Microstructure Formation and Mechanical Properties of AZ31 Magnesium Alloy Solidified with a Novel Mechanical Vibration Technique

Abstract

A novel mechanical vibration for refining microstructure is reported where vibration energy was directly exerted into a molten alloy by a vibrating horn, and the vibrating horn was melted during vibration. Effects of vibration intensity and melt superheat on the microstructure and mechanical properties of AZ31 magnesium alloy were investigated. It is confirmed that the melting of the vibrating horn could effectively extract the superheat and latent heat from the interior of the molten alloy, leading to rapid cooling during the initial stage of solidification, and the cooling rate is strongly dependent on the vibration acceleration and melt superheat. This study showed that it was difficult to refine the solidified microstructure when the treated alloy was kept in the full liquid state within the entire vibrating duration. A significantly refined microstructure was obtained by applying mechanical vibration during the nucleation stage, and a globular microstructure could form in a few seconds after solidification. When the molten alloy was treated from 920 K to 903 K (647 °C to 630 °C), with increasing vibration acceleration from 2.5 to 19 m s−2, the coarse dendritic microstructure of the produced AZ31 billets transformed into a well-refined, reasonably uniform, and non-dendritic one, and mechanical properties were improved significantly. Moreover, the mechanisms of microstructure formation are discussed.