Effect of nitrogen-assisted ultrasonic vibration treatment on microstructure and mechanical properties of near-liquidus squeeze-cast A356.2 aluminum alloy

The synergistic effects of nitrogen-assisted ultrasonic vibration treatment coupled with near-liquidus squeeze casting on the microstructure and mechanical properties of A356.2 aluminum alloy were investigated. Experimental and theoretical analyses were conducted to evaluate the role of process parameters, specifically pouring temperature (630 °C), ultrasonic vibration treatment duration (12 s), ultrasonic power (1,280 W), and nitrogen flow rate (6 L·min −1 ) in enhancing grain refinement and alloy performance. Results demonstrate that nitrogen-assisted ultrasonic vibration treatment achieves a 68.9% reduction in grain size (average equivalent circle diameter) alongside increases of 16.1%, 18.8%, and 150.0% in tensile strength, yield strength, and elongation, respectively. The cavitation effect induced by ultrasound disrupts dendrites and promotes nucleation, while the acoustic streaming effect ensures uniform temperature and concentration fields within the melt, improving the uniformity of the microstructure. The combined action of nitrogen and ultrasound at near-liquidus temperatures intensifies the effect of grain refinement by generating additional cavitation nuclei and inducing localized cooling and stirring effects and producing more intense cavitation. These findings demonstrate the potential of integrating nitrogen injection with ultrasonic vibration treatment to optimize squeeze casting process of high-performance aluminum alloys.