Development and Validation of a Calculation Routine for the Precise Determination of Pulse Overlap and Accumulated Fluence in Pulsed Laser Surface Treatment

In laser material processing, a variety of parameters like pulse fluence, total dose, step size, and pulse-to-pulse overlap are used to define and compare laser processes. Of these parameters, the pulse-to-pulse overlap can be the hardest to access as it is not implemented directly but instead depends on the spot diameter, its shape, and the respective scanning path that is used to cover the surface. This article shows that existing calculation routes overestimate the actual overlap by up to 21%. A novel calculation route is developed that greatly facilitates the determination of the pulse overlap and thereby the average number of laser pulses that interact with a given point on the surface. This approach makes it possible to achieve more reliable and comparable laser processes, which in return leads to better control of the procedure as the effect of individual parameters on a given output can be determined with greater precision

Neue Möglichkeiten der objektiven Graphitklassifizierung in Gusseisen durch Nano-Tomographie und internetbasierte Online-Verfahren

Gusseisen spielt eine wichtige Rolle als Konstruktionswerkstoff auch bei Hochleistungsanwendungen,wenn man in der Lage ist, die Ausbildung der Graphitmorphologie zu beherrschen. Dabei unterscheidet man nach DIN EN ISO 945 sechs Typen von Graphitausbildungen die in Gusseisen vorkommen. Diese unterscheiden sich nicht nur in der Form und Anordnung der jeweiligen Graphiteinlagerungen voneinander, sie differieren auch grundlegend bezüglich ihrer mechanischen und physikalischen Eigenschaften. Da die traditionelle Begutachtung der Graphitmorphologie nach Richtreihenbildern nur qualitativ ist und einen breiten subjektiven Interpretationsspielraum zulässt, erfordern Entwicklung und Einsatz von "maßgeschneiderten" Gefügen im Gusseisen auch objektive und quantifizierbare automatische Klassifikationsverfahren. Eine 2D-Formanalyse der Graphiteinschlüsse über die konventionellen Verfahren der quantitativen Gefügeanalyse ermöglicht die Auswahl der gefügecharakteristischen Parameter für die reproduzierbare Graphitklassifizierung. Mit Hilfe von 3D-Untersuchungen der realen Graphitmorphologie mittels FIB-Nanotomographie und der Simulation möglicher 2D-Schnitte konnten erstmals die objektiven Grenzen der bildanalytischen 2D-Klassifizierung beurteilt werden. Auf dieser Basis konnten auch diejenigen bildanalytischen Messgrößen ermittelt werden, die eine Klassifizierung der Graphitmorphologie optimieren können. Auf dieser Basis wurde ein internetbasiertes Klassifizierungsverfahren entwickelt (www.materialography.net), das mit Hilfe des Stützvektorverfahrens diese Vielzahl bildanalytischer Messgrößen gleichzeitig in die Klassifizierung einbezieht und eine nachprüfbar hohe Klassifikationsgüte der komplexen Graphitteilchen in Gusseisen durch Angabe der Wahrscheinlichkeiten für die jeweilige Zuordnung gewährleistet

Shape analysis and classification of irregular graphite morphology in cast iron

Die Unterscheidung der verschiedenen Graphitmorphologien in Gusseisen erfolgte bisher im Wesentlichen qualitativ durch subjektiven Vergleich mit Richtreihenbildern. In Hochleistungsanwendungen im Motorenbau werden nunmehr objektive quantitative Messungen der jeweiligen Graphitanteile benötigt, damit die mechanischen Eigenschaften reproduzierbar erreicht werden können. Vorgestellt werden bildanalytische Untersuchungen zur automatischen Klassifizierung von Lamellen-, Vermikular- und Kugelgraphitformen, sowie Graphitformen in Temperguss. Dabei wurden insbesondere teilchenbasierte Meßmethoden angewendet. Unterschiedliche Formparameter wurden in dieser Arbeit zusammengefasst, und ihre Relevanz für die Charakterisierung und Klassifizierung irregulärer, komplexer Graphitteilchen in Gusseisen wurde bestimmt. Es zeigte sich, dass die Kombination der Formparameter Rundheit und Kompaktheit die präziseste Klassifizierung liefert. Ein Klassifizierungsalgorithmus wurde mit Berücksichtigung der Abhängigkeit der Formparameter von der Teilchengröße entwickelt. Die traditionelle subjektive Analyse der Gusseisengefüge anhand EN ISO 945 kann durch die Anwendung der neu vorgeschlagenen Methode mit einer automatischen reproduzierbaren Graphitformcharakterisierung und Klassifizierung deutlich verbessert werden

