Investigation On Effects Of Ni And Cu Interlayers On Interface Microstructures And Properties Of TC4/AZ91D Bimetallic Materials By Solid-Liquid Compound Casting

Ti/Mg bimetals show advantages of titanium and magnesium,which have potential applications in the fields of automobile,aerospace,biomedical and so on.Solid-liquid compound casting has become an important method for bimetallic material preparation because of its advantages such as flexible technical means,the ability to prepare complex components and good economic property.In this study,the surface treatment of TC4 titanium alloy was carried out firstly by electrodeposition technique,and then the TC4/AZ91D bimetals were prepared with electrodeposited Ni,Cu and Cu/Ni coating as interlayer via the solid-liquid compound casting process,aiming to overcome the difficulties of weak reaction and low mutual solubility of titanium/magnesium interface and improve interface bonding performance.Then the microstructure and properties of TC4/AZ91D bimetals were observed and studied by a series of characterization methods,such as SEM,EDS,XRD,TEM,microhardness,nanoindentation and shear strength tests.Combined with the results of casting simulation and thermodynamic calculation（by Miedema-Toop）,the effects of some process factors such as types of interlayer metal,pre-vacuum heat treatment and sequence of composite interlayer on interfacial microstructure evolution and shear strength were studied,respectively,and the formation mechanism of interface were discussed.The main conclusions are as follow:（1）When Ni was chosen as interlayer to fabricate TC4/AZ91D bimetals,according to its functional characteristics,the effects of pouring temperature on interface microstructure and properties were investigated.When the pouring temperature was not less than 690 ℃,the interface zone of the TC4/AZ91D bimetals which composed of（Mg₂Ni+α-Mg）,Ni₂Mg₃Al and Al₃Ni compounds was acquired.When the pouring temperature rose to 750 ℃,the Al₃Ni binary phase appeared in the interface zone and formed a continuous reaction layer and the width of the interface reaction layer also reached the maximum.Due to the generation of Al₃Ni reaction layer,the maximum microhardness value of interface zone reached 334 HV.When the pouring temperature was 720 ℃,the TC4/AZ91D bimetals achieved the highest shear strength of～97.35 MPa,and the rupture occurred at the interface of（Mg₂Ni+α-Mg）/TC4 or interior of（Mg₂Ni+α-Mg）eutectic structure.Furthermore,an excessive increase in the pouring temperature of 750 ℃ resulted in an increase of thickness of interface zone and led to the generation of ribbon-like Al₃Ni binary phase which could increase the possibility of the brittle fracture,thus the shear strength went down.（2）When Cu was chosen as interlayer to fabricate TC4/AZ91D bimetals,according to its functional characteristics,the effects of thickness of Cu interlays on interface microstructure and properties were investigated.When the thickness of Cu interlayer was not less than 36.7μm,the interface zone of the TC4/AZ91D bimetals which composed of（Mg₂Cu+α-Mg）,Mg₂Cu,Cu₂Mg and Mg（Al,Cu）₂ compoundswas acquired.The amount of Mg₂Cu compounds increased with the increase of thickness of Cu interlayer,and the distribution of Mg₂Cu also changed from the scattered island to a complete reaction layer.When the thickness of Cu interlayer increased to 51.4μm,the（Mg₂Cu+Cu₂Mg）mixed structure generated at the interface zone with a relatively maximum value of microhardness of 349 HV and resulting in the most violent fluctuations of microhardness in the interface zone.The TC4/AZ91D bimetals achieved the highest shear strength of 65 MPa,when the thickness of Cu interlayer was36.7μm,and the rupture occurred at the interface of（Mg₂Cu+α-Mg）/Mg（Al,Cu）₂.（3）Under proper pre-vacuum heat treatment time,the interface zone of the TC4/AZ91D bimetals with Ni interlayer could be divided into three categories:when the preheat time was 0.5 h,the interface was composed of Ni Ti₂,Ni Ti,Ni₃Ti and（（Mg₂Ni+α-Mg）+Ni₂Mg₃Al）mixed structure;when the preheat time was 5 h,the interface composed of Ni Ti₂,Ni Ti,Ni₃Ti and granular（Al₃Ti+Ni₂Mg₃Al+Al₃Ni）mixed structure;when the preheat time was 9 h,the interface was composed of Ni Ti₂,Ni Ti and Ni₂Ti₄O（Mg）reaction layers.In the interface zone,the microhardness of Ni₂Ti₄O（Mg）reaction layer remained at the highest level.When the preheat time was5 h,without any（Mg-Ni）eutectic structure appeared in the interface zone and the granular（Al₃Ti+Ni₂Mg₃Al+Al₃Ni）mixed structure dispersed in AZ91D matrix,so the shear strength of TC4/AZ91D bimetals was increased to 105.2 MPa,and the brittle rupture occurred at Ni Ti₂ reaction layer.（4）When Cu-Ni was chosen as interlayer,the interface microstructure and properties of TC4/AZ91D bimetals were closely related to the order of Cu-Ni.When Cu/Ni order was used,the interface zone of TC4/（Cu/Ni）/AZ91D bimetals which composed of Cu-Ti compounds,Cu（Ni）solid solution,Cu layer and（Mg₂Ni+α-Mg）+Ni₂Mg₃Al)mixed structure had lower factor of plastic deformation resistance(H³/E^*2)and higher microhardness dissipation parameter（MDP）,indicating that the interface zone had relatively good plasticity and toughness,which is beneficial to interface bonding.When Cu/Ni order was used,the Ni-Ti compounds in the interface zone of TC4/（Ni/Cu）/AZ91D bimetals had higher factor of plastic deformation resistance(H³/E^*2)and lower microhardness dissipation parameter（MDP）,indicating that the interface zone had the tendency of stress concentration and brittleness,which is harmful to interface bonding.Results of shear tests also indicated that TC4/（Cu/Ni）/AZ91D bimetals had higher shear strength.Therefore,when Cu-Ni composite interlayer was used to fabricate TC4/AZ91D bimetals,the sequence of Cu/Ni should be selected preferentially.（5）Under proper pre-vacuum heat treatment temperature,the interface zone of TC4/（Cu/Ni）/AZ91D bimetals were optimized further,and could be divided into three categories:the first was composed of Cu-Ti,Cu（Ni）solid solution,Ti Ni₂Cu,Mg₂（Cu,Ni）,（Mg₂Cu+α-Mg）and Ni₂Mg₃Al structures;the second was composed of Cu-Ti,Cu（Ni）solid solution,Ni₃Ti,Mg₂（Cu,Ni）and Ni₂Mg₃Al structures;the third was composed of Cu-Ti,Ti Ni Cu,（Al₃Ti+Ni₂Mg₃Al）（slight）and Mg（Al,Cu）₂ structures.When pre-vaccum heat treatment temperature was 700 ℃,the shear strength of TC4/AZ91D bimetals reached 112 MPa and the shear fracture occurred at Cu（Ni）solid solution reaction layer which exhibited certain plastic characteristics.When pre-vaccum heat treatment temperature was 600 ℃ and 800 ℃,the rupture occurred at Mg₂（Cu,Ni）and Mg₁₇（Al,Cu）₁₂ reaction layers and presented features of brittle fracture.