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One of the main advantages of metal additive manufacturing (MAM) techniques is their ability to produce components with site-specific microstructural features. Nevertheless, microstructural defects and lack of repeatability are still major concerns in MAM. In this study, a laser powder bed fusion (PBF-LB/M) IN718 material, produced using two different scan length vectors, is investigated using Bragg-edge neutron 2D imaging (BENI) combined with electron backscatter diffraction (EBSD) analysis. BENI is able to detect, on a macroscopic scale, process-induced changes in texture in a large field of view covering the entire sample (20×80 mm2). In addition, high-resolution BENI (HR-BENI), with a pixel size of 12.8 µm, provides a micro-scale examination of the local variations of texture and grain morphology, otherwise undistinguishable using the standard resolution. As such, HR-BENI offers a straightforward and detailed way of screening the integrity of MAM parts at cm-length scales.
Neutron cameras allow visualizing hydrogen distributions with radiographic or tomographic imaging methods in iron (and steel) and many other metals. The necessary contrast between hydrogen and these metals stems from the high difference in the total neutron cross section of both elements. This allows, e.g., the in situ measurement of hydrogen mass flow inside cm thick metal samples with a temporal resolution of at best 10 s using neutron radiography as well as the quantitative measurement of hydrogen accumulations, e.g., at the crack’s inner surfaces in hydrogen embrittled iron samples with neutron tomography. This new quality of the information on a micrometer scale allows new insights for the analysis of hydrogen-assisted damage mechanisms. Further, this method is non-destructive and provides local information in situ and in three dimensions with a spatial resolution of 20 μm - 30 μm. In this contribution, we give a short historical overview of neutron imaging and show examples that demonstrate the spatial and temporal resolution of the neutron radiography and tomography methods in order to visualize and quantify hydrogen accumulations, absorption processes, and diffusion. The examples are taken from the works of researchers dealing with titanium, palladium, zirconium, and iron or steel. More detailed descriptions of the experimental and analytic procedures are given for hydrogen detection using radiography and tomography on iron and steel samples.
Neutron cameras allow visualizing hydrogen distributions with radiographic or tomographic imaging methods in iron and steel. The necessary contrast between hydrogen and iron stems from the high difference in the total neutron cross section of both elements. This allows e.g. the in situ measurement of hydrogen mass flow inside cm thick steel samples with a temporal resolution of 20 s using neutron radiography as well as the quantitative measurement of hydrogen accumulations at the crack’s inner surfaces in hydrogen embrittled iron samples with neutron tomography. We could detect directly gaseous hydrogen in the crack cavities and we measured the gas pressure. This new quality of the information on a micrometer scale allows new insights for the analysis of hydrogen-induced damage mechanisms. Further, this method is non-destructive and provides local information in situ and in three dimensions with a spatial resolution of 20-30 µm. In this contribution, we show examples that demonstrate the spatial and temporal resolution of the neutron radiography and tomography methods in order to visualize and quantify hydrogen accumulations at cracks. The measurements were performed at the research reactor BER II of the HZB in Berlin and at the FRM II reactor of the neutron source Heinz Maier-Leibnitz in Garching.
Neutron cameras allow visualizing hydrogen distributions with radiographic or tomographic imaging methods in iron and steel.
The necessary contrast between hydrogen and iron stems from the high difference in the total neutron cross section of both elements. This allows e.g. the in situ measurement of hydrogen mass flow inside cm thick steel samples with a temporal resolution of 20 s using neutron radiography as well as the quantitative measurement of hydrogen accumulations at the crack’s inner surfaces in hydrogen embrittled iron samples with neutron tomography. We could detect directly gaseous hydrogen in the crack cavities and we measured the gas pressure. This new quality of the information on a micrometer scale allows new insights for the analysis of hydrogen-induced damage mechanisms. Further, this method is non-destructive and provides local information in situ and in three dimensions with a spatial resolution of 20-30 μm.
In this contribution, we show examples that demonstrate the spatial and temporal resolution of the neutron radiography and tomography methods in order to visualize and quantify hydrogen accumulations at cracks. The measurements were performed
at the research reactor BER II of the HZB in Berlin and at the FRM II reactor of the neutron source Heinz Maier-Leibnitz in
Garching.
In situ chemical analysis of duplex stainless steel weld by laser induced breakdown spectroscopy
(2024)
The high corrosion resistance and good mechanical properties of duplex stainless steel (DSS) are due to its special chemical composition, which is a balanced phase ratio of ferrite (α) and austenite (γ). Many industrial applications require the integration of DSS components. For this, Gas tungsten arc welding (GTAW) is an excellent choice, as it allows an automated operation with high reproducibility. However, when the weld pool solidifies, critical ratios of α- and γ- phases can occur, which lead to solidification cracking, increased susceptibility to corrosion, and a decrease in ductility and critical strength. Previous studies have shown that these defects can be caused by the accumulation of manganese and chromium in the heat affected zone (HAZ), requiring ongoing monitoring of this accumulation. A suitable method for such monitoring is laser-induced breakdown spectroscopy (LIBS), which can be used in two operating modes: calibration using standard reference samples and calibration-free. Unlike conventional quantitative LIBS measurements, which require reference samples to generate a calibration curve, calibration-free LIBS (CF-LIBS) allows chemical compositions to be determined solely from the emission spectrum of the plasma. Numerous publications show that CF-LIBS is a fast and efficient analytical method for the quantitative analysis of metal samples. In this work, CF-LIBS is applied to spectra obtained during GTAW DSS welding and the result is compared with those obtained by PLS analysis. A good correlation was found between both types of analysis, demonstrating the suitability of the CF-LIBS method for this application. The CF-LIBS method has a significant advantage over conventional LIBS due to the rapid in situ measurement of concentrations of major alloying elements without calibration procedure. This, combined with fast feedback and appropriate adjustment of welding parameters, helps prevent welding defects.
