Scientific direction Development of key enabling technologies
Transfer of knowledge to industry

PhD : selection by topics

Technological challenges >> Additive manufacturing, new routes for saving materials
6 proposition(s).

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Advanced nanocomposites for additive manufacturing

Département des Technologies des NanoMatériaux (LITEN)

Laboratoire Synthèse et Intégration des Nanomatériaux

01-10-2020

SL-DRT-20-0419

thomas.pietri@cea.fr

Additive manufacturing, new routes for saving materials (.pdf)

The proposed scientific objectives are at the crossroads of nanomaterials and additive manufacturing. Various 3D printing technologies of polymeric matrices have been developed, allowing a conversion of a numerical model with a great precision. But, due to the very recent development of these technologies, the currently available materials appear insufficiently mature and require significant improvements. A great chance of success for properties enhancement could certainly come from the fabrication of advanced nanocomposites (through inclusion of nanomaterials within a polymeric matrix). The work that will be carried out during his PhD will take advantage of the synthesis and functionalization of one-dimensionnal nanomaterials (nanowires, nanotubes). After characterization of the intrinsic properties of the nanocomposites, printable wires will be produced and used with 3D printers. High performance nanocomposites will be used for the fabrication of 3D elements with high conduction of electricity and/or heat. Applications for health will also be considered.

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Microstructural changes in additive manufacturing materials during Hot Isostatic Pressing: modelling and experimental study

Département Thermique Biomasse et Hydrogène (LITEN)

Laboratoire Conception et Assemblages

01-10-2020

SL-DRT-20-0470

emmanuel.rigal@cea.fr

Additive manufacturing, new routes for saving materials (.pdf)

Additive manufacturing (AM) processes are promising techniques for manufacturing metallic components from powder or wire feedstock. AM materials exhibit microstructures very different from cast or forged equivalent materials. They are out of equilibrium, sometimes anisotropic, with specific features like a high dislocation density and defects (unmelted particles, pores) which may be detrimental to mechanical properties (creep, fatigue resistance). Defects can be mitigated using a heat treatment under high gas pressure (hot isostatic pressing HIP), at the expense of material softening. The objective of the PhD thesis is to model the microstructural évolutions during HIP in order to optimise the HIP cycle for a given AM microstructure: defects shall be decreased enough while softening shall be limited. A detailed characterisation of the initial microstructure will be done (defects, grain size, dislocation density, precipitates, texture?) in order to provide data for the DIGIMU software. This software uses the level set method to simulate, by finite element calculation, the evolution of a volumic element representative of a microstructure during thermomechanical loading. This software will be enriched. The comparison between modelled evolution and experimentally observed ones will be used to assess the relevancy of the modelling (HIP will be applied on samples). Furthermore, attention will be paid to the evaluation of the impact of the HIP treatment on mechanical properties of AM material (316L steel will be used).

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Embedding of high temperature resistant Fiber Bragg Gratings into metal structures obtained by additive manufacturing processes

Département Métrologie Instrumentation et Information (LIST)

Laboratoire Capteurs Fibres Optiques

01-10-2020

SL-DRT-20-0645

guillaume.laffont@cea.fr

Additive manufacturing, new routes for saving materials (.pdf)

LCFO laboratory from the Technological Research Division at CEA List, in partnership with the LISL laboratory from the CEA DEN, specialized in metal additive layer manufacturing processes, proposes a PhD thesis aiming at developing methods to integrate optical fiber sensors (OFS) based on high temperature resistant Fiber Bragg Gratings (FBGs) in metallic components obtained thanks to metal additive layer manufacturing processes either for the aerospace or for the nuclear industry. Thanks to recent developments, ultra-stable FBGs have been realized using direct writing processes into silica optical fibers with femtosecond lasers. These temperature and strain transducers combined with special optical fibers designed for very high temperature environments will be considered for the instrumentation of components obtained by metal additive layer manufacturing. This project aims at contributing to the adoption of in situ monitoring of 3D-printed metallic components, paving the way for their Structural Health Monitoring (SHM) to anticipate failures in the fabrication process and to optimize operating costs thanks to the development of predictive and conditional maintenance-based procedures.

