Assessment of local residual stress distributions at the local repair of components by means of cold gas spraying

• Applicants: Dr.-Ing. Jens Gibmeier; Professor Dr. Robert Vaßen
• Subject Area: Coating and Surface Technology
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 452606919

The aim of the project is to continue to extend the application spectrum of repairs of components made of Inconel 718 (IN718) that are damaged close to the surface using cold gas spraying by creating a basic understanding of the process with regard to coating quality and the formation of local residual stress distributions in the area of the repair. In the first funding period, it was shown that, depending on the coating parameters, high residual stress amounts are sometimes introduced into the repair area, which leads to large residual stress gradients due to the formation of a residual stress equilibrium in the direct interface region. In order to avoid premature damage caused by residual stress, it seems favorable to minimize the residual stresses in the repair region by using appropriate spray parameters without negatively affecting the coating quality. This optimization problem will be supported by a novel numerical model for predicting the residual stresses during cold gas spraying. When transferring the results to more complex geometries with curved surfaces, CFD (Computational Fluid Dynamics) flow simulations, which were already established in the first funding period, are used specifically to optimize the geometry of the cavities for an existing component geometry, taking into account the gas flow. In the continuation of the project, the adhesion in the edge area of the cavity is in the focus and will be analyzed in detail. At the same time, the tensile adhesion test according to DIN EN ISO 14916 is being adapted in the continuation of the project so that not only flat, coated substrates can be examined for their adhesion strength, but the geometry of the repair cavity is also taken into account. These investigations are supplemented by local determination of the load transfer at the interface between the substrate material and the repair filling using in situ neutron diffraction analyses of samples under bending stress. With regard to the determination of residual stress distributions in the area of the repair site, laboratory methods will be used more frequently in the future in order to become as independent as possible from analyses at the few large scale (neutron) research facilities. To this end, the contour method for residual stress analyses on the repair geometries will be improved. The mechanical method allows the mapping of residual stresses in the repair cavity and in the surrounding substrate. The results will be validated by applying complementary laboratory methods (X-ray diffraction analyses, incremental hole drilling method) and the neutron diffraction analyses that have already been carried out and are still planned within the continuation. After successful validation of the new approach, a much larger number of variants can be analyzed and used for evaluation with less effort in a much shorter period of time.

• DFG Programme: Research Grants