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As opposed to reviewing results on experimental determination of residual stress by diffraction, this paper discusses the open issues when dealing with residual stress determination in additive manufactured parts, in particular those manufactured with laser powder bed fusion techniques. Three points are addressed in detail: (a) the proper determination of the strain-free reference d0, (b) the problem of the determination of the principal axes, and (c) the use of the correct diffraction elastic constants. It is shown that all methods to determine the strain-free reference d0 suffer from caveats, and care must be taken in evaluating the most suitable for the problem being tackled. In addition, it is shown that, in some systems, the principal axes do correspond to the geometrical axes of the specimen, but this needs to be systematically checked, especially in the case of uni- or bidirectional hatching strategies. Finally, the need to experimentally determine the proper diffraction elastic constants is underlined, especially in the case of strongly textured specimens, which again depends on the deposition strategy.
Laser powder bed fusion (LPBF) is an additive manufacturing process for materials which inherently tends to yield various degrees of metastable hierarchical microstructures, defects and high residual stresses in the as-built condition depending on the process parameters. The understanding of the evolution of these typical features during heat treatment and subsequent thermal and mechanical ageing is crucial for the wider acceptance for safety critical structures. A multi-disciplinary research project at BAM studying the development of the microstructure, defects, residual stresses typical of LPBF 316L and their evolution during thermal and mechanical ageing has led to insights into the stability of these inherent features. This presentation aims to give a broad overview of the project with a few specific cases of investigation. Firstly, the formation of residual stresses, the nature of the initial microstructure, the tensile properties and a modelling approach to understand the anisotropy will be presented. This will be followed by examples of studies of their evolution during heat treatment, long term thermal exposure, and room temperature and high temperature mechanical testing compared to a baseline of conventional wrought variant of the same alloy.
An overview of the BAM funed Focus Area Materials Project "AGIL" will be presented. AGIL focussed on the stdiy of the ageing characteristics of additively manufactured austenitic stainless steel with a "powder to mechanical failure" Approach. Recent Highlights are presented and a perspective for future studies.
In Hinblick auf den austenitischen Stahl AISI 316L erzeugt das pulverbettbasierte selektive Laserstrahlschmelzen, als ein additives Fertigungsverfahren, kristallographisch texturierte und Multiskalige Mikrostruktur. Einerseits können diese Mikrostrukturen zu einer Verbesserung der statischen mechanischen Eigenschaften führen (z. B. zu einer höheren Streckgrenze). Andererseits stehen diesen Verbesserungen der mechanischen Eigenschaften hohe Eigenspannungen gegenüber, die sich nachteilig auf das Ermüdungsverhalten auswirken können. Zur Reduzierung der Eigenspannungen und der daraus resultierenden negativen Auswirkungen auf die Ermüdungseigenschaften, werden Bauteile nach der Herstellung typischerweise wärmebehandelt. In dieser Studie wurde eine niedrige Wärmebehandlungstemperatur von 450°C höher temperierten Behandlungen bei 800 °C und 900 °C gegenübergestellt. Diese Wärmebehandlungstemperaturen bilden die oberen und die untere Grenze ein Spannungsarmglühendes Materials, ohne die prozessinduzierte Mikrostruktur signifikant zu verändern. Zusätzlich bieten diese Temperaturen den Vorteil, dass sie eine übermäßige intergranulare Ausscheidung von Karbiden und TCP-Phasen vermeiden, die zu einer Sensibilisierung des Werkstoffes gegen korrosive Umgebungen führen würden. Die Auswirkungen der Wärmebehandlung auf das Gefüge wurden mittels Rasterelektronenmikroskopie (BSE und EBSD) untersucht. Die Relaxation der Eigenspannungen wurde vor und nach den jeweiligen Wärmebehandlungen bei 800°C und 900°C mittels Neutronenbeugung charakterisiert. Die Ergebnisse zeigen, dass die Proben nach der Wärmebehandlung bei 900 °C nahezu spannungsfrei sind, was mit der Auflösung der zellularen Substruktur korreliert.
