Study On Grain Refinement And Refining Mechanism Of Mg- Al Alloys Inoculated By Oxide Inclusions

Mg-Al alloys have been widely paid attention due to their remarkably low density and high specific strengths.Controlling and refining their microstructures,especially grain size,is crucial to the improvement of mechanical properties and quality of components.Various oxide inclusions such as MgO and MgAl₂O₄ are inevitably generated in the casting process of the Mg-Al alloy since Mg is very active and easily oxidized.The structure and properties of the oxide inclusions are thermodynamically stable.Their behaviors during the solidification of Mg alloys kept ambiguous.Specially,the issue that whether the oxide inclusions can participate in the nucleation ofα-Mg phase and refine the grains is deserved to be further explored.In the present study,the MgO and MgAl₂O₄ powders were used as oxide inclusions to inoculate the Mg-Al alloy melt to evaluate their grain refining effects.The influence of factors such as the amount of addition,holding time and Al content on the refining effect were systematically studied.The refinement mechanism of oxidation inclusions was revealed by nuclei structure observation,theoretical calculation and thermal analysis techniques.The MgO particles could effectively refine the Mg-3%Al alloy.The optimum addition amount and holding time were 1.5%and 10 min,respectively.Experimental results showed that MgO could not serve as the effective heterogeneous nuclei of the primaryα-Mg phase and Al was an indispensable element to ensure the grain refining efficiency of MgO addition on Mg alloys.Only when the Al content exceeded 2%in the Mg-Al melt,the Mg-Al alloys were effectively refined by MgO and the refining ratio was the highest when the Al content was 3%.MgO was transformed into MgAl₂O₄ phase by reaction with Al during the inoculation process.Further experiments showed that MgAl₂O₄ inclusion powder could effectively refine pure Mg and Mg-Al alloys with different Al content.The optimum addition amount and holding time were 2%and 10 min,respectively.The mismatch calculation results showed that MgAl₂O₄ was more suitable for the heterogeneous nuclei ofα-Mg grains than MgO.The grain refinement on Mg-Al（Al%≥2%）alloys inoculated by MgO was attributed to the newly formed MgAl₂O₄particles,which could act as the effective heterogeneous nuclei of primary a-Mg phase.The MgAl₂O₄ inclusion powder could effectively refine the commercial AZ31B alloy.The grain refining efficiency could be further improved by MgAl₂O₄ after being ball-milled.MgAl₂O₄ particles were observed in theα-grains of Mg alloy inoculated by MgAl₂O₄ inclusion powder.This results supported that the grain refinement effect was attributed to the fact that MgAl₂O₄ served as the heterogeneous nuclei ofα-Mg grains.After being inoculated by MgAl₂O₄,the mechanical properties of AZ31B alloy were significantly improved and the tensile strength and elongation to fracture increased by more than 50%.The solidification characteristics of Mg alloy melts changed significantly after being effectively inoculated by the oxide inclusions.The nucleation starting temperature ofα-Mg increased.The recalescence temperature decreased and the recalescence time was shortened.Compared with the melt being inoculated by MgO,the change of characteristic parameters after being inoculated by MgAl₂O₄was more significant.After being inoculated by oxide inclusions,there existed a correlation between the grain refinement and the change of solidification characteristics.If the nucleation starting temperature rose and the recalescence phenomenon was weakened or even disappeared,it indicated that the melt was effectively inoculated.The analysis of the cooling curve facilitate the rapid judgment of the grain refinement effect.To sum up,we can conclude that the oxidation inclusions generated during the casting process of Mg alloys can affect the solidification process.MgAl₂O₄ can serve as the effective heterogeneous nuclei of primaryα-Mg grains.This study is of great significance to the beneficial utilization of inclusions to refine grains of Mg-Al alloys.