A Low-Temperature Joining Technology Suitable for High-Temperature Applications Developed by Establishing Chemical Potential and Supercooling Gradients

Abstract

We designed a composite filler by building chemical potential and supercooling gradients to control the diffusion of metallic atoms but facilitate that of light metals and shortened the diffusion distance, which helped to accelerate the isothermal solidification period and control the excessive compounds. Eventually, a transient liquid phase bonding (TLP) composite joint with hedgehog-like reinforcements and a ductile matrix was thereby obtained at 430°C~490°C for 10min~30min. The “hedgehog-like” structure progressively formed on the NiTi surface: (i) Ni 4 Si 7 Ti 4 whiskers, (ii) gold+Ni 4 Si 7 Ti 4 , (iii) Ni 4 Si 7 Ti 4 , (iv) NiSiTi, and (v) diffusion region; and it then changed into (i) Ni 49 Si 37 Ti 14 and Au 4 Ti, (ii) NiSiTi, and (iii) diffusion region, with the increase in temperatures. Surprisingly, the maximum joint strength reached 84MPa at room temperature and reached ~30MPa in the air at 600°C. This study offered a novel method feasible for most ceramic/metal systems to obtain the goal of low-temperature joining and high-temperature use.

Keywords: Sapphire, NiTi, reaction-composite brazing, Transient liquid phase bonding, Supercooling.