9.6 Additive Fertigung metallischer Komponenten
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Paper des Monats
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A large explosion occurred at an oil refinery after a desulfurization reactor cracked, releasing hot, pressurized gasoline. The resulting explosions and fire caused extensive damage and injured some employees.
A 1.4 m crack adjacent to a weld seam at a reactor support bracket caused the release. The reactor shell exhibited distinctive circumferential bulging. The reactor was erected using mild steel. All material properties of the reactor shell complied with the regulations in effect at that time. Fractographic analysis of the main crack and smaller ones at the other support brackets revealed stepwise ductile fracture resulting from static loading. All well-known failure mechanisms for pressure vessel burst had proven wrong: overpressure, pressure cycles, overtemperature, creep, corrosion, external (cyclic) mechanical loads… Due to small spherical indentations on the inside of the reactor shell, a completely new failure mechanism was established and investigated:
During service, the reactor was partially filled with ceramic ball grading and catalyst. Gasoline was processed at 150–250 °C and ∼ 24 bar. Each of the reactor’s ∼ 20 operating cycles ran for several months until the catalyst was spent, after which the reactor was cooled, depressurized, emptied, and refilled. Upon heating, the thermal expansion of the steel shell (∼3 times greater than ceramic) created gaps that were filled as ceramic balls settled. During operation, the catalyst degraded and agglomerated with the ceramic balls, forming a rigid mass. Upon cooling, the rigid mass resisted the vessel’s thermal contraction, inducing circumferential tensile stresses and plastic deformation, resulting in permanent bulging of the vessel. Charpy impact energy near the welds was significantly reduced, attributed to thermal and strain aging in the bulged region. Progressive embrittlement and increasing plastic strain led to crack initiation and incremental ductile crack propagation at the support bracket welds over successive cycles. The fracture features were reproduced in laboratory tests at 200 °C. FEA analysis confirmed the proposed mechanism. Two identical vessels showed similar damage but had not yet failed. To prevent future damage in pressure vessels, this new failure mechanism needs to be incorporated into design, operating and inspection codes for pressure vessels possibly/partially filled with solids.
Inconel 718 (IN718) is the most commonly used nickel-based superalloy for high-temperature structural applications due to its remarkable strength, as well as its resistance to creep, fatigue, and corrosion up to 650 °C. This study investigated the room- and high-temperature (650 °C) tensile and low-cycle-fatigue (LCF) behavior of IN718 produced by laser powder bed fusion (PBF-LB/M). A bidirectional scanning strategy with 90° rotation after each layer and a four-step heat treatment was applied, and the results were compared to the conventional wrought material. The hierarchical microstructure after heat treatment was characterized on different length scales using microscopic methods.
Distinct microstructural characteristics generated during the PBF-LB/M process, such as grain size and morphology, and the periodically graded arrangement of stacked columnar grains interspersed with regions of elongated grains aligned with the build direction, are largely preserved following the applied heat treatment. Additionally, the heat treatment reduces microsegregation and dislocation density associated with the cellular structure, promoting a more uniform precipitation of γ’/γ’’ strengthening phases.
At both room and elevated temperatures, the elastic and yield properties of the PBF-LB/M material are comparable to those of the wrought variant. However, the additively manufactured material shows slightly reduced tensile strength, ductility, and strain hardening capability. As a result, it exhibits slightly lower inelastic strain under LCF conditions at both temperatures. While the fatigue life of the PBF-LB/M material is slightly lower than that of the wrought alloy at room temperature, it is vice versa at 650 °C. Both materials demonstrate cyclic softening behavior, which becomes more pronounced at the higher test temperature.
