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

Cold gas spraying is an established process for coating substrates with similar or dissimilar materials. In this process, solid material particles are accelerated onto a substrate at high velocities by means of a high pressure process gas, typically heated to a temperature in the range of 373 K up to 1200 K. The bonding of material to the substrate is achieved by interlocking solid particles to the substrate through significant plastic deformation. In addition, the particle-particle and particle-substrate surfaces are subjected to high, localised shear stress. As a result, thin oxide layers on the surface break up and the particles are bonded metallurgically to the substrate or to other particles. Due to the high process speed the heat, generated during deformation, cannot dissipate fast enough, which leads to a local temperature increase.This adiabatic shear instability further promotes the process.The method is particularly suited for repair applications, since there are neither structual changes nor oxidation occuring during the process. Cold spray coating usually induces residual compressive stresses in the coating system, which can be be advantageous for repair applications. Especially for repair applications there is no reliable literature data available as to which geometry, of the cavities to be worked out, is most suitable and to what extent the feature geometry and the location of the repair site in the component are influencing the local residual stress distribution and therefore also impact the mechanical integrity of a component. By creating a fundamental understanding with regard to coating quality, local micro structure and the formation of residual stress distributions at the repair site, this project aims at extending cold gas spraying to the repair of near-surface damaged components, made of Inconel 718. The stability of the microstructure and residual stresses in the course of thermal loading and the warping associated with relaxation of local residual stresses is a key subject of the investigation as well. These findings are important for e.g. the evaluation of the shakedown behaviour of the components during commissioning in the medium or high temperature range.For the investigations, substitute geometries are chosen, which are relevant to practice and in which the local induction of residual stresses and their change in the course of a global static or a local cyclical thermal stress can have a significant effect. A sufficiently stiff, flat plate serves as a reference condition. The evaluation of the coating quality and the stability of the generated states is carried out by means of metallographic analyses, tomographic analyses, local residual stress analyses using complementary methods and the determination of the component distortions by means of 3D coordinate measurement technology.

• DFG Programme: Research Grants