A comparative study of the role of solute, potent particles and ultrasonic treatment during solidification of pure Mg, Mg–Zn and Mg–Zr alloys

Ultrasonic treatment (UST) applied during the solidification of pure Mg, eutectic (Mg – Zn) and peritectic (Mg – Zr) alloys was investigated in order to explore the grain refinement mechanisms. Temperature dependent grain refinement is observed in pure Mg where decreasing the superheat temperature (at which UST is applied from above the melting temperature, TM) from 100 °C to 40 °C produces significant refinement with a uniform grain structure. The presence of solute reduces the temperature dependence of the UST refinement and excellent grain refinement is obtained regardless of the superheat temperature (100 °C or 40 °C) and even with the use of preheated sonotrode in the Mg – 6 wt.% Zn alloy. A further improvement in grain refinement is achieved when the alloy contains potent particles that introduce additional nucleation of grains in Mg – 0.5 and 1.0 wt.% Zr alloys (producing an average grain size of ≤100 µm). At 40 °C superheat, UST of Mg – Zn alloys produces excellent refinement (average grain size <200 µm) with non-dendritic grains, which is normally achieved only with the addition of grain refining master alloy in the as-cast condition. The enhanced refinement observed in the eutectic alloy is explained through the undercooling imposed by a relatively cold sonotrode combined with high frequency vibrations and acoustic streaming. The advantages of using a cold sonotrode, a low superheat and solute are demonstrated for achieving significant refinement during solidification of Mg alloys under UST without or with a lower addition of grain refining master alloys.