Additive manufacturing, commonly referred to as 3D printing, is a highly promising technology for the manufacture of components with extremely complex shapes, unfeasible with conventional machining. Using the Laser Powder Bed Fusion (LPBF) process, an object is manufactured from a 3D digital model by successive stacking of powder layers fused by a laser. The use of alloy powders yields parts manufactured using this process unprecedented thermo-mechanical and topological properties, while the possibility of recycling unfused powders enables manufacturers to make gains in productivity and manufacturing costs. As a result, additive manufacturing is proving extremely popular with research establishments and the aerospace, energy, automotive and medical sectors. Although additive manufacturing techniques have made spectacular advances in recent years, their deployment remains conditional on mastery of the process, which is still highly perfectible [1,2]. Indeed, the thermo-hydro-dynamic phenomena involved in laser-material interaction and the kinetics implemented induce numerous defects: porosities, ejecta, geometrical defects, surface defects, residual stresses, cracks, delamination and inhomogeneity of the final structure. These local melting defects affect the mechanical properties of the manufactured parts. It is crucial to identify these defects at individual layer level, and to correct them before the upper powder layers solidify and the defect cannot be corrected. Holographic methods offer the prospect of in-situ, on-line optical diagnostics. Partners ONERA (Chatillon) and LAUM (Le Mans University) have recently developed a method of in-situ 3D visualization of themelting zone using digital holography, which is applicable to metal powders. The proof-of-concept has been patented [3] and has now been demonstrated [4]. The aim of this PhD progam is to bring the holography bench to pre-industrial maturity, in order to characterize the metallurgy of parts based on the properties of the melt bath, leading to in-situ control of laser-material interaction.
Objectives
1- Optimization of holographic set-up
Compaction of the holographic bench will be undertaken with new, more powerful laser sources integrated into the bench, whose architecture will be reviewed. An innovative hologram processing method will be developed to overcome the difficulties associated with the high reflectivity of certain areas of the melt. The performance of the algorithm in terms of spatial resolution and topographic
measurement sensitivity will be analyzed.
2- Tests and validations with materials AlSi7 and IN718
In close synergy with the PIMM laboratory (ENSAM Paris), tests will be carried out on AlSi7 and IN718 materials. Estimation of the reflectivity mapping of melting zones will be developed. Based on the analysis of holographic measurement sequences, their correlation with thermal radiation measurements and the analysis of defects observed post-fabrication, characteristic figures will be extracted with a view to creating a defect library.
3- Algorithm for default detection in the melt pool
A defect library will be built up by analyzing laser fabrications with different parameters to identify defects and determine associated singularities in holographic measurements. The development of an AI algorithm based on unsupervised deep learning will be carried out, which should make it possible to detect future structural damage, without the need for labeling work on voluminous training data. This
part will be carried out in close collaboration with the PIMM and MATEIS laboratories (INSA Lyon).
Candidate profile
This thesis is aimed at a motivated and inquisitive graduated (master degree) with higher education, with a good knowledge of (general) photonics and/or instrumentation, and a solid background in signal and image processing. The candidate will need to demonstrate autonomy and creativity to successfully complete the job.
Contract conditions
The PhD student will be based at LAUM, Le Mans, France, but will work very closely with ONERA (Chatillon, close to Paris). Missions to PIMM (Paris) and MATEIS (Lyon) will be planned during the program.
Recruitment from December 2024, funding from the French National Agency for Research over 36 months (salaries, missions, equipment)
Supervision and contacts
Address Curriculum Vitae and motivation letter to:
P. Picart (LAUM CNRS);
pascal.picart@univ-lemans.fr
B. Sorrente (ONERA Chatillon);
beatrice.sorrente@onera.fr
M. Thomas (ONERA Chatillon);
marc.thomas@onera.fr
S. Montrésor (LAUM CNRS);
silvio.montresor@univ-lemans.fr
K. Hassan (LAUM CNRS);
kais.hassan@univ-lemans.fr
Bibliographic references
[1] S. K. Everton, et al, “Review of in-situ process monitoring and in-situ metrology for metal additive
manufacturing,” Materials & Design 95, 431-445 (2016).
[2] R. Fabbro, et al “Experimental study of the dynamical coupling between the induced vapour plume
and the melt pool for Nd–Yag CW laser welding”, J. Phys. D: Appl. Phys. 39, 394-400 (2006).
[3] M. Piniard, B. Sorrente, G. Fleury, P. Picart, “Dispositif de contrô le pour fabrication d’une piè ce
avec ajout de matiè re”, PCT/FR2021/051249.
[4] M. Piniard, B. Sorrente, G. Hug, P. Picart, “Melt pool monitoring in Laser Beam Melting with
two-wavelength holographic imaging,” Light : Advanced Manufacturing 3(11) (2022).
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