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Localized laser surface treatments are able to produce tailor-made surface properties to fulfill requirements of a variety of technical applications. Especially micrometric surface topologies can be beneficial for optimizing tribological contact situations. Structures with lowered surface features are already utilized for bearings or cylinders of combustion engines. There are also other fields of application, where the potential of protruding surface features is known, e. g. for metal forming tools.
A promising approach for a tailored surface treatment working in the microsecond range is the localized dispersing of hard ceramic particles by pulsed laser radiation, the so-called laser implantation. This technique is able to produce micrometric surface structures and to improve simultaneously the wear resistance by creating metal matrix composites.
In this talk, the laser implantation technique is described and compared to the state of the art. The potential to adjust the geometry as well as the mechanical properties of laser implanted surfaces is demonstrated by means of microstructural and topographical investigations of different ceramic materials and steel substrates. Finally, results of research projects are presented aiming on the application of such structured surfaces. Their capability to change and optimize friction and wear are demonstrated for fully lubricated contacts, tools for hot sheet metal forming and tools for cold rolling of sheets for automotive applications.
The laser implantation–named technique aims to address the tribological problems frequently seen on tool surfaces during hot stamping. It is based on the creation of elevated dome- or ring-shaped hard structures on the surface of tool steels by a localized dispersing of hard particles. Therefore, a combination of the two distinct approaches that are normally used in surface Technology for optimizing friction and wear, i.e., surface texturing and surface material optimization, are realized in one processing step. In experimental studies, a localized dispersing of TiB2 particles in the surface layer of the hot work tool steel X38CrMoV5-3 was considered and compared with punctual laser–remelted textures. The structures (micro-) hardness was measured at top- and cross-sections. With the aid of a scanning electron microscope, energy dispersive X-ray spectroscopy and X-ray diffraction the interaction between the hard particles and the substrate material were studied. From the results, an optimal parameter range was identified for laser implantation. To the investigation’s end, the implant geometry was measured by optical microscopy and White light microscopy. Furthermore, a mathematic model was introduced, which allows a prediction of the implant geometry as a response to the laser parameters. It was shown that the implantation of TiB2 particles leads to a significant hardness increase up to 1600 HV1 due to the dispersion of initial particles and an in situ precipitation of new titanium-rich phases. It was possible to create defect-free dome- and ring-shaped microstructures on the surfaces. It was also shown that the implants geometry highly depends on the applied laser parameters. The applied central composite design shows a good agreement with the experimental results.
Das Presshärteverfahren hat sich zur ressourceneffizienten Verarbeitung von höchstfesten Stahlwerkstoffen im Karosserieleichtbau weltweit etabliert. Die fehlerfreie Bauteilproduktion wird jedoch durch Reibungs- und Verschleißerscheinungen aufgrund hoher thermo-mechanischer Werkzeugbelastungen und fehlender Schmierstoffsysteme limitiert. Als Lösungsansatz wird eine Modifikation der Werkzeugoberfläche mittels Laserimplantation angestrebt, um folglich deren Verschleißbeständigkeit nachhaltig zu erhöhen. Das Verfahren basiert auf einem lokalen Dispergieren keramischer Hartstoffpartikel in die Werkzeugoberfläche, infolgedessen hochfeste und erhabene Strukturen im Mikrometerbereich entstehen. Aufgrund der signifikanten Reduzierung der Kontaktfläche sowie der hohen Verschleißbeständigkeit der eingesetzten TiB2-Hartstoffe wird ein verbessertes tribologisches Einsatzverhalten unter presshärtetypischen Prozessbedingungen erwartet. Zur Verifizierung dieser Annahmen wurden im Rahmen dieser Arbeit modifizierte Pin-on-Disk Tests durchgeführt, um das Reib- und Verschleißverhalten der laserimplantierten Werkzeugoberflächen unter Presshärtebedingungen zu untersuchen. Zur weiteren Vertiefung des Prozess-verständnisses wurden die verschlissenen Platinen via Tastschnittmessungen und Querschliffaufnahmen analysiert. Durch den Ergebnisvergleich mit konventionellen Werkzeugoberflächen erfolgte eine abschließende Bewertung des tribologischen Einsatzverhaltens der laserimplantierten Strukturen.
Laser powder bed fusion (L-PBF) is the most prominent additive manufacturing (AM) technology for metal part production. Among the high number of factors influencing part quality and mechanical properties, the inter layer time (ILT) between iterative melting of volume elements in subsequent layers is almost completely unappreciated in the relevant literature on L-PBF. This study investigates the effect of ILT with respect to build height and under distinct levels of volumetric energy density (VED) using the example of 316L stainless steel. In-situ thermography is used to gather information on cooling conditions during the process, which is followed by an extensive metallographic analysis. Significant effects of ILT and build height on heat accumulation, sub-grain sizes, melt pool geometries and hardness are presented. Furthermore, the rise of defect densities can be attributed to a mutual interplay of build height and ILT. Hence, ILT has been identified as a crucial factor for L-PBF of real part components especially for those with small cross sections.
