Brazing: A Definition and Guide of Joining Process

What is Brazing?

Brazing is a metal-joining process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, with the filler metal having a lower melting point than the adjoining metal.

Brazing differs from welding in that it does not involve melting the workpieces. Brazing differs from soldering through the use of a higher temperature and much more closely fitted parts than when soldering.

During the brazing process, the filler metal flows into the gap between close-fitting parts by capillary action. The filler metal is brought slightly above its melting (liquidus) temperature while protected by a suitable atmosphere, usually a flux.

It then flows over the base metal (in a process known as wetting) and is then cooled to join the workpieces together. A major advantage of brazing is the ability to join the same or different metals with considerable strength.

Key Takeaway

Brazing is the process of joining two independent pieces of metal to form one strong load-bearing joint.

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  • Brazing is similar to soldering, but at higher temperatures.
  • Use the right brazing rod material for the metal used in your project.
  • The brazing rod should be melted by the heat of the metal pieces being joined, not by direct contact with the flame of the torch.
  • Use a torch that produces a high intensity flame.

Brazing Rods Materials

How To Braze Metal?

Brazing is a joining process traditionally applied to metals (but also to ceramics) in which molten filler metal (the braze alloy) flows into the joint.

Steps by step:

  • Use the wire brush or emery cloth to scuff the surface of the metal. Then clean the surfaces using soapy water or a degreaser.
  • Position the metal as desired. In most cases, an overlapped joint will be stronger and braze together easier than a gapped joint. If needed, use clamps to secure the pieces in position.
  • Heat the joint where the two pieces of metal will meet until the joint glows.
  • Apply the brazing rod to the joint while continuing to heat the metal surfaces. For large areas, heat portions of the joint to temperature and then move to the next adjacent area.
  • After brazing, use a wire brush to clean the brazed surface to remove any oxidation or residue.

The melting point of the filler metal is above 450°C, but always below the melting temperature of the parts to be joined, which distinguishes the process from welding where high temperatures are used to melt the base metals together.

The filler metal, while heated slightly above the melting point, is protected by a suitable atmosphere which is often a flux. The molten filler metal cools to join the workpieces together providing a strong join between similar or dissimilar metals.

The atmospheres in which the brazing process can be undertaken include air, combusted fuel gas, ammonia, nitrogen, hydrogen, noble gases, inorganic vapors, and vacuum, using a variety of heating sources such as a torch, furnace, and induction coil.

To achieve a sound brazed joint, the filler and parent materials should be metallurgically compatible, and the joint design should incorporate a gap into which the molten braze filler can be drawn or distributed by capillary action. The required joint gap is dependent on many factors, including the brazing atmosphere and the composition of the base material and brazes alloy.

Ideal for joining dissimilar metals, brazing is a commercially accepted process used in a wide range of industries due to its flexibility and the high integrity to which joints may be produced. This makes it reliable in critical and non-critical applications, and it is one of the most widely used joining methods.

What Are the Different Methods of Brazing?

Brazing, which utilizes a wide variety of heat sources, is often classified by the heating method used. To achieve brazing temperature, some methods heat locally (only the joint area), others heat the entire assembly (diffuse heating).

Some of the more widely used methods are highlighted below:

Localized Heating Techniques

1. Torch Brazing

In this method, the heat required to melt and flow filler metal is supplied by a fuel gas flame. The fuel gas can be acetylene, hydrogen, or propane and is combined with oxygen or air to form a flame. This process is readily automated and requires low capital investment. Torch brazing requires the use of a flux, so a post-braze clean is often required.

2. Induction Brazing

High-frequency induction heating for brazing is clean and rapid, giving close control of temperature and location of heat. Heat is created by a rapidly alternating current which is induced into the workpiece by an adjacent coil.

3. Resistance Brazing

This is a process in which heat is generated from resistance to an electrical current (as for induction brazing) flowing in a circuit that includes the workpieces. The process is most applicable to relatively simple joints in metals that have high electrical conductivity.

Diffuse Heating Techniques

1. Furnace Brazing

Furnace brazing offers two prime advantages: protective atmosphere brazing (where high purity gases or vacuum negate the need for flux) and the ability to control accurately every stage of the heating and cooling cycles. Heating is either through elements or by gas firing.

2. Dip Brazing

This involves immersion of the entire assembly into a bath of molten braze alloy or molten flux. In both cases, the bath temperature is below the solidification point of the parent metal, but above the melting point of the filler metal.

Brazing vs. Welding

Brazing is also classified by the AWS as a liquid-solid phase bonding process. Liquid means that the filler metal is melted, and solid means that the base material or materials are not melted.

Unlike welding, brazing does not involve melting the workpieces. The main difference between brazing and arc welding is the heat source. Brazing is applied via torch, furnace, induction, dipped, or resistance as heat sources occurring at a temperature above 840°F (450°C) whereas arc welding uses electricity as a heat source reaching temperatures of roughly 10,000 degrees Fahrenheit.

Brazing vs. Soldering

The only difference between brazing and soldering is the temperature at which each process takes place. Soldering takes place at a temperature below 840°F (450°C), and brazing occurs at a temperature above 840°F (450°C).

Advantages of Brazing and Soldering

Some advantages of brazing and soldering as compared to other methods of joining include the following:

  • Low temperature
  • May be permanently or temporarily joined
  • Dissimilar materials can be joined
  • Speed of joining
  • Less chance of damaging parts
  • Slow rate of heating and cooling
  • Parts of varying thicknesses can be joined
  • Easy realignment

Brazing and soldering are processes that have many great advantages but are often overlooked when a joining process is being selected. The ability to join many different materials with a limited variety of fluxes and filler metals reduces the need for a large inventory of materials, which can result in great cost savings for a small business, home shop, or farm.