264 



HANDBOOK OF MECHANICAL DESIGN 



FURNACE BRAZING 



Brazing Metal. — For steel parts, copper or brass in the form of wire, paste, or 

 powder. For inaccessible surfaces, use copper-coated steel or make a spray consisting 

 of copper or brass powder mixed with lacquer, and thin. This mixture is sprayed or 

 brushed on the surfaces to be brazed. 



For aluminum, a special flux and brazing metal is required. For inaccessible 

 places, the aluminum sheet can be obtained with the brazing material rolled in along 

 the edges. 



Furnace Temperature. — For brass wire or powder, 1740 to 2000°F.; for copper 

 brazing, about 2100°F. Furnace temperature can be anything reasonably higher that 

 wUl not be detrimental to the parts. 



Furnace Atmosphere. — Percentage composition carbon dioxide, 5.6; hydrogen, 

 11.9; carbon monoxide, 10.3; methane, 0.2; nitrogen, 72.0. No oxygen. 



Heating for anneahng or hardening can be simultaneous with the furnace brazing. 

 All heating operations subsequent to the furnace brazing must be at temperatures 

 below the melting temperatures of the brazing metal used. 



Fit of Part. — ^Light press fits are desirable. Tight fits increase the flow of the 

 brazing material into the joint, the tighter the joint the farther the molten metal will 

 flow. Void spots or gaps are difficult to seal because clearance is too great to permit 

 capillary attraction drawing the molten metal into the joints. 



FURNACE BRAZING CHARACTERISTICS OF ALUMINUM ALLOYS 



Note: Thickness range for alloys listed is 0.006 to J-2 in. 



Design Precautions. — The brazing metals flow by virtue of capillary attraction, 

 but if the gap between the surfaces to be brazed is greater than about 0.005 in., the 

 capfllary attraction is destroyed. Therefore, at no point in the path of intended flow 

 of brazmg metal should the gap between the surfaces to be brazed be more than about 

 0.005 in. as this would stop the further penetration of the brazing metal. 



