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
* Heating the joint and applying the alloy
* Removal of flux residues
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.