Entwicklung der Mikrostruktur der mechanischen Eigenschaften und der Eigenspannungen in L-PBF 316L
(2022)
Die additive Fertigung (AM) metallischer Werkstoffe mittels Laser Powder Bed Fusion (L-PBF) ermöglicht einzigartige hierarchische Mikrostrukturen, die zu Verbesserungen bestimmter mechanischer Eigenschaften gegenüber konventionell hergestellten Varianten derselben Legierung führen können. Allerdings ist das L-PBF-Verfahren häufig durch das Vorhandensein hoher Eigenspannungen gekennzeichnet, die es zu verstehen und zu mindern gilt. Daher ist das Verständnis der Mikrostrukturen, der Eigenspannungen und der daraus resultierenden mechanischen Eigenschaften entscheidend für eine breite Akzeptanz bei sicherheitskritischen Anwendungen. Die BAM hat ein multidisziplinäres Forschungsprogramm gestartet, um diese Aspekte bei LPBF 316L zu untersuchen. Der vorliegende Beitrag stellt einige der wichtigsten Ergebnisse vor: der Einfluss von Prozessparametern auf die Mikrostruktur, der Einfluss von Mikrostruktur und Textur auf die Festigkeit, Kriechverhalten und Schädigung und die Stabilität von Eigenspannungen und Mikrostruktur unter Wärmebehandlungsbedingungen.
The European Commission has identified Advanced Manufacturing and Advanced Materials as two of six Key Enabling Technologies (KETs). It is considered that Metrology is a key enabler for the advancement of these KETs. Consequently, EURAMET, the association of metrology institutes in Europe, has strengthened the role of Metrology for these KETs by enabling the creation of a European Metrology Network (EMN) for Advanced Manufacturing. The EMN is comprised of National Metrology Institutes (NMIs) and Designated Institutes (DIs) from across Europe and was formally established in October 2021. The aim of the EMN is to provide a high-level coordination of European metrology activities for the Advanced Manufacturing community.
The EMN itself is organized in three sections representing the major stages of the manufacturing chain: 1) Advanced Materials, 2) Smart Manufacturing Systems, and 3) Manufactured Components & Products. The EMN for Advanced Manufacturing is engaging with stakeholders in the field of Advanced Manufacturing (large companies & SMEs, industry organisations, existing networks, and academia), as well as the wider Metrology community, including Technical Committees, to provide input for the Strategic Research Agenda (SRA) on Metrology for Advanced Manufacturing.
This contribution will give an overview about the first version of the SRA prepared by the EMN for Advanced Manufacturing
The progress of Advanced Manufacturing, which has been identified by the European Commission as a Key Enabling Technology (KET) for future economic and societal progress is strongly reliant on the development of metrology capabilities. EURAMET, the association of metrology institutes in Europe, has established metrology research programs to address the metrology requirements across a spectrum of different thematic areas. In order to leverage the benefits of these developments on the wider industrial landscape, a high-level coordination of the metrology community supporting the Advanced Manufacturing landscape is required. This coordination is aimed to be achieved by the establishment of European Metrology Networks (EMNs), which are intended by EURAMET to provide a sustainable structure for stakeholder engagement and support. The joint networking project 19NET01 AdvManuNet funded by EMPIR for 4 years, started in June 2020 and aims to accelerate the process of establishing an EMN to strengthen Europe’s position in Advanced Manufacturing. The AdvManuNet project aims to support the establishment of an EMN on Advanced Manufacturing via the following specific aims:
1. Creation of a single hub for stakeholder engagement across the landscape of various industrial sectors including relevant societies and standardization bodies.
2. Development of a Strategic Research Agenda (SRA) and roadmaps for Advanced Manufacturing metrology based on the stakeholder engagement activities, considering current gaps in metrological capabilities existing networks and roadmaps.
3. Establish a knowledge-sharing program for Advanced Manufacturing stakeholders, promoting the dissemination and exploitation of the results of the project, including those from previous EU funded research projects.
4. Development of a sustainable web-based platform and service desk for Advanced Manufacturing stakeholders to allow for easy access to European metrology capabilities and support the wider advanced manufacturing community with metrology-based requirements.
5. Develop a plan for a coordinated and sustainable European metrology infrastructure for Advanced Manufacturing via a European Metrology Network.
The project concept followed by the scope and definition of Advanced Manufacturing will be described. The analysis of the current capability of metrology for Advanced Manufacturing and the preliminary concepts for the strategic research agenda will be presented with a focus on dimensional metrology.
AbstractIn this study, we compare the residual stress state in a laser powder bed fusion (PBF‐LB/M) AlSi10Mg alloy in the as‐built (AB) condition with that after two different heat treatments (265 °C for 1 h, HT1; and 300 °C for 2 h, HT2). The bulk residual stress (RS) is determined using synchrotron X‐ray diffraction (SXRD), and near‐surface profiles are determined using laboratory energy‐dispersive X‐ray diffraction (EDXRD). The EDXRD results do not reveal any notable difference between the conditions at a depth of 350 μm, suggesting that the machining process yields a comparable residual stress state in the near‐surface regions. On the other hand, the SXRD results show that HT1 is more effective in relieving the bulk RS. It is observed that HT1 reduces the RS state in both the aluminium matrix and the silicon network. In addtion, HT2 does not have a significant impact on relaxing the RS as‐built state of the matrix, although it does induce a reduction in the RS magnitudes of the Si phase. It is concluded that the heat treatment stress relieving is effective as long as the Si‐network is not disaggregated.
