Additive manufacturing technology is expanding rapidly in several branches of industry. This process allows one to turn a numerical object into a physical one, layer by layer, by addition of the matter.
Tremendous progress has been made over the last few years and many industries such as nuclear, shipbuilding, marine and wind energies have shown an interest in these techniques which may serve a wide variety of applications. In fact, by contrast with standard shaping methods by matter removal, additive manufacturing offers new possibilities to repair worn out or damaged parts. This method proceeds by melting down metallic powders on top of a part using a laser. It is compatible with a vast array of metals and permits metallic depositions with complex geometries and a good metallurgical quality. Rather than changing parts, reparation presents a considerable advantage in terms of cost and timeline. Indeed, a number of components with complex geometry are unique and are difficult to reproduce in a timely manner leading to a more global asset. Indeed, a number of components with complex geometry are unique and the difficulty reproducing them in a timely manner results in a more global unavailability.
The proposed materials are AISI 316L for marine and nuclear industries. However, for the latter case, the stainless steel is coated with cobalt-based alloy to overcome the problems of friction. The typical defects on these hard coatings are a loss of matter because of wear or cracks due to friction movements. For naval applications, the reparation of both cracks and pits from marine corrosion are identified.
Laboratory experiments have revealed the importance of machining on repaired parts. Extensive thermal cycling inherent to the process is responsible for inducing strain inside the material, which may lead to cracks in the coating or the part itself. The success of the reparation is tightly linked to machining before, during and after reparation. Machining before reparation must be defined considering a geometry facilitating the reparation and a reduced strain introduction.
The influence of laser scanning strategy on strain introduction could be simulated with the finite element model (CAST3M/COMSOL) such as crack propagation. Experimentally, the effect of in situ machining during the reparation and post machining on strain introduction and consequently on crack formation will be compared.
The thesis will aim at evaluating the possibility of the reparation of parts by laser projection.As a first step, a parameter study in laser projection will be carried out in order to identify a range in which defect reparation is possible, meaning a dense deposition, minimum porosity and no crack.
Therefore, a second part of the thesis will be dedicated to optimizing machining in order to obtain repaired parts conforming to the requirements of the applications considered . The use of both the OPTOMEC MR-7 laser projection facility at CEA and the hybrid DMG Mori facility at NTU Singapore will allow us to compare in-situ and ex-situ machining and to understand its consequences on deformations and residual stress which may induce delamination and the problems of cracks.
Also, an approach of understanding is necessary to highlight the relations existing between the thermal cycles, the microstructure and the mechanical properties. For this purpose, a panel of characterizations must be identified, from mechanical tests to tomography. Micro-electrochemistry is also proposed to investigate local corrosion behavior in fixed zones. Finally, a research work on the qualification of the process must be done. This is an essential step to validate the reparation. According to the applications, various standards have to be identified.
The ultimate goal is to propose an approach that relies on setting up a digital chain allowing eventually to automate a large part of the reparation process. A 3D digital image of the defect to be patched allows to numerically determine the geometry of the deposition zone. Starting from the defect digital data and a comparison with the CAD model of the part in its original state, a 3D model is built. Based on the latter model a generation of a cladding trajectory is applied to the zone to be fixed. The cladding trajectory depends on the process parameters. This step is the most difficult to automate as many elements must be taken into account, such as the geometric complexity of the cladding zone, its accessibility and the material itself. The great flexibility of the projection process regarding projected materials opens the door to reparations with composition gradients, heterogeneous materials or ODS steels.
The thesis is a CEA/NTU collaborative project and will take place at both CEA Saclay (for 2 years) and at NTU Singapore (for 1 year). It will be an opportunity to share facilities and expertise of both entities.
CEA: Wilfried PACQUENTIN, firstname.lastname@example.org, 01 69 08 95 62