Controlled Atmosphere Aluminum Brazing

The Theory

Controlled Atmosphere Brazing was developed by Alcan, under their trade name NOCOLOK®*, as a non-corrosive flux brazing process and is now established as the accepted process for brazing of aluminum heat exchangers.

During furnace brazing, a brazing sheet, fig. 2, of aluminum silicon alloy plate (cladding) is heated to a liquid state and flows to form aluminum joints or fillets.  A sample fillet is shown in fig. 3

Aluminum brazing involves joining of components with a brazing alloy (cladding) whose melting point is appreciably lower than that of the parent material (base alloy). The cladding is typically placed adjacent to or in between the components to be joined and the assembly is heated to a temperature where the cladding material melts and the parent material does not.

Upon cooling, the cladding forms a metallurgical bond between the joining surfaces of the component.  The brazing process occurs in a furnace under the following parameters:

• Operating Temperature 580 degrees C to 620 degrees C
• Part Temperature Uniformity of ± 3 degrees C, Oxygen free
• Nitrogen Atmosphere of -40 degrees C and 100 ppm of O2 content

The cladding is supplied via a thin sheet on the base alloy. The base alloy provides the structural integrity while the low melting point cladding melts to form the brazed joints.

Why the need of flux? Once the flux is molten inside the furnace, the flux works to dissolve the tenacious oxide layer that is present on the aluminum surface and prevent further oxidation. By dissolving this oxide layer the flux wets the faying surface of the components to be joined allowing the filler metal (cladding) to be drawn freely into the joints by capillary action. Upon cooling the flux remains on the surface as a thin film.

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