Using non-optimum combination of manufacturing parameters in selective laser melting (SLM) may lead to reduction of quality of component: defects generation, distortion of geometry and even cracking. Usually, the optimization of parameters is performed by changing volumetric energy density (Ev) and selecting parameters giving low porosity values. However, not only low porosity but also stable microstructure and low residual stresses will help to achieve advanced mechanical behavior of the component.
In present work, we investigated cuboid-shaped Ti-6Al-4V samples produced with different manufacturing parameters. The parameters leading to the same Ev were considered as well as parameters which are not included in Ev. Residual stresses in subsurface region were investigated by synchrotron X-ray diffraction, which allows to penetrate around 100 µm from the surface therefore overcome the problem of high roughness of SLM components without additional sample preparation. Only tensile stresses were found along the building direction, that can play critical role especially during cyclic loading. In parallel, using X-ray computed tomography we also observe that porosity is mainly concentrated in the contour region, except in case where the laser speed is small. However, by using some process parameters it was possible to decrease residual stresses and obtain uniform α+β Ti microstructure and relatively low porosity. Additionally, it was found that not included in Ev (e.g., base plate position, focus distance) should be considered as additional manufacturing parameters during SLM process.
Introduction to AGIL
(2021)
Metal Additive Manufacturing (AM) technologies such as Laser Powder Bed Fusion (LPBF) are characterized by layer wise construction, which enable advancements of component design, with associated potential gains in performance and efficiency. However, high magnitude residual stresses (RS) are often a product of the rapid thermal cycles typical of the layerwise process. Therefore, a deep understanding of the formation of RS, the influence of process parameters on their magnitude and the impact on mechanical performance is crucial for widespread application. The experimental characterisation of these RS is essential for safety related engineering application and supports the development of reliable numerical models. Diffraction-based methods for RS analysis using neutrons and high energy X-rays enable non-destructive spatially resolved characterisation of both surface and bulk residual stresses in complex components. This presentation will provide an overview of recent research by the BAM at large scale facilities for the characterization of residual stresses in LPBF metallic alloys as a function of process parameters. In addition, the challenges posed by the textured and hierarchical microstructures of LPBF materials on diffraction-based RS analysis in AM materials will be discussed. This will include the question of the d0 reference lattice spacing and the appropriate choice of the diffraction elastic constants (DECs) to calculate the level of RS in LPBF manufactured alloys.
In this presentation we discuss the online monitoring of metallic AM parts produced by the Laser Powder Bed Fusion (LPBF) process by using optical, thermographic and electromagnetic methods. In a first approach we present the detection of defects generated during the process and discuss how to improve these methods for the optimization of design and production of metallic AM parts.
Residual stresses Analysis in Additively Manufactured alloys using neutron diffraction (L-PBF)
(2023)
An overview of recent progress at BAM of residual stress analysis in additively manufactured, in particular Laser Powder Bed Fusion of metallics materials, using neutron diffraction will be presented. This will cover important topics of the stress-free reference, the diffraction elastic moduli and principal stress determination.
Additive manufacturing (AM) technologies are experiencing an exceedingly rapid growth, driven by their potential through layer wise deposition for transformational improvements of engineering design, leading to efficiency and performance improvements. Laser Powder Bed Fusion (LPBF) is an Additive Manufacturing (AM) method which permits the fabrication of complex structures that cannot otherwise be produced via conventional subtractive manufacturing methods. Nevertheless, the rapid cooling rates associated with this process results in the formation of significant and complex residual stress (RS) fields. A large body of both experimental and simulation research has been dedicated in recent years to the control and mitigation of RS in AM. In order to validate simulations with the end goal of being able to model the residual stress state in AM components and to devise strategies for their reduction during manufacturing, experimental methods need to be able to accurately determine 3D residual stresses fields in complex geometries. Several destructive and non-destructive methods can be used to analyze the RS state, the choice of which depends on the geometry and the information required. Diffraction-based methods using penetrating neutron and synchrotron X-rays at large scale facilities offer the possibility to non-destructively spatially resolve both surface and bulk residual stresses in complex components and track their changes following applied thermal or mechanical loads. This presentation will overview the success stories of using large scale facilities by the BAM for the characterization of residual stresses in additively manufactured metallic alloys. In particular, the study of the influence of process parameters on the residual stress state and the relaxation of these stresses through heat treatment will be presented. However there remains challenges to overcome particularly of the hypotheses underlying the experimental determination of residual stresses, which will be discussed.
