Fraunhofer IWS Research Articles

Use of the Laser Fragmentation Method in the Extraction of LFCM from the Shelter

The Project to transform the Shelter into an environmentally safe system has achieved a significant milestone in the process of transitioning Chornobyl NPP Unit 4 to a safe environmental state. However, the program does not include the measures for long-term safe management of the nuclear stockpile at Chornobyl NPP Unit 4 that are essential. A considerable amount of fuel, approximately 200 tons, has been mixed with reactor debris and structural components, resulting in the formation of fuel-containing materials spanning hundreds of cubic meters. These fuel-containing materials are located in the lower levels of Chornobyl NPP Unit 4.
 Given the degradation and natural deterioration of fuel-containing materials leading to generation of radioactive aerosols, as well as recent changes in neutron flux density observed after the installation of the New Safe Confinement, there is an urgent need to enhance the development and implementation of appropriate technology. One promising approach is application of the dry fragmentation technology, which uses a high-power solid-state laser developed at the Fraunhofer IWS laboratory in cooperation with the Institute for Safety Problems of Nuclear Power Plants NAS of Ukraine and Plejades GmbH. This technology offers advantages such as minimizing the dynamic and static loads on local structures, efficient energy transfer (heat), and reducing dust formation.
 This article presents the main aspects of the laser fragmentation method of LFCM. The article describes the details of the controlled destruction technology of thermally stressed material, developed as a result of tests with simulated material at the Fraunhofer IWS laboratory. The work is focused on the measures required for implementing the dry fragmentation technology at the Shelter. It presents pre-conceptual approaches for retrieval and handling of fuel-containing materials located in the lower floors of Chornobyl NPP. The article outlines an approach for accessing fuel-containing materials, a strategy for their fragmentation, subsequent packaging, and safe storage within the Chornobyl NPP until final disposal. Additionally, the creation of a tool system and means of delivery are also described.

New possibilities for laser welding of highly loaded transmission components by strategic use of simulation methods

Laser welding in transmission manufacturing opens up completely new kinds of product solutions with excellent properties in terms of wear, corrosion resistance, and service life. Current welding designs are characterized in particular by difficult-to-weld material combinations (e.g., steel versus cast iron) and a high component stiffness, which is correlated with high residual welding stresses. The major challenge for these mass-produced components remains both their crack-free weldability and their complex cyclic load capacity [U. Stamm, “Rissfreies Laserstrahlschweißen von Mischverbindungen aus Gusseisen und Einsatzstahl, Jahresbericht Fraunhofer IWS 2006” (2006); X. Shu, “Untersuchungen zum Laserstrahlschweißen von Werkstoffkombinationen aus Gußeisen und Stahl,” Dissertation, Shaker, Band 4/94 (1994), ISBN: 3-8265-0098-9; G. Göbel, “Erweiterung der Prozessgrenzen beim Laserstrahlschweißen heißrissgefährdeter Werkstoffe,” Dissertation, Fraunhofer-IRB-Verlag (2007), ISBN: 978-3-8167-7671-0]. Therefore, this contribution presents practical solutions for weld-compatible joint constructions and the reduction of residual stresses on the basis of a representative transmission design. Specifically, a systematic study is being conducted to understand and qualitatively evaluate effective methods for reducing residual weld stresses in circumferential welds. The recommendations developed as part of this study take particular account of the influence of process modifications, material conditions, and geometric aspects on weldability and component distortion. Here, structural welding simulations are performed and verified by experimental welding trials, including metallographic examinations. To ensure the required component fatigue strength, a practical concept for determining Wöhler curves is presented, which is based on structural mechanical simulations and multiaxial fatigue strength tests on simplified test specimens. The adaptations developed in this way facilitate the production of difficult-to-weld and highly stressed transmission components. In particular, time-consuming and cost-intensive iterations of laser welding tests can be significantly reduced or even eliminated [Brenner et al., “Neuere Ergebnisse zum Schweißen von Eisenbasiswerkstoffen mit Faserlasern, 5. Laser-Anwenderforum,” Strahltechnik Band 28 (2006), pp. 139–148, ISBN: 3-933762-18-9, S]. Furthermore, the systematic investigations provide effective recommendations for phenomenological understanding and solving typical welding challenges in practice [J. Standfuß, “Ganzheitliche innovative fügetechnische Konzepte am Beispiel des PKW-Antriebsstranges,” Fraunhofer Verlag (2010), ISBN: 978-3-8396-0125-9]. This applies, in particular, to load-transmitting components in the fields of e-mobility, aerospace, and industrial engineering [Wagner et al., “Entwickeln und Auslegen von lasergeschweißten Getriebe-komponenten für die Luftfahrt,” DVS-Berichte Band 365 (2020), pp. 62–69, ISBN: 978-3-96144-098-6].

