Research On Mechanism Of Grain Refinement By Alloying And External Field Of AZ31 Magnesium Alloys

Magnesium alloys are structural metallic materials with lowest density, and have wide applications in different fields for their high specific strength, improved damping capability, good machinability and recyclability. However, compared with aluminium alloys, magnesium alloys have worse plastic processing ability and absolute strength because of their HCP structure, and grain refinement can lead to improved strength and plasticity. According to metal solidification and grain refinement theory, this study will be focused on the ways of grain refinement and mechanism of grain refinement of Mg-Al based alloys, building grain refinement model for choosing the grain refiner and optimizing the process factors.In this paper, AZ31 magnesium alloys is chose as the base alloys, the microalloying (Gd, Nd, AlN particles) and external field are employed to study the effects on the microstructure and mechanical properties, and a grain refinement model will be built to describe the effects of grain refiner which based on constitutional supercooling (CS)-driven nucleation model. The results of the study are showed bellow:The additions of Gd and Nd can have great effects on the microstructure and mechanical properties of AZ31 alloy:the grain sizes of AZ31 alloys were increasing with the additions of alloying element until 0.6%Gd or 0.8%Nd, respectively; the formation and distribution ofβ-Mg17Al12 phase was changed because of the addition of Gd or Nd, and new phases(Al2Nd, AlNd3, Al2Gd) were formed between the alloying elements and Al element; the additions of Gd or Nd play influence the mechanical properties and change the fracture mechanism from quasi-cleavage crack into ductile fracture.AlN can act as a very effective, stable and reliable grain refiner for AZ31 alloys, SEM observation and EDS results show that AlN particles act as heterogeneous nuclei to refine the microstructure; the disregistry between AlN andα-Mg in the low index planes is 3.05%, which validate that AlN particles are useful heterogeneous nuclei.Spiral magnetic field, which is composed of rotating and traveling magnetic field, can refine the grains of magnesium alloys by improving the nucleation rate of AZ31 alloys, the grains in the border and center of samples is refined from about 400μm and 321μm to 251μm and 263μm, respectively; the Lorentz force which generated in the melt by electromagnetic field play a important role in the formation of precipitated phases, and make the solute dispersive and discontinuous.Following the CS-driven nucleation mechanism and in view of the reaction between alloying elements and Al element in melt, a new grain refinement model was proposed to describe the relationship between alloying element and grain sizes, which based on the experiments of grain refining of AZ31 alloys with additions of Gd and Nd, and the model was tested on the grain refinement of AZ31 alloys with compound additions of Nd and Zr.