Microstructure and mechanical properties of TiAl/42CrMo joint brazed with Ti_(70-x)Zr_xCu₁₅Ni₁₅ filler metals

To address the difficulty in reliably joining TiAl alloy and 42CrMo steel, Zr was introduced into conventional Ti‑based brazing filler metal (BFM) to restrain the formation of brittle phases and improve the mechanical properties of brazed joints. The effects of Zr content on the microstructure, mechanical properties, and fracture behavior of TiAl/42CrMo brazed joints were systematically investigated. The results indicate that the typical microstructure of the joint obtained with the Zr-free Ti-15Cu-15Ni BFM is TiAl/α₂ + Ti(Ni,Cu)Al + (α₂ + Ti(Cu,Ni)₃)/α₂ + Ti(Cu,Ni)₃/τ₃ + Ti(Ni,Cu)Al + TiC/42CrMo. As the Zr content increases, the acicular structure of α₂ + Ti(Cu,Ni)₃ gradually disappears. When the Zr content exceeds 10 wt.%, a lamellar eutectoid structure of (Ti,Zr)₂(Cu,Ni) + (Ti,Zr)₃Al appears within the joint. Meanwhile, Zr atoms from the BFM partially substitute Ti in TiC to form (Ti,Zr)C phase, which reduces the brittleness of the interfacial carbide layer. The joint brazed with Ti-20Zr-15Cu-15Ni BFM achieves the highest average room temperature shear strength of 221.1 MPa. The crack propagation path analysis reveals that all joints undergo brittle fracture along the (Ti,Zr)C layer, indicating that the interfacial carbide layer is the critical factor determining the strength of the TiAl/42CrMo brazed joint. The addition of Zr effectively enhances the joint strength by forming the tougher (Ti,Zr)C phase and the lamellar eutectoid structure. This study provides theoretical guidance and experimental support for the composition optimization of Ti-based BFM in dissimilar brazing of TiAl alloy and steel.