Nondestructive characterization of aluminum grain size using a ring-shaped laser ultrasonic method

In this study, a nondestructive ring-shaped laser ultrasonic method with a thermoelastic excitation regime was used to determine the grain size of metal materials. This method was proposed in order to evaluate the quality of metal in a fast online nondestructive manner. Normally, laser ultrasonic is used to detect grain size in the ablation excitation regime. The laser excites high energy longitudinal waves but causes damage to the surface of metal materials. To achieve strict online nondestructive testing, the thermoelastic regime was used in this work. The ring-shaped laser was converted from a circular collimated laser by an axicon and irradiated on the surface of the aluminum sample to induce ultrasonic waves and enhance the signal amplitude. The directivity pattern was analyzed to find a suitable detection parameter by the finite element method before performing laser ultrasound experiments. Quantitative analysis of the converging waves with different deviations from the center via laser ultrasound experiments demonstrated the enhancement effect of signal energy using a ring-shaped laser. The issues of low signal energy and the generation of a directivity pattern were solved by this ring-shaped laser ultrasonic method aimed at nondestructive grain size inspection. Aluminum samples with different mean grain sizes were detected by ring-shaped laser ultrasonic technology. A grain size characterization model was built with mean grain sizes and ultrasonic signals. Laser-generated ultrasound technology in the thermoelastic regime is a promising online detection method and can be used to detect material properties nondestructively with a ring-shaped laser.