Microstructures And Properties Control Of Ti/mg Bimetal Interface Strengthed By Al Interlayer With Solid-liquid Compound Casting

Ti/Mg bimetallic composites have both the advantages of titanium alloys and magnesium alloys,which have the widely application prospect in aerospace,deep sea,automobile and other fields.The process of solid-liquid compound casting（SLCC）has the advantages of high efficiency,short process,and is suitable for the connection between dissimilar metals for the atructures with complex shapes.However,the process of SLCC cannot be used to achieved the metallurgical bonding between Ti and Mg for their enormous differences in physical and chemical properties.As an important alloying element of both titanium alloy and magnesium alloy,Al can react with with Ti and Mg matrix at low temperature,which provides the basic of research of Al as the strengthed interlayer for the connection of Ti/Mg bimetal by solid-liquid compound casting.The interfacial structure of Ti/Mg bimetal was regulated and strengthed by optomizing the parameters of casting process,Al-Si alloying and the composite interlayer was prepared on the surface of TC4.The better casting parameters and the preparation scheme of interlayer were obtained.The microstructures,compositions and properties of Ti/Mg bimetals were observed and investigated by varieties of characterization methods,such as XRD,SEM,EDS,TEM,microhardness and shear strength tests.Combined the results of thermodynamic simulation into account,the mechanism of the interfacial bonding was investigated.The main conclusions are as follows:（1）The effects of casting temperature on the microstructures and properties of Ti/Mg bimetal by solid-liquid compound casting were studied by single factor experiment.The results showed that the metallurgical reaction between Al interlayer and Mg melt could taken place at the casting temperature of 720 ^oC and 740 ^oC under the premise of liquid-to-solid volume ratio of 48.However,Ti Al₃ interlayer generated during the hot dip process was falled off from TC4 matrix,resulting in low shear strength.Based on the research of the casting temperature was 720 ^oC,the influence of liquid-to-solid volume ratio on the microstructures and thickness of the interface between Ti and Mg was studied.The results showed that with the increases of liquid-to-solid volume ratio from 3 to 48,the microstructurs of the interface were evolved fromα-Al to Al₁₂Mg₁₇+δ-Mg eutectic structure,at lastδ-Mg.When the liquid-to-solid volume ratio was 24,there were obvious Al₁₂Mg₁₇+δ-Mg eutectic structure appeared at the interface with the morphology of tiny honeycomb-like and feather-like.The thickness of the interface was decreased about 10μm.When the casting temperature was 720 ^oC and the liquid-to-solid volume ratio was 24,the shear strength of Al-coated Ti/Mg bimetal was reached to 48.5 MPa,however,Ti/Mg bimetal without Al interlayer was only 19.1 MPa.The fracture of Al-coated Ti/Mg bimetal by SLCC produced and developed at the position of brittle Al₁₂Mg₁₇ intermetallic compound.（2）To prevent the formation of brittle Al₁₂Mg₁₇ intermetallic compounds at the interface,Ti-Al-Si and Al-Si interlayer was prepared on the surface of TC4 by hot dipped Al-15Si.The results showed that both Ti-Al-Si and Al-Si interlayers can react with AZ91D melt during the process of SLCC.When the casting temperature was 700^oC,and the liquid-to-solid volume ratio was 48,the interface of Ti/Mg bimetal was composed ofα-Al+Mg₂Si andτ-Ti Al Si ternary phase,the shear strength was 59.4 MPa.The fracture occurred atα-Al+Mg₂Si interfacial layer.However,when the csating temperature is 720 ^oC and the liquid-to-solid volume ratio is 48,a mixed region composed of Ti₅Si₃,Ti（Al,Si）₃,Ti₇Al₅Si₁₂,Ti₅Si₃/τ-Ti₇Al₅Si₁₂ twin crystals and Mg₂Si was formed at the interface.The shear strength is reach to 73.1 MPa.The interface fracture occurred at the casting defects caused by the accumulation of the granularτ-Ti Al Si ternary phase in Mg matrix near the interface.In order to eliminated the casting defects caused by granularτ-Ti Al Si phases,the uniform interfacial structures of Ti（Al,Si）₃ andτ-Ti Al Si were obtained on the surface of TC4 by vacuum heat treatment.The shear strength of Ti/Mg bimetal after heat treatment is reach to 80.3 MPa.（3）The application of Zn-Al composite layer was used to control the microstructure and properties of Ti/Mg bimetal by SLCC.The wettability between Al and Mg was improved by Zn coating and the metallurgical reaction was occurred at lower casting temperature,which inhibited the formation of Al₁₂Mg₁₇+δ-Mg eutectic structure at the interface.The casting temperature mainly affected the diffusion of Mg and Zn element into Al interlayer.On the basis of the optimal liquid-to-solid volume ratio of Al-coated Ti/Mg bimetal,when the casting temperature was 720 ^oC,the structure of the interface was composed ofα-Al（Zn）+Mg₂₁（Al,Zn）₁₇,and the shear strength was reached to 67.4 MPa.The fracture occurred between different layers caused by uneven diftribution of elements at the interface.（4）The application of high entropy alloys-Al composite layer was used to controlled the diffusion rate of Mg to Al interlayer of Al-coated Ti/Mg bimetal by SLCC,as to control the microstructures and mechanical properties of Ti/Mg bimetal.The high entropy alloy coating has the obviouly‘hysteretic diffusion’effect,which could effectively inhibite the diffusion between Mg melt and Al interlayer.On the basis of the optimal liquid-to-solid volume ratio of Al-coated Ti/Mg bimetal,with the casting increased from 720 ^oC to 750 ^oC,the interface structures ofα-Al（Mg）,α-Al（Mg）+Al₃Mg₂,Al₃Mg₂+Al₁₂Mg₁₇ and Al₁₂Mg₁₇+δ-Mg were formed between Ti and Mg.When the coating thickness of high entropy alloy is 800 nm and the casting temperature is 720 ^oC,the interface structure was solid solution structure,and the bonding strength of Ti/Mg bimetal fabricated by SLCC was reached to 93.6 MPa,and the fracture occurs between magnesium matrix and high entropy alloy（Mg）solid solution.