What is Brazing, its process, materials and techniques? (R)

Brazing is the process of joining two or more metal surfaces by pouring molten metal into the joint. To prevent the melting of the workpiece, the melting temperature of the filler material is lower than the parts to be joined.

Soldering is one of the oldest connection methods, but there's a good reason it's still used.

What Is Brazing?

Brazing joins metal surfaces together with a filler metal that has a low melting point. This process uses capillary action, which causes a uniform fluid flow of the filler to combine with the base metal. A unique property of the brazing process is that it preserves the mechanical properties of the metal, which is useful in applications such as silver brazing or other similar metal brazing.

Brazing Process

One of the most important steps in the metal joining process is cleaning the base metal surface. A file or wire brush is best for removing dirt. After the bond gap of the liquid filler metal is calculated to achieve surface tension with the workpiece, the soldering process begins with the proper positioning of the assembly. Typically, a torch is used to slowly heat the metal surface of the workpiece and the filler metal to brazing temperature.

When the filler metal liquefies, it passes through narrow spaces by capillary action and forms bonds between the surfaces of the base metal.

The brazed joints are shaped because they cool down with the assembly.

Materials

These are the metals that are often joined by brazing:

• Cast iron.
• Copper and copper alloys
• Silver
• Aluminium
• Magnesium

Filler metal requirements

Once the molten flux and filler metal have solidified, the solder joint should exhibit the expected mechanical properties.

The brazing temperature must ensure proper fluid flow from the molten solder alloy to the joint.

To form a strong bond, the filler metal must achieve suitable wetting conditions. 

Difference Between Brazing, Soldering & Welding

Brazing uses capillary action to join dissimilar metal surfaces. A process called wetting is used to bond the base metal to the molten filler material. The melting point of brazing is above 450 °C.

When soldering, welding materials with a melting point of 450℃ or less are used. Soldering uses the same concept as brazing, but the main difference lies in the working temperature and therefore the strength of the connection created.

During welding, not only the filler metal but also the workpiece melts. This allows for a stronger connection than brazing. Processes such as TIG, MIG, and stick welding operate at much higher temperatures.

Brazing is a type of MIG/MAG welding. The difference lies in the melting point of the additional wire, which is significantly lower than that of the base metal. Filler metal is deposited to fill the gap by capillary action. There is no significant fusion of base metals, but it can occur to a limited extent.

Different Methods of Brazing

Brazing uses various heating methods depending on the purpose and application. Heat can be applied directly to the joint (localized) or throughout the workpiece (diffused heating).

Localized Heating Techniques

High-Frequency Brazing – High-frequency alternating current is passed through the coil to reach the brazing temperature, heating the workpiece, and melting the filler material.

Torch Brazing – Combustion fuel gas is produced by burning acetylene, propane, or hydrogen with oxygen to heat and melt filler metal. This technique requires flux to protect the connections, which must be cleaned afterward. Torch brazing is primarily used for small-scale production assemblies with uneven metal weights. This process is often performed using a gas welder.

Resistance soldering – The electrical resistance of the solder alloy generates heat, making it ideal for highly conductive metals. This heating technique is ideal for creating simple bonds between metals.

Diffuse Heating Techniques

Furnace Brazing - A gas-fired, or heating element is used to bring the furnace to the desired temperature. Apply brazing solder to the surfaces to be joined and place the entire assembly in an oven to bring it to soldering temperature. Furnace brazing allows for precise control of the heating and cooling cycles of the metal. This process is often performed in a vacuum to protect the solder from atmospheric conditions. This also eliminates the need for flux protection.

Dip brazing – The workpiece or assembly is immersed in a bath of molten filler metal (molten metal bath brazing) or molten salt (chemical bath immersion brazing). Soldering flux is applied to the components to prevent oxidation. Once the molten braze has solidified, the assembly can be removed.

Advantages

• Unlike most welding methods, it can join dissimilar metals.
• High production rate.
• It consumes less power than welding.
• Produces cleaner joints compared to most welding processes.
• Base metals do not melt and retain their shape and mechanical properties.

Disadvantages

• Weak results compared to welded joints.
• Components that operate at high temperatures cannot be connected.
• A tight and uniform joint gap is required to achieve capillary action.
• Impure or contaminated metals can cause leaks in the connections.

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