It has been said that it is the capillary action and not the operator skill that ensures the dispersal of filler metal into a brazed joint. The actual skill of brazing metals is in the engineering and design of the joint. However, even the best designed joint will not turn out perfect without following the proper procedure.
The procedure can be broken down into six basic steps. While some of the steps will only take a few seconds to complete, none should be ignored. The end result will be a strong, attractive joint
The act of brazing metals follows the principle of capillary action for the distribution of molten filler metal between base metal surfaces. During this operation, it is crucial to keep adequate clearance between those metals and so the capillary action can work effectively. The tensile strength of brazed joints vary with the amount of clearance.
Clean surfaces are key to the capillary action. If contamination from dirt, scale, grease, rust or oil is present, it must be removed. Otherwise, the contaminants will create a barrier between the brazing material and base metal.
The chemical compound that is applied to the joint surface prior to brazing metals is called flux. This is vital to the process of brazing with only a few exceptions. Heating metal causes accelerated oxidation, which will inhibit the bonding. However, a coating of flux will shield the surface from air to prevent oxidation.
Once the parts have been cleaned and fluxed, they need to be held in positon. They must remain aligned throughout the heating and cooling process to allow the capillary action to work. If the parts permit, gravity is the best way to hold them together. However, in some cases, additional weight or a brazing support fixture may be used.
Brazing the Joint
The next step is the actual brazing of the joint. This involves the heating of the assembly to the brazing temperature. A number of different fuels can be used in brazing metals, such as acetylene, natural gas or propane. However, the most popular combination is acetylene and oxygen. Both metals must be heated uniformly so that they both reach brazing temperature at the same time.
Cleaning the Joint
Once the assembly has been blazed, the joint must be properly cleaned. First, any flux residue is removed. Then, pickling is done to remove any oxide scale that may have formed during the process of brazing metals.
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