Latest developments in coaxial multiwire high-power laser cladding

Laser cladding is widely used in the industry to precisely apply tailored surface coatings, as well as three-dimensional deposits for repair and additive manufacturing of metallic parts. However, the processing of larger components is economically challenging mainly because of low deposition rates. At Fraunhofer IWS, a Laserline fiber-coupled diode laser with 20 kW power has been employed for over a decade to develop competitive coating solutions with powder-based laser cladding. The deposition rates achieved with this technology is comparable to common PTA technique at the same time bringing significant advantages in terms of reduced heat affected zone, distortion, and savings in material resources. While high-power powder-based laser cladding is an industrially established coating technology, for example, to coat hydraulic cylinders or most recently brake discs, a high-productivity solution for wire-based processes is still challenging. Fraunhofer IWS has developed a new nozzle for high-power high-productivity laser wire cladding for coating and additive manufacturing, the so-called COAXquattro. This system enables to feed at the same time four wires into the melt pool, reaching deposition efficiencies in the same range as a powder-based laser process. For selected materials, the improvement in coating quality compared to powder laser cladding is achieved. Furthermore, with COAXquattro system simultaneous feeding of powder particles to wire cladding presents a great potential for in situ alloying and cost-effective production of new compositions on material alloying or hardmetal-reinforced composites for coating application and 3D additive manufacturing.

Advanced Characterization by Using Coupled Operando Mass Spectrometry and Operando Dilatometry

Mechanical modifications or harmful side reactions are undesirable ageing effects that can occur during cycling. These phenomena have a negative impact on cell performance and consequently limit cycle life. The focus here is on investigations of ageing processes such as volume changes and electrolyte decomposition during cycling.Combined thickness measurements and gas analytical studies of Li-Graphite half cells with a carbonate-based electrolyte are presented. In addition, the same measurements on Li-O2 cells with an ether-based electrolyte are introduced. The electrolytes consist of 1M LiPF6 in EC:DMC (1:1, wt) and 1M LiTFSI in DEGDME, respectively. All measurements were carried out using specially developed multifunctional test setups and accordingly modified test cells. The different cell specifications were examined by operando mass spectrometry (MS) and operando dilatometry. In addition, post-mortem gas chromatography-mass spectrometry (GC-MS) was conducted.Application-oriented analysis such as gas analysis, thickness measurements or both combined are suitable methods to define operating parameter, to benchmark and to identify decomposition reactions. Operando electrochemical dilatometry was conducted to check whether an irreversible increase in thickness occurred. On the corresponding cell configurations, the dilatation was recorded over the entire electrode stack as well as of the separate working electrode only. Furthermore, operando MS can be used to detect gaseous substances produced by (electro)chemical processes as a function of the state of charge. With the GC-MS, a post-mortem analysis was performed to identify the individual substances qualitatively.Results of the different cell specifications are shown and can be compared with each other. A detailed relationship can be demonstrated between the change in thickness and the potential profile. Moreover, the cyclic formation of several degradation products is detected. Possible correlations between gas evolution and thickness changes can be shown. In conclusion, these analytical studies provide a new important contribution to get a more detailed insight into ageing processes that take place. The possible coupling of two operando techniques enables an even more holistic examination to analyze cell performance.Parts of the presented work are supported by the Fraunhofer and Max-Planck cooperation program in the project “Clusterbatt” together with Fraunhofer IWS, Dresden and MPIKG, Potsdam. Parts of the presented work are funded by the German Federal Ministry of Education and Research (BMBF) in the joint German-Japanese project “OsabanPlus” (03XP0469C) together with Justus-Liebig-University Giessen, TU Braunschweig and University of Kyoto. Figure 1

