Active metal brazing is used mainly for joining ceramics that are not wetted by 'conventional' brazes. In active metal brazing, a metal (usually titanium) is added to the braze alloy to promote reaction and wetting with a ceramic substrate. The addition of titanium to several braze alloy compositions results in increased reactivity and considerable improvement in wetting behaviour. The ceramic is wet by the formation of an intermetallic interfacial reaction product which can then form a joint with the braze alloy.
The most well-established active metal brazes are based on the 72Ag-28Cu eutectic alloy, to which 1-5 wt% titanium is added to produce sheets or cored wires. Indium is sometimes added to lower the eutectic temperature. These form the basis of a range of commercially available brazes which wet alumina and other ceramics such as silicon carbide, silicon nitride and diamond.
The reactivity of these active metal braze alloys requires the brazing operation to be carried out in a vacuum, or an inert gas atmosphere. Brazing temperatures are typically 50-100°C above the liquidus of the braze alloy.