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Microstructure control of additive manufacturing parts by generation and detection of ultrasound by laser

Département Imagerie Simulation pour le Contrôle (LIST)

Laboratoire Instrumentation et Capteurs

01-01-2020

SL-DRT-20-0757

jerome.laurent2@cea.fr

Additive manufacturing, new routes for saving materials (.pdf)

The metal additive manufacturing (AM) processes show a great potential still growing, and those in very varied applications fields. However, existing systems have limitations, in particular on the ability to adapt the microstructures and to detect online the melting defects [1]. To overcome these limitations, it is necessary to develop new manufacturing strategies that could make it possible to adapt the solidification conditions as well as online non-destructive testing (NDT) inspection methods. Direct energy deposition (DED) or selective laser melting (SLM) processes use metallic powder and a locally concentrated energy source, which generates strong thermal gradients, which most often lead to highly oriented microstructures and relatively rough surfaces, both making NDT inspection and physical interpretations more tricky. The microstructures produced are out of thermodynamic equilibrium and possess coarse grains structure; they are characterized by the entanglement of columnar and equiaxed grains. This type of microstructure influences significantly the mechanical behavior and elastic waves propagation; the size distribution of heterogeneities are close to the acoustic wavelengths, having for effect the attenuation and scattering of elastic waves. One of the major challenges in AM is to reduce/prevent the formation of columnar grains during manufacturing, as their presence within the microstructure is the most unfavorable for the use properties. By controlling the thermal conditions during the solidification/crystallization (cooling rate, temperature gradients), it is a priori possible to favorably induce the formation of equiaxed grains. It is also known that, by insonification of the molten metal with high intensity ultrasound, it is possible to perform a ?grain refinement-like?, or also to generate other effects (cavitation, flow, mixing, spraying, dislocation, scattering and phase transformation [2]). Indeed, when an elastic vibration is applied directly to molten metal, it would be possible to control-like the solidified grain structure, i.e. to change locally the direction of growth and morphology of the microstructure during the solidification phase. Therby perturbating the solidification conditions, then, it is conceivable to cause the formation of equiaxial grains, and, potentially, the number of flaws (microporosities, cracks). This observation sets the objective of this thesis, which aims to shape more optimally the AM microstructures by vibrating the melting pool and to conduct either offline or online monitoring by ultrasonic-laser (LU) technique. On the one hand, the work, in the CEA-DEN-LISL Lab. [3], will consist of controlling the microstructural evolution of AM parts by contactless vibration of the melting pool. Thus, it will seek to modify the dynamics of the melt, for example, by destabilizing the dendritic growth in the solidification front due to elastic waves generation with a continuous-wave modulated or pulsed laser source. The control parameters will be assessed in laboratory conditions using an existing experimental prototype, which will be improve by adding several other instruments (fast/thermal/Schlieren cameras, pyrometer) to generate ?enhanced microstructures? and by improving the coaxial manufacturing nozzle. On the other hand, the work in the CEA-DRT-LIST Lab., will consist to inspect online AM samples by employing a LU method. Thus, it will seek to generate and detect ultrasound by laser in the melt, to monitored, for example, the evolution of solidification front, keyhole outbreak, optical penetration evolution, and so on, by measuring acoustic precursors [4]. Ultrasonic characterization measurements, under laboratory conditions, will also be carried out in order to determine the elastic properties by LU [5], whether using surface waves (Rayleigh) or zero-group velocity modes (local Poisson's ratio, anisotropy, thickness), and other NDT methods available from the LIST lab., which can then be compared to EBSD (homogenization method) and metallurgical cuts. FDTD or Finite Elements simulations of ultrasound in these rough and heterogeneous media will also be considered. References: [1] Zhao et al, Phys. Rev. X, 9, 02052, (2019), Wolff et al, Sci. Rep., 9, 962, (2019), Martin et al., Nat. Com., 10, 1987, (2019), Wei, Mazumder & DebRoy, Sci. Rep., 5, 16446, (2015). [2] G. I. Eskin & D. G. Eskin, ?Ultrasonic melt treatment of light alloy melts', 2nd edn, Boca Raton, FL, CRC Press, (2014), M. C. Flemings, ?Solidification processing', McGraw-HilI press, (1974), T.T. Roehling et al., Acta Materialia 128, 197, (2017), M.J. Matthews et al., Optics Express 25, 11788, (2017). [3] P. Aubry et al., J. Laser Appl., 29(2), (2017) [4] Walter & Telschow, QNDE, 15, (1996), Walter, Telschow & Haun, Proc COM, (1999), Carlson and Johnson, WJ, (1998), He, Wu, Li & Hao, Appl. Phys. Lett., 89, (2006). [5] Clorennec, Prada & Royer, Murray, Appl. Phys. Lett., 89, (2006), Laurent, Royer & Prada, Wave Motion 51(6), (2014) Laurent, Royer, Hussain, Ahmad & Prada, J. Acoust. Soc. Am. 137(6), (2015).

