Chapter 7 - Brazing: A Subclassification of Welding

Abstract

Brazing is a process for producing permanent, mechanically acceptable, and leaktight joints comparable to fusion welding but without necessitating melting of the base materials. It is accomplished by using a filler material that melts above 450 °C but below the solidus temperature of the substrate materials and that flows by capillary action into a properly designed joint, which consists of a controlled gap or clearance. The brazing is capable of producing strong, continuous joints over large areas or long lengths with little or no alteration of the microstructure or chemistry of the substrates. This allows dissimilar materials, by type or composition within a type, to be readily joined. The surface bonding mechanism spreads loading so thin and thick sections can be joined in any combination under relatively stress-free conditions, producing close-tolerance structures or brazements. A variety of brazing process embodiments allows either manual or automatic operation with either localized heating of the joint or uniform heating of the entire braze assembly. The classification of brazing processes is normally by the method of heating, including torch brazing, furnace brazing, induction brazing, resistance brazing, dip brazing, infrared brazing, and laser or electron-beam brazing. Braze filler can be metallic or ceramic. Metallic fillers are used for joining metals or ceramics to themselves or to one another, while ceramic fillers are used exclusively for joining ceramics. The important characteristics of brazing filler materials are chemical, mechanical, and physical compatibility with the base materials, melting temperature and fluidity, tendency for liquid and solid phases to separate or liquate, and the ability to wet and bond the substrates.