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Description
Casting is a manufacturing process in which a molten material is usually poured into a mold which contains a hollow cavity of the desired shape and then allowed to solidify. The casting process involves fluid flow, heat transfer, and solidification phenomena which together determine the microstructure, mechanical properties and dimensional accuracy of the final product. Casting is used for the fabrication of complex shaped parts such as turbine blades. Gas holes and shrinkages are the common defects that can occur during the process. They are closed or open spaces not filled with the casting material. These micrometer sized porosities degrade the mechanical performance of parts and assemblies.
To ensure the quality of the components, non-destructive testing (NDT) is performed throughout the manufacturing process. The detection of micrometer scale surface defects currently relies mainly on fluorescent penetrant testing, which involves chemical products, multiple processing steps and provides no digital data. In this context, Safran is investigating advanced non-destructive evaluation techniques, in particular active thermography using induction and laser heating, capable of delivering the digital data. This work focuses on induction heating based active thermography. While induction thermography is widely reported for detecting millimeter-long cracks, its application to porosities with diameters of a few micrometers remains sparsely documented. As the dimension of the defect becomes smaller, all the experiment parameters must be re-investigated for the successful implementation of induction thermography.
This study demonstrates the capability of induction thermography in detecting microporosities. Numerical modelling using COMSOL is utilized to understand the detection mechanism and to evaluate the influence of various experiment parameters in the detection of microporosities. These results, together with those obtained using laser thermography, will be used to assess the potential of active thermography to replace penetrant testing for the industrial inspection of turbine blades.
