RESUMEN
The dissolution of metals, influenced by mechanical and chemical factors, plays a crucial role in various applications. Ultrasonic irradiation has been explored for its ability to enhance dissolution rates and modify surface characteristics. In this study, we investigate the dissolution of magnesium (Mg) and magnesium alloys under high-intensity focused ultrasound (HIFU) conditions with frequency sweeping (wobbling). Our findings reveal distinct effects of cavitation and acoustic streaming on the dissolution process. For pure magnesium, ultrasonic treatment significantly increases dissolution rates compared to silent conditions. Negative frequency sweeps result in the highest dissolution rates, linked to increased cavitation activity, while positive sweeps reduce dissolution rates but maintain acoustic streaming effects. The removal of surface oxides is accelerated in all sonication conditions. Macro- and micro-roughness patterns on the surface correspond to the wobbling frequency range, with wavelengths matching the average ultrasonic frequency. However, dissolution is not uniform across the sample, and preferential attack occurs at the focal point during negative frequency sweeps. In contrast, magnesium alloys exhibit lower dissolution rates than pure Mg. The alloy's mechanical properties make it less susceptible to cavitation erosion but more sensitive to acoustic streaming-induced dissolution. Grain boundaries are preferentially attacked, revealing differences between ductile pure Mg and the harder, more cavitation-resistant, alloy. This study highlights the complex interplay between cavitation and acoustic streaming in the dissolution of magnesium and its alloys under HIFU conditions, shedding light on the limits and potential applications of this technique, particularly in microstructure analysis.
RESUMEN
Use of sweeping mode with a 3.6 MHz High Intensity Focused Ultrasound (HIFU) allows cavitation activity to be controlled. This is especially true in the pre-focal zone where the high concentration of bubbles acts as an acoustic reflector and quenches cavitation above this area. Previous studies attributed the enhancement of cavitation activity under negative sweep to the activation of more bubble nuclei, requiring deeper investigations. After mapping this activity with SCL measurements, cavitation noise spectra were recorded. The behavior of the acoustic broadband noise follows the sonochemical one i.e., showing the same attenuation (positive scan) or intensification (negative scan) of cavitational activity. In 1 M NaCl 3.7 mM 2-propanol solution saturated by a mixture of Ar-15.5%O2-2.2%N2, intensities of SL spectra are high enough to allow detection of several molecular emissions (OH, NH, C2, Na) under negative frequency sweeps. This is the first report of molecular emissions at such high frequency. Their intensities are low, and they are very broad, following the trend obtained at fixed frequency up to 1 MHz. Under optimized conditions, CN emission chosen as a spectroscopic probe is strong enough to be simulated, which is reported for the first time at such high frequency. The resulting characteristics of the plasma do not show any spectral difference, so bubble nature is the same in the pre-and post-focal zone under different sweeping parameters. Consequently, SL and SCL intensification was not related to a change in plasma nature inside the bubbles but to the number of cavitation bubbles.
