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In vielen Industriebranchen werden zunehmend hochfeste Feinkornbaustähle mit Streckgrenzen ≥ 690 MPa eingesetzt. Allerdings werden mit zunehmender Festigkeit deutlich höhere Anforderungen an deren schweißtechnische Verarbeitung gestellt, da gerade die Sensibilität gegenüber einer Degradation der mechanischen Eigenschaften, durch den beim Schweißen aufgenommenen Wasserstoff, mit steigender Festigkeit stark zunimmt. In der vorliegenden Präsentation sind Ergebnisse aus vergleichenden Untersuchungen mit konventionellem Übergangslichtbogen und modifiziertem Sprühlichtbogen mit reduziertem Nahtöffnungswinkel hinsichtlich der Wasserstoffaufnahme beim Schweißen unter Variation von Streckenenergie, Arbeitstemperatur und Nachwärmprozedur dargestellt. Grundsätzlich ist den erarbeiteten Ergebnissen zu entnehmen, dass im Schweißgut von Stumpfstoßverbindungen mit reduziertem Nahtöffnungswinkel erhöhte mittlere Wasserstoffkonzentrationen vorliegen. Geeignete Wärmeführungen führten zu einer signifikanten Reduzierung der Wasserstoffkonzentration beim Einsatz der Nahtkonfiguration mit abgesenktem Nahtöffnungswinkel. Dabei erwies sich eine Nachwärmprozedur aus der Schweißwärme heraus als zielführend. Des weiteren sind Gradienten der Wasserstoffkonzentration in Blechdickenrichtung aufgezeigt.
Two-dimensional residual stress mapping of multilayer LTT weld joints using the contour method
(2018)
Low transformation temperature (LTT) weld filler materials offer an attractive alternative to cost intensive postweld treatments as they can mitigate detrimental welding residual stresses during the welding process. Compared to conventional weld filler materials, LTT alloys are characterized by a delayed martensite transformation at low temperatures, which can result in compressive residual stresses in the weld. The high strength of these filler materials makes them potentially applicable to high-strength steels as well as for a large amount of requested repair works in steel structures. The focus of the study is on the confirmation of the LTT idea with regard to the residual stress state for multipass weld lines processed by metal active gas welding. It is demonstrated that the contour method is a well-suited technique for measuring the residual stress in the weld joint as it gives an entire two-dimensional map of the residual stress state in the weld line, heat affected zone (HAZ), and base material. The technique was applied at different LTT alloys with varying chemical compositions. Additionally, the results are compared to residual stress maps that were determined by Neutron diffraction using the Strain Analyzer for Large Scale Engineering Applications, an
instrument referred to as SALSA, at the Institut Laue-Langevin in Grenoble. For all investigated specimens, compressive residual stress distributions were determined in the area of the weld joint and the HAZ. They are balanced by tensile residual stresses in the surrounding base material. However, it is shown that the size of theregion exhibiting compressive residual stresses and the absolute values of the compressive residual stresses depend on the chemical composition of the weld filler material.
The residual stress distribution in IN718 elongated prisms produced by Selective Laser Melting was studied by means of neutron (bulk) and laboratory X-ray (surface) diffraction.
Two deposition hatch lengths were considered. A horizontal plane near the top surface (perpendicular to the building direction) and a vertical plane near the lateral surface (parallel to the building direction) were investigated. Samples both in as-built (AB) condition and removed (RE) from the base plate were characterized.
While surface stress fields seem constant for AB condition, X-ray diffraction shows stress gradients along the hatch direction in the RE condition. The stress profiles correlate with the distortion maps obtained by tactile probe measurements.
Neutron diffraction shows bulk stress gradients for all principal components along the main sample directions. We correlate the observed stress patterns with the hatch length, i.e. with its effect on temperature gradients and heat flow. The bulk stress gradients partially disappear after removal from the baseplate.
