Three orthogonally arranged ultrasounds modulate solidification microstructures and mechanical properties for AZ91 magnesium alloy

Three dimensional (3D) power ultrasound sources with 20 kHz frequency and 14∼22 μm amplitudes were introduced into the solidification process of AZ91 magnesium alloy, and the acoustic spectra versus temperature were synchronously examined. The measured sound pressure monotonically increased with the rise of both ultrasound dimension and amplitude, whereas the transient cavitation intensity firstly increased to its maximum under 3D ultrasound sources at 18 μm amplitude and then unexpectedly declined if further raising the amplitude to 22 μm. The grain size of primary α-Mg phase which was refined down to 16% of that under static solidification mainly depended on the transient cavitation intensity, indicating that the transient cavitation enhanced the nucleation rate by over two orders of magnitudes and dominated the grain refinement. The dislocation density within α-Mg phase increased owing to the intense vibration under semi-solid state. A more homogeneous distribution of divorced eutectic β-Mg₁₄Al₁₂ phase and a smaller amount of β precipitation were induced due to the grain refinement and suppressed segregation for primary α-Mg phase. The microstructural evolution improved the mechanical properties for AZ91 alloy, where its elongation rate and tensile strength were enhanced by the factors of 2.4 and 1.7, respectively. It is demonstrated that applying 3D ultrasound sources provides an effective approach to modulate solidified microstructures and enhance mechanical performances for magnesium alloy.