Carbon Nanotube (CNT)-Reinforced Metal Matrix Bulk Composites: Manufacturing and Evaluation

This chapter deals with the blending and processing methods of CNT-reinforced metal matrix bulk composites (Al/CNT, Cu/CNT and Ni/CNT) in terms of solid-state processing, referring mainly to the research works of the last ten years in this research field. The main methods are depicted in a brief way, and the pros and cons of each method are discussed. Furthermore, a tabular summary of the research work of the mentioned three systems is given, including the blending methods, sintering methods, the used amount of CNTs and the finally achieved relative density of the composite. Finally, a brief discussion of each system is attached, which deals with the distribution and interaction of the CNTs with the matrix material

Objective homogeneity quantification of a periodic surface using the Gini coefficient

The significance of periodic surface structuring methods, such as direct laser interference patterning, is growing steadily. Thus, the ability to objectively and consistently evaluate these surfaces is increasingly important. Standard parameters such as surface roughness or the arithmetic average height are meant to quantify the deviation of a real surface from an ideally flat one. Periodically patterned surfaces, however, are an intentional deviation from that ideal. Therefore, their surface profile has to be separated into a periodic and a non-periodic part. The latter can then be analyzed using the established surface parameters and the periodic nature allows a quantification of structure homogeneity, e.g. based on Gini coefficient. This work presents a new combination of established methods to reliably and objectively evaluate periodic surface quality. For this purpose, the periodicity of a given surface is extracted by Fourier analysis, and its homogeneity with respect to a particular property is determined for the repeating element via a Gini analysis. The proposed method provides an objective and reliable instrument for evaluating the surface quality for the selected attribute regardless of the user. Additionally, this technique can potentially be used to both identify a suitable surface structuring technique and determine the optimal process parameters

Electrical Characterization of Carbon Nanotube Reinforced Silver and Copper Composites for Switching Contacts

Carbon nanotube (CNT)-reinforced silver and copper metal matrix composites—at three different reinforcement phase concentrations (0.5 wt.%, 0.75 wt.%, and 1 wt.%)—were produced via powder metallurgy and sintered via hot uniaxial pressing. Optical and electron microscopy techniques were used to characterize the powder mixtures and sintered composites. The latter were also electrically characterized via load-dependent electrical contact resistance (ECR) and surface fatigue tests. Particle size and morphology play a crucial role in CNT deposition onto the metallic powder. CNT were deposited exceptionally well onto the dendritic copper powder regardless of its larger size (compared with the silver flakes) due to the higher surface area caused by the grooves and edges of the dendritic structures. The addition of CNT to the metallic matrices improved their electrical performance, in general outperforming the reference material. Higher CNT concentrations produced consistently low ECR values. In addition, high CNT concentrations (i.e., 1 wt.%) show exceptional contact repeatability due to the elastic restitutive properties of the CNT. The reproducibility of the contact surface was further evaluated by the fatigue tests, where the composites also showed lower ECR than the reference material, rapidly reaching steady-state ECR within the 20 fatigue cycles analyzed

Wear Reduction via CNT Coatings in Electrical Contacts Subjected to Fretting

Carbon nanotubes (CNT) are of great interest to the research community due to their outstanding mechanical, transport, and optical properties. These nanoparticles have also shown exceptional lubricating capabilities, which coupled with their electrical conductivity show promising results as solid lubricants in electrical contacts. In this study, three diferent CNT coatings were deposited over copper platelets via electrophoretic deposition and subsequently tribo-electrically characterized including electrical contact resistance evolution during fretting wear, wear protection, chemical analysis of fretting marks, as well as infuence of CNT coating thickness, duration and normal load applied during fretting, and atmospheric humidity. Thicker CNT coatings show improved wear protection while retaining similar electrical behavior as uncoated copper, or even improving its electrical contact resistance. Moreover, the compaction of the porous CNT coating is crucial for optimal electrical performance at low humidity. For longer fretting tests (150,000 and 500,000 cycles), the coatings are displaced thus afecting the wear protection ofered. However, the coatings stabilize and reduce ECR compared to uncoated samples. Furthermore, thicker CNT coatings can bear higher loads during fretting due to the increased lubricant reservoir, with carbonaceous triboflm remaining at the contacting interface after 5,000 fretting cycles regardless of normal load

