Effect Of Rare Earth On Microstructure And Properties Of Casting Mg-Al-Zn Alloy Under Ultrasonic Condition

Since electromagnetic radiation?EMR?appears as a new form of pollution,it is important to develop lightweight structural materials with high electromagnetic shielding performance,in order to protect people or systems from unnecessary electromagnetic radiation.Magnesium alloys are expected to be widely used as lightweight electromagnetic shielding materials in the fields of electronic communications,aerospace,military fields and so on,due to their light weight,excellent electromagnetic shielding performance and good machinability.In this paper,Mg-Al-Zn-xY-yCe alloys with different additions of rare earth were prepared by ordinary casting method and optimized the alloy compositions.Ultrasonic processing was applied to the alloys with good microstructure and excellent properties.The microstructure and morphology of the alloy were observed and analyzed by Optical metallographic analysis?OM?,Scanning electron microanalysis?SEM?,Spectrum analysis?EDS?and X-ray diffraction analysis?XRD?.The mechanical properties of the alloy at room temperature and electromagnetic shielding performance were tested by universal tensile tester,electronic Brinell hardness tester,flange coaxial tester and D60K digital conductivity tester.The effect mechanism of rare earth elements Y,Ce and ultrasonic treatment on the structure and properties of Mg-Al-Zn alloy was revealed.Mg-Al-Zn-xY-yCe alloys with different contents of rare earth Y and Ce were prepared by metal mould casting,and their microstructures were observed.The results show that the microstructures of Mg-Al-Zn alloys without rare earth elements are mainly composed of?-Mg matrix and?-Mg₁₇Al₁₂2 phase,in which the reticulated?-Mg₁₇Al₁₂2 phase is relatively thick.At the same time,?-Mg₁₇Al₁₂2 phase densely intertwines with each other and distributes on and within the grain boundaries.The?-Mg₁₇Al₁₂2 phase of Mg-Al-Zn-xY-yCe alloy with the addition of the rare earth element Y and Ce are obviously refined,and the rare earth phases Al₂Y and Al₂Ce are formed.When the rare earth content is 0.6 wt.%Y+1.2 wt.%Ce,the optimal refinement of the?-Mg₁₇Al₁₂2 phase in the alloys is achieved,which changed from continuous network distribution to semi-continuous distribution on the matrix.The morphology and distribution of the alloy are further improved by different ultrasonic treatment applied to the Mg-Al-Zn-0.6Y-1.2Ce alloy melt.When the ultrasonic power and time are 1500 W and 90 s respectively,the best improvement of alloy microstructure is achieved.The fracture level of the?-Mg₁₇Al₁₂2 phase is the highest,and changing from semi-continuous to dispersive distribution.The bright white strip phase Al₂Ce tends to needle-like,the average length-diameter ratio is26:1,and the average size of the irregular block phase Al₂Y decreases to 4?m.The effect of rare earth elements Y and Ce on room temperature mechanical properties of Mg-Al-Zn alloy increases first and then decreases with the increase of total rare earth addition.The mechanical properties of the alloy decrease because the excessive addition of rare earth elements will lead to the agglomeration of the rare earth phases in the alloy structure,the roughening of the?-Mg₁₇Al₁₂2 phase.When the rare earth content is 0.6 wt.%Y+1.2 wt.%Ce,the mechanical properties of the alloy were optimal at room temperature.At this time,the tensile strength,post-fracture elongation and hardness of the alloy are 233.8 MPa,7.2%and87.6 HB,respectively.Compared with the Mg-Al-Zn-0Y-0Ce alloy,the room temperature tensile strength,post-fracture elongation and hardness increased by 18.74%,79.10%and13.03%,respectively.Ultrasonic treatment can further improve the microstructure of the alloy,reduce the segregation of the second phase and improve the mechanical properties of the alloy.After ultrasonic treatment with 1500 W power and 90 s time applied to mg-al-zn-0.6Y-1.2Ce alloy melt,the room temperature tensile strength,elongation after fracture and hardness value of the alloy increased to 264.8 MPa,8.03%and 92.1 HB.The electrical conductivity and electromagnetic shielding efficiency?SE?of Mg-Al-Zn alloy can be improved by improving the morphology of the alloy and reducing the alloying elements dissolved in the alloy matrix.The conductivity of magnesium alloys without rare earth elements is 14.8%IACS and the minimum SE value is 74.24 dB?1500 MHz?.When the mixed rare earth element content is 0.6 wt.%Y+1.2 wt.%Ce,the conductivity of rare earth magnesium alloys reaches 16.5%IACS and the minimum SE value is 84.96 dB?1500 MHz?.After ultrasonic treatment of Mg-Al-Zn-0.6Y-1.2Ce alloy melt,the conductivity of the alloy varies in the range of 16.5-17.5%IACS and the minimum SE varies in the range of 84.96-87.86dB?1500 MHz?.