Brazing Technology | How To Brazing | Brazing Joints | Brazing Filler and Flux

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:

1. Any metals can be joined including dissimilar metals
2. Certain brazing methods can be performed quickly and consistently, thus permitting high cycle rates and automated production
3. Some methods allow multiple joints to be brazed simultaneously
4. Brazing can be applied to join thin walled parts (But welding doesn’t)
5. Less heat and power are required than in fusion welding
6. Brazed joint’s never penetrate the base metal (were the welded joint penetrate the base metal)

Limitations of Brazing:

Brazing also has some disadvantages:

1. Brazing’s joint strength is normally less than that of a welded joint
2. 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
3. High operational temperatures may weaken the brazed joint
4. 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 Joints:

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:

1. Melting temperature must be compatible with base metal
2. Low surface tension in liquid phase for good weldability
3. High fluidity for penetration into the interface
4. Capability of being brazed into a joint of adequate strength for the application
5. 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 Flux:

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:

1. Paste
2. Powder
3. Liquid

Characteristics of a good flux include:

1. Flux has low melting temperature
2. It has low viscosity so that it can be displaced by the filler metal
3. Fluxes facilitates wetting
4. It protects the joint until solidification of the filler metal starts. The flux should also be easy to remove after brazing operations.
5. Chemically cleans the metal surfaces
6. It shields the process from oxidation and atmospheric contamination