Parametric analysis of the coating thickness development of electrophoretically deposited carbon nanotube coatings

In this study, the coating thickness evolution of pristine and oxidized carbon nanotubes (CNT) on stainless steel substrates is investigated. Potentiostatic electrophoretic deposition (EPD) is used as a coating technique with two different additives, triethylamine (TEA) and magnesium nitrate hexahydrate (Mg-Nit). Moreover, the depositions are conducted at different voltages (50, 100 and 150 V). Confocal laser scanning microscopy is used to determine the thickness of the CNT depositions after 1, 2, 5, 10, 20 and 30 min. Furthermore, the ability of Hamaker’s law to accurately predict coating thickness development is investigated for the thickness evolution on stainless steel. Independent of the additive, the results show that higher voltages lead to increased deposition rates. Comparing the two additives, Mg-Nit generally allows for a higher CNT deposition rate than TEA and forms thicker layers. Coating thickness development can be approximated as linear during the initial 5 min with Mg-Nit and during the initial 20 min with TEA. Finally, Hamaker’s law allows for a fairly accurate approximation for the thickness development of CNT coatings with TEA on stainless steel

Time-Dependant Microstructural Evolution and Tribological Behaviour of a 26 wt% Cr White Cast Iron Subjected to a Destabilization Heat Treatment

By employing destabilization heat treatments (HT), it is possible to create microstructures possessing diferent fractions of carbides, martensite, and austenite, which lead to varying tribological responses in abrasion-resistant high-chromium white cast irons. In the current work, the destabilization temperature was kept constant at 980 °C, whereas the time was varied from 0 to 90 min. As a result, the microstructure of the 26 wt% Cr white cast iron had a mixture of M23C6 secondary carbides (SC), martensite, and a decrease in the amount of retained austenite (RA) with increasing destabilization holding time. The microstructures as well as their tribological characteristics were evaluated by combining confocal laser scanning microscopy, SEM, XRD, and EBSD, together with dry-sliding linear reciprocating wear tests. Results show that the volume fraction of SC were statistically comparable in samples destabilized for 0 and 90 min, although the average size was almost two-fold in the latter. This had direct implications on the wear properties where a decrease of up to 50% in the wear rate of destabilized samples compared to the non-treated material was observed. Furthermore, the sample with the lowest increase in the matrix hardness (~20% higher than non-treated), showed the highest wear resistance. This was attributed to a favourable distribution of the RA (~10%) and SC volume fraction (~5%), in combination with the harder martensitic matrix. Finally, the results obtained from this study shed light on the ability to alter the HT parameters to tune the microstructure depending upon the application prerequisite

Precipitate number density determination in microalloyed steels by complementary atom probe tomography and matrix dissolution

Particle number densities are a crucial parameter in the microstructure engineering of microalloyed steels. We introduce a new method to determine nanoscale precipitate number densities of macroscopic samples that is based on the matrix dissolution technique (MDT) and combine it with atom probe tomography (APT). APT counts precipitates in microscopic samples of niobium and niobium-titanium microalloyed steels. The new method uses MDT combined with analytical ultracentrifugation (AUC) of extracted precipitates, inductively coupled plasma–optical emission spectrometry, and APT. We compare the precipitate number density ranges from APT of 137.81 to 193.56 × 1021 m−3 for the niobium steel and 104.90 to 129.62 × 1021 m−3 for the niobium-titanium steel to the values from MDT of 2.08 × 1021 m−3 and 2.48 × 1021 m−3. We find that systematic errors due to undesired particle loss during extraction and statistical uncertainties due to the small APT volumes explain the differences. The size ranges of precipitates that can be detected via APT and AUC are investigated by comparison of the obtained precipitate size distributions with transmission electron microscopy analyses of carbon extraction replicas. The methods provide overlapping resulting ranges. MDT probes very large numbers of small particles but is limited by errors due to particle etching, while APT can detect particles with diameters below 10 nm but is limited by small-number statistics. The combination of APT and MDT provides comprehensive data which allows for an improved understanding of the interrelation between thermo-mechanical controlled processing parameters, precipitate number densities, and resulting mechanical-technological material properties. Graphical abstract: [Figure not available: see fulltext.