The increasingly severe sets of requirements resulting from material applications lead to multifunctional and often conflicting properties. To face this challenge, one has to operate on the association of different materials, on their shape, on their spatial arrangement, i.e. to deal with architectured materials. The variety of possible solutions requires to use numerical modelling strategies, including optimization procedures.
The objective is to determine, for a given set of requirements, the most favourable architectures. Shape optimization as well as parametric optimization have to be coupled with numerical approaches. These approaches have to include the specific constraints related to the fabrication process of these architectured materials.
The fields of application are various (weight saving in aeronautics, thermal management of buildings, osseointegration …). A special attention is paid to architectures obtainable by additive manufacturing and/or powder metallurgy.
(a) Effusion cooling through a multi perforated wall
(b) Transpiration cooling through a porous wall (partially sintered metallic powder)
Figure 1: Enhanced heat transfer cooling technologies.
During transpiration cooling, the cooling air flows through a porous liner in which a large amount of heat can be exchanged by convection. Partially sintered metallic materials are potential candidates to form these porous liners. Flows and heat transfers are governed by some effective material properties which depend on the porous architecture: the effective solid phase thermal conductivity, the volumetric heat transfer coefficient and the permeability properties. Thanks to experimental works and numerical studies on samples digitized by X-ray tomography, simple relationships have been developed for partially sintered materials. These relationships have been integrated into a heat transfer model predicting the thermal performance of a design at working engine conditions. A multi-objective optimization and an optimal design analysis highlight some designs as being potentially interesting for transpiration cooling. Materials with a low porosity and made of large irregular powders seem to ensure the best trade-off among the different criteria taken into consideration (PhD thesis of S. Pinson, funded by CEMAM Labex).
Coatings as polyelectrolyte multilayer films (PEM) are currently developed as carriers for bioactive molecules able to induce bone regeneration, like the Bone Morphogenetic Proteins (BMP). In a closed collaboration with C. Picart team (PhD of A. Ho-Shui-Ling), we develop TA6V scaffolds with a tuneable macroporosity and an osteoinductive surface. We study their respective influence on the bone regeneration at the bone-material interface. 3D scaffolds have been designed and built by EBM. Their surface have successfully coated with the PEM films previously developed. Cell seeding was optimized. First results show the possibility, via osteoinductive coatings, to trigger the formation of bone at the surface and in the bulk of the scaffolds.
(a) EBM fabricated TA6V scaffolds with tuneable porosity and architecture. (b) Fluorescence confirms the presence of PEM inside the scaffolds (credit A. Ho-Shui-Ling, J. Vollaire). (c) After 1 week, an actin filament carpet is visible at the surface and inside the scaffold (nuclei in blue and actin filaments in green)
We have recently developed corrective functions in order to take into account the geometrical characteristics inherited from EBM fabrication of thin structures. Depending on the thickness, the orientation of single struts, we are for example able to optimize the orientation of the parts during the fabrication, for a given desired property (cf M. Suard PhD thesis).
Such results can be used into a process we define as “grey-level optimization”. The idea is to stop a classical topological optimisation process and to replace the intermediate densities by optimized lattice bricks (PhD of P.T. Doutre, in close collaboration with Polyshape).
Replacing an intermediate density by an optimized lattice structure.
Date of update January 16, 2019