What is Brazing?
Brazing Technology is a joining process in which a filler metal is melted and dispersed by capillary action between the faying surfaces of the metal parts being joined. No melting of the base metals occurs in brazing; only the filler metals are melted. In brazing, the filler metal (also called as brazing metal), has a melting point is above 840° F (450° C) but below the melting point of the base metal to be joined. Tensile strength of the joint is about 40000 psi.
Advantages of Brazing:
Brazing has numerous advantages compared to welding:
- Any metals can be joined including dissimilar metals
- Certain brazing methods can be performed quickly and consistently, thus permitting high cycle rates and automated production
- Some methods allow multiple joints to be brazed simultaneously
- Brazing can be applied to join thin walled parts (But welding doesn’t)
- Less heat and power are required than in fusion welding
- Brazed joint’s never penetrate the base metal (were the welded joint penetrate the base metal)
Limitations of Brazing:
Brazing also has some disadvantages:
- Brazing’s joint strength is normally less than that of a welded joint
- Even though the strength of a good brazed joint is better than that of the filler metal, it is likely to be less than that of the base metals
- High operational temperatures may weaken the brazed joint
- The color of the metal in the brazed joint may not match the color of the base metal parts. It is an aesthetic disadvantage.
Brazing Filler metal:
- Copper brazing filler metal
- Silver brazing filler metal
- Coated brass brazing filler metal
- Nickel brazing
- Aluminum brazing
- Mild steel brazing
- Aluminium – silicon brazing
- Copper – phosphorus brazing
- Magnesium brazing
To qualify as a brazing metal, the following characteristics are needed:
- Melting temperature must be compatible with base metal
- Low surface tension in liquid phase for good weldability
- High fluidity for penetration into the interface
- Capability of being brazed into a joint of adequate strength for the application
- Avoidance of chemical and physical interactions with base metal
Filler metals practiced to the brazing operation in a variety of ways. The brazed metal comes as wire, rod, sheets, strips, powders, pastes and preformed parts made of braze metal. They designed to fit a particular joining configuration and cladding on one surface to be brazed.
Brazing fluxes (aluminum brazing flux, silver brazing flux) serve as a similar purpose as in welding. They dissolve or combine or slow down the formation of oxides and other unwanted by products in the brazing process. Use of a flux doesn’t substitute for the cleaning steps described previously.
Brazing Fluxes are available in three phases. They are:
Characteristics of a good flux include:
- Flux has low melting temperature
- It has low viscosity so that it can be displaced by the filler metal
- Fluxes facilitates wetting
- It protects the joint until solidification of the filler metal starts. The flux should also be easy to remove after brazing operations.
- Chemically cleans the metal surfaces
- It shields the process from oxidation and atmospheric contamination