Crack propagation is primarily influenced by grain orientation, morphology, and the presence of δ phase at grain boundaries. Under LCF loading at both room and elevated temperatures, multiple crack initiation sites are observed on the surfaces of PBF-LB/M specimens. In microstructurally heterogeneous regions, pronounced crack branching and deflection occur, suggesting that crack paths are shaped by sharp micromechanical gradients and localized clusters of grains with ⟨001⟩ orientation, which are favored for crack growth. The tendency of the PBF-LB/M material to exhibit systematic crack branching and deflection results in irregular crack fronts and mixed-mode propagation behavior. This crack path complexity contributes to additional energy dissipation during fatigue loading. Consequently, despite the relatively large average grain size and a pronounced formation of slip bands, the fatigue life at room temperature approaches that of the wrought material.
Global decarbonization targets are driving expansion of green hydrogen infrastructure, yet high-pressure hydrogen exposure can degrade the mechanical performance of critical stainless-steel components, typically made of the common steel 316L. This study evaluates the strength and fracture behavior of both cold-drawn and heat-treated 316L austenitic stainless-steel tubular specimens fabricated from 1/2″ (12.7 mm) and 3/8″ (9.53 mm) tube after gaseous high-pressure pre-charging with wall thickness 0.5 mm. Hydrogen concentration in the range between 76 and 100 ppm was measured in precharged sample in both states and the recorded quantity of 2–5 ppm in the non-precharged specimen. Hydrostatic burst tests at ambient tem perature were carried out with precharged specimens and revealed an increase in burst pressure for both material states, as compared to non-precharged specimens as a reference. In addition, fractographic analysis by scanning electron microscopy has identified deeper dimples and wider microvoids in hydrogen-precharged specimens, indicating also some softening of the material in the cold-drawn as well as in the heat-treated state. As the time to failure also increased, it is anticipated that the hydrogen introduced in the specimens caused a respective increase in both, ductility in terms of the straining and deformation capacity, as well as an increase in strength. For engineering applications, the findings indicate some beneficial effect that, despite exposure to extreme hydrogen conditions, 316L tubes retain, and even modestly enhance, their structural integrity, supporting a safe deployment in green hydrogen transport and storage systems, at least at ambient temperature.
Influence of initial powder oxidation on mechanical properties of components fabricated via PBF-LB/M
(2026)
The reuse of feedstock in laser powder bed fusion of metals (PBF-LB/M) enhances sustainability and reduces production costs but is limited by progressive degradation of the material [1]. One contributor to powder degradation is oxidation, even in protective atmospheres [2]. This study investigates how the initial oxidation state of Haynes 282, a nickel-based alloy for high-temperature applications, influences its performance at both ambient and elevated temperatures (850 °C). For this purpose, four feedstock batches with oxygen contents ranging from approximately 140 ppm to 1400 ppm were produced by controlled oxidation of virgin powder in a laboratory furnace.
Microscopy analysis revealed that higher oxidation levels promoted irregular melt pool morphology and increased surface roughness, accompanied by a reduction in grain size. Electron probe microanalysis revealed that the oxide layers observed on the surface of the manufactured specimens in the as-build condition consisted primarily of aluminium and titanium oxides. Oxygen analysis via inert gas fusion on heat-treated specimens measured approximately 90 ppm in material from virgin feedstock and approximately 600 ppm in material from the most oxidized batch. Notwithstanding these discrepancies, the part porosity and the room-temperature tensile properties, including ultimate strength and elongation at break, remained largely unaltered. Conversely, creep testing at elevated temperatures under application-relevant conditions showed a pronounced decline in creep resistance and a corresponding increase in creep strain with rising oxygen content. However, both the tensile strength and the deformation mechanisms observed in high-temperature tensile tests remained comparable across all batches. These findings indicate that while the use of oxidized powder is viable for less demanding service conditions, strict powder quality control is imperative when manufacturing components for high-temperature applications.
Additive manufacturing (AM) has seen rapid growth in recent decades, with Laser Powder Bed Fusion (PBF-LB/M) emerging as the leading technique for producing high-density, geometrically complex metal parts. Austenitic stainless steel 316L is one of the most studied alloys for PBF-LB/M due to its excellent strength, ductility, and corrosion resistance [1]. The microstructure formed during PBF-LB/M processing can improve certain mechanical properties compared to conventionally manufactured 316L [2].
