NON-DESTRUCTIVE ANALYSIS OF CHEMICAL TREATMENT OF INTERNAL SHAPED CHANNELS IN LPBF INCONEL 718

Abstract

The laser powder bed fusion (LPBF) method can offer significant technological advantages in the aerospace sector for the production of complex and integrated parts, such as turbine blades or combustion chambers with an internal networks of film cooling holes [1]. However, the build direction of parts in LPBF causes sizable differences in the achievable surface qualities. Most critical are the downward facing areas, which suffer from excessive heat due to poor thermal conductivity of the powder underneath, resulting in fusion defects, growth of melt conglomerates and powder particle adhesions. Defects and poor surface quality are particularly problematic in internal cavities difficult to access for post-operative machining. One way to improve the quality of internal surfaces are chemical etching techniques using pressurized fluids. Their evaluation and optimization require careful control by comparative analysis of the original and treated surfaces. The best method for non-destructive evaluation is an X-ray scanning using tomographs or synchrotrons, followed by 3D reconstruction of objects before and after etching and their accurate quantitative representation. Fragments with internal channels were scanned using the ESRF synchrotron (Grenoble, France) before and after applying two different post-processing methods. The presented method allowed a qualitative analysis of film cooling holes with different diameters ranging from 0.4 to 1 mm, inclinations (from horizontal to diagonal) and configurations (from annular to fan-shaped widening or narrowing into a slit). The comparative equivalent diameters and surface curvature distributions allowed high precision diagnostics of the posttreatment efficiency of LPBF-derived Inconel 718 internal channels.