Grain size distribution measurement of Ti-6Al-4V plate using laser-ultrasonics

As the most widely used titanium alloy, Ti-6Al-4V has wide range of applications in aerospace industry and medical prostheses. Grain size is a significant parameter that should be controlled during material processing for the reason that it can affect the strength and toughness directly. Laser-ultrasonics is an appropriate method for in-situ measurement of the grain size and distribution during heat treatment of Ti-6Al-4V as it is a nondestructive and non-contact technique. In this study, a laser-ultrasonic system was established combining an 8 ns width pulsed laser for ultrasound generation and a two-wave mixing (TWM) interferometer for detection. Several Ti-6Al-4V samples were heat treated variously to get different grain sizes but with the same phase ratios. Attenuation rates of ultrasonic waves in the different specimens were precisely calculated using exponential fitting for various frequencies. The true value of grain size and distribution were observed via scanning electron microscopy and electron backscatter diffraction (SEM-EBSD). The frequency dependent attenuation of ultrasonic waves was obtained for various grain sizes of Ti-6Al-4V. A statistical correlation between the grain size distribution and the frequency-dependent ultrasonic attenuation was established.