling conditions and resulting microstructure were analyzed for varied heat control parameters. The application of a unique 3d-operating testing facility and X-ray diffraction measurements allowed the analysis of local stresses in the weld while welding and cooling under component relevant shrinkage restraints. As a result, correlations between material behavior, welding, and cooling condition and the arising multi-axial stresses and forces were found. Based on this study, statements for the development of adapted heat control concepts were derived, which are presented by means of specific analysis examples.
Welding residual stresses have an impact on the performance of welded structures, on their fracture resistance, their resistance against fatigue crack propagation and, most important, their fatigue strength and fatigue lifetime. The present paper provides an overview on the issue mainly from the point of view of the application of fracture mechanics to the determination of the fatigue strength as the topic of this Special issue. Besides own experimental and theoretical data a comprehensive discussion is provided in that context which includes the definition and interaction of short- and long-range (or reaction) residual stresses, the effect of cyclic mechanical loading and its treatment in fracture and fatigue analyses.
High-entropy alloys (HEAs) and compositionally complex alloys (CCAs) represent new classes of materials containing five or more alloying elements (concentration of each element ranging from 5 to 35 at. %). In the present study, HEAs are defined as single-phase solid solutions; CCAs contain at least two phases. The alloy concept of HEAs/CCAs is fundamentally different from most conventional alloys and promises interesting properties for industrial applications (e.g., to overcome the strength-ductility trade-off). To date, little attention has been paid to the weldability of HEAs/CCAs encompassing effects on the welding metallurgy. It remains open whether welding of HEAs/CCAs may lead to the formation of brittle intermetallics and promote elemental segregation at crystalline defects. The effect on the weld joint properties (strength, corrosion resistance) must be investigated. The weld metal and heat-affected zone in conventional alloys are characterized by non-equilibrium microstructural evolutions that most probably occur in HEAs/CCAs. The corresponding weldability has not yet been studied in detail in the literature, and the existing information is not documented in a comprehensive way. Therefore, this study summarizes the most important results on the welding of HEAs/CCAs and their weld joint properties, classified by HEA/CCA type (focused on CoCrFeMnNi and AlxCoCrCuyFeNi system) and welding process.