Flux Transformations

As manufactured, a non-corrosvie K-Al-F-type flux typically is a mixture of potassium tetra-fluoroaluminate (KAlF₄), and also contains potassium penta-fluoroaluminate (K₂AlF₅). K₂AlF₅ exists in different modifications: potassium penta-fluoroaluminate hydrate (K₂AlF₅ · H₂O), and hydrate-free (K₂AlF₅).
During the brazing process, the material undergoes essential physico-chemical alterations. While the chief component, KAlF₄, is simply heated up, the compound K₂AlF₅ · H₂O begins to lose its crystal water from 90°C (195°F) on. When the temperature is further increased within the ranges of 90° – 150°C (195°F – 302°F), and 290°C – 330°C (554°F – 626°F), two different crystallographic (structural) modifications of K₂AlF₅ are formatted.

When the furnace temperature is raised above 490°C (914°F), K₂AlF₅ begins to react chemically. According to the equation:

2 K₂AlF₅ → KAlF₄ + K₃AlF₆ (Equation 1)

the exact amount of potassium hexa-fluoroaluminate (K₃AlF₆) necessary for a eutectic flux composition (i.e. mixture of two or more substances which has the lowest melting point; see phase diagram) is obtained from the original K₂AlF₅ content. At brazing temperature, the resulting flux composition has a clearly defined melting range of 565°C to 572°C (1049°F – 1062°F). The flux melts to a colorless liquid.
Due to a vapor pressure of 0.06 mbar at 600°C, some of the KAlF₄ evaporates during the brazing cycle, particularly once melting temperature is reached. The total content of KAlF₄ contained in the exhaust is depending on time and temperature. Based on results from TGA analysis (with a heating rate of 20°C/min), the quantity of volatile compounds in Flux between 250°C and 550°C (482°F and 1022°F) is approximately 0.2 to 0.5%. These flux fumes contain fluorides and have the potential to react with the furnace atmosphere, especially moisture, to form hydrogen fluoride according to the equation:

3 KAlF₄ + 3 H₂O → K₃AlF₆ + Al₂O₃ + 6 HF (Equation 2)

This is one of the reasons, why the brazing process should take place in a controlled atmosphere (nitrogen) with low dew point and low oxygen level (another reason is to minimize re-oxidization effects on the aluminum surfaces).

Directly after brazing has been completed, flux residues consist mainly of KAlF₄ and K₃AlF₆. In the presence of moisture from the surrounding atmosphere, the K₃AlF₆ is converted back to K₂AlF₅ · H₂O over time (several days) in a reaction reverse to the one described in equation 1 followed by a re-hydration step.

The schematic below illustrates the transformations that occur as the flux is heated to brazing temperature. Note that these phases are unstable outside the furnace atmosphere.