Tailored Pre-Lithiation of Carbon Anodes Using Melt Deposited Thin Lithium

The user demands for Lithium ion batteries in mobile applications and electric vehicles request steady improvement in terms of capacity and cycle life. State-of-the-art lithium ion batteries irreversibly loose about 10 % of the capacity during the first cycles due to SEI formation. Therefore, 10 % of the cathode material is not used during the rest of the cells life which results in a waste of cathode material and leads to a lowered range of battery electric vehicles. Therefore, these losses must be compensated to ensure efficient resource utilization and to further increase the capacity of the lithium ion battery.We have developed a method to compensate these capacity losses by pre-lithiation: A fast and simple approach of electrolyte-free direct contact pre-lithiation leads to targeted degrees of pre-lithiation in a range of 7-50 % for state-of-the-art graphite electrodes. The parameters pressure (0-40 MPa), temperature (20-150°C) and time (0-60 min) are used to transfer lithium from a temporary substrate to the graphite anode. This process uses 1-5 µm thin lithium films made by the Fraunhofer IWS lithium melt deposition process as a lithium source.[1] The degree of pre-lithiation can be controlled by the lithium loading of the temporary substrate. NCM full cells built with pre-lithiated graphite electrodes where electrochemically evaluated in comparison to unlithiated electrodes. They show 6.5 % capacity increase after the first cycles using the additional lithium to form the SEI during the first cycles. The parameter influence of the pre-lithiation process on the initial coulomb efficiency of cells with pre-lithiated graphite electrodes is examined in half cells showing an ICE of 99.91 % for the best parameters.This method of electrolyte-free direct contact pre-lithiation shows an efficient way to further increase the capacity of state-of-the-art LIBs with graphite electrodes by increasing the cathode material utilization. It can potentially be scaled up to a roll-to-roll process using a heated calender nip. The temporary substrates with tailored lithium loading can be processed by lithium melt deposition in an efficient way with high lithium yield and high precision.[1] K. Schönherr, B. Schumm, F. Hippauf, R. Lissy, H. Althues, C. Leyens, S. Kaskel, Chemical Engineering Journal Advances 2022, 9, 100218. Figure 1

Metal‐Organic Frameworks: Synthesis, Structures, and Applications

Metal‐Organic Frameworks: Synthesis, Structures, and Applications

Solution for real time sensor guided programmable logic controller of galvanometer scanners

This paper introduces PLC (programmable logic controller)-based real-time motion control of 2D galvanometer scanners under consideration of different sensor information. The Fraunhofer IWS developed ESL2-100 module works as a locally separated client of a higher-level machine control to drive the scanner. The EtherCAT field bus supports extensive and constant exchange of information between the control levels. An application example, therefore, is the spatially resolved surface structuring of large areas using a continuous wave laser. In this application, the scanner must move the laser beam to predefined positions, stop there during the processing time, and jump to the next position. Frequencies up to 2 kHz are applicable depending on the scanner performance. The working area of a scanner can be extended by moving the scanner head with Cartesian machine axes. In the presented solution, the scanner control records these movements via encoder signals and captures the geometrical boundaries of the workpiece, i.e., previous operated areas, via cameras. The obtained data are included in the real-time calculation of scanner position data and laser emission signals. These measures maintain precision in positioning the patterns for surface structuring and adapt them to existing contours. The processing of the control data, the 2D-on-the-fly correction, and the correction of the target positions are done on the PLC level. Then, within one control cycle, multiple position data are sent in blocks from the PLC to the ESL2-100 module within one control cycle. At the end, the ESL2-100 module converts the field bus data into the scanner control protocol (here SL2-100 protocol). The approach described enables the deterministic calculation of the scan movement and responds dynamically to changing conditions by linking all necessary sensor information on one control platform.

