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

Abstract

Airfoil structures such as fan blades have free form geometry which require a high level of precision in order to create a uniform finish for ideal gas path flow. Challenges in machining of such parts have led to rework in order to remove defects and conform to dimensional requirements at the same time.Mechanical polishing is the most common method to remove surface irregularities on fan blades such as scallop height, while maintaining the required dimensions. 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 pre-treated with rough, uniform textures in order to promote good surface-to-surface adhesion at the end of the manufacturing cycle.Generally, the polishing process is assisted by an external cooling medium applied on the part surface at intervals. This method of removing heat is not effective, as the polished surface may experience scratches or distortion, especially around thin-walled sections of the leading edge in fan blade. The existing polishing method uses a single-axis rotary tool can produce average surface roughness, Ra, of 1.2 µm that satisfies the requirement. However, this form of aggressive polishing has a high material removal rate, resulting in excessive reduction in material thickness which leads to the rejection of costly fan blade.This study a new localized polishing method and examines its effect on the surface topography of an airfoil component made of aluminum alloy. The area of interest is focused on the leading edge of a fan blade at which polishing is carried out using a random-orbital polishing tool with modified features to incorporate internal cooling capability. Experimental trials are conducted to study the effects of surface finish with fixed grain abrasive disks under four conditions. A cold gun is connected in-line to guide cold air inside the internal passages of the tool and out onto the surface of the part directly. A secondary cooling source by water nozzle spray is integrated in the tool to mix with the cold air jet and form an aerosol mist during tool activation.Surface topography of the samples are determined by arithmetic mean deviation, maximum height and root mean square of the profile. Surface roughness was performed using an optical profilometer. The localized polishing method achieved a desirable surface roughness, Ra of 0.8 µm, while removing all traces of scallop height and maintaining the leading-edge thickness within tolerance. The study showed that the new method produced a topography that is uniformly textured. This method can improve the manufacturing cycle time.