Thermochemical Calculations Regarding Fluoride Flux-Mg-Al2O3-Li Interactions in Aluminum Brazing

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.). The active components of the flux are consumed in producing the stable compound MgF₂, the formation of which reduces the ability of the flux to dissolve the oxide, thus preventing the molten metal from wetting the surfaces to be joined. Li poisoning of the flux will also occur in a similar manner; LiF is the stable product formed in this instance. Calculations indicate that when the starting Li level is approximately 0.1 % to 0.2%, flux poisoning can result.