Non-destructive Detection of Small Blowholes in Aluminum by Using Laser Ultrasonics Technique

In this paper, blowhole defects of sub-millimeter diameter in an aluminum alloy are successfully detected by the laser ultrasonic (LU) technique. A Q-switched and pulsed Nd:YAG laser is used for ultrasonic generation, and a laser Doppler vibration meter is used for detection of ultrasound waves on the sample surface. Through adding a thin and transparent film on the sample, a bulk wave penetrating method can be used for evaluating the position and size of the blowhole defects using the LU system. And the directivities of a longitudinal wave generated by a laser at different conditions are discussed. By two-dimensional moving of the sample with a precise motorized translation platform, the ultrasonic waves can be detected at different positions of the sample for evaluating the defects. The C-scan images were obtained to analyze the blowholes' position and size. Furthermore, a numerical simulation is also used to research the propagation properties of ultrasound in the specimen with internal holes. Results from the experiment and numerical simulation are discussed and compared to demonstrate the reliability and accuracy of the method.