Brazing

 

Brazing is similar to both welding and soldering, and is a process of fitting two parts together by melting a filler metal between them.
This filler metal, brass or another alloy, is heated by a torch to a melting temperature of 450 degrees C or above, at which point it flows by capillary action into the space between the two parts. Flux is normally used to enhance this capillary action. Once the filler metal cools, the two pieces are joined together as if welded or soldered.

The most common method of manual brazing is torch brazing. Manual torch brazing is a process where a gas flame is used to heat the joint to be brazed. This is a relatively slow, piece by piece process that is most often used on small production or specialied operations. When larger production or assembly-line work is called for, automated brazing equipment is typically used.

In order for brazing to work, the parts being joined must fit closely together, and the base metals need to be well cleaned and free of any oxides. This is important because any contamination can cause poor wetting, with a weak joint the result.

There are two primary methods of cleaning base metals prior to brazing – mechanical cleaning by an abrasive material, or chemical cleaning to remove oxides. Another cosideration when mechanically cleaning a part is to leave a rough surface at the area to be brazed, as this roughness allows for much better wetting and thus a stronger joint.

Note – a brazing rod differs from a typical filler rod in that it does not mix with the steel parts being brazed. Instead it adheres to them, and can produce up to an 85,000 pound-per-square-inch bond.

 

Time & temperature factors when brazing

The other important factor when brazing is the effect that time and temperature have on the quality of the brazed joints. Generally, the higher the brazing temperature, the better the wetting action of the filler metal. There are other considerations, however, as a brazing temperature that’s too high can cause melting of the parts being joined.

In general, the temperature is usually selected by: the lowest possible braze temperature; the least amount of heat effects on the assembly; and that keeps any base metal/filler metal interactions to a minimum.

There are a variety of alloys that are commonly used as filler metals when brazing, depending on the application or intended use. As a rule, a braze alloy is comprised of 3 or more metals combined to achieve the desired properties.

These braze alloys are available as ribbons, wires, pastes, powders, or creams. How the brazing allow is applied depends on the application – if you’re performing a manual braze, for example, rod or wire is typically applied during the heating cycle.

If you plan on brazing in environments that aren’t contained within an inert or reduced atmosphere, you’ll need to use flux to prevent harmful oxides from forming as the metal is being heated. Flux serves a secondary purpose of cleansing unwanted contamination the might otherwise be left on the brazing surfaces.

When brazing, a flux is usually selected according to its performance on a particular base metal. This is important, as the flux must be chemically compatable with both the filler metal and the base metal of the two components.

Brazing aluminum

Some welder’s aren’t aware that many aluminum alloys can in fact be brazed. It takes some skill and practice, but when done properly, aluminum brazing is a way to add corrosion resistance and additional strength to all-aluminum structures.

Since the melting point of the materials being joined is relatively close to the melting point of the brazing filler metal, careful temperature control is necessary when brazing aluminum. The base metal should never be melted during the brazing process, and as a result the brazing temperature is determined by the melting point of the brazing filler metal, not the base metal.


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