Joining of partially sintered alumina to alumina, titanium, Hastealloy and C–SiC composite using Ag–Cu brazes

Abstract

The joining behavior of polycrystalline alumina sintered at different temperatures (1473–1773 K) and times (0.5–4 h) to itself and to Ti, Hastealloy and a CVI C–SiC composite using two Ag–Cu active metal brazes (Cusil–ABA and Ticusil) containing Ti has been evaluated. Partial densification and the resulting high porosity content of Al 2O 3 substrates sintered at low temperatures led to braze infiltration of Al 2O 3 pores in 20 min contact but no penetration occurred in dense Al 2O 3 substrates sintered at high temperatures. The scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) examination of the joints showed that chemical reactions between Al 2O 3 and braze constituents formed a Ti-rich reaction layer at braze/Al 2O 3 interface regardless of the sintering conditions and degree of densification of the Al 2O 3. Some redistribution of the substrate and braze constituents across the joint interfaces was observed due to dissolution and interdiffusion. The Knoop microhardness of sintered alumina was consistent with the degree of Al 2O 3 densification achieved, and microhardness profiles across the joint regions displayed a minimum (∼70–160 KHN) in the braze region and a maximum (1900–2012 KHN) in the alumina region. The polished CVI C–SiC composites led to better quality Al 2O 3/C–SiC composite joints and greater segregation of Ti at the composite/braze interface than unpolished composite specimens.