Directional dependence of aluminum grain size measurement by laser-ultrasonic technique

A blade method was used to identify the relative position of pulsed and detect lasers when measuring aluminum grain size via laser-ultrasonic technique. Aluminum specimens received a variety of treatments to obtain different grain sizes. Electron backscatter diffraction data were used to obtain the parameters of the grain size calculation model. The finite element method model was established by COMSOL Multiphysics to simulate the generation and propagation of ultrasonic waves in aluminum. Both simulation and experimental data revealed that directional pattern has a significant effect on the results of grain size measurement when unpredictable deviations occur. The direction angle of the first and third ultrasonic echoes differs at the detection point when deviation occurs. Thus, the error of grain size measurement using laser-ultrasonic technique will increase when deviation increases. For the grain size measurement of aluminum using laser-ultrasonic technique, the accuracy is acceptable when deviation is no more than 0.2 mm when the thickness of the specimen and diameter of the pulsed laser are set to 2 mm and 1 mm respectively.