In-Process Monitoring of Surface Roughness of Internal Channels Using

Abstract

Additively manufactured (AM) parts generally have a higher surface roughness than parts manufactured using conventional methods. In most applications, a smooth surface finishing is preferred since rough surfaces are prone to corrosion attacks and fatigue crack incubation. Therefore, surface finishing is necessary to reduce the surface roughness of AM parts before they are used. The roughness of outer surfaces can be measured directly, using either contact stylus profilometer or non-contact optical microscope. However, there are still challenges to quantify the roughness of the internal surfaces using these conventional techniques of surface metrology. In this paper, we present a method to measure the roughness of internal channels by analyzing ultrasonic signals from the backwall reflection. A frequency-domain technique based on phase-screen approximation is used to reconstruct the root mean square ($R_{q}$) value from the ultrasonic signal scattered from the rough surfaces. Finite element simulations are developed to demonstrate the method, showing excellent accuracy when the $R_{q}$ value is within 1/15 of the wavelength of the incident ultrasound. The method is then applied to monitor the surface roughness of the internal channels during the Abrasive Flow Machining (AFM) process. The reconstructed roughness value shows a clear and steady downward trend during the polishing process, quantitatively indicating the polishing rate. This demonstrates that such ultrasonic method can be used as a tool to provide feedback controls in the polishing process.