Search Results

The effects of temperature, pressure, and atmosphere composition were examined to determine how each influenced vacuum brazed aluminum radiator tube-to-header joints. Brazing at 1 x 10-5 Torr (1.3 x 10-3 Pa), while varying the braze temperature from 1070°F (577°C) to 1125°F (607°C), showed that acceptable joints were produced in the 1080°F (582°C) to 1110°F (590°C) range. Below 1080°F (582°C), insufficient melting caused poor filleting. Above 1110°F (590°C) severe tube core dissolution occurred. Increasing the pressure (constant temperature) by introducing air into the furnace caused extremely poor fillets above 2.0 x 10-4 Torr (3 x 10-2 Pa). Acceptable fillets were formed in N2 saturated with 10,000 ppm of H2O at 9 x 10-4 Torr (0.12 Pa). XPS analyses of the air generated oxides showed that they were 700 A thick or less, and composed of MgO and Al2O3. Techniques were developed which allow the operating temperature and pressure of the furnace to be estimated from brazed radiators.

Analysis of the NOCOLOK™ aluminum brazing process is difficult because of the multiple reactions which can occur at high temperature between the components of this complex system: flux (KAIF4 and K3AIF6), oxides (principally Al2O3), and reactive alloying elements in the core or filler, such as Mg and Li. The “Mg-poisoning” phenomenon, in which the oxide removal properties of the flux are reduced, is a key concern of those using the NOCOLOK process. Thermochemical calculations demonstrate that an initial Mg content of 0.2%-0.4% coming into contact with the flux is sufficient to inhibit the desired oxide dissolution (i.e., the flux is “poisoned”). Based on thermodynamic calculations, the principal “poisoning” reaction appears to be: 3Mg (liq. soln.) + 3KAIF4 = 3MgF2 + K3AIF6 + 2AI (liq. soln.).