TY - JOUR A1 - Mohr, Gunther A1 - Sommer, Konstantin A1 - Knobloch, Tim A1 - Altenburg, Simon A1 - Recknagel, Sebastian A1 - Bettge, Dirk A1 - Hilgenberg, Kai T1 - Process Induced Preheating in Laser Powder Bed Fusion Monitored by Thermography and Its Influence on the Microstructure of 316L Stainless Steel Parts JF - Metals N2 - Undetected and undesired microstructural variations in components produced by laser powder bed fusion are a major challenge, especially for safety-critical components. In this study, an in-depth analysis of the microstructural features of 316L specimens produced by laser powder bed fusion at different levels of volumetric energy density and different levels of inter layer time is reported. The study has been conducted on specimens with an application relevant build height (>100 mm). Furthermore, the evolution of the intrinsic preheating temperature during the build-up of specimens was monitored using a thermographic in-situ monitoring set-up. By applying recently determined emissivity values of 316L powder layers, real temperatures could be quantified. Heat accumulation led to preheating temperatures of up to about 600 °C. Significant differences in the preheating temperatures were discussed with respect to the individual process parameter combinations, including the build height. A strong effect of the inter layer time on the heat accumulation was observed. A shorter inter layer time resulted in an increase of the preheating temperature by more than a factor of 2 in the upper part of the specimens compared to longer inter layer times. This, in turn, resulted in heterogeneity of the microstructure and differences in material properties within individual specimens. The resulting differences in the microstructure were analyzed using electron back scatter diffraction and scanning electron microscopy. Results from chemical analysis as well as electron back scatter diffraction measurements indicated stable conditions in terms of chemical alloy composition and austenite phase content for the used set of parameter combinations. However, an increase of the average grain size by more than a factor of 2.5 could be revealed within individual specimens. Additionally, differences in feature size of the solidification cellular substructure were examined and a trend of increasing cell sizes was observed. This trend was attributed to differences in solidification rate and thermal gradients induced by differences in scanning velocity and preheating temperature. A change of the thermal history due to intrinsic preheating could be identified as the main cause of this heterogeneity. It was induced by critical combinations of the energy input and differences in heat transfer conditions by variations of the inter layer time. The microstructural variations were directly correlated to differences in hardness. KW - Additive manufacturing KW - Laser powder bed fusion KW - In-situ process monitoring KW - Thermography KW - Heat accumulation KW - Inter layer time KW - Cellular substructure PY - 2021 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-529240 DO - https://doi.org/10.3390/met11071063 VL - 11 IS - 7 SP - 1063 PB - MDPI CY - Basel, Schweiz AN - OPUS4-52924 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Schneider, M. A1 - Bettge, Dirk A1 - Hilgenberg, Kai A1 - Binder, M. A1 - Dollmeier, K. A1 - Dreyer, M. A1 - Klöden, B. A1 - Schlingmann, T. A1 - Schmidt, J. T1 - Reproducibility and Scattering in Additive Manufacturing: Results from a Round Robin on PBF-LB/M AlSi10Mg Alloy T1 - Reproduzierbarkeit und Streuung bei der additiven Fertigung: Ergebnisse eines Ringversuchs mit einer PBF-LB/M AlSi10Mg-Legierung JF - Practical Metallography N2 - The round robin test investigated the reliability users can expect for AlSi10Mg additive manufactured specimens by laser powder bed fusion through examining powder quality, process parameter, microstructure defects, strength and fatigue. Besides for one outlier, expected static material properties could be found. Optical microstructure inspection was beneficial to determine true porosity and porosity types to explain the occurring scatter in properties. Fractographic analyses reveal that the fatigue crack propagation starts at the rough as-built surface for all specimens. Statistical analysis of the scatter in fatigue using statistical derived safety factors concludes that at a stress of 36.87 MPa the fatigue limit of 107 cycles could be reached for all specimen with a survival probability of 99.999 %. N2 - Im Rahmen eines Ringversuchs wurde durch die Untersuchung der Pulverqualität, der Prozessparameter, der Gefügefehler, der Festigkeit und der Ermüdung die Zuverlässigkeit bestimmt, die Nutzer von AlSi10Mg-Proben erwarten können, die mit pulverbettbasiertes Schmelzen mittels Laser (engl. Laser Powder Bed Fusion) gefertigt worden sind. Abgesehen von einem Ausreißer wurden die erwarteten statischen Materialeigenschaften erreicht. Eine optische Gefügeprüfung diente dazu, die tatsächliche Porosität und Arten von Porosität zu ermitteln, um die bei den Eigenschaften auftretende Streuung zu erklären. Fraktographische Unterschungen zeigen eine bei allen Proben von der rauen Oberfläche im As-built-Zustand ausgehende Ermüdungsrissausbreitung. Aus der statistischen Analyse der Streuung bezüglich der Ermüdung unter Anwendung von statistischen abgeleiteten Sicherheitsfaktoren geht hervor, dass alle Proben die Dauerfestigkeit von 107 Zyklen bei einer Spannung von 36,87 MPa mit einer Überlebenswahrscheinlichkeit von 99,999 % erreichten. KW - Additive manufacturing KW - Reproducibility KW - Reliability PY - 2022 DO - https://doi.org/10.1515/pm-2022-1018 SN - 2195-8599 VL - 59 IS - 10 SP - 580 EP - 614 PB - De Gruyter AN - OPUS4-55935 LA - mul AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Sommer, Konstantin A1 - Agudo Jácome, Leonardo A1 - Hesse, René A1 - Bettge, Dirk T1 - Revealing the nature of melt pool boundaries in additively manufactured stainless steel by nano-sized modulation JF - Advanced engineering materials N2 - In the current study, the 3D nature of the melt pool boundaries (MPBs) in a 316 L austenitic steel additively manufactured by laser-based powder bed fusion (L-PBF) is investigated. The change of the cell growth direction and its relationship to the MPBs is investigated by transmission electron microscopy. A hitherto unreported modulated substructure with a periodicity of 21 nm is further discovered within the cell cores of the cellular substructure, which results from a partial transformation of the austenite, which is induced by a Ga+ focused ion beam. While the cell cores show the modulated substructure, cell boundaries do not. The diffraction pattern of the modulated substructure is exploited to show a thickness ≥200 nm for the MPB. At MPBs, the cell walls are suppressed, leading to continuously connecting cell cores across the MPB. This continuous MPB is described either as overlapping regions of cells of different growing directions when a new melt pool solidifies or as a narrow planar growth preceding the new melt pool. KW - Additive manufacturing KW - Austenitic steel 316L KW - Melt pool boundary KW - Microstructural characterization KW - Transmission electron microscopy PY - 2022 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-547295 DO - https://doi.org/10.1002/adem.202101699 SN - 1527-2648 VL - 24 IS - 6 SP - 1 EP - 11 PB - Wiley-VCH CY - Weinheim AN - OPUS4-54729 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -