Kornfeinung in Aluminium-WIG-Schweißnähten

Kornfeinung ist ein wichtiger Ansatz, um die mechanischen Eigenschaften von Schmelzschweißnähten zu verbessern. Außerdem kann Kornfeinung im Schweißgut die Heißrissanfälligkeit verringern und damit die Schweißeignung des Grundwerkstoffs erhöhen. In dieser Studie wurde Kornfeinung in Aluminium-Schweißnähten durch das Zugeben eines Kornfeiners erzeugt. Hierfür wurden in einem Gießverfahren stäbchenförmige Einlagen hergestellt. Diese bestanden aus einer Mischung des jeweiligen Grundwerkstoffs und Zusät-zen an kommerziellem Al Ti5B1-Kornfeiner. Jede Einlage wurde in eine in ein Blech aus Grundwerkstoff eingebrachte Nut eingelegt und mit einem WIG-Schweißprozess umgeschmolzen. Die Untersuchungen zeigten, dass steigende Al Ti5B1-Zusätze zu einer deutlichen Verringerung der mittleren Korngröße im Schweißgut führen (bis zu 86%). Dabei wurde jeweils an einer bestimmten Stelle zwischen Schmelzlinie und Schweißnahtmitte ein Übergang von überwiegend länglicher zu überwiegend globulithischer Kornstruktur (= Columnar to Equiaxed Transition, CET) beobachtet. Die Mindestkornfeinergehalte für eine maximale Kornfeinung waren stark von der chemischen Zusammensetzung des Grundwerkstoffs abhängig. Demnach war die Kornfeinungseffizienz in Reinaluminium (Al 99,5 bzw. Al-Legierung 1050A) am höchsten, gefolgt von den Al-Legierungen Al Si1MgMn (6082) und Al Mg4,5Mn0,7 (5083). Zur genaueren Untersuchung dieses Effekts wurden die Unterkühlungsparameter P und Q für jede Kombination aus Grundwerkstoff und Kornfeinergehalt berechnet. P und Q beschreiben den Einfluss der Legierungselemente auf die konstitutionelle Unterkühlung einer Legierung während der Erstarrung und damit auf die resultierende Korngröße. Darüber hinaus wurden WDX- (wellenlängendispersive Röntgenspektroskopie) und TEM-Analysen (Transmissionselektronenmikroskopie) durchgeführt. Hierbei wurden im Schweißgut Ti- und B-reiche Partikel gefunden, die wahrscheinlich während der Erstarrung als Keime gedient haben, insbesondere TiB2-Partikel, die mit einer Schicht aus Al3Ti bedeckt waren. Es wurde beobachtet, dass steigende Schweißgeschwindigkeiten die Bildung von sehr kleinen, globulithischen Körnern fördern und damit den CET-Effekt verstärken. Deswegen wurden die Abkühlbedingungen während der Erstarrung der Schweißnähte bestimmt. Dies geschah anhand einer intensiven thermischen Analyse mit Thermoelementen. Aus den gemessenen Temperatur-Zeit-Kurven wurden die Abkühlrate, die Wachstumsgeschwindigkeit der Erstarrungsfront, der (lokale) thermische Gradient und das Erstarrungsinter-vall berechnet. Die Analyse zeigt, dass diese thermischen Parameter sehr stark entlang der Erstarrungsfront (zwischen Schmelzlinie und Schweißnahtmitte) variieren. In einem weitergehenden Schritt wurden die Ergebnisse mit der zugehörigen Schweißnahtkornstruktur verknüpft. So konnten für jeden thermischen Parameter die kritischen Werte bestimmt werden, die den CET-Übergang repräsentieren. Diese Daten wurden schließlich verwendet, um einen vom Gießen bekannten, empirischen Ansatz zur Beschreibung des CET-Effekts auf die Erstarrung in Schmelzschweißnähten zu übertragen. Der Einfluss der Kornstruktur auf die mechanischen Schweißnahteigenschaften wurde zunächst in Zugver-suchen untersucht. Demnach konnte für Al Mg4,5Mn0,7 (5083) die Schweißnahtduktilität durch Kornfeinung gesteigert werden wohingegen die Festigkeit unverändert blieb. Rissfortschrittsversuche mit Reinaluminium (Al99,5, 1050A) zeigten eine deutlich erhöhte Zähigkeit in feinkörnigen Schweißnähten. Außerdem wurde die Bildung von Mittenheißrissen beim Schweißen von Al Si1MgMn (6082) durch Schweißnahtkornfeinung ver-hindert, was einer deutlichen Verbesserung der Schweißeignung entspricht. Auf Basis der obigen Ergebnisse konnten schließlich Mindest-Ti/B-Gehalte für kommerzielle Schweißzusätze definiert werden, die in der Schweißnaht einen maximalen Kornfeinungseffekt erlauben. Diese Werte be-rücksichtigen den Einfluss von Grundwerkstoff und Schweißprozess. Sie machen deutlich, dass die in den bestehenden Normen vorgesehenen Ti/B-Gehalte in der Regel nicht ausreichen, um Schweißnahtkornfeinung zu gewährleisten.Grain refinement is an important opportunity to improve mechanical properties of fusion welds and the weldability (cracking sensitivity) of the base metal. In this thesis, grain refinement was achieved for aluminium welds by additions of a grain refiner. For this purpose, inserts consisting of aluminium base metal and small additions of commercial Al Ti5B1 grain refiner were cast, deposited in base metal plates, and fused in a gas tungsten arc (GTA) welding process. As a result, higher grain refiner additions increased the weld’s titanium and boron content resulting in a significant decrease in the weld metal mean grain size up to 86%. This grain size reduction led to a transition from predominantly columnar to equiaxed grain shape (columnar to equiaxed transition CET). The grain refinement was thereby found to be strongly dependent upon the base metal chemical composition. Accordingly, the grain refining efficiency was the highest in commercial pure Al (Alloy 1050A, Al 99.5), followed by Alloy 6082 (Al Si1MgMn) and Alloy 5083 (Al Mg4.5Mn0.7). In this regard, the parameters P and Q were applied to investigate the influence of alloying elements on the supply of constitutional undercooling during solidification and on final grain size. Also, WDS (wavelength dispersive x-ray spectroscopy) and TEM (transmission electron microscopy) analysis found an increasing number of particles rich in Ti and B. These substrates are probably TiB2 particles coated by Al3Ti likely nucleating Al grains during solidification. The variation in torch speed showed that increasing torch speeds support the CET effect leading to many small and equiaxed grains at high torch speed. To give explanations for this observation, the thermal conditions, that are controlled by welding parameters such as torch speed, were determined with temperature measurements via thermocouples. These measurements revealed that solidification parameters like solidification growth rate, cooling rate, (local) thermal gradient and solidification time vary significantly along the solidification front (from weld centreline to weld fusion line). In a further step, the solidification parameters were related to the corresponding grain size and shape. On the basis of this comparison, an analytical approach was used to model the CET. This allowed the prediction of critical values for both solidification growth rate and thermal gradient, at which the CET occurs in aluminium weld metal. The influence of grain refinement on the weld mechanical properties was investigated in tensile tests. Accordingly, the ductility of Alloy 5083 welds was increased through grain refinement whereas no improvement in weld metal strength was observed. Furthermore, tear tests with notched specimens revealed for Alloy 1050A that the resistance against initiation and propagation of cracks in the weld metal can be enhanced through grain refinement. In addition, when welding Alloy 6082, weld metal grain refinement prevented the formation of centreline solidification cracking that was present only in welds with unrefined grain structure. On the basis of the above experiments, the Ti/B contents needed in commercial filler wires or rods to allow optimum weld metal grain refinement were estimated. Accordingly, this work gives specific recommendations to filler material producers through a simple calculation that considers the influence of base alloy and welding process. The results show that the Ti/B contents defined by the corresponding standards for filler alloys are too low to allow weld metal grain refinement

