The method where the identical or different metals are not melted, but jointed with an additional metal (brazing alloy) melting at temperatures over 450°C is called "brazing". If the parts to be jointed are on top of the other, butt to butt or nested the brazing alloy capillary flows through the joint point and provides a solid and sealed joint. This brazing method is called "capillary soldering". However, if the bends of the parts to be jointed are opened such as V-, U-, X- and the jointing is performed by filling with brazing alloy then this brazing method is called "braze welding". The braze welding method may also be applied for surface coating.
Any of the heat sources such as oxyacetylene torch, furnace, induction, soldering gun, electric resistance may be used in braze welding method. The heat source most suitable can be selected depending on the joint design, ease of application and sped of production. The braze welding method is most commonly used in manufacturing of heating, cooling and ventilation systems, bikes and motorcycles, automotive, metal furniture and white goods.
The brazing materials are the pure metals or alloys classified in relevant standards such as DIN 8512, DIN 8513, AWS A5.8 and ISO 3677. These standards classify the brazing alloys according to their chemical compositions, melting range, type of additional metal etc. The detailed technical data of OERLIKON brazing alloys are available in technical data sheets.
Preparing the Brazing Surfaces
The surfaces to be jointed should be perfectly clean to obtain a high-quality brazing. The surfaces should be smooth and completely removed from oxide, burrs, oil, grease etc. Sharp edges and corners should be avoided during surface preparation. Surfaces may be prepared and cleaned with mechanical and chemical methods. The cleaned surfaces should be taken in brazing process as quick as possible. Otherwise the surface may be re-oxidized and should be re-cleaned.
The special chemicals applied on the surface and brazing material before the brazing process are called "brazing fluxes". The brazing fluxes are liquefied at 50-100°C lower than the melting point of brazing alloy and perform the following duties:
- Chemically dissolve the oxides on the surface,
- Prevent possible oxidation during pre-annealing process,
- Reduce the surface tension of brazing alloy in liquid form and ensure that it is easily spread on and wet the workpiece,
- Ensure the slow cooling-down of the joint area in some cases,
- Melt before the brazing alloy and notify that the brazing process began. This is particularly important in aluminum and its alloys that don't reveal an annealing color.
The amount of brazing flux to be used in brazing should be carefully selected. If the amount of brazing flux is not sufficient then the abovementioned tasks are not performed efficiently, while excessive brazing flux causes excessive flux residues after the brazing process.
The flux residues should be removed after the brazing is completed, otherwise they can cause corrosion on the application area depending on the properties of the flux. This removal process may be performed by washing with water, brushing (mechanically), submerging the brazed part into the water when it is hot or by means of chemical methods.