Investigation on High Power Laser Removal of Thermal Barrier Coating (TBC) and Bond Layer (MCrAlY) from Inconel 718 Alloy
J. C. Hernandez-Castaneda, W. Y. Chi, F. Yongwei, N. F. Lan, M. C. C. Khin and Z. Hongyu
This paper reports the development of a precise laser removal process of thermal barrier coating (TBC) and bond layer (BL) from aero engine components as an alternative method to hazardous chemical bath processes. A high power Nd:YAG laser (λ=1064 nm, τ=38 ns) system was successfully employed to remove yttria-stabilised zirconia (TBC) coating and bond layer (MCrAlY) from equiaxial grain Inconel 718 alloy. Investigation on the different assisting media and process conditions to remove TBC coating by high power pulsed laser are reported. A parametric investigation of direct laser removal process of the TBC coating established optimal conditions to achieve 140 mm³/min removal rate. A novel twostep laser scanning method was developed to remove the bond layer coating at 10 mm³/min and with a minimal recast layer of 4 μm thickness. A wavelength-dispersive X-ray spectroscopy (WDS) analysis identified the chemical composition of the laser cleaned surfaces for the complete set of conditions tested. It was found that low oxidation levels were attained when the laser beam was scanned at 14 kHz followed by a 6 kHz laser beam scan which is also the optimal condition for the highest material removal rate. An X-ray diffraction (XRD) analysis identified the type and magnitude of residual stresses after the laser cleaning process. The measured residual stresses are tensile in nature with an average magnitude of 547 MPa, which are comparable to those observed in laser additive processes.
Keywords: laser removal, thermal barrier coating, bond layer, MCrAlY, Inconel 718