The influence of texture and phase distortion on ultrasonic attenuation in Ti-6Al-4V

A high resolution experimental capability has been developed to map the phase and magnitude of ultrasonic waves transmitted in a solid. The advancement presented in this paper is provided by laser detection of the ultrasonic energy over a microscopic aperture of approximately 50 mum. The system is built around a computer controlled scanner and a confocal Fabry-Perot interferometer, which uses a diode pumped, frequency-doubled Nd:YAG laser as a light source. Wave propagation in the axial and radial directions of a 2.5" diameter bar of textured Ti-6Al-4V was investigated in this study. Measurements were also taken on samples cut with angles between the surface normals and the axis of the bar of 0, 30, 45, 60, and 90 degrees. The work was motivated by the observation of unusually high apparent attenuation in the axial direction of the as-received bar, thought to be associated with phase distortion rather than actual energy loss. The current phase mapping results, using a focused laser spot, show relatively high wavefront distortion and more nonuniform distribution of the transmitted energy in the axial direction. The contribution to attenuation associated with phase cancellation loss was also investigated. These measurements show the laser detected attenuation to be substantially lower than the piezoelectrically measured attenuation. However, even the relative phase insensitivity of focused laser detection approach clearly indicates the attenuation to be strongest in the axial direction. This paper demonstrates the orientation dependence of attenuation stems from scattering effects associated with texturing and the elongated macroscopic grain structure in the mill annealed Ti-6Al-4V bar generated during processing, which may also affect diffraction and beam divergence.