Study On Microstructure And Properties Of Re-containing Mg Alloys Fabricated By Ultrasonic Vibration And Rheo-squeeze Casting

As the lightest structural metallic materials,Mg and its alloys have wide prospects for applications in the fields of automobile,electronic communication and aerospace.Compared with cast Al alloys,cast Mg alloys generally exhibit lower mechanical performance both at room temperature and elevated temperature,so it is necessary to further improve the strength and heat resistance of Mg alloys.One of the important ways of strengthening and toughening the Mg alloys is Rare Earth(RE)alloying.However,there are also some serious problems for RE-containing Mg alloys,such as the non-uniform distribution of coarse RE-containing phases.Therefore,it is desiderate to make a breakthrough in the aspects of alloy composition design and forming technology.In this research,a new kind of multi-components RE-containing Mg alloy(Mg-Zn-Y-Ce/La alloy)was designed,and an ultrasonic vibration and rheo-squeeze casting process was proposed to fabricate the RE-containing Mg alloys.The effects of RE(both Ce/La and Y)elements contents,ultrasonic vibration,squeeze pressure and heat treatment on micro structure and mechanical properties were systematically investigated,which provided theoretical and experimental basis for the development and application of new RE-containing Mg alloys and their forming processes.With the addition of low-cost misch metal Ce/La,a new series of Mg-Zn-Y-Ce/La alloys were designed.The influence of Ce/La content on the microstructure evolution,transformation behavior and mechanical properties both at room and elevated temperatures was systematically investigated.The addition of Ce/La contributed to the grain refinement and the formation of T-phase((Ce,La)(Mg1-xZnx)11)with a high melting point.The formation of T-phase would decrease the Zn/Y ratio in the residual melt,consequently changing the phase transformation of quasicrystal I-phase(Mg3Zn6Y)and W-phase(Mg3Zn3Y2).The general precipitation sequence of Mg-Zn-Y-Ce/La alloy could be concluded as:Priamry ?-Mg?T-phase?W-phase?I-phase.T-phase possessed a high thermal stability,which contributed to the improvement of elevated-temperature mechanical properties.However,excessive T-phase was easy to be enriched at the grain boundaries,leading to the high stress concentration and premature fracture during the room-temperature tensile process.ZWE611(Mg-6Zn-1.4Y-1 Ce/La-0.6Zr)alloy was obtained by the optimization of Ce/La content,and it possessed relatively high mechanical performance both at room and elevated temperatures due to the synergistic reinforcement of quasicrystal I-phase and high-temperature stable T-phase.The effects of different parameters(the vibration time,the ratio of vibration time to intermittent time in a cycle,and the volumetric power density)on the micro structure evolution of ZW61 semisolid slurry were investigated.Based on orthogonal experiments,a set of parameters for preparing good semisolid slurry of ZW61 alloy was obtained,which was also applicable for Mg-Zn-Y-Ce/La alloy.In the semisolid microstructure,primary a-Mg grains were fine and spherical,with an average grain size of 28?33?m and average shape factor above 0.73.Owing to the effects of ultrasonic cavitation and acoustic streaming,two different-sized a-Mg grains,including ?1-Mg grains nucleated at ultrasonic processing and ?2-Mg grains formed at the subsequent solidification,were significantly refined and spheroidized in the rheo-squeezing casting micro structure.Besides,the application of ultrasonic vibration could change the distribution of solute and temperature,consequently suppressing the enrichment of eutectic RE-containing phases at grain boundaries and promoting the precipitation of numerous quasicrystal I-phase particles in a2-Mg grains.The effects of squeeze pressure on microstructure and mechanical properties were investigated.With the increase of squeeze pressure from 0 MPa to 200 MPa,the thermodynamic and kinetic conditions during the solidification process were changed.Therefore,the grain sizes of both ?1-Mg and a2-Mg grains were decreased,and the morphology of a2-Mg grains would change from equiaxed dendritic to spherical.Owing to the squeeze pressure,the eutectic structure at the grain boundaries changed from irregular morphology to typical lamellar eutectic.Besides,the volume fraction of the second phases was decreased,especially the quasicrystal I-phase particles.The increase of density and the decrease of casting defects were the main reasons for the significant improvement of tensile properties when the applied pressure is 50 MPa,and the microstructure refinement contributed to the further improvement with the pressure increasing to 200 MPa.The effects of T6 heat treatment with a high solution temperature on the microstructure evolution and mechanical properties of quasicrystal-reinforced Mg alloys were investigated,and the phase transformation behavior and strengthening mechanism were also discussed.When the solution temperature was 460?,I-phase was no longer stable and would transform into W-phase.Besides,a part of Zn atoms would be dissolved into the matrix and a large number of ?1' and ?2' precipitated during the subsequent ageing treatment.Compared with the normal solution temperatures,the high solution temperature brought better effects of solution and aging strengthening,and the yield strength and ultimate tensile strength of ZW61 alloy were increased by 61.7%and 30.7%after T6 heat treatment.The effects of ultrasonic vibration and heat treatment on the microstructures and properties of four Mg-Zn-Y-Ce/La alloys with different Y contents were studied.The results showed that the variation of Y content(0?2.8 wt.%)would lead to the change of phase compositions.When the Y content was 2.8 wt.%,the precipitation sequence was also changed as:Primary ?-Mg?W-phase?T-phase?I-phase.With the application of ultrasonic vibration to the four alloys,RE-containing phases(I-phase,W-phase and T-phase)were refined and the enrichment at the grain boundaries was suppressed,which contributed to the improvements of mechanical properties.The strengthening effects of heat treatment on the four alloys were obviously different,which were mainly caused by the different thermal stability of the second phases during the solution process.