The chemical composition of a weld metal determines the resulting solidification mode of stainless steel and the consequent weld metal quality. In this work tungsten inert gas (TIG) welding of EN grade 1.4435 austenitic stainless steel was monitored using laser-induced breakdown spectroscopy (LIBS) for the in situ measurement of chemical composition changes. This research aims to prototype a real-time chemical composition analysis system for welding applications and prove the feasibility of such quality control loop. LIBS was used to investigate in situ the monitoring of metal vaporization during TIG welding. We found Mn vapor formation above the weld pool and subsequent condensation of Mn on the weld metal surface using LIBS. Post-weld line scans were conducted by LIBS on various welds produced with different welding currents. Local changes of Ni and Mn were observed at higher welding currents. The results are in good agreement with the literature and proved that LIBS can be used in situ to inspect the TIG welding process.
A laser-induced breakdown spectroscopy (LIBS) system was combined with a bead-on-plate Tungsten Inert Gas (TIG) welding process for the in situ measurement of chemical compositions in austenitic stainless steels during welding.Monitoring the weld pool's chemical composition allows governing the weld pool solidification behavior, and thus enables the reduction of susceptibility to weld defects. Conventional inspection methods for weld seams (e.g. ultrasonic inspection) cannot be performed during the welding process. The analysis system also allows in situ study of the correlation between the occurrence ofweld defects and changes in the chemical composition in theweld pool or in the two-phase regionwhere solid and liquid phase coexist. First experiments showed that both the shielding Ar gas and the welding arc plasma have a significant effect on the selected Cr II, Ni II and MnII characteristicemissions, namely an artificial increase of intensity values via unspecific emission in the spectra.
In situ investigations showed that this artificial intensity increase reached a maximum in presence of weld plume. Moreover, an explicit decay has been observedwith the termination of thewelding plumedue to infrared radiation during sample cooling. Furthermore, LIBS can be used after welding to map element distribution. For austenitic stainless steels,Mnaccumulations on both sides of theweld could be detected between the heat affected zone (HAZ) and the base material.
The ISO 3690 standard “Determination of hydrogen content in arc weld metal” requires a thermal activation of the diffusible hydrogen in a piece of weld metal for the subsequent ex situ concentration measurement by carrier gas hot extraction CGHE or thermal desorption spectroscopy (TCD). Laser-induced breakdown spectroscopy (LIBS) offers a time and spatially resolved, almost non-destructive, in situ measurement of hydrogen at surfaces without sample preparation. We measured hydrogen in steels, which were charged either electrochemically or by high-pressure hydrogen gas, and compared the results. Further, the feasibility of quantitative hydrogen line scan measurements with LIBS was demonstrated by measuring hydrogen at water jet cut surfaces. The hydrogen concentrations measured with the help of LIBS were compared with CGHE measurements. It was observed that hydrogen can be reliably measured with LIBS for concentrations larger than 2 wt.-ppm. The maximum hydrogen concentration achieved using electrochemical charging was 85.1 ppm. The results show that LIBS is a promising technique for time- and spatially resolved measurements of hydrogen in steels.
Die dehnungsinduzierte Martensitbildung wurde mittels Röntgen-Diffraktion und -Radiographie mit dem EDDI-Instrument am BESSY II in supermartensitischem Stahl mit Anlassaustenit untersucht. Die Bildung wird durch Wasserstoff im Gefüge unterdrückt. Die Duktilität von wasserstofffreiem SMSS korreliert mit dem Anteil an dehnungs-induziertem Martensit
Neutron imaging is a valuable tool for measuring hydrogen distributions qualitatively and quantitatively in metals. Time-resolved neutron radiography allows to measure hydrogen mass flow inside cm thick steel samples with ~10 s temporal resolution. Hydrogen accumulations around cracks in embrittled iron samples can be visualized three-dimensionally by neutron tomography. This quality of information allows new insights for the analysis of damage mechanisms on a micrometer scale, e.g. of hydrogen blistering. Even the gas pressure of molecular hydrogen in crack cavities has been measured from tomographic reconstructions to be in the range of 5 MPa to 15 MPa for technical iron. Further, this method is non-destructive and provides local information in situ and in all three dimensions with a spatial resolution of 20 - 30 µm. The combination with other methods gives a new quality of information, e.g. of the hydrogen allocation on fractured surfaces.