Acoustic Properties of a Metal Close to Its Melting Point, as Measured by Laser Ultrasonics

This study investigates the acoustic properties of a metal near its phase-change temperature using laser ultrasonics. A Pb-free solder sample was prepared, and its temperature was increased stepwise from 40 degrees C to 210 degrees C with concurrent measurement of laser ultrasonic waves on the solder surface. Noticeable change in the waveforms was observed at 203 degrees C and 209 degrees C. These sudden changes occurred with a 1 degrees C change in temperature. The change at 203 degrees C was attributed to an acoustic mode transformation from Rayleigh-like to Lamb-like as we also found that part of the solid solder became thin as a result of partial melting from the bottom and rim. Only the sound through the air was detected at temperature of 209 degrees C and higher, where this sound was due to ablations. This phenomenon was responsible for local melting at the laser irradiation point, generating the ultrasonic waves.