However, the current 316L standards allow broad ranges for key alloying elements, particularly Cr and Ni. While such variations have little effect on conventionally produced alloys, emerging evidence shows that they can markedly alter the microstructure and mechanical properties in PBF-LB/316L—even within specification limits [3]. This study investigates through microstructural and thermodynamical assessment, how two powders of nominally standard 316L composition (Alloys A and B) respond to identical PBF-LB/M processing parameters. Despite identical printing conditions, Alloy A exhibited
twice the grain size and five times higher low-angle grain boundary (LAGB) density compared to Alloy B . Conversely, Alloy B showed a significantly higher density of Σ3 twin boundaries, nearly absent in Alloy A. These microstructural differences are attributed primarily to variations in Cr and Ni content in the liquid, which may influence icosahedral short-range ordering (ISRO) mechanism [4]. ISRO potentially facilitates twin boundary formation, ultimately refining grain structure [5]. This work highlights the critical impact of compositional control on final part microstructures and consequent mechanical properties and emphasizes the need to reassess compositional tolerances for AM-specific applications.
Im Rahmen des AM Forums Berlin 2026 wird durch diesen Vortrag ein Einblick in FuE-Aktivitäten der BAM auf dem Gebiet der additiven Fertigung für Hochtemperaturanwendungen gegeben. Es werden dabei insbesondere Aspekte der nachhaltigen Verwendung von Pulverausgangswerkstoff und der Einfluss von Sauerstoff im Pulverwerkstoff auf die Degradation resultierender mechanischer Eigenschaften von Probekörpern beleuchtet. Die präsentierten Arbeiten sind im Rahmen des Pro FIT Projektes HTA2.0 entstanden.
Laser powder bed fusion of metals (PBF-LB/M) offers great potential for the production of new and spare parts for stationary gas turbines made of nickel superalloys such as Inconel 939 (IN939). In order to enable integration into existing assemblies and overcome design limitations, the additive manufacturing process chain must be expanded by suitable joining techniques. This study compares the electron beam welding of cast IN939 sheets and sheets produced additively using PBF-LB/M. The investigation focuses on the achievable seam quality with regard to geometric irregularities and internal defects in the form of liquation cracks on the microscale in the heat-affected zone. The evaluation of the welded samples shows no differences in the formation of the seam shape between the additively manufactured material and the cast material. For both materials, the highest quality category for beam-welded seams according to DIN EN ISO 13,919–1 was achieved at high welding speeds of 20 mm/s. Regardless of the manufacturing method, both materials show an increase in crack formation with increasing welding speed. However, due to its microstructure, the PBF-LB/M material exhibits significantly fewer microcracks overall. Final crack propagation tests on welded PBF-LB/M samples that were treated using HIP also show stable crack growth without sudden failure, which opens up potential for practical application.
Die Zielstellung des laufenden Kooperationsprojektes MCGUSS zwischen der BAM Berlin und der MPA Stuttgart ist es, das probabilistische Master Curve (MC)-Konzept hinsichtlich seiner Übertragbarkeit auf ferritisches Gusseisen mit Kugelgraphit (DCI) zu untersuchen. Über Teilergebnisse der Arbeiten wurde bereits auf den Jahrestagungen 2023 bis 2025 des DVM-AK Bruchmechanik und Bauteilsicherheit berichtet. Der vorliegende Beitrag hat zwei Schwerpunkte.
Zum einen wurde die Datenbasis der dynamischen Bruchzähigkeit für SE(B)-Proben der Dicken 10 mm, 25 mm und 140 mm auf insgesamt 101 Versuche bei Temperaturen von -40 °C bis -140 °C erweitert, statistisch ausgewertet sowie MC-Analysen unterzogen. Die Ergebnisse zeigen, dass die Daten sich zwar mit einer Weibullverteilung gut beschreiben lassen, die MC-Methode nach ASTM E1921 jedoch nicht ohne Weiteres anwendbar ist, sondern Modifikationen erforderlich sind.