Die Beschaffung und Verarbeitung von Werkstoffen für hochbelastete Komponenten sind meist kostenintensiv. Bestrebungen zur Kosten- und Ressourceneffizienz führen zu komplexeren Strukturen bzw. Konturen, sodass generative Fertigungsschritte zur Bauteilreparatur und -fertigung deutliche ökonomische Vorteile bieten. Hierfür sind generative und abtragende Fertigungsschritte komplementär und gezielt aufeinander abzustimmen, um beanspruchungsgerechte Funktionsflächen herzustellen. Hinsichtlich Inhomogenität und Anisotropie der Gefüge und Eigenschaften sowie fertigungsbedingter Beanspruchungen sind für den wirtschaftlichen Einsatz bei KMU, gerade für drahtbasierte Fertigungsverfahren und Wechselwirkungen nachfolgender Zerspanung dieser schwer spanbaren Werkstoffe noch viele Kenntnisse notwendig. Deshalb sind Untersuchungen zu diesen Einflüssen und Wechselwirkungen unter Nutzung innovativer Ansätze geplant. Mit typischen kostenintensiven Ni- und Co-Cr-Legierungen werden generativ Bauteile, Auftrag- und Reparaturschweißungen hergestellt und Schweißzusätze für das PTA-Verfahren modifiziert, um die Erstarrungsmorphologie und das Eigenschaftsprofil zu optimieren. Die Übertragung auf MSG-Verfahren sichert einen breiten industriellen Einsatz für hohe Auftragraten ab. Die wirtschaftliche spanende Bearbeitbarkeit wird mit Zerspanbarkeitsanalysen für Schlichtfräsen und vergleichend für ultraschallunterstütze Fräsprozesse sichergestellt. Instrumentierte Experimente und Werkstoffanalytik hinsichtlich der Einflüsse auf Schmelzbad, Gefüge und Ausscheidungen sowie auf Randzoneneigenschaften und Eigenspannungen ermöglichen umfassende Erkenntnisse zur kombinierten generativen und abtragenden Fertigung. Durch Bauteilversuche und -prüfungen, Übertragbarkeitsstudien weiterer innovativer Werkstoffe verbunden mit Handlungsempfehlungen und der Zusammenarbeit mit den jeweiligen Normungsgremien profitieren KMU von einer hochwirtschaftlichen Herstellung und Reparatur kostenintensiver Komponenten.
Influence of support configurations on the characteristics of selective laser-melted Inconel 718
(2018)
Samples fabricated using two different support configurations by following identical scan strategies during selective laser melting of superalloy Inconel 718 were characterized in this study. Characterization methods included optical microscopy, electron back-scattered diffraction and x-ray diffraction residual stress measurement. For the scan strategy considered, microstructure and residual stress development in the samples were influenced by the support structures. However, crystallographic texture intensity and the texture components formed within the core part of the samples were almost independent of the support. The formation of finer grains closer to the support as well as within the columnar grain boundaries resulted in randomization and texture intensity reduction by nearly half for the sample built on a lattice support. Heat transfer rates dictated by the support configurations in addition to the scan strategy influenced the microstructure and residual stress development in selective laser-melted Inconel 718 samples.
The Topic of the presentationis a discussion on defects which can cause failure in cyclically loaded metallic components. Although also touching Features such as material defects such as pores or micro-shrinkages, etc. and geometric defects such as surface roughness and secondary notches (which are not considered in the design process) which origin in manufacturing, and others the presentation concentrates on non-metallic inclusions. It is prefaced by an introduction to the life cycle of a fatigue crack from initiation up to fracture. Special emphasis is put on the fact that only cracks which are not arrested during one of their distinct Propagation stages can grow to a critical size.
Additive Manufacturing (AM) through the Selective Laser Melting (SLM) route offers ample scope for producing geometrically complex parts compared to the conventional subtractive manufacturing strategies. Nevertheless, the residual stresses which develop during the fabrication can limit application of the SLM components by reducing the load bearing capacity and by inducing unwanted distortion, depending on the boundary conditions specified during manufacturing. The present study aims at characterizing the residual stress states in the SLM parts using different diffraction methods. The material used is the nickel based superalloy Inconel 718. Microstructure as well as the surface and bulk residual stresses were characterized.
For the residual stress analysis, X-ray, synchrotron and neutron diffraction methods were used. The measurements were performed at BAM, at the EDDI beamline of -BESSY II synchrotronand the E3 line -BER II neutron reactor- of the Helmholtz-Zentrum für Materialien und Energie (HZB) Berlin. The results reveal significant differences in the residual stress states for the different characterization techniques employed, which indicates the dependence of the residual state on the penetration depth in the sample. For the surface residual stresses, longitudinal and transverse stress components from X-ray and synchrotron agree well and the obtained values were around the yield strength of the material. Furthermore, synchrotron mapping disclosed gradients along the width and length of the sample for the longitudinal and transverse stress components. On the other hand, lower residual stresses were found in the bulk of the material measured using neutron diffraction. The longitudinal component was tensile and decreased towards the boundary of the sample. In contrast, the normal component was nearly constant and compressive in nature. The transversal component was almost negligible. The results indicate that a stress re-distribution takes place during the deposition of the consecutive layers. Further investigations are planned to study the phenomenon in detail.