Laser-Ultrasonic Study of the Local Porosity of Reactive Cast Aluminum Matrix Composites

One of the most critical manufacturing defects of cast metal-matrix composites is a nonuniform porosity distribution over the composite volume. This nonuniformity not only leads to local softening, but also plays a key role in the evolution of the damage process under external loads. In this work, we present the results of the study of a local porosity in disperse-strengthened reactive cast aluminum matrix composites. The studies were performed using a laser-ultrasonic method based on statistical analysis of the amplitude distribution of backscattered broadband pulses of longitudinal ultrasonic waves in composites. Laser excitation and piezoelectric detection of ultrasound were performed using a laser-ultrasonic transducer. Two series of reactive cast aluminum matrix composites were analyzed: reinforced by in situ synthesized Al₃Ti intermetallic particles in different volume concentrations and by Al₃Ti particles with the addition of synthetic diamond nanoparticles. It has been found that, for both series of composites, the amplitude distribution of backscattered ultrasonic pulses is approximated by a Gaussian probability distribution applicable for statistics of a large number of independent random variables. The empirical dependence of the half-width of this distribution on the local porosity in composites is approximated by the same nearly linear function regardless of the size and concentration of reinforcing particles. This function was used to derive the calculating formula for determining the local porosity in the studied materials. The obtained results can be used to reveal potentially dangerous domains with a higher porosity in reactive cast metal matrix composites.