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In-situ characterisation of the temperature field in a Powder Bed Fusion additive manufacturing fabrication process

Département Imagerie Simulation pour le Contrôle (LIST)

Laboratoire Simulation et Modélisation en Electro-magnétisme

01-10-2020

SL-DRT-20-0768

florian.lebourdais@cea.fr

Additive manufacturing, new routes for saving materials (.pdf)

Additive Manufacturing is a field that is currently in full swing. Among the processes used, those based on the use of a powder bed designated under the acronym PBF (Powder Bed Fusion) appear to be the most promising for the production of complex parts at the quality standard of the most demanding manufacturers. Many interesting results have already been obtained, but there is still a need to understand the solidification and annealing mechanisms involved in the manufacturing of the parts. To this end, it is necessary to develop high-performance in situ diagnostics to control the process. Among the many parameters to be controlled, temperature plays a particular role because it is featured in all the physical mechanisms involved. However, to date, there is no possibility to estimate the temperature field within the powder bed. Preliminary studies conducted within our team have made it possible to evaluate an original technique, based on the measurement of a time of flight and the variation of the propagation velocity of an ultrasonic wave (US) with temperature. The results obtained have shown that the technique has a clear potential. However, compared to experiments performed ex situ on a reference sample, ultrasonic signals are noisier on an additive manufacturing raw material. Beyond the research of the physical causes leading to this behaviour, and of any information to be derived from this noise, the PhD work should make it possible to specify the conditions that will allow effective measurements to be made. The purpose of this thesis with a mixed experimental / signal processing character will therefore be to implement the ultrasonic measurement developed in an Additive Manufacturing machine located on-site at the Additive Factory Hub platform (CEA Saclay). The subject is intended for a motivated student with ideally a training of the type "Instrumentation / Physical Measurements". Basic knowledge of materials science and real-time computing would be a plus.

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Improvement of Diffusion Welding assembly process for large exchangers manufacturing : identification of critical welding defects and modeling of their closure kinetics

Département Thermique Biomasse et Hydrogène (LITEN)

Laboratoire Conception et Assemblages

01-09-2020

SL-DRT-20-0841

isabelle.moro@cea.fr

Additive manufacturing, new routes for saving materials (.pdf)

The diffusion bonding method has been used for many years in order to manufacture heat exchangers. Concretely, these exchangers consist of machined plates, stacked, then put under vacuum in a container and then introduced into a Hot Isostatic Compression (CIC) furnace which, by the simultaneous application of a high pressure and a high temperature, allows the plates to be assembled together by atomic diffusion. The main objectives of the thesis are on the one hand the improvement of the model allowing to describe the kinetics for closing the pores at the interfaces of welding, using both experimental and numerical approaches, and on the other hand the quantification of the influence of different geometries and residual pore levels on the mechanical strength of these interfaces. This will ultimately allow "degraded" CIC cycles to be defined so that they allow a minimum of macroscopic deformation of the exchanger during its assembly by CIC, even if it means accepting defects. However, it should be possible to distinguish allowable defects from those that are not, and via modeling to predict their frequency, their exact geometries and their preferential locations. This thesis presents both experimental and numerical aspects in a context of a strong technological challenge. As a first step, the study will focus on a reference material of type 316L that has already been the subject of numerous preliminary studies. Its behavior has already been studied in a temperature range from ambient to more than 1000 ° C, and a suitable constitutive law and associated parameters have been determined and validated. Numerous structures of 316L type material have already been assembled by CIC, and important feedback already exists within the laboratory concerning the assembly of this type of material. The work to be done during the thesis will consist in the realization of assemblies by CIC on sheets having different types of defects and varied geometries, these defects having been carefully characterized. A follow-up of the progressive elimination of these defects during the welding diffusion and the modeling of this process via the model of Hill and Wallach will make it possible to highlight shortcomings or inadequacies of the different mechanisms constitutive of this same model, which did not never been realized until then. This work will lead to an improvement of the Hill and Wallach model currently used. In a second step, we will seek to identify the harmfulness of different defects according to their geometry, their frequency and their location. To do this, typical assemblies will be manufactured and tested. It will also be necessary to identify the most relevant mechanical test (s), which may also be a function of the location of the defect in the assembly. Thus, a fault located in a side bank of a heat exchanger does not see during the operation of the exchanger the same thermo-mechanical loading as a fault located in an isthmus. This work will eventually make it possible to differentiate in an assembly the welding defects with prohibitive interfaces, from those acceptable. In the end, these two lines of research will allow, on the one hand, via modeling to predict the geometry of the residual pores at the interfaces after assembly by CIC, and on the other hand to be able to predict if these defects after diffusion welding are critical for the structure or not. It will thus be possible to define, in terms of pressure and temperature evolution during assembly by CIC, an optimized cycle which will allow both the minimization of the macroscopic deformations of the structure, and the obtaining of a welding sufficient to ensure the operation of the exchanger.

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