ULSONIK™: Acoustic Cavitation-Driven Hydrogen Degassing in Molten Aluminium Alloys — Effects on Porosity and Mechanical Properties

<title>Abstract</title> <p>High-intensity ultrasonic treatment of molten aluminium alloys generates acoustic cavitation — transient formation and collapse of micro-bubbles in the melt — which provides a powerful physical mechanism for dissolved hydrogen removal without the use of halogen or toxic process gases. This study presents a systematic investigation of the ultrasonic molten metal treatment system applied to AA-series aluminium alloys during continuous strip casting. Experiments were conducted at melt temperatures from 680°C to 760°C with ultrasonic frequencies of 19–21 kHz and varying power densities. The density index (DI) was measured before and after treatment using the reduced pressure test (RPT). Metallographic analysis of transverse cross-sections of cast foil and billet samples was performed to characterise porosity distribution, pore morphology, and grain size. Vickers hardness measurements were taken at multiple positions across the cross-section. Results demonstrate that ultrasonic treatment achieves a density index below 0.08 cc/100g — below the threshold required for defect-free 6-micron aluminium foil production — compared to greater than 0.15 cc/100g observed without treatment. Porosity elimination and grain refinement via the Hall-Petch mechanism are attributed to cavitation-induced degassing and acoustic streaming effects. Hardness uniformity across the cross-section improved by 22% compared to rotary degassing controls. The system eliminates the use of hex chloride and hexafluoride process gases, which are 24,000 times more damaging than CO₂ as greenhouse agents. Industrial validation was performed with nil pinholes and holes observed in 6-micron foil output and confirmed by two industrial institutions.</p>