Comparative analysis of the potential of state-of-the-art lasers and new prototypic high-power beam sources for cutting nonmetals

At Fraunhofer IWS Dresden, different, partially prototypic high-power continuous wave (cw) beam sources for the remote laser processing of nonmetals with galvanometric-driven scanner systems are available. The emitted wavelengths of the lasers cover the range from visible light to mid-infrared. A spectroscopic analysis was performed to gain insight into the processability of typical nonmetal materials with single wavelengths. The wavelength-depending absorption was calculated based on the measured reflectivity and transmissivity of the selected materials. Relating to these results, the theoretic potential of the beam sources was verified in a comparable cutting research. The investigated work pieces are representatives of the groups of thermoplastics and thermosets, fiber-reinforced polymers, glasses, natural fiber textiles, and cellulosic based materials. The laser systems were divided in two groups for the investigation. To get a reference of the processability with state-of-the-art cw systems, a CO2 laser with a wavelength of 10.6 μm and a solid-state Yb:YAG fiber laser with 1.07 μm were used. The new prototypic beam sources were a green laser with a wavelength of 0.515 μm, a CO laser with a range of 5.3–5.8 μm, and a CO2 laser with 9.3 μm in a power range from 1 to 1.5 kW. Differences in the possible focusability are caused by the wavelengths and given beam qualities. For a direct comparison, these differences needed to be compensated. To get comparable intensities, the spot diameter in the working distance for each laser was set to 250 μm. This allows to process the materials with almost the same intensities. Since the laser sources with the short wavelengths offer a better focusability, experiments with the smallest possible focus diameters for the used optics were performed to get an optimized ablation result. Finally, the aim of the research was to evaluate the suitability of the available beam sources for new fields of application. The latest results of experimental and theoretical investigations on the remote laser processing of nonmetals with a wide range of laser wavelength will be presented and discussed.

Advanced manufacturing approach via the combination of selective laser melting and laser metal deposition

Additive manufacturing processes are frequently discussed in a competitive manner instead of being considered synergetically. This is particularly unfavorable since advanced machining processes in combination with additive manufacturing can be brought to the point that the results could not be achieved with the individual constituent processes in isolation [K. Gupta, R. F. Laubscher, and N. K. Jain, Hybrid Machining Processes—Perspectives on Machining and Finishing (Springer, New York, 2016), p. 68]. On that basis, boundary conditions from selective laser melting (SLM) and laser metal deposition (LMD) are considered in mutual contemplation [A. Seidel et al., in Proceedings of 36th International Congress on Applications of Laser & Electro-Optics, Atlanta, GA, 22–26 October 2017 (Fraunhofer IWS, Dresden, 2017), pp. 6–8]. The present approach interlinks the enormous geometrical freedom of powder-bed processing with the scalability of the LMD process. To demonstrate the potential of this approach, two different strategies are pursued. Firstly, a hollow structure demonstrator is manufactured layer wise via LMD with powder and subsequently joined with geometrically complex elements produced via SLM. Afterward, possibilities for a microstructural tailoring within the joining zone via the modification of process parameters are theoretically and practically discussed. Therefore, hybrid sample materials have been manufactured and interface areas are subjected to microstructural analysis and hardness tests. The feasibility of the introduced approach has been demonstrated by both fields of observation. The process combination illustrates a comprehensive way of transferring the high geometric freedom of powder-bed processing to the LMD process. The adjustment of process parameters between both techniques seems to be one promising way for an alignment on a microstructural and mechanical scale.

Extended DC arc atmospheric pressure plasma source for large scale surface cleaning and functionalization

Plasma technologies at atmospheric pressure (AP) promise efficient integration into industrial manufacturing technology and save considerable investment and maintenance costs. The development of new fields of application of AP techniques increasingly requires the provision of adapted, large‐area plasma sources. The linearly extended direct current (DC) arc plasma source, long arc generator principle (LARGE) for short, with a working width of up to 350 mm was developed at the Fraunhofer IWS for the large area plasma surface treatment. In contrast to other jet‐based AP sources arrays, the LARGE plasma source has a very long afterglow flame length, which makes it possible to efficiently treat surfaces over the range ±40 mm. Especially for large‐area plasma applications, commercial jet nozzle arrays are very complex to control and require more than one power generators which increases the equipment cost. In this paper we discuss selected application fields, namely the rapid (up to 50 m/min) removal of oil films on aluminium sheets, the large‐area functionalization (removal of release agents) of carbon‐fibre reinforced plastic (CFRP) materials, and the deposition of SiO2 adhesive layers on titanium to improve adhesion in the structural bonding of these materials.

