Investigation On The Interfacial Control And Reinforcement Of The Al/Mg Bimetals Prepared By Lost Foam Casting With Solid-liquid Compound

Al/Mg bimetallic components have the advantages of aluminum and magnesium,which have broad application prospects in the fields of electronic 3C,automobiles,weaponry and aerospace and so on.Lost foam casting(LFC)is a near-net forming process,which is suitable for low-cost preparation of complex Al/Mg bimetallic components.However,the solid-liquid-gas three-phase interaction makes the filling and solidification process as well as interface behavior complex in the process of LFC,and the interface exists many brittle phases,which obviously weakens the bonding strength of the Al/Mg bimetal castings.Therefore,how to control and strengthen the interface of the Al/Mg bimetal effectively is essential for the preparation of high-performance Al/Mg bimetal castings.This article aims at preparing complex Al/Mg bimetallic castings.The interfacial structure of the Al/Mg bimetals was regulated and strengthened by optimizing the parameters of LFC,inlay surface treatment and heat treatment to improve the bonding performance of bimetal.It provides the theoretical and practical basis for low cost Al/Mg bimetal parts prepared by LFC.(1)The effects of three key process parameters,pouring temperature,liquid-solid volume ratio and vacuum degree,on the structure and properties of the Al/Mg bimetal castings were systematically studied.As the pouring temperature and liquid-solid volume ratio increased,the thickness of the interface layer of the bimetallic casting gradually increased.The vacuum degree mainly affected the proportion of defects in the interface area.As the vacuum degree increased,the defects increased at first,and then decreased.The influence relationship of pouring temperature,liquid-solid volume ratio and vacuum degree on the shear strength of Al/Mg bimetal was obtained by response surface design method fitting and optimization.The optimal preparation parameters of Al/Mg bimetal in LFC were:pouring temperature 730?,liquid-solid product ratio 14.6,vacuum degree 0.03MPa.(2)Based on experimental analysis,numerical simulation,thermodynamics and kinetic calculations,the formation mechanism of the interface layer of the Al/Mg bimetallic casting in LFC was ascertained.The experimental analysis showed that the interface layer of the Al/Mg bimetallic casting was mainly composed of three reaction layers,which were the Al₃Mg₂+Mg₂Si(? layer)near the aluminum side,the intermediate reaction layer Al₁₂Mg₁₇+Mg₂Si(? layer)and Al₁₂Mg₁₇+Mg eutectic structure(layer ?)closed to Mg base.Numerical simulations and thermodynamic and kinetic calculations indicated that the formation mechanism of the interface layer was a combination of melting and diffusion.The specific process was as follows:When the magnesium alloy fluid melted the foam pattern and encountered the Al inlays,a shock zone(?-Mg)was formed firstly,and elemental diffusion occured between the Al inlays and the Mg melt.Under the joint action of latent heat release of crystallization,high temperature of magnesium melt and concentration gradient,surface melting of Al inlay and remelting of the shock zone occurred,and reaction layer ?,? and ? were successively formed.During this time,the latent heat of crystallization released by the eutectic reaction caused the?-Al₁₂Mg₁₇ dendrite to remelt.(3)The effects of Mg(AZ91D)and Al(A356)as solid inlays,respectively,on the structure and properties of the Al/Mg bimetal castings were compared,which provided a theoretical basis for the selection of inlays in the preparation of the Al/Mg bimetals.The interface layer of the Al/Mg bimetallic casting was uniform and defect-free when A356 was as the solid inlay.The interface layer of the Al/Mg bimetallic casting had crack defects with AZ91D as the solid inlay.There were two main reasons for the cracks:one was that the linear shrinkage of AZ91D was greater than that of A356;the other was that the stress at the interface of the Al/Mg bimetallic casting with AZ91D as a solid inlay was greater than that of A356 as a solid inlay.Thus,the Al/Mg bimetallic casting prepared with A356 as an inlay had significantly higher shear strength than that with AZ91D as an inlay.(4)The effects of different coatings(Zn,Ni and Ni-Cu coatings)on the structure and properties of Al/Mg bimetal castings were systematically studied,and the mechanism of the adjustment and strengthening of the interface structure with different coatings was revealed.The Zn coating can eliminated the oxide layer on the surface of Al alloy and improved the wettability between Mg liquid and Al inset.However,due to the low melting point and thin coating thickness of Zn,it was easy to dissolve and reduce the melting point of aluminum alloy during pouring.As a result,the interfacial area finally solidified and cannot be filled,resulting in gap defects,which reduced the shear strength of bimetal casting.Three different Ni coatings prepared by electroplate,electroless plating and plasma spray can prevent the generation of Al-Mg intermetallic compounds and reduce the thickness of the interface layer.The interface layer was mainly composed of Mg₂Ni,Nickel-based solid solution(SS)and Al₃Ni.The shear strength of plasma sprayed Ni for Al/Mg bimetal castings was superior to that with electroplated Ni and electroless Ni plating,which was increased by 69%compared to that untreated bimetal castings.Ni-Cu composite coating can reduce the thickness of the interface layer of bimetallic components significantly.The phase composition of the interface layer was Al₃Ni,Ni(Cu),Cu(Ni),Mg₂Cu and Al₇Cu₃Mg₆ mainly.The shear strength of the Al/Mg bimetal castings coated with the Ni-Cu composite coating was increased by 20.3%compared with the untreated castings.(5)The effects of different heat treatment methods(solution treatment,aging treatment,homogenization annealing,multi-level homogenization annealing)on the structure and properties of Al/Mg bimetal castings were systematically studied.With solution treatment,the Al/Mg bimetal produced a new diffusion layer composed of Al(Mg)solid solution+Mg₂Si between the Al matrix and reaction layer I.However,because the solution treatment was quenched by rapid cooling,a gap defect was generated in the interface area due to the rapid temperature change.Artificial aging had almost no effect on the structure of reaction layers I and II.With homogenization annealing,crack defects were prone to generat at the interface when the cooling method is air cooling,but furnace cooling did not.As the homogenization annealing time increased,the thickness of the diffusion layer between the Al substrate and the Al₃Mg₂+Mg₂Si reaction layer continued to increase.Multi-level homogenization treatment can make the structure and composition of the bimetal interface layer more uniform,and promote the Al₁₂Mg₁₇ phase in the magnesium matrix to precipitate in a small layer,which was beneficial to improve the bimetal shear performance.