Surface texturing of fan-blade body by random-orbital polishing with in-line aqueous mist

Abstract

Mechanical polishing is the most common process to remove surface irregularities on fan blades such as scallop height features, while maintaining the required dimensional limits and textures. After the polishing process, the part will undergo shot peening, vibratory finishing, and later painting and coating at the final stages. It is therefore essential for the fan blade surface to be pre-treated with appropriate surface conditions to promote good surface-to-surface adhesion at the end of the manufacturing cycle. The existing method uses a single-axis rotary polishing tool where an external water coolant source is performed ad-hoc. This polishing method can produce average surface roughness, Ra, of 1.0 μm that satisfies the requirement. However, this form is aggressive and has a high material removal rate, resulting in excessive reduction in material thickness, and thus warrants rejection of costly fan blade. The main objective of this study is to better understand the influence of water and air coolant-induced surface texturing and its characterization by a proposed random-orbital polishing method. The criteria require surface roughness, Ra, between 0.8 and 1.0 μm and maximum height profile, Rz, between 5.0 and 6.0 μm for post-polishing condition, while maintaining low material removal rate to prevent under-thickness of the fan blade. Experimental trials are performed on fan blade specimens at the leading-edge sections and its surface topography characterized by coherence correlation interferometry. A range of different abrasive grit sizes is tested for suitability of media selection. Integrating in-line cooled air with deionized water to the process achieved desirable surface roughness, Ra, of 0.8 μm and height profile, Rz, of 5.8 μm, while simultaneously removing all traces of scallop height features and maintaining the leading-edge thickness within tolerance. Taking into consideration that the surface texture measurements are random in nature, the corresponding polishing methods were also analyzed with statistical functions by fast Fourier transform (FFT) and power spectral density (PSD). Comparison between the surface texture parameters or statistical functions with the corresponding polishing methods are then established. The surface integrity of the polished fan blade and wear condition of the abrasive disk are also reported in this study.