Laser beam welding of atmosphere aluminum die cast material using high frequency beam oscillation and brilliant beam sources

In the serial production of components for automotive applications such as cooling and air-conditioning systems, aluminum die-cast materials are frequently used due to their excellent castability. The aim of providing light weight components can be approached with thin walled cross sections even for complex structural parts. However, cast components are usually connected to semifinished products such as profiles or tubes. The connections have to be mostly pressure tight. The joining technique for these applications has to be highly productive to obtain high component outcome and cost-efficient. Laser beam welding techniques are especially suitable for these tasks. Die-cast components have limited or no weldability due to their manufacturing process. This is due to entrapped gases within pores or cavities under high pressure conditions. Furthermore, the mold release agents for the die-cast process are inappropriate for obtaining homogeneous and sound weld seams. Consequently, this results in a larger number of pores in the weld seam and stochastic melt pool blow-outs, which prohibit mostly the use of the component. To solve these issues, a new welding technique, remoweld®T, has been developed at Fraunhofer IWS. This unique method has been extensively tested and used for serial-production. The decisive step was to use laser sources with brilliant beam quality in combination with a high frequency beam oscillation within the melt pool. In this paper, the technological approach will be presented. With the remoweld®T method, it was possible to obtain homogeneous weld seams with low porosity and a strongly reduced distortion for the first time. Minor component tolerances and a reproducible joining technique with a high output for serial production can be achieved.

Fabrication and efficiency measurement of a Mo/C/Si/C three material system multilayer Laue lens

In this letter, we report on the manufacturing of a multilayer Laue lens (MLL) consisting of a multilayer stack with three materials: molybdenum and silicon as the absorber and spacer layer, respectively, and carbon as transition layers. The design has four layers per period: Mo/C/Si/C. It yields 6000 zones and provides an aperture of 50 μm. This allows the MLL structure to accept a large portion of the coherent part of the beam and to achieve a small spot size. The MLL deposition was made by magnetron sputtering at the Fraunhofer IWS, and the sectioning was done by laser cutting and subsequent focused ion beam milling to a thickness that provides a good efficiency for a photon energy of 12 keV. The diffraction efficiency as a function of the tilting angle has been measured at beamline 1-BM of the Advanced Photon Source. An efficiency of almost 40% has been achieved. This shows that the material system performs well compared to MLLs made of two-materials and that it is in excellent agreement with the numerically calculated efficiency for a comparable molybdenum/silicon bilayer system lens. We conclude that the three material system offers high efficiencies and is advantageous for stress reduction in MLLs.

Friction Stir welding of Light Metals for Industrial Applications

The high potential of Friction Stir Welding (FSW) is already widely used in industrial applications. In combination with a suitable machine concept, this technology is appropriate for small components and can be extended to applications on structures of several meters in length. Using a 3D-capable system that is stiff enough to handle the high process forces, machining and friction stir welding of complex geometries can be carried out directly in one set-up. In order to ensure reproducible, high quality welds, appropriate control strategies are needed. Furthermore the use of various tool types such as standard or bobbin tools can extend the range of weldable geometries tremendously.The aforementioned aspects have been part of process development work for FSW at Fraunhofer IWS Dresden in the last few years. Within the current paper interesting findings are discussed in detail. This includes the realization of a fully 3D-capable FSW machine based on a so-called Pentapod concept. This parallel-kinematic system is suitable for large and complex three-dimensional structures up to 7 meters in length. The FSW research is focused on the welding of aircraft structures and mixed-material joints; examples of research projects and their results are given. Selected welding examples of wrought and cast aluminum, high strength alloys and copper will be presented as well as experiences, which have been made in terms of process development and process control.