Des Weiteren werden neue Ergebnisse zum werkstoffspezifischen Bruchmechanismus bei dynamischer Belastung im unteren Übergangsbereich der Zähigkeit diskutiert. Die fraktographische Datenbank wurde durch REM-Analysen weiterer Bruchflächen von SE(B)25-Proben bei -40 °C, -60 °C und -80 °C ergänzt. Die Ergebnisse unterstreichen die prägende Rolle, die Rissarrestereignissen im Verlauf des dynamischen Sprödbruchversagens bei DCI zukommt. Der in MCGUSS zur Erklärung eines gegenüber Stahl inversen Probengrößeneinflusses vorgeschlagene Mechanismus „Specimen size-dependent arrest of local brittle fractures before global brittle failure by weakest link“ wird dadurch weiter unterstützt.
Die Zielstellung des laufenden Kooperationsprojektes MCGUSS zwischen der BAM Berlin und der MPA Stuttgart ist es, das probabilistische Master Curve (MC)-Konzept hinsichtlich seiner Übertragbarkeit auf ferritisches Gusseisen mit Kugelgraphit (DCI) zu untersuchen. Über Teilergebnisse der Arbeiten wurde bereits auf den Jahrestagungen 2023 bis 2025 des DVM-AK Bruchmechanik und Bauteilsicherheit berichtet. Der vorliegende Beitrag hat zwei Schwerpunkte.
Zum einen wurde die Datenbasis der dynamischen Bruchzähigkeit für SE(B)-Proben der Dicken 10 mm, 25 mm und 140 mm auf insgesamt 101 Versuche bei Temperaturen von -40 °C bis -140 °C erweitert, statistisch ausgewertet sowie MC-Analysen unterzogen. Die Ergebnisse zeigen, dass die Daten sich zwar mit einer Weibullverteilung gut beschreiben lassen, die MC-Methode nach ASTM E1921 jedoch nicht ohne Weiteres anwendbar ist, sondern Modifikationen erforderlich sind.
Des Weiteren werden neue Ergebnisse zum werkstoffspezifischen Bruchmechanismus bei dynamischer Belastung im unteren Übergangsbereich der Zähigkeit diskutiert. Die fraktographische Datenbank wurde durch REM-Analysen weiterer Bruchflächen von SE(B)25-Proben bei -40 °C, -60 °C und -80 °C ergänzt. Die Ergebnisse unterstreichen die prägende Rolle, die Rissarrestereignissen im Verlauf des dynamischen Sprödbruchversagens bei DCI zukommt. Der in MCGUSS zur Erklärung eines gegenüber Stahl inversen Probengrößeneinflusses vorgeschlagene Mechanismus „Specimen size-dependent arrest of local brittle fractures before global brittle failure by weakest link“ wird dadurch weiter unterstützt.
The use of laser-additively manufactured metallic components in the industrial sector is gaining traction, with the process now entering its application phase. Consequently, the materials are exposed to corrosive microbes, which this study examines in order to determine the implications of this exposure. Specifically, the study examines the exposure of laser powder bed fused AlSi10Mg (PBF-LB/AlSi10Mg) components to sulphate-reducing bacteria in an anaerobic environment. Is the specific PBF-LB microstructure a potential factor in determining or aiding biocorrosion?
A prismatic specimen was exposed to sulphate-reducing bacteria (SRB) for two weeks. The sample was scanned by X-ray computed tomography (XCT) before and after SRB exposure. The two data sets were registered to each other (using the internal void distribution as markers, due to the corrosion of the specimen’s surface) to enable the evaluation of the corrosion damage.
Virtual cuts of the 3D XCT reconstructions show a loss of volume and localised damage. The damage distribution aligns with the hatching pattern of laser exposure from laser powder bed fusion.
