Control And Strengthening Mechanism Of Vibration And Rare Earth Gd On Interface Of Mg/Al Bimetal Prepared By Lost Foam Casting

With the rapid development of modern industry,parts produced by a single material have been difficult to meet the increasingly high requirements for lightweight,integrated,and comprehensive performance.Mg/Al bimetallic materials have both the advantages of aluminum alloy and magnesium alloy.It has very broad application prospects in the fields of automobiles,weaponry,and aerospace.Lost foam compound casting is a low-cost near-net-shape process suitable for preparing complex Mg/Al bimetallic products.But there is a three-phase interaction between the solid,liquid,and gas in the lost foam compound casting process,which makes the filling,solidification,and interface formation process becomes complex.In addition,there are many brittle intermetallic compound phases in the interface,and the solidification structure of the interface is coarse.These problems greatly weaken the bonding strength of the Mg/Al bimetal.To solve the problem of poor bonding quality of the Mg/Al bimetal,vibration,and rare-earth element Gd was used to strengthen the Mg/Al bimetallic interface.As a result,the microstructure and properties of the interface were improved,and the strengthening mechanism of the Mg/Al bimetallic interface was revealed.These investigations lay the foundation for the preparation of high-performance,low-cost Mg/Al bimetallic products,by lost foam compound casting and have important theoretical and practical meaning.（1）The effect of mechanical vibration on the filling and solidification process of the lost foam solid-liquid compound casting process was investigated.The results show that the mechanical vibration can accelerate the filling rate of the melt and improve the uniformity of the temperature field of it.The application of mechanical vibration can significantly reduce the filling time of the bimetal and improve the wettability between the magnesium alloy melt and the aluminum insert.When the vibration frequency is 35 Hz and the amplitude is 0.2 mm,the filling time is the shortest,about 1.6 s,and the filling rate is increased by 27%.During the filling process,both the mold wall and the surface of the insert have a wall attachment phenomenon due to the influence of the vacuum degree.During the solidification process,the natural convection in the molten metal was transformed into forced convection,the flow velocity of the melt was significantly increased,and the uniformity of the temperature distribution inside the casting was significantly improved,under the influence of mechanical vibration.So that the temperature gradient at the interface was significantly reduced.（2）The influence of mechanical vibration on the microstructure and properties of the Mg/Al interface was studied,and the strengthening mechanism of mechanical vibration was analyzed.The Mg/Al bimetallic interface can be divided into the intermetallic compounds（IMCs）layer(mainly composed of Al₃Mg₂ and Al₁₂Mg₁₇ substrate and Mg₂Si phase)and eutectic layer(mainly composed of Al₁₂Mg₁₇+δ-Mg eutectic structure).After applying the mechanical vibration,the thickness of the IMCs layer decreased,the grain size of the Al₃Mg₂ substrate in the IMCs layer was refined,and the size of the Mg₂Si second phase was significantly reduced.In the control group,the average size of the Mg₂Si phase in the Al₃Mg₂ and Al₁₂Mg₁₇ substrate were 4.34μm and 4.66μm,respectively,which can be reduced to 1.75μm and 2.65μm after applying mechanical vibration.With the increase in amplitude and frequency,the average shear strength of the bimetal gradually increased and reached a maximum of 46.83 MPa,which was about 47%higher than that of the control group.The improvement of the microstructure of the IMCs layer is the main reason for the improvement of the shear performance.（3）The microstructure and properties of the Mg/Al bimetal are improved by the recombination of mechanical vibration and rare earth Gd.The microstructure of the interface is refined,the uniformity of the Mg₂Si phase distribution is improved,and the bonding strength of the AZ91D/A356 bimetal is further improved.With the increasing amount of added Gd,the shear strength of the bimetal first increases and then decreases.When the addition amount was 0.6 wt.%,the shear strength of the bimetal reached the maximum value of 46.30 MPa,which was about 39%higher than that of the control group.At the same time,the average size of the primary Al₁₂Mg₁₇ phase in the eutectic layer was reduced from 131μm to 75μm,which was about 42.7%lower than that of the control group,the eutectic cell size was reduced from 89μm to 49μm,which was about 44.9%reduction.The strengthening effect of Gd on the Mg/Al bimetallic interface is mainly due to the improvement of the microstructure of the eutectic layer,including three aspects:1)the continuous Al₂O₃ film in the interface,which fragments the microstructure of the interface,can be broken and eliminated by adding Gd,thereby promoting the migration and diffusion of the Mg₂Si phase;2)after adding Gd,a large amount of Al₂Gd and Al₈Mn₄Gd phases precipitated in the eutectic layer,which plays a role of second phase strengthening;3)the Gd-rich phase can serve as a heterogeneous nucleation substrate,thereby refining the microstructure of the eutectic layer.The combination of mechanical vibration and rare earth Gd has a better strengthening effect on the Mg/Al bimetallic interface than using a strengthening method alone.When adding 0.6 wt.%Gd and applying mechanical vibration（frequency 50 Hz,amplitude 0.4mm）,the shear strength of the bimetal reached 54.25MPa,which was about 64.3%higher than that of the control group.The reason is that the combination of mechanical vibration and Gd can simultaneously improve the microstructure of the IMCs and eutectic layer and can solve the problem that the Gd-rich precipitates are easy to segregate in the eutectic layer when Gd is added alone.（4）A new ultrasonic-assisted lost foam solid-liquid composite casting process for the preparation of Mg/Al bimetal was proposed,and the effect of ultrasonic vibration on the microstructure and properties of the bimetallic interface was studied.The cavitation and acoustic flow effects of ultrasonic vibration can greatly refine the solidification structure of the bimetallic interface,eliminate the continuous Al₂O₃ film in the interface,significantly improve the uniformity of the interface microstructure,and strengthen the shear strength of the bimetal to 69.23 MPa,with an increase of 86.5%.Ultrasonic vibration enhances the shear strength of the bimetal mainly through fine-grain strengthening.In addition,the substantial improvement of the uniformity of the microstructure in the bimetallic interface and the elimination of oxidized inclusion defects in the bimetallic interface also has a positive effect on the improvement of the shear strength of the Mg/Al bimetal.