Brazing is a versatile joining process. It is used to join most metals and alloys commonly used in engineering.
It is a thermal joining process in which a molten brazing filler metal is drawn into a capillary gap between the metals being joined. Brazing filler metals have a melting point above 450˚C but always below the melting point of the metals being joined.
The Properties of a Successfully Brazed Joint
Strong and ductile - well made brazed joints can be at least as strong as the parent metals being joined and will withstand demanding service conditions.
Leak tight - brazing produces leak tight joints widely used on liquid and gas pipe-work installations.
Electrical conductivity - brazed joints offer good electrical conductivity and are used in applications where this property is important.
Joint appearance - brazed joints have smooth, neat fillets.
Some Features of Brazing
An outstanding feature of brazing is its ability to join different metals and components of dissimilar size and mass.
Brazing is also capable of joining tungsten carbide, ceramics and similar non-metallic materials.
Successful silver brazing takes place at relatively low temperatures (600˚C-900˚C). Unlike welding the parent materials are not melted.
The Brazing Process
Brazed joints can be made using a wide range of heating methods.
It is easy to set up for brazing and by following the correct procedures good results will be achieved.
Brazing is an economical process for joining small numbers of components through to mass production.
Many brazing processes can be automated allowing reliability and control as well as reducing costs.
Thermal Joining Processes - Where Brazing Fits In
Soldering and Brazing
Thermal joining processes in which the molten filler metal is drawn into a capillary gap between two closely fitting surfaces. By definition:- Soldering takes place at temperatures below 450˚C and brazing above 450˚C. Both processes occur below the melting point of the metals being joined.
Is a type of brazing using filler metals containing silver that melt between 600˚C and 900˚C. Silver soldering/Hard soldering are terms synonymous with Silver Brazing (not soldering).
A thermal joining process in which the mating faces are softened or made liquid by the application of heat or pressure or by both. Filler metal with a similar composition to the parent metal may or may not be added.
Braze or Bronze Welding
A thermal joining process using a hand torch and filler metal with a lower melting point than the parent metals. The process uses copper based filler metals melting above 850˚C and does not rely on capillary attraction.
Typical Joint Designs for Brazing vs Welding
Typical joint designs for brazed and welded components are shown below.
Terms Associated with the Brazing Process and Brazing Filler Metals
Brazing Alloy / Filler Metal
A filler metal which melts above 450˚C but below the metals being joined.
A chemical compound applied to the parent metals to protect them from oxide formation during heating and to promote flow and wetting of the brazing filler metal.
The materials to be joined by brazing.
When molten brazing alloy flows over and adheres to the parent metals.
The gap between the parent metals to be joined by the brazing filler metal.
The force that draws a molten brazing alloy into the joint gap.
The highest temperature at which a filler metal is completely solid.
The lowest temperature at which a filler metal is completely liquid.
The temperature range over which the filler metal melts.
An alloy (filler metal) with a single melting point rather than a melting range is known as a eutectic alloy.
When a brazing filler metal with a long melting range is heated too slowly, the phase with the lowest melting point begins to flow first. The material left behind has a changed composition and a higher melting point. It will not flow readily. An unsound and unsightly joint is the usual result of liquation.
This is a form of galvanic corrosion, generally associated with two phase brass alloys, in which the zinc rich beta phase is selectively leached out of the brass, leaving a sponge like matrix of the copper rich phase. This can occur when brazed joints are exposed to salt or seawater.
Nickel and nickel-based alloys are prone to cracking during brazing with silver brazing filler metals. The cracking is known as intergranular penetration or stress cracking. It usually follows the grain boundaries and only occurs when components are subject to high stresses either in the material or created through the brazing process.
Crevice or Interfacial Corrosion
When silver brazed joints in stainless steel are exposed to water or humidity in service, joint failure may result from a specific form of interfacial or crevice corrosion along the stainless steel - brazing filler metal interface.