Improvement of Joint Strength of TC4/AZ91D Bimetal in Solid-liquid Compound Casting Process Using Cu-Ni Composite Interlayer

In the present study, the liquid-solid compound casting process has been developed for fabricating AZ91D/TC4 bimetal by adapting a Ni/Cu or Cu/Ni composite intermediate. The effects of interlayer sequence (Ni/Cu and Cu/Ni) on interface microstructure evolution and mechanical properties were investigated in detail. In particular, in order to promote inter-diffusion of Cu and Ti elements at the Cu/Ti interface or Ni and Ti elements at the Ni/Ti interface, the vacuum heat-treat method was adapted before the liquid-solid compound casting process. The results showed that both the Ni/Cu and Cu/Ni composite interlayer realized metallurgical bonding between TC4 and AZ91D. The interface reaction layers of the TC4/AZ91D bimetal using Cu/Ni composite interlayer were composed of Mg₂(Ni, Cu), Ni₂Mg₃Al, Cu(Ni) solid solution and Mg-Ni eutectic structure. However, the interface reaction layers of TC4/AZ91D bimetal using the Ni/Cu composite interlayer were mainly composed of Mg₂(Ni, Cu), (Al₃Ni + Ni₂Mg₃Al) and Ni-Ti phases. Nano-indentation tests show that Ni-Ti intermetallic compounds has the highest nano-hardness at interface region, which leading to a poor shear strength at interface. When using Cu/Ni composite interlayer, the TC4/AZ91D bimetal had the highest shear strength of 97 MPa.