Zr hydrogenation by cathodic charging and its application in TC4 alloy diffusion bonding

A hydrogenated Zr interlayer was formed by cathodic charging and was subsequently used to facilitate the diffusion bonding of TC4 alloy. The microstructure and phase composition of the hydrogenated Zr were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermogravimetry (TG). The hydride γ phase precipitated from the supersaturated αH phase when the αH phase was charged with 0.15 wt.% hydrogen. When the hydrogen content reached 0.3 wt.%, the γ phase completely transformed into the ε phase. The effects of hydrogen on the microstructure evolution and on the mechanical properties of the diffusion bonds were investigated by XRD and by scanning electron microscopy in conjunction with energy-dispersive spectroscopy (SEM/EDS). The interdiffusion coefficient and activation energy of the Ti-hydrogenated Zr diffusion couple were calculated using the Boltzmann-Matano method and Arrhenius' law. According to the experimental and computational results, the thickness of the diffusion layer and the shear strength of the diffusion bond increased drastically with increasing hydrogen content. Specifically, when 0.3 wt.% hydrogen was added into the Zr interlayer, the thicknesses of the diffusion layers of the joints bonded at 650 °C and 700 °C increased by 5 times and 1.2 times, respectively. The activation energy at the Zr-rich end was reduced by approximately 40 kJ mol−1, and the interdiffusion coefficient increased accordingly by two orders of magnitude. Mechanisms for the hydrogen-induced enhancement of diffusion bonding are proposed.