Effect Of Rare Earth Y,Sm And Melt Ultrasonic On Microstructure And Mechanical Properties Of AZ91D Magnesium Alloy

In this paper,the effects of different contents of single rare earth Y and compound rare earth Y and Sm on the microstructure and properties of AZ91D magnesium alloy were studied.The mechanism of rare earth on AZ91D alloy was analyzed by means of optical microscope(OM),scanning electron microscope(SEM),X-ray diffraction(XRD)and other test techniques,as well as melt ultrasonic process and heat treatment process.The optimal heat treatment process system and the addition amount under the optimal performance were explored.The results show that the effect of composite addition of Y and Sm on AZ91D alloy is significantly better than that of single addition of Y.After adding Y and Sm into AZ91D alloy,bright white block phase Al₂Y phase and needle phase Al₂Sm phase are formed,which can be used as effective heterogeneous nucleation sites of?-Mg.When the addition of 0.8wt%Y+1.0wt%Sm,the properties of AZ91D alloy are the best,the grain size of?-Mg is the smallest and the distribution is the most uniform.At this time,the grain size of?-Mg phase is 101.38?m,which is 66.9%finer than that of the alloy without rare earth addition,and is 66.37%better than that of the alloy with single rare earth addition.In addition,the addition of Y and Sm can also improve the room temperature mechanical properties of as-cast AZ91D alloy.After adding 0.8wt%Y+1.0wt%Sm,the hardness,tensile strength and elongation of the alloy are 67.42 HV,153.37 MPa and 3.62%,respectively,which are 3.5%,20.86%and 25.2%higher than that of the single addition of rare earth Y,respectively.However,the mechanical properties of the alloy at room temperature will decrease after exceeding this optimal addition.The AZ91D alloy with 0.8wt% Y and 1.0wt% Sm addition was subjected to melt ultrasonic treatment.The results show that the grain size of the alloy decreases first and then increases,and the mechanical properties of the alloy increase first and then decrease.When the ultrasonic power is 700 W,the grain size of?-Mg reaches the minimum value of 81.38?m by cavitation and acoustic flow effect,which is15.28%smaller than that of 96.06?m without ultrasonic.The?-Mg₁₇Al₁₂,Al₂Y and Al₂Sm phases are also broken by ultrasonic,and they are uniformly dispersed in the solution by acoustic flow effect,and finally the uniform and fine intermetallic precipitates are formed.Finally,the alloy containing composite rare earth after ultrasonic 700 W was heat treated.The optimal solution treatment process was410°C+6 h,and the aging process was 200°C+25 h.The mechanism of composite rare earth Y and Sm on AZ91D alloy is:Al₂Y phase and Al₂Sm phase formed by eutectic reaction can be used as effective heterogeneous nucleation point of?-Mg solidification,improve nucleation rate and refine grain;In addition,the network?-Mg₁₇Al₁₂ phase can be broken into short rod-like,thus improving the performance.