Metal Additive manufacturing (AM) technologies such as Laser Powder Bed Fusion (LPBF) enable the fabrication of complex structures, giving rise to potential improvements in component and manufacturing efficiency. However, the processes are typically characterized by the generation of high magnitude residual stress (RS) which can have detrimental consequences for subsequent applications. Therefore, the characterization of these RS fields and the understanding of their formation and mitigation through optimized processing is crucial for the wider uptake of the technology. Due to the potential complex nature and high value of components manufactured by LPBF, it is important to have suitable characterisation methods which can determine the spatial variations of RS in a non-destructive manner. Neutron diffraction is considered to be the best suited for these requirements. However, the microstructures developed in the complex thermal cycles experience in the production can pose challenges to the ND method for RS analysis. The BAM has conducted significant research over the past years to overcome these obstacles, enabling higher confidence in the RS determined in LPBF materials by neutron diffraction. This contribution will overview some of these advancements made recently at European neutron sources including on Stress-Spec at FRM2/MLZ.
Residual stresses in Laser Beam Melting (LBM) – Critical Review and outlook of activities at BAM
(2019)
Additive manufacturing (AM) technologies have experienced an exceedingly rapid growth, which is coupled with the knowledge about the resulting material properties and performance. In particular, residual stress (RS) was soon recognized as an important issue in AM parts, such that parts are usually subjected to a post build-heat-treated. Significant effort has been spent on simulations of RS in AM, especially using finite element methods. As a consequence, the experimental determination of RS has thereby become increasingly important as a validation tool for simulations, as well as a method for assessing the influence of process parameters. In particular, diffraction methods, which are fundamentally non-destructive, offer enormous possibilities to gain knowledge on the residual stress state in real components, since synchrotron radiation and neutrons can penetrate even heavy metals up to several millimeters or centimeters, respectively. Indeed, significant progress has been achieved, in the understanding of the origins of the RS fields as a function of process parameters, as well as their stability under thermal and/or mechanical exposure.
In this paper, a few success stories will be outlined. It will be shown how the determination of RS in metallic parts (with the focus on those produced by laser powder bed fusion) has even revealed that process parameters that were previously considered unimportant (e.g. the position and orientation on the base plate) play a major role in the onset of residual stress accumulation.
However, while RS characterization is starting to be considered in the component design, deposition strategy (e.g. build plate temperature), and even in the definition of the relevant metric to assess the quality of a part, much is still to be investigates about the hypotheses underlying its experimental determination. Therefore, some aspects to be aware of, or even those which to date are unclear, will also be discussed. These include the determination of the stress-free reference and of the principal axes of stress. All of these aspects will lead towards a comprehensive understanding of the process-structure-performance relationships in AM materials and parts.
Advanced Manufacturing and Advanced Materials have been identified by the European Commission as one of six Key Enabling Technologies (KETs), the full exploitation of which will create advanced and sustainable economies. Metrology is a key enabler for progress of these KETs. EURAMET, which is the association of metrology institutes in Europe, has addressed the vital importance of Metrology for these KETs through the support for the creation of a European Metrology Network for Advanced Manufacturing. The EMN for Advanced Manufacturing (AdvanceManu) was approved in June 2021 and held the formal kick-of meeting in October 2022. The EMN comprises both National Metrology Institutes (NMIs) from across Europe and other designated Institutes (Dis). The EMN is organized in three sections; Advanced Materials, Smart Manufacturing Systems and Manufactured components and products. The aim of the EMN is to engage with stakeholders in the field of Advanced Manufacturing and Advanced Materials (Large & SMEs, industry organisations, existing networks and academia) with the aim to prepare a Strategic Research Agenda (SRA) for Metrology for Advanced Manufacturing. In the shorter term, an orientation paper is aimed to be produce to in the context of the European Partnership for Metrology. In addition to the SRA, the EMN will establish knowledge and technology transfer and promotion plan. This includes leveraging the existing research results from the completed and running EMPIR JRP projects funded through EURAMET.
This presentation will outline the EMN for Advanced Manufacturing, describing the structures and goals, the route to the production of the SRA and the progress made to date identifying the key metrology challenges across the related Key Industrial Sectors (KICs). In particular, the presentation aims to inform the community on how to be involved in the shaping of the strategic research agenda for the future of Metrology for Advanced Manufacturing and Advanced Materials.