Investigation of Cu Interlayer on Joint Formation of Ti/Mg Bimetal Fabricated by Liquid–Solid Compound Casting Process

In this paper, TC4/AZ91D bimetallic composites has been prepared by liquid–solid compound casting process adapting various thickness of Cu coating. The effects of Cu coating thickness on the evolution of interface microstructures and joint mechanical properties are investigated. The results indicate that metallurgical bonded joints are obtained with Cu coating thickness ranging from 36.7 to 51.4 μm. With the increase of Cu coating thickness, the interface microstructure evolves from δ-Mg + Mg2Cu eutectic structure to Mg–Cu intermetallic compound (IMC) Cu2Mg + Mg2Cu and Mg–Al–Cu ternary intermetallic compound. The calculation results of formation enthalpy and chemical potential of Mg–Al–Cu system suggest that Cu element prefers to react with Mg element and formed Mg–Cu IMC. In particular, when Cu coating thickness reaches 36.7 μm, the average shear strength of the bimetal reaches a maximum of 65.3 MPa. Further increasing Cu coating thickness leads to the generation of thick Mg2Cu IMC layer and Mg2Cu + Cu2Mg mixed IMC layer which are proved to be harmful to the shear strength of TC4/AZ91D bimetals. All the fracture surfaces of the bimetallic composites exhibite to have a brittle fracture morphology. However, the fracture location is different with each other. For Cu coating thickness of 36.7 μm, the interface fractures at the δ-Mg + Mg2Cu eutectic structure, while the interface fractures at the Cu2Mg + Mg–Al–Cu ternary intermetallic layers when Cu coating thickness is 44.2 μm and 51.4 μm.