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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.
Additive Manufacturing (AM) of metals has become industrially viable for a large variety of applications, including aerospace, automotive and medicine. Powder bed techniques such as Selective Laser Melting (SLM) based on layer-by-layer deposition and laser melt enable numerous degrees of freedom for the geometrical design. Developing during the manufacturing process, residual stresses may limit the application of SLM parts by reducing the load bearing capacity as well as induce unwanted distortion depending on the boundary conditions specified in manufacturing.
The residual stress distribution in the bulk of IN718 elongated prisms produced by SLM was studied non-destructively by means of neutron diffraction. The samples with different scanning strategies, i.e. hatching length, were measured in as-build condition (on a build plate) and after removal from the build plate.
The absolute values of all stress components decreased after removal from the build plate. Together with surface scan utilizing a coordinate-measuring machine (CMM), it is possible to link the stress release to the sample distortion. Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component depending on the thermal gradient in the respective direction.
Having been introduced almost two decades ago, Additive Manufacturing (AM) of metals has become industrially viable for a large variety of applications, including aerospace, automotive and medicine. Powder bed techniques such as Selective Laser Melting (SLM) based on layer-by-layer deposition and laser melt enable numerous degrees of freedom for the geometrical design. Developing during the manufacturing process, residual stresses may limit the application of SLM parts by reducing the load bearing capacity as well as induce unwanted distortion depending on the boundary conditions specified in manufacturing.
The residual stress distribution in IN718 elongated prisms produced by SLM was studied non-destructively by means of neutron (bulk) and laboratory X-ray (surface) diffraction. The samples with different scanning strategies, i.e. hatching length, were measured in as-build condition (on a build plate) and after removal from the build plate.
The absolute values of all stress components decreased after removal from the build plate. Together with surface scan utilizing a coordinate-measuring machine (CMM), it is possible to link the stress release to the sample distortion. Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component depending on the thermal gradient in the respective direction.
Having been introduced almost two decades ago, Additive Manufacturing (AM) of metals has become industrially viable for a large variety of applications, including aerospace, automotive and medicine. Powder bed techniques such as Selective Laser Melting (SLM) based on layer-by-layer deposition and laser melt enable numerous degrees of freedom for the geometrical design. Developing during the manufacturing process, residual stresses may limit the application of SLM parts by reducing the load bearing capacity as well as induce unwanted distortion depending on the boundary conditions specified in manufacturing.
The residual stress distribution in IN718 elongated prisms produced by SLM was studied non-destructively by means of neutron (bulk) and laboratory X-ray (surface) diffraction. The samples with different scanning strategies, i.e. hatching length, were measured in as-build condition (on a build plate) and after removal from the build plate.
The absolute values of all stress components decreased after removal from the build plate. Together with surface scan utilizing a coordinate-measuring machine (CMM), it is possible to link the stress release to the sample distortion. Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component depending on the thermal gradient in the respective direction.
Additively Manufactured (AM) parts are still far from being used in safety-relevant applications, mainly due to a lack of understanding of the feedstock-process-propertiesperformance relationship. This work aims at providing a characterization of the fatigue behavior of the additively manufactured AISI 316L austenitic stainless steel and a direct comparison with the fatigue performance of the wrought steel. A set of specimens has been produced by laser powder bed fusion (L-PBF) and a second set of specimens has been machined out of hot-rolled plates. The L-PBF material shows a higher fatigue limit and better finite life performance compared to the wrought material, accompanied by an extensive amount of cyclic softening.
Damage tolerance counts as one of the most widespread approach to fatigue assessment and surely as one of the most promising in understanding the process-structure-property-performance relationships in additively manufactured metallic materials. Manufacturing defects, surface roughness, microstructural features, short and long crack fatigue propagation, residual stresses and applied loads can be taken into consideration in a fracture mechanics-based fatigue assessment. Many aspects are crucial to the reliable component life prediction. Among those a prominent role is played by an accurate measurement and modelling of the short crack fatigue behavior, and reliable statistical characterization of defects and residual stresses.
This work aims at addressing the issues related to both experimental testing, fatigue and fatigue crack propagation, and fracture mechanics-based modelling of fatigue lives. Examples will be provided on an additively manufactured AISI 316 L.
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.
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.
Das zyklische plastische Verformungsverhalten von additiv gefertigtem Edelstahl AISI 316L wurde in dehnungsgeregelten Kurzzeitfestigkeitsversuchen (LCF-Versuchen) untersucht. Dabei wurden zwei Wärmebehandlungszustände betrachtet: Beim ersten Zustand war die fertigungsbedingte zelluläre Struktur vorhanden, während sie durch die zweite Wärmebehandlung aufgelöst wurde. Untersuchungen im Transmissions-Elektronen-Mikroskop (TEM) zeigten, dass die zyklische Verformung die Zellstruktur lokal zerstörte und Gleitbänder entstanden, was die Entfestigung in diesem Fall erklärt. Für Material ohne Zellstrukturen resultierten nach zyklischer Verformung ähnliche Versetzungsstrukturen wie in konventionell gefertigtem (warmgewalzten) Material.