The Effect Of Al-Ti-C-Gd Master Alloy On Grain Refinement Of AZ31

Magnesium alloy has the lowest density,high specific strength,specific stiffness,good impact shock absorption and cutting performance,and has the advantages of strong thermal conductivity,good electromagnetic shielding function and recycling which is widely used in industry,electronics,national defense,aerospace and other fields.However,magnesium alloy is close-packed hexagonal crystal structure with less slip systems,and its absolute strength and plasticity are not as good as those of aluminum alloy.Fine crystal reinforcing can simultaneously increase the absolute strength and plasticity of magnesium alloy and broaden the application range of magnesium alloy.Therefore,it is of great significance for the development of magnesium alloys to develop and prepare a kind of high effective grain refiner.In addition,the deformed magnesium alloys have superior comprehensive mechanical properties and are more widely used than the cast magnesium alloy.Therefore,Al-Ti-C-Gd master alloy refiner was prepared for the improvement of AZ31 deformed magnesium alloy,and the effect of Al-Ti-C-Gd master alloy on the refining effect and refinement mechanism of AZ31 was analyzed in this paper.The Al-Ti-C-Gd master alloy refiner was prepared by self-propagating high temperature synthesis?SHS?.The effect of different Gd contents on the microstructure of Al-Ti-C-Gd master alloy was analyzed.And Al-Ti-C-Gd master alloy was applied to the AZ31 magnesium alloy to reveal its effect on the grain size of AZ31,and the refinement mechanism was also analyzed.The results show that the Al-Ti-C-Gd master alloy refiner with different Gd content has well refining effect on AZ31,among which Al-3.3Ti-1.9C-2.8Gd is the most excellent.Adding1.5%Al-3.3Ti-1.9C-2.8Gd refiner can refine the average grain size of AZ31 alloy from 393?m to 92?m,which refining efficiency is 76.6%,and the mechanical properties of as-cast AZ31alloy has obvious improvement.After the refinement,the tensile strength,yield strength and elongation of the AZ31 alloy were increased from 43.5 MPa,128.25 MPa,and 7.9%to 63 MPa,205.45 MPa,and 16.05%,respectively.In addition,the average grain size of the as-rolled AZ31alloy also changed significantly.After rolling treatment,the original average grain size of AZ31is about 30?m.After adding 1%and 1.5%Al-3.3Ti-1.9C-2.8Gd master alloy,the average grain size of the as-rolled AZ31 is 26?m and 19?m,respectively.The texture of as-rolled AZ31 also weakened slightly.Grain refinement also has an effect on the mechanical properties of the as-rolled AZ31.After adding 1.5%Al-3.3Ti-1.9C-2.8Gd to AZ31 alloy,the tensile strength of the as-rolled AZ31 increases from 217.5MPa to 278MPa,the yield strength is increased from 144.1MPa to 201 MPa and the elongation increased from 8.5%to 13.95%.The main reason for the remarkable grain refining efficiency of Al-Ti-C-Gd master alloy refiner for AZ31 alloy is the heterogeneous nucleation of TiC particles and the restriction of?-Mg grain growth by Al-Gd compounds.TiC particle clusters accumulated at the?-Mg grain boundaries which formed a solute supercooled layer also restrict the grain growth of?-Mg grain.And the particle cluster containing Al,Gd,Ti elements accumulates at the?-Mg grain boundary to make the?-Mg₁₇Al₁₂distribute more uniformly in AZ31.The phase composition and microstructure of Al-Ti-C-Gd master alloys with different Gd contents were studied.The XRD results show that TiAl₃,TiC,Ti₂Al₂₀Gd and?-Al phases were present in the master alloy.The Gd element promoted the wettability between all elements and changed the morphology of the TiAl₃ phase.After adding rare earth Gd,a special core-shell structure TiAl₃/T_i2Al₂₀Gd is produced in the imaster alloy.And with the increase of rare earth element Gd,the outer layer of Ti₂Al₂₀Gd of the core-shell structure became thicker and the inner TiAl₃ phase gradually became smaller.After the shell core structure was added to AZ31,its special structure can release solute elements step by step,affecting the refinement and improving the refining efficiency.