Guide to Metal Brazing
Introduction to Metal Brazing
Brazing is a metal-joining process in which two or more metal pieces are joined together by melting a filler metal into the joint. The filler metal typically has a lower melting point than the base metals being joined, and is distributed between the joint by capillary action. The resulting joint is strong, leak-proof, and able to withstand high temperatures and pressures.
Benefits of Brazing
- Brazing allows for the joining of dissimilar metals.
- Brazed joints are strong and durable.
- Brazing is a cost-effective alternative to welding.
- Brazing can be used to join thin materials.
- Brazing does not melt the base metals being joined, resulting in minimal distortion or warping.
The Brazing Process
The brazing process typically involves the following steps:
- Cleaning the surfaces to be joined to remove any dirt, grease, or other contaminants.
- Fitting the parts together to be brazed.
- Applying heat to the joint area using a torch, furnace, or other heating source.
- Placing the filler metal in the joint.
- The filler metal will melt and flow into the joint due to capillary action.
- Allowing the joint to cool down and solidify.
- Removing any excess flux residue.
Choosing the right brazing alloy is critical for a strong, durable joint. Brazing alloys are typically composed of a base metal and one or more alloying elements, which can affect the strength, ductility, and corrosion resistance of the joint. Some common brazing alloys include:
- Silver-based alloys
- Copper-based alloys
- Nickel-based alloys
- Aluminum-based alloys
Brazing fluxes are used to clean and prepare the surfaces of the metal that will be brazed, and also to prevent oxidation during the brazing process. Fluxes can come in the form of paste, powder, or liquid, and there are different types of fluxes that are suited for different types of metals and brazing methods.
Types of Brazing Fluxes
Some of the commonly used brazing fluxes include:
- Borax-based flux: This is the most commonly used flux and is suitable for use with copper, brass, and other non-ferrous metals.
- Aluminum-based flux: This type of flux is suitable for use with aluminum and aluminum alloys.
- Silver-based flux: This type of flux is suitable for use with silver and silver alloys.
- Copper-based flux: This type of flux is suitable for use with copper and copper alloys.
- Phosphorus-based flux: This type of flux is suitable for use with copper and copper alloys.
Applying Brazing Flux
Brazing flux can be applied to the surface of the metal using a brush or by dipping the metal into the flux. Care should be taken to ensure that the flux is applied evenly and covers the entire area that will be brazed. Excess flux should be removed before brazing, as it can interfere with the brazing process and weaken the joint.
There are several techniques that can be used for brazing, depending on the type of metal and the size and shape of the parts to be joined. Some of the commonly used brazing techniques include:
- Torch brazing: This is the most common brazing technique and involves using a torch to heat the metal and melt the filler metal into the joint.
- Dip brazing: In this technique, the metal is dipped into a bath of molten filler metal, which then solidifies to form the joint.
- Furnace brazing: This technique involves placing the metal and the filler metal in a furnace and heating them to a specific temperature for a set amount of time.
- Induction brazing: This technique involves using an induction coil to heat the metal and melt the filler metal into the joint.
- Resistance brazing: In this technique, an electric current is passed through the metal, heating it and melting the filler metal into the joint.
What metals Cannot be brazed?
Not all metals can be brazed due to their chemical and physical properties. Some metals cannot be brazed together because they may form brittle intermetallic compounds, have poor wetting properties, or have significantly different melting temperatures.
The following metals cannot be brazed:
- Aluminum and magnesium: These metals have a very strong oxide layer that is difficult to remove, which prevents good wetting and bonding.
- Titanium: It has a strong affinity for oxygen, nitrogen, and carbon, which can form brittle intermetallic compounds and make brazing difficult.
- Zinc and galvanized steel: These metals contain a coating that produces toxic fumes when heated, making brazing dangerous.
- Cast iron: It has a high carbon content that can form brittle compounds during brazing.
- Certain high-strength steels and exotic alloys: These metals can have poor wetting properties, form brittle intermetallics, or require specialized techniques for brazing.
It is important to carefully consider the properties of the metals to be joined before attempting to braze them together.