Beam Characterization of the MYRRHA-RFQ

The Linear Accelerator for the MYRRHA project* is under construction. In a first step the linac up to 100 MeV will be realized. The LEBT section has been set into operation in Belgium and the RFQ is installed in summer 2018. A system to analyze the ion beam consisting of a slit-grid emittance scanner, a beam dump and a momentum spectrometer, called diagnostic train descripted in [2], will be set on the rails to characterize the beam at the RFQ injection point. The results will be used to adjust the optimal matching for the RFQ. After the measurements downstream the LEBT, the diagnostic train begins its journey along the beam line and at the first station the RFQ is installed. The accelerated beam of the RFQ is then analyzed and optimized. In addition to optimization of transmission the artificial production of beam offsets in the LEBT is of special interest. These will be measured at the injection point to estimate the range of possible offsets. In the following measurements these offsets will be used to study the influence of the offsets on the RFQ performance. Furthermore, the RFQ parameters are varied to see their influence on the beam transport, transmission and beam quality

Space-Charge Compensation in the Transition Area Between LEBT and RFQ

The transition from a space charge compensated beam in the LEBT to an uncompensated beam in the RFQ will influence the beam parameters. To investigate the impact of the electric fields on the space charge compensation, an insulated cone is used as a repeller electrode in front of the RFQ. Depending on the time dependent potential of the RFQ rods respectively to the beam potential, the compensation electrons may be prevented from moving into the RF field which oozes out of the RFQ entrance. The simulation studies are performed with the particle-in-cell code bender*. The simulations may substantiate measurements at the CW-operated RFQ in Frankfurt University** as well as at the foreseen MYRRHA LEBT-RFQ interface.*** In this contribution, a study on a LEBT-RFQ interface is shown. Results of numerical and experimental investigations will be compared

The MYRRHA-RFQ - Status and First Measurements

The MYRRHA project requires a proton linac with an energy of 600 MeV with a beam current of 4 mA in cw operation. As first RF structure a 176 MHz 4-Rod RFQ has been chosen because of tuning possibilities, maintenance, lower capital costs and technological risk compared to a 4-Vane-RFQ. The aim of beam dynamics design was to preserve excellent beam quality and to avoid the creation of halo particles especially in the longitudinal plane. Using the NFSP (New Four-Section Procedure) with a soft and symmetric pre-bunching with full 360° acceptance it was possible to reach the requirements. The simulated transmission of the 4 m long RFQ is close to 100%. The electrode voltage has been chosen to 44 kV which gives enough transverse focusing but limits the required RF losses to about 25 kW/m. The cooling has been optimized for reliable operation and a new method of dipole compensation has been applied. The RFQ has been built and tuned with respect to field flatness. The paper describes the status of the RFQ and first measurements

Contact sensitizers induce skin inflammation via ROS production and hyaluronic acid degradation.

