Laser-ultrasonic nondestructive evaluation of porosity in particulate reinforced metal-matrix composites.

In this work, we propose the laser-ultrasonic method for nondestructive evaluation of porosity in particulate reinforced metal-matrix composites fabricated by stir and in-situ reactive casting techniques. The method is based on the influence of porosity on dispersion of the phase velocity of longitudinal acoustic waves, which is measured by the broadband acoustic spectroscopy with laser excitation of ultrasound (laser-ultrasonic spectroscopy). We studied stir cast hypereutectic aluminum-silicon alloy A336 matrix composites reinforced with the SiC micro particles (3.3-13.5 vol%) and in-situ reactive cast Al/AlTi composites reinforced with the AlTi intermetallic particles (4-11.5 vol%). In the spectral range of 3-40 MHz, the phase-velocity dispersion in both types of composites was observed: the high-frequency velocity in the range of 20-40 MHz increases with the increase of the reinforcement content independent of porosity, whereas the low-frequency velocity in the range of 3-10 MHz decreases with the increase of porosity independent of the reinforcement content. As a result, the relative dispersion grows up with the increase in the composite porosity independent of the variation in the reinforcement content. The empirical dependence between the porosity in a scanning composite region and the relative phase-velocity dispersion in this region is approximated by the same unified function. For the first time, such unified porosity-phase velocity functional relationship is obtained for particulate reinforced metal-matrix composites completely different in fabrication techniques as well as in chemical composition and elastic properties of reinforcing particles.