Influence of Surface Roughness from Additive Manufacturing on Laser Ultrasonics Measurements

Additive manufacturing (AM) is viewed as a revolutionary technique as it offers numerous appealing capabilities such as complex geometries, functionally graded properties, build-upon-demand, repairs, etc. However, in order to attain the full potential of AM, nondestructive testing for quality assurance of AM parts is essential. Laser ultrasound is of particular interest as a nondestructive technique for AM as it provides a viable means of in-situ process monitoring that could ultimately provide feedback for process control. Rayleigh waves generated by a pulsed laser could interrogate the current layer in the AM build and be received by a laser interferometer. The surface roughness is one challenge that must be overcome if Rayleigh waves are to be used for in-situ monitoring. Surface roughness has detrimental effects on the quality of measurements of laser ultrasonics due to factors such as speckle noise, non-uniform reflectivity of the surface, and wave scattering. In this research, we have studied the effects of surface roughness on generation, ultrasonic wave propagation and reception of laser-generated Rayleigh waves. Further investigations on the effects of surface roughness on nonlinear ultrasonic waves are also being carried out.