Active brazed Invar-SiO2f/SiO2 joint using a low-expansion composite interlayer

An AgCu-4.5 wt.%Ti/W/AgCu-1 wt.%Ti composite interlayer was designed to braze the Invar-SiO2f/SiO2 joints. Ti contents in the interlayer between the tungsten foil and the Invar alloy was optimized. The effects of the tungsten-foil thickness on the microstructure, elemental distribution and mechanical properties of the brazed joints were investigated. Compared with a single AgCu-4.5 wt.%Ti interlayer, a sufficient metallurgical reaction happened on the SiO2f/SiO2 composite side and the formation of Fe2Ti, Ni3Ti compounds which are detrimental to the joint properties, was greatly inhibited by using a low-expansion composite interlayer (LECI). The tungsten interlayer can reduce the CTE (coefficient of thermal expansion) mismatch in the Invar-SiO2f/SiO2 joint. The highest shear strength of joints brazed with an LECI was 33 MPa, which was 1.75 times higher than the joints brazed with a single AgCu-4.5 wt.%Ti interlayer.