The information on these pages will be advantageous to the average brazing operator.
Select Brazing Alloy
When brazing Copper to Copper, the best choices are Aufhauser PhosCopper 15 or PhosCopper 5. These alloys contain phosphorus and are self-fluxing on copper. When brazing brass or bronze fittings, Aufhauser White SilverFlux is required. When brazing iron, steel or other ferrous metals, select a high silver brazing alloy and use White SilverFlux flux.
For more information on selection of brazing alloys please refer to our proper alloys selection chart.
Select Proper Flux
The use of some flux is essential in the brazing process, because heating a metal surface accelerates the formation of oxides, and these oxides inhibit the ability of the brazing filler metal to wet and bond with the base metals, and also interfere with the integrity of the joint. A coating of flux on the joint area, however, will shield the surfaces from the air, preventing oxide formation. The flux will also dissolve and absorb any oxides that form during heating or that were not completely removed in the cleaning process. When using White SilverFlux, apply it only with a brush. To prevent excess flux residue inside refrigeration lines, apply a thin layer of flux to only the male tubing and, if possible, rotate the fitting once or twice on the tube to ensure uniform coverage.
For more information on the selection of brazing fluxes please refer to our flux selection page.
Proper Clearances / Proper Joint Design
Joint clearance is a principal factor in determining the mechanical strength of brazed joints. It is also a factor in eliminating harmful voids in the joint area and in establishing the capillary force required to fill the joint.
During the brazing process, two closely fitted surfaces or parent metals are heated and a filler metal is introduced. As the filler metal becomes liquid, capillary action draws the molten filler between the surfaces of the parent metals, forming, as it cools, a strong, void-free braze joint. The joint integrity will be maximized by maintaining good fits or clearances between the parent metals. Some alloys are better at "closing" wide gaps than others, and you should consider this in selecting the brazing filler metal.
Metals change their volume when heat is added or subtracted, depending on their thermal coefficient of expansion. When joining dissimilar metals, the expansion rate of each parent metal must be calculated and introduced into the joint design. A good joint should be neither too tight nor too wide; joint strength may suffer from bad design.
Aufhauser LayerMet (a silver-copper-silver “sandwich”) should be used to braze large carbides to steels or stainless steels. ( By “large” we mean bigger than 0.5 sq. inches or 12.7 mm².) When brazing with LayerMet, the silver on each side of the sandwich bonds with the carbide and the steel. The the copper in the middle (which melts at a higher temperature), acts to relieve the stresses caused by unequal rates of thermal expansion of the joined metals.
Cleaning of Metals to be Welded
The joint surface areas should be clean and free from oil, grease, or oxide contamination. Use a stainless steel wire brush, or rub with emery cloth. To clean oil or grease, use a commercial solvent. Just prior to brazing, wipe any remaining particles away with a clean, dry cloth.
Assembly and Fixturing
Maintaining alignment of the base metals during the heating cycle will assist capillary action. You will often have to design a "fixture" to keep the parts steady during the brazing process. The best fixtures are made with materials that are poor heat conductors, such as stainless steel, Inconel or ceramics. Also keep in mind the thermal expansion characteristics of the fixture.
Fluxing of Parent Metals
Since most Aufhauser flux is conventionally made in a paste consistency, it is usually most convenient to brush it on. But as production quantities increase, it may be more efficient to apply the flux by dipping. When using White SilverFlux, however, apply it only with a brush. To prevent excess flux residue inside refrigeration lines, apply a thin layer of flux to only the male tubing and, if possible, rotate the fitting once or twice on the tube to ensure uniform coverage.
Brazing the Assembly
This is the point where heat is introduced. Torch Brazing using a fossil fuel such as oxy-acetylene is a reliable method. This is most common in single assemblies or smaller production levels. It involves heating the assembly to brazing temperature, and flowing the filler metal through the joint. In larger operations, multiple station turntables with multi-tip torches can increase production levels. Automation can replace with preforms, introduce heat and post-clean the assembly reducing labor costs, for more information of Aufhauser performs please visit our preforms product page, or contact a knowledgeable Aufhauser customer representative for more details.
The heat must be applied uniformly. Mass differences and conductivity of the base metals will affect the amount of heat and how much time is required. The heat is directed to a broad area surrounding the joint. Because filler metals follow the greater heat source, the key is getting the interior facing surfaces to proper temperature. If you are brazing a small assembly, you may heat the entire assembly to the flow point of the brazing filler metal. If you are brazing a large assembly, heat a broad area around the joint. DO NOT direct heat solely on the joint surface as it can lead to premature flow of the alloy but not necessarily into the length of the joint. The joint might look adequate but it will have little strength. When using preforms, the alloy is preplaced as close as possible to the joint.
Post Braze Cleanup
After completing the brazed assembly, it must be cleaned, and the flux residues must come off. Fluxes are corrosive. If not removed they can eventually weaken a braze joint. The quickest and most economical method is a water quench. Once the filler has solidified, place the warm assembly in a hot water bath. This will normally "crack" the residue off. For more tenacious residues, agitate the water bath or use a jet spray to knock the flux off. Or simply wire-brush the assembly while submerged in the bath. If the flux has been saturated during the heating cycle, the assembly will have a blackish discoloration. In most cases, an acid bath will be needed to assist the flux removal. Care must be taken in choosing a mild acid to avoid etching the joint.