Tips for Successful Brazing

It is easy to produce successful, leak-tight, high strength brazed joints if six simple steps are considered in the design and manufacture of the joints.

The six fundamental points are:

* Joint design
* Choice of filler metal
* Pre-cleaning of the parent materials
* Fluxing
* Heating the joint and applying the alloy
* Removal of flux residues

Joint Design

The best-brazed joints are those, which have a capillary joint gap into which the molten filler metal can flow. A comparison of the different joint designs used in welding and brazing is shown below.

The most common type of joint used for brazing is the lap joint, or the sleeve joint in the case of tubular components.

To design a good lap joint, two criteria should be considered:

1. The joint gap.
2. The degree of overlap.

It is these two parameters that determine the ultimate joint strength, and not necessarily the properties of the filler metal. A correctly designed brazed joint will often be stronger than the parent materials from which it is constructed. The best degree of overlap for a brazed joint is 3-4 t where t is the thickness of the thinnest parent metal part making up the joint.

The general rule for tubular parts is that the overlap should be one pipe diameter for sizes up to 25 mm diameter tube.

The most suitable joint gap depends mainly on the flow characteristics of the filler metal. The joint gaps for the various alloys listed in the following section have been indicated. The gaps quoted are those which should be present at the brazing temperature, the cold clearances being adjusted as necessary to account for any difference in the expansion properties of the parent materials.

Choice of Filler Metal

An alloy is normally selected for its melting and flow characteristics. The easiest to use filler materials are the high silver, free flowing alloys, because of their low melting temperatures and narrow melting ranges. The higher the brazing temperature and the longer the melting range of the alloy the more difficult the brazing operation will be.

Pre-cleaning of the Parent Materials

It is important that the mating surfaces of the components to be brazed are free from oil, grease and any surface oxide layer prior to joining. Most engineering components require nothing more than degreasing before assembly.

Oxide removal can be accomplished either chemically or mechanically. Mechanical removal is preferable because the surface is roughened, and excellent bonding is obtained. A medium emery cloth provides about the right amount of surface roughness.

Oil and grease removal is best carried out using a solvent-degreasing agent, but hot, soapy water is better than nothing at all.


The choice of the correct flux is just as important as the choice of filler material.

There are three desirable properties of a flux:

The flux must melt and become active below the melting point of the brazing alloy. Borax or borax based fluxes are not sufficiently molten and active at the low temperatures at which silver brazing alloys are used. A low temperature fluoride based flux such as Easy-flo needs to be employed.

The flux must be capable of removing the oxides found on the parent materials. Easy-flo flux will remove the oxides found on most of the common engineering materials such as mild steel, brass and copper. Special fluxes may be required on certain types of highly alloyed steel and tungsten carbide tool tips. It is also necessary to use a specially formulated flux on aluminium bronze or aluminium brasses containing more than 2% aluminium.

The flux must remain active at the brazing temperature for long enough to allow the brazing operation to be carried out. Fluxes are chemical compounds, which dissolve oxides formed in heating. Like most chemical compounds a flux eventually reaches the point where it is saturated, and becomes unable to dissolve any more oxide. If the flux residues appear blackened and glassy the flux has very likely been exhausted during heating, and a flux with higher time/ temperature stability should be used.

For most engineering requirements there are two Probraze Metals fluxes which will take care of most needs. These are Easy-flo flux powder and Tenacity No.5 flux powder.

Flux Applications

The best way to apply a flux is to paint it onto the joint as a paste. It is common to see operators heating the rod end and dipping it into the flux, and then applying both to the joint. This “hot rodding” technique has the disadvantages that the flux does not protect the joint during the heating cycle, and that the limited amount of flux applied does not prevent oxidation of the parent metals in the vicinity of the joint. Alloys penetration into the capillary gap is also restricted.

A flux powder should be mixed to a double cream consistency with water and a few drops of detergent. It should be applied to the joint by means of a paintbrush. Too much flux will rarely result in a bad joint, but too little flux will invariably give joints of poor quality.

Heating the joint and applying the alloy

When heating a joint for brazing it is essential that it is slowly and evenly heated to the brazing temperature.

The type and size of flame used will depend on the parent materials and the mass of the components. Gas/Air, Air/Acetylene and Oxy/Acetylene are commonly used, but care should be taken with the latter because of the high flame temperature, which may melt the parent materials.

If the mass of metal is very large, more than one torch should be used to raise the components to temperature before the flux becomes exhausted.

As a temperature guide, either the colour of the metal or the condition of the flux may be used. The flux on a joint that has reached the correct temperature for brazing should be clear, fluid and run and flow over the joint area like water. When flux is not being used, for example with phosphorus bearing filler materials, the colour of the metal should be a dull cherry red.

When brazing temperature is reached, the filler metal can be applied by touching the joint mouth with the filler metal rod and applying some indirect or splash heat from the torch. The brazing alloy should be applied according to its flow characteristics; an alloy with free flowing characteristics such as Silver-flo 55 should be touched at one point on the joint, from where it will flow into and around the joint by capillary action, whereas a less-free flowing alloy such as Silver- flo 40 should be applied along or around the entire joint, building up a fillet of alloy.

Once brazing has been completed the heating should be discontinued, as excess heating may cause metallurgical problems with the parent materials, and porosity in the filler material.

When the alloy has solidified the joint can be quenched in water to help remove flux residues.

Quenching should only be carried out when it will not damage the properties of the parent metals, or cause cracking because of stresses caused by the thermal shock (e.g. in the case of tungsten Carbide pieces).

Removal of Flux residues

The method of residue removal depends on the type of flux which has been used. Easy-flo flux residues can be quite simply removed by soaking in hot water, provided they are not in a burnt and blackened condition. Complete flux residue removal is usually possible within 10 to 15 minutes of soaking in water with a temperature of 60ÂșC or above. After soaking, the joints should be scrubbed under running water to ensure complete cleanliness.

Tenacity No.5 flux residues are not water soluble and are best removed by some mechanical means, e.g. shot blasting, although boiling in 10% caustic soda solution may be satisfactory.

Acid pickling is not effective in removing flux unless the residues are in burnt and blackened condition. If pickling is necessary it should be carried out after the flux residue removal operation.