BACKGROUND: Allergic contact dermatitis (ACD) represents a severe health problem with increasing worldwide prevalence. It is a T cell-mediated skin disease induced by protein-reactive organic and inorganic chemicals. A key feature of contact allergens is their ability to trigger an innate immune response that leads to skin inflammation. Previous evidence from the mouse contact hypersensitivity (CHS) model suggests a role for endogenous activators of innate immune signaling. Here, we analyzed the role of contact sensitizer induced ROS production and concomitant changes in hyaluronic acid metabolism on CHS responses. METHODOLOGY/PRINCIPAL FINDINGS: We analyzed in vitro and in vivo ROS production using fluorescent ROS detection reagents. HA fragmentation was determined by gel electrophoresis. The influence of blocking ROS production and HA degradation by antioxidants, hyaluronidase-inhibitor or p38 MAPK inhibitor was analyzed in the murine CHS model. Here, we demonstrate that organic contact sensitizers induce production of reactive oxygen species (ROS) and a concomitant breakdown of the extracellular matrix (ECM) component hyaluronic acid (HA) to pro-inflammatory low molecular weight fragments in the skin. Importantly, inhibition of either ROS-mediated or enzymatic HA breakdown prevents sensitization as well as elicitation of CHS. CONCLUSIONS/SIGNIFICANCE: These data identify an indirect mechanism of contact sensitizer induced innate inflammatory signaling involving the breakdown of the ECM and generation of endogenous danger signals. Our findings suggest a beneficial role for anti-oxidants and hyaluronidase inhibitors in prevention and treatment of ACD

Antioxidant treatment inhibits HA degradation by ROS <i>in vitro</i> and <i>in vivo.</i>

<p>(A) High molecular weight HA was incubated with ROS inducing compounds in combination with RF-40s (R) or solvent controls (S) at corresponding concentrations. MW of HA after incubation was analyzed by SDS gel electrophoresis and staining with Stains all. The untreated HA control is shown in lane 1, CuSO₄ or H₂O₂ only treated HA controls are shown in lanes 2 and 3. One representative gel out of three is shown. (B) High molecular weight HA was incubated with ROS inducing compounds as in (A) with or without addition of different concentrations of the antioxidant NAC. MW of HA after incubation was analyzed by SDS gel electrophoresis and staining with Stains all. One representative gel of three is shown. (C) Staining of HA in murine ears treated topically with acetone, TNCB or TNCB and NAC on the back side of the ear skin (upper side in panels). Samples from ears were fixed as described and paraffin sections (3 µm) were stained with bHABP with subsequent AEC (3-Amino-9-ethylcarbazole) staining and haematoxylin counterstaining. HA is stained brown/red and cell nuclei in blue. Pictures are representative of three independent experiments with three mice each. Magnification = 200×; scale bars  = 50 µm.</p

Contact sensitizers induce ROS production <i>in vivo</i> and in mitochondria <i>in vitro.</i>

<p>(A) Mice were pre-treated by topical application of antioxidants (NAC 5 mM or PBS as solvent, RF-40s 5.24 mM or solvent) on the ears. NAC/PBS was applied 1 h before and RF-40s/solvent 15 min before induction of ROS production by topical application of TNCB (7%). Acetone treatment served as solvent control for TNCB. 15 min later, ears were taken after euthanasia and incubated <i>ex vivo</i> with DHE (5 mM) in DMSO for 30 min before analysis of ROS production by fluorescence microscopy. Fluorescence was set for minimal background staining with the acetone control to optimize visualization of differences in ROS production in the other samples. Same acquisition times were used for all samples of one experiment. Results shown are representative of three independent experiments. Magnification  = 200×, scale bar  = 50 µm. (B) Pam212 or L929 cells were incubated with TNCB, LPS or left untreated for 1 h before addition of MitoSOX™. ROS production was observed by red/orange fluorescence of MitoSOX™ by fluorescence microscopy. Nuclei were visualized by DAPI staining (blue). Pictures shown are representative of three independent experiments. Magnification  = 400×, scale bar  = 20 µm. (C) Primary human fibroblasts or keratinocytes were incubated with DNCB or left untreated for 1 h before addition of MitoSOX™. ROS production was observed by red/orange fluorescence of MitoSOX™ by fluorescence microscopy. Pictures shown are representative of three independent experiments. Magnification  = 1000×, scale bar  = 50 µm.</p