Fabrication Of Rapid-cooled Mg-Al-Y Alloy With Fine Grain And Subsequent Heat Treatment Process

The spontaneous solidification of magnesium alloy at conventional cooling rate results in coarse grain size,large volume shrinkage,and poor deformation ability to limit the development of magnesium alloy.Grain refinement can significantly improve material plasticity,high corrosion resistance,optimize distribution and morphology.Therefore,it is an effective way to expand application of magnesium alloy by studying high-efficiency modifier and process to achieve alloy refinement.The unique extra nuclear electron distribution characteristics of rare earth elements make it beneficial to refine the magnesium alloy as a modifier and improve the strength of the magnesium alloy.it combines with the elements in the alloy to form a long strip of high melting point compound to improve the high temperature creep performance,at the same time.Increasing the solidification rate of the alloy accelerates the solid/liquid interface transition rate,effectively inhibits segregation of components,increases the solid solubility of the element in the matrix,and eliminates the formation of harmful phase in the grain boundary caused by segregation of the elements under near equilibrium solidification.Therefore,the combination of rare earth addition and rapid cooling technology is expected to produce a fine-grained structure with good thermal stability provides reference for the development of high-strength magnesium alloy.In this paper,the combination of stepped copper mold casting and rare earth elements was chosen to study the microstructure,grain size and phase structure evolution of Mg-6Al alloy under different cooling rates and rare earth contents.The rare earth phase was discussed.The effect of formation and size on the refinement of solidified structure.the systematic study on the heat treatment transformation of the fast-cooled Mg-6Al alloy before and after the rare earth metamorphism,The dynamic transformation process of the volume fraction and distribution of high temperature softening?-Mg₁₇Al₁₂2 phase under isothermal at 200°C and 400°C was compared and analyzed.The hardness of the fast-cooled magnesium alloy prepared by different processes was determined.The main findings are as follows:The results of modification experiments show that the average grain size of the alloy decreases first and then increases 1wt%Y.the average grain size decreases from58?m to 21?m with the addition of rare earth Y and lots of granular grains appear.The refinement mechanism is that the enrich of rare earth elements causes the super cooling of the solid/liquid interface front component to inhibit the growth of the?-Mg and stabilize the formation of the rare earth phase to promote nucleation.Fast cooling experiments show that The Al₂Y phase is formed in the nearly equilibrium solidification and rapid cooling Mg-6Al-xY alloys,but the alloys with different compositions will cause significant changes in the morphology and volume fraction of the rare earth phase.As the inner diameter decreases,the grain refining effect increases.Compared with the average grain size of 118?m in the as-cast Mg-6Al-1Y alloy,the alloy has the largest cooling rate when the inner diameter of the copper mold is 2 mm,and the formed grain size is only 5?m.At the same time,the increase of the cooling rate leads to the continuous decrease of the?-Mg₁₇Al₁₂2 phase content,which is favorable for the formation of supersaturated solid solution.By comparing and analyzing the effect of tissue refinement under the above different conditions,it is found that the refining ability caused by accelerating the cooling rate is superior to that of rare earth addition.The refining effect combined with the modification of the copper mold and the rapid cooling technique of the copper mold is better than the single method and can significantly reduce the volume fraction of the eutectic structure in the alloy.Although the refining mechanism of the alloy prepared under different process conditions is different,the hardness change is consistent with the change of the average grain size.The results of thermal stability test show that the cold die Mg-6Al alloy is treated at 200?.The morphology of the grain changes little with time,but the Al element will precipitate from the high saturated matrix,resulting in?-Mg₁₇Al₁₂.The volume fraction of the phase continues to increase.The copper mold rapid cooling Mg-6Al-xY alloy at 200?,and the precipitated phase precipitated at the grain boundary was changed from a layer with no Y added to a narrow shape,that is,the addition of Y was beneficial to disconnect the?phase.After Mg-6Al alloy of copper mold is treated by400?+2h solution,the grain boundary is more clearly visible with time.After solution-casting Mg-6Al-xY alloy was treated with 400?+2h solution,the grain boundary was almost never appeared with time,?-Mg₁₇Al₁₂2 was dissolved or even eliminated,the trace residue was uniform,The floc is in the matrix and at the grain boundaries.The formed rare earth phase can effectively inhibit grain growth at high temperatures,and no change is found at low temperature and high temperature.