AlSi10Mg is one of the most applied alloys for laser powder bed fusion (LPBF) technology, due to its great possibilities for implementing new lightweight concepts such as in automotive industries. For the component design it is necessary to know about the mechanical properties and the mechanical behaviour. The many published strength properties of LPBF processed AlSi10Mg show significant differences up to approximately 225 MPa in ultimate tensile strength (UTS) and 195 MPa in yield strength (YS). To understand these varying properties, a ring trial was carried out manufacturing specimens on 6 LPBF machines with different parameters and build-up strategies. They were studied in the as-built (AB) condition and after heat treatment at 300 °C for 30 min, respectively. For examining the mechanical properties, tensile tests and hardness measurements were carried out. The microstructure was characterized by optical light microscopy (OM), field emission scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). The identified differences in strength properties were discussed based on the 4 strengthening mechanism known for metallic materials and at the background of material defects. It was found that the size of the typical sub-cell structure of LPBF AlSi10Mg affected substantially the mechanical properties in the AB condition, in which with decreasing sub-cell size strength increased. If heat treatment was applied, the strength properties decreased and did not differ anymore. Since annealing led to coarsened sub-cells, whereas the grains itself did not change in size, the influence of sub-cell structure on strength was further confirmed. In addition, acicular precipitates in the AB condition were observed at specimens from one LPBF machine showing the lowest tensile elongation.
Among additive manufacturing (AM) technologies, the laser powder bed fusion (L-PBF) is one of the most important technologies to produce metallic components. The layer-wise build-up of components and the complex process conditions increase the probability of the occurrence of defects. However, due to the iterative nature of its manufacturing process and in contrast to conventional manufacturing technologies such as casting, L-PBF offers unique opportunities for in-situ monitoring. In this study, two cameras were successfully tested simultaneously as a machine manufacturer independent process monitoring setup: a high-frequency infrared camera and a camera for long time exposure, working in the visible and infrared spectrum and equipped with a near infrared filter. An AISI 316L stainless steel specimen with integrated artificial defects has been monitored during the build. The acquired camera data was compared to data obtained by computed tomography. A promising and easy to use examination method for data analysis was developed and correlations between measured signals and defects were identified. Moreover, sources of possible data misinterpretation were specified. Lastly, attempts for automatic data analysis by data Integration are presented.
This article is an outcome of a workshop on Fatigue of Additive Manufactured Metallic Components jointly organized by the Federal Institute for Materials Research and Testing (BAM) Berlin, Germany and the National Institute of Standards and Technology (NIST) Boulder, CO, U.S.A. The aim of the workshop was a comprehensive discussion of the specific aspects of additively manufactured (AM) components in regard to failure under cyclic loading. Undoubtedly, a better understanding and the further development of approaches for damage tolerant component design of AM parts are among the most significant challenges currently facing the use of these new technologies.
This article presents a thorough overview of the workshop discussions. It aims to provide a review of the parameters affecting the damage tolerance of AM parts with special emphasis on the process parameters intrinsic to the AM technologies, the resulting defects and residual stresses. Based on these aspects, concepts for damage tolerant component design for AM are reviewed and critically discussed.
Thermography is one on the most promising techniques for in-situ monitoring of metal additive manufacturing processes. Especially in laser powder bed fusion processes, the high process dynamics and the strong focus of the laser beam cause a very complex thermal history within the produced specimens, such as multiple heating cycles within single layer expositions. This complicates data interpretation, e.g., in terms of cooling rates. A quantity that is easily calculated is the time a specific area of the specimen is at a temperature above a chosen threshold value (TOT). Here, we discuss variations occurring in time-over-threshold-maps during manufacturing of an almost defect free cuboid specimen.
Unterschiedliche Werkstoffeigenschaften reagieren auf jeweils spezifische Weise auf Gittertyp, Gefüge und Defekte. Für die schadenstolerante Betrachtung von Werkstoff und Bauteil ist das Verständnis dieser Zusammenhänge essentiell. Der Beitrag gibt mit Hinblick auf additiv gefertigte metallische Bauteile mittels Selective Laser Melting einen kurzen, keineswegs vollständigen Überblick über Faktoren, die die Steifigkeit, Festigkeit, Duktilität, Zähigkeit, Ermüdungsrissausbreitung und Schwingfestigkeit beeinflussen. Es wird aufgezeigt, wie die bruchmechanische Betrachtung zur Quantifizierung der Zusammenhänge beitragen kann.
The laser implantation technique bases on a localized dispersing of ceramic particles in metallic surface layers for changing their micro geometry as well as their material properties. Using TiB2 and NbC hard particles, tailored microtextures consisting of elevated dome-, ring-, or line-shaped Features can be manufactured. Especially TiB2 implants feature high hardness values up to 1800 HV1 and ceramic particle contents up to 55 %, indicating superior wear resistant properties.