How to Braze 

Image credit: Wikipedia  

 

Brazing is a process used to adjoin metal materials together by the use of a consumable metal filler. This process is similar to soldering, except temperatures required to melt the filler metal are higher (above 840° F). The molten metal filler is wicked across the seam of the metal joint, and when cooled it produces a solid bond between the workpieces.

 

Brazing Operation

Brazing is an efficient and easy means of adhering components, and workpieces are able to maintain their structural integrity. Items of dissimilar composition can be brazed with the correct alloys, and brazing is particularly useful when base metals may contain toxic components such as lead or cadmium. Brazing is also preferable if the assembly contains brittle compounds, extensive liquation and segregation, or residual stress and porosity. The brazing process is not limited to metal components, as ceramic components can be brazed. When electrical contacts need to be connected, brazing does not result in the increased resistivity between contacts that welding creates.

Brazing capillary action

Brazing's capillary action

Image credit: Abbott Furnace

 

To begin, workpieces need to be clean, free of oxides, and relatively close fitting. For the best brazed joints, it is recommended to keep the expanse between workpieces less than 1 mm, though many gap sizes can be accommodated. Once components are in place, the filler metal is applied to the seam of the workpieces. The filler metal is brought to its melting point, at which time it is wicked into the cavity by capillary action. A flux is often used to prevent oxides or other contaminants from compromising the braze, but it can also be accomplished in an inert or reduced environment. The flux may even be provided within the filler material for convenience. Flux should be cleaned from the base metal after the brazing process.

Video credit: Panofish via Youtube

 

Brazing Advantages and Disadvantages

 

 

Filler Materials

Braze fillers come in a variety of forms, such as:

Image credit: Sulzer

 

Fillers are chosen based upon their ability to flow across the base metal, ability to withhold a bond while in service, and the temperature at which they melt (below that of the base metal).

 

The most versatile brazing alloys are made up of three or more metals, called a trifoil. They consist of a copper base layer that is coated on each side with a brazing alloy. The copper absorbs mechanical stresses and acts as a diffusion barrier. The resulting joint minimizes the amount of cracking during cooling. Trifoils are useful for conjoining metal-to-cermet, metal-to-carbide, and metal-to-ceramic components.

Trifoil brazing alloy

Trifoil brazing alloy

Image credit: Aufhauser Corp.

 

Some of the most common types of filler materials, and their suitability determined upon the brazing method and application, are as follows.

 

Image credit: Sulzer

 

Ceramics are best adjoined by using filler materials of titanium, zirconium, hafnium, or vanadium. When making electrical connections, silver and copper-based brazes provide the best thermal and electrical conductivity.

 

Obviously, different alloys exhibit different characteristics when being heated. When bonding base metals with a relatively large gap, it is helpful to use an alloy where only a small portion of the filler is molten at a lower melting temperature. On the contrary, alloys with a tendency to liquefy easily are best used on workpieces that have narrow tolerances. The viscosity of the filler material as it is heated through its melting range must be carefully monitored to ensure an adequate union between workpieces. However, eutectic alloys melt at a single temperature and are therefore very ductile. They are useful for strong bonds between items with very little clearance for the filler.  

 

The following table lists the best filler materials based upon the composition of the base metal.

Table credit: Sulzer

 

 

Technique

Many methods exist to accomplish brazing, but the most important consideration is the efficient transfer of heat to filler materials without damaging the base metal. Depending upon the application, the aesthetic and volume of brazing may be considered as well.

  • Torch brazing

Heat is applied by an oxyfuel gas torch. It is a time consuming but precise heating method. The torch may be controlled by a human operator, machine, or an automated brazing system.

 

  • Furnace brazing

Assemblies are passed through a furnace where the materials are prepositioned. This allows a high volume of brazing to be completed, but equipment and energy demands can be costly. 

 

  • Induction brazing

Each part is placed in an induction coil with a limited amount of filler material. Heat is precisely delivered to the brazing area and may I/O options are available. Induction brazing can be accomplished in a furnace as well.

 

  • Dip brazing

This is best used on small components that can be dipped in a molten salt flux. Since air is excluded, oxides are not formed.

 

  • Resistance brazing

Used for joining small, highly conductive parts, the filler material is heated by the resistance of the parts to an electrical current.

 

  • Infrared brazing

A technique that focuses light from quartz bulbs to heat components to the correct temperature.

 

  • Exothermic brazing

An exothermic reaction generates heat that is applied to the filler material and base metals.

 

  • Blanket brazing

In this technique, a blanket is resistance heated and the energy is transferred to the parts by conduction and radiation. It is a very economical process in regards to other brazing processes.

 

  • Electron beam/laser brazing

Accomplished in a vacuum, this method is used to conjoin dissimilar materials by preferentially heating a filler metal between two lower atomic number materials, thereby creating new types of brazed joints.

 

  • Braze welding

A process that uses acetylene or MAPP gas to supply a larger quantity of heat to the filler material. No capillary action is used to promote the bond between workpieces.

 

 

Brazing Safety

Brazing can pose an extreme danger to employees within the vicinity of the brazing process. Notably, fumes and smoke emanated by the materials, material coatings (such as cadmium and zinc), shielding gases, fluxes (which release fluoride), and contaminants can be diluted by the use of proper ventilation equipment. The extreme heat and light from brazing can cause burns and damage retinas, respectively, so proper PPE such as gloves, helmets and goggles, and clothing should be worn.

 

Brazing Standards

The American Welding Society has a collection of documents regarding the specifications for brazing; they are available through the IHS industry standards page. Other selected standards are linked below.

 

-ASD-STAN PREN 3878 Aerospace brazing

-ISO 18279 Imperfections in brazed joints

-ANSI B212.1A Brazing carbide tips

-AN 785 American military specifications on brazing

 

Resources

 

Wikipedia - Brazing; List of brazing alloys

 

Brazco Manufacturing - Products

 

Torch Brazing - Heat Sources of Brazing

 

Joining Tech - Micro-Joining Techniques (.pdf)

 

Aufhauser - Brazing

 

Sulzer - Introduction to Brazing (.pdf)


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