Bridging a Cu layer enables decent metallurgical bonding for wire-arc directed energy deposited Mg-Ti bimetals

In this study, TA15/AZ91D bimetallic materials with Cu as the intermediate layer were prepared by wire arc directed energy deposition. The interfacial microstructe and mechanical properties were systematically studied. The results show that under the best process parameters, an intermetallic compound (IMC) layer of 179 μm thickness (mainly TiCu and Ti₂Cu phases) was formed at the Ti-Cu interface, and an IMC layer of 44 μm thickness (mainly TiAl, α-Mg, MgCu and MgAlCu ternary phases) was formed at the Cu-Mg interface, realizing a good metallurgical bonding. Due to the lower temperature difference, the residual stress between Ti and Cu is reduced, the thickness of the IMC layer decreases, thereby eliminating cracks. The thermodynamic analysis reveals the reaction mechanism of Cu-Mg interface: TiAl phase and MgCu phase tend to form preferentially, and Cu element promotes the formation of TiAl phase. The increase of TiAl phase is related to the activation of more Cu atoms when the heat input increases. Due to the increase of heat input, the concentration of Ti atoms in the Cu layer increases. The Cu dendrites in the Cu layer disappear and the TiCu phases are generate, and the microhardness reaches 480 ± 6.4 HV (10 times that of pure Cu). It is particularly noted that the strength of the bimetal increased by 27 MPa because of the increase in the thickness of the Cu layer (from 1357.90 μm to 1851.96 μm). The fracture mechanism of the bimetal is brittle fracture. Under the best process, the tensile strength of the bimetal can reach up to 106 MPa, and the bending strength can reach up to 138.9 MPa. This study provides important process references for the manufacturing of Mg-Ti bimetals.