High-Strength Diffusion Bonding of TC4/17-4PH Using an Nb/Cu Multilayer Interlayer: Microstructural Evolution and Fracture Mechanism
摘要
Achieving high-strength diffusion bonding between titanium alloys and steels remains challenging because brittle Ti–Fe reaction products can form at the interface and provide preferential paths for brittle fracture. In this study, TC4/17-4PH joints with an Nb/Cu multilayer interlayer were investigated to clarify the temperature-dependent evolution of interfacial microstructure, mechanical performance, and fracture behavior at 850 °C to 950 °C. Microstructural and chemical analyses were combined with tensile and lap-shear testing and fractographic characterization. At 850 °C and 900 °C, interfacial diffusion remained moderate and did not lead to the formation of a continuous brittle reaction layer. The joints were mainly bonded through diffusion-assisted solid-solution-type interfacial interactions. Increasing the bonding temperature from 850 °C to 900 °C improved the joint strength, and the best overall performance was obtained at 900 °C, with ultimate tensile and lap-shear strengths of 849.13 ± 20.24 and 410.89 ± 24.19 MPa, respectively. At 950 °C, the Cu interlayer was markedly consumed, accompanied by formation of Ti–Fe reaction products and a more continuous Nb-rich reaction region, leading to a sharp decrease in the strength. Fractographic analysis further showed a temperature-dependent transition of the fracture path. At 850 °C to 900 °C, both tensile and shear fractures occurred within or near the Cu interlayer region and were accompanied by plastic deformation and tearing. By contrast, fracture at 950 °C shifted toward regions containing Ti–Fe intermetallics and Nb-rich constituents, resulting in predominantly brittle failure. These results indicate that the effective bonding window of the Nb/Cu-interlayered TC4/17-4PH joints is governed by the continuity of brittle interfacial products and the resulting fracture-mode change.