Challenges in Joining Aluminium with Copper for Applications in Electro Mobility

The effective joining of aluminium with copper is one of the central technical goals involved in electro mobility. However, the joining of both metals by conventional fusion welding is challenging because of poor weldability arising from different chemical, mechanical and thermal properties of the materials and especially from the massive formation of hard and brittle intermetallic compounds (IMC) weld interface. In order to accomplish the difficult task of joining aluminium and copper several new joining technologies and strategies such as Laser Beam Welding (LBW) using highly dynamic beam deflection, Friction Stir Welding (FSW), Laser Induction Roll Plating (LIRP) and Electromagnetic Pulse Welding (EMPW) are under development at the Fraunhofer IWS. The current work describes the different technological approaches to the dissimilar joining of aluminium and copper. Thereby, the different joining technologies are compared with respect to weld quality. Special consideration is given to the study of interface morphology and microstructure of the welding zone. It will be shown that, depending on the joining method chosen the kind and extension of intermetallic phase formation differs considerably. Conclusions are drawn with respect to the applicability of the different joining methods.

EHMG WINTEREV 06: 'Property maps and near-surface characterisation'

The EPMA European Hard Materials Group (EHMG) held the third of its themed WINTEREV meetings at Fraunhofer IWS, Dresden on 23–24 February 2006, report BRYAN ROEBUCK and PETER BREWIN. Hosted by Dr Lutz-Michael Berger of the Institute for Materials and Beam Technology at Fraunhofer IWS, the meeting was attended by 17 people from 11 organisations from industry, academia and RTOs, who debated issues associated with structure–property relationships in an informal setting. Dr Berger also led, with colleagues, an informative tour of the Fraunhofer laboratories, particularly to see their advanced HVOF and laser deposition equipment.

Ultra hydrophobic wetting behaviour of amorphous carbon films

Carbon films are distinguished by their broad structural variability and a correspondingly wide range of properties varying from diamond-like to graphite-like. This variability is determined by the relation of diamond bonds (sp3) to graphitic bonds (sp2) and can be influenced by addition of other elements. The highest degree of diamond-likeness, e.g. expressed by high hardness, is achieved in pure carbon films. Such super hard films, so-called tetrahedral bonded amorphous carbon films (ta-C), reach a hardness from 40 GPa up to 80 GPa and a Young's modulus from 400 GPa up to 800 GPa.By incorporation of additional elements, not only the mechanical properties but also the surface energy can be varied. PTFE-like properties can be obtained by addition of fluorine or other elements. At Fraunhofer IWS Dresden such doped amorphous carbon films (a-C:X) have been deposited by a special pulsed vacuum-arc technique, the laser-induced pulsed arc (Laser-Arc). The deposition process is characterized by producing a high activated carbon plasma from a pure or doped graphite cathode and its deposition on substrates under vacuum or reactive conditions.The mechanical properties of the doped a-C:X-films were investigated in dependence on the content of the doped elements. Furthermore, the surface energy was determined by means of contact angle measurements.To get ultra hydrophobic properties of a surface additionally to a low surface energy a defined structuring of the surface topography is necessary. Different kinds of such structuring methods and their effects in conjunction with a doped a-C:X-film on the super hydrophobic behaviour are shown.

Lasergaslegieren – ein Verfahren zur Erzeugung verschleißfester Randschichten auf Titanwerkstoffen

AbstractTitanlegierungen besitzen hohe spezifische Festigkeiten, sind korrosionsbeständig und haben eine gute Biokompatibilität. Daraus resultiert ihr Einsatz im Bereich Luft‐ und Raumfahrt, der chemischen Industrie, der Medizin und zunehmend auch im Straßenfahrzeugbau. Ein Nachteil der Titanlegierungen ist ihr geringer Widerstand gegen Abrasiv‐ und Gleitverschleiß. Durch Erhöhung der Verschleißfestigkeit können den Titanlegierungen neue Anwendungsfelder erschlossen werden. Mit der im Fraunhofer IWS Dresden entwickelten Methode des Lasergaslegierens gelingt es, sehr tragfähige und verschleißfeste Randschichten auf Titanlegierungen in einem technologisch sicheren Prozeß zu erzeugen. Durch eine zusätzlich mit dem Laser‐Arc‐Verfahren aufgebrachte harte amorphe Kohlenstoffschicht lassen sich die Gleitverschleißeigenschaften noch einmal erheblich verbessern. Nach einer kurzen Vorstellungen der neuen Methoden des Lasergaslegierens werden die enormen Verbesserungen des Verschleßverhaltens anhand von Ergebnissen verschiedener Verschleißversuche nachgewiesen.