Nielsen, Sören

Material deposition in cold spraying occurs in solid state and thus avoids undesired effects of melting and solidification. However, residual stress conditions in cold sprayed coatings could limit possible part performance. The temperature distribution and thermal history of the cold sprayed components has significant influence on stress distribution and thus deposition and part quality. The present study investigates the effect of substrate material and nozzle traverse speed (as a secondary parameter) on effective temperatures and residual stress distributions of titanium-grade 1 deposits. The results demonstrate that substrate material properties and nozzle traverse speeds have significant influence on residual stresses of the cold spray deposit. It is understood that coefficient of thermal expansion (CTE) difference of the coating and substrate materials has significant effect on residual stress state. On the other hand, the residual stresses change from more compressive to more tensile state as the temperature of the components increases by decreasing the nozzle traverse speed. These findings indicate that thermal parameters affect residual stresses substantially. Thus, by adjusting the kinematic parameters and reducing maximum reached local temperatures within the part, more favorable stress states of the finished component can be obtained. The attained knowledge is essential for the development of high-quality deposits and the selection of the best strategies for repair and additive manufacturing applications.

In recent years, cold spraying (CS) has emerged as a promising technology for repair applications, particularly for oxidation-sensitive materials. In order to obtain an optimum repair result that fulfills the highest requirements regarding material properties, simple geometric shape restoration is not sufficient. Any additive manufacturing process results in particular features in microstructure, possible defects and respective – potentially even anisotropic – mechanical properties. To systematically tailor these microstructures and properties to the specific component and geometry requires complex routines. This work proposes the design of a knowledge-based cold spray repair system that facilitates a complete individual repair procedure for aircraft components. This system includes the elements of (i) reverse engineering to analyze, classify and generate digital data of the damaged component, (ii) pre-processing to obtain the ideal conditions for the CS process, (iii) toolpath planning to optimize robotics for the CS process, (iv) on-line monitoring to ensure process quality, (v) post-processing and (vi) performance testing of the material properties to meet the challenging requirements of the aerospace industry. By using an industrial robot and computer-aided planning of the trajectories, components are to be repaired under cold spray and geometrical conditions for ideal material deposition. The goal is to obtain repaired components that fulfill the required property profile equally well as respective new parts.