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>Méthodes morphologique et par éléments finis combinées pour une nouvelle approche de la modélisation 3D du dépôt par projection dynamique par gaz froid (« cold spray »)
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Méthodes morphologique et par éléments finis combinées pour une nouvelle approche de la modélisation 3D du dépôt par projection dynamique par gaz froid (« cold spray »)
This study on the cold spray process aimed at achieving an original coating build-up model, capable of predicting the resulting microstructure as a function of powder morphology and process parameters. The work focused on three main interrelated subjects: 3D powder characterization, simulation of individual impacts on a flat substrate by the finite element method and deposition build-up modeling.An innovative method based on microtomographical observations was used for 3D characterization of the powder. Image analysis allowed to separate single powder particles and to gather them into a 3D collection containing approximatively 18 000 objects. Their size and shape were quantitatively measured. A cluster analysis method (K-means) was then applied to this data set to divide the particles into 7 classes based on their shape.The second main research topic consisted in performing particle impact simulations on a flat substrate by the finite element method (using the commercial software Abaqus). The use of dedicated meshing tools allowed to simulate the impact of real particles, as observed by microtomography. Scripting techniques were used to carry out a large number of these simulations but, due to limited robustness of the procedure, only few of them were successfully conducted.The third research area focused on the development of a deposition build-up model (in 2D to allow a simpler implementation). Data from finite element results were interpolated and used in an iterative simulation, where impacting particles were deposited one by one. Different approaches were tested but the development of the model could not be completed in the framework of this thesis.Model validation could be performed on finite element simulations. The two kinds of splats (Ta on Cu and Ta on Ta) were considered separately. Concerning the first, direct microtomographical imaging could be applied, due to the heterogeneity of materials. Splats were observed, individually separated and gathered in a 3D collection as done before with powder particles. Simulated and observed splats could then be compared on a statistical basis. No particular discrepancy was observed, confirming the impact simulation method used. The second kind of splats (Ta on Ta) was complicated by the homogeneity of the materials, preventing the use of microtomography. The deposition (before spraying) of a contrast layer between Ta substrate and Ta particle was tried by different techniques. The only method giving exploitable results was the chemical vapor deposition of a Fe layer onto the powder particles. However, the small number of adherent particles and the weak contrast obtained in the images prevented the use of the methods already applied to powder particles and Ta splats onto Cu.The optimization of powder granulometry and shape (towards a specific application) is one of the main expected applications of the deposition build-up model, together with the simulation of composite powders (typically, metal and oxide). The involvement of phase transformation phenomena into the model could extend its application to the whole family of thermal spray processes (plasma, HVOF, etc.) or to other additive manufacturing techniques. In general, the philosophy behind our modeling approach could be applied to every manufacturing/coating technique where the supply material is in powder form and undergoes a certain transformation during the process. Finally, the coupling of such a model with homogenization techniques would allow the prediction of macroscopic properties depending on deposit microstructure (e.g. thermal or electrical conductivity).
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