Sensitization in Aluminum Alloys: Effect on Acoustic Parameters

Velocity and attenuation of shear and longitudinal sound waves were measured as a function of sensitization level in AA5083-H116 and AA5456-H116 aluminum alloys using two different techniques: resonant ultrasound spectroscopy (RUS) and pulse echo (PE). The frequency range for RUS was 0.3 MHz to 1 MHz. For PE, longitudinal 3.5 MHz and shear 1 MHz transducers were used. Sensitization was artificially induced in the laboratory by isothermally heating specimens at temperatures of 120°C, 175°C, 185°C, and 240°C for fixed periods of time (12 h to 96 h, the longer times for the lower temperatures). Measurements were repeated after each heating sequence. The overall changes are reported as percent difference between the as received and the fully sensitized states. The quantity that was found to be most sensitive to the degree of sensitization in this study is the attenuation coefficient of longitudinal waves, α_L. The measured change in α_L exceeds 20%, with error bars within 1% to 4%. Despite the magnitude of the error bars, the α_L vs. degree-of-sensitization curve is clear and smooth. The shear wave velocity changed by 1.2% to 1.5%, with error bars within 0.01% to 0.1%. The larger change was observed for the AA5456 alloy, which contains more Mg. The longitudinal wave velocity was found to change by 0.5% for the 5456 alloy, with error bars within 0.004% to 0.02%. The attenuation coefficient for shear waves was not measured. This study identifies multiple ultrasonic parameters that can be combined into a new, on-site, nondestructive tool for quantitatively monitoring the sensitization level in Mg-rich aluminum alloys, with possible extensions to other materials.