Speaker
Description
Casting is widely used to produce complex-geometry metallic aerospace components such as turbine blades. Mold filling, heat transfer and solidification control the microstructure, mechanical properties and final geometry. Typical defects include gas porosities (open or closed cavities generated by degassing of superalloys during solidification) and shrinkage porosities (cavities created by local solidification shrinkage when the remaining liquid cannot feed the contracting solid). These micrometer-sized pores degrade the mechanical performance of parts and assemblies [1].
To ensure the quality and traceability of aero-engine components, non-destructive testing (NDT) is performed throughout manufacturing. Detection of micrometric surface defects currently relies mainly on penetrant testing, which involves chemical products, multiple processing steps and no digital data. In this context, Safran is investigating non-conventional inspection methods, in particular active thermography using induction and laser heating, capable of delivering digital data. This work focuses on laser-heated active thermography. While laser thermography is widely reported for detecting millimeter-long cracks with micrometric openings [2–3], its application to microporosities with diameters of a few tens of micrometers remains sparsely documented, especially for cast components [4].
The technique consists in locally scanning the surface with a focused laser beam while measuring the induced temperature field using an infrared camera. A surface-breaking or near-surface pore increases the local thermal resistance, slows down heat diffusion and generates a local surface overtemperature. Subsequent processing of the infrared image sequences (spatial filtering and Fourier-based analysis) enhances defect-to-background contrast and enables accurate localization of indications.
The study combines numerical modelling in COMSOL with experiments on cut turbine blades previously inspected by penetrant testing. The heat transfer module is used to build a realistic model of the laser surface heat source, accounting for the local angle of incidence and for the spatial and temporal distributions of the heat flux. The influence of the laser beam cross-section on heating homogeneity and defect detectability is analyzed for different excitation modes (continuous scanning, pulsed and lock-in).
Experimentally, a 938 nm point laser mounted on an optical scanning device is coupled to a FLIR X6581sc infrared camera. Comparisons of thermal responses obtained on sound areas and on areas exhibiting penetrant-testing indications show that microporosities are detectable using active laser thermography. These results, together with those obtained using induction heating, will be used to assess the potential of active thermography to replace penetrant testing for the industrial inspection of turbine blades.
[1] S. Roskosz, “Evaluation of porosity of precision castings made of high-temperature creep resisting nickel superalloys”, Praktische Metallographie / Practical Metallography, 50(8), 527–547, 2013.
[2] N. Puthiyaveettil et al., “Laser line scanning thermography for surface breaking crack detection: modeling and experimental study”, Infrared Physics & Technology, 104, 2020.
[3] N. W. Pech-May et al., “Robot-assisted crack detection on complex shaped components using constant-speed scanning infrared thermography with laser line excitation”, Applied Research, 4, 2024.
[4] M. I. Silva et al., “Review of conventional and advanced non-destructive testing techniques for detection and characterization of small-scale defects”, Progress in Materials Science, 138, 2023.
