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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.

In its simplest form, a slurry is held in a reservoir tank and continuously agitated to prevent settling. The slurry is pumped, usually with air-diaphragm pumps to the flux slurry cabinet where the heat exchangers moving on a conveyor are sprayed with the slurry. After spraying, the excess flux slurry is blown off in a separate chamber with high volume air. The over spray and blown off slurry is recycled back to the reservoir tanks, again using air-diaphragm pumps.

Depending on the sophistication desired, a second flux spray chamber may be installed after the first chamber to deliver a higher concentration slurry to problem areas such as tube to header joints in condensers and radiators. This second spray chamber would have a separate flux delivery system and a separate reservoir tank to contain the higher concentration flux slurry.

The components of the flux delivery system including reservoir and agitators should all be constructed of stainless steel or chemically resistant plastics (nozzles for instance). There should be no mild steel or copper containing components – includes brass or bronze – in contact with the flux slurry. The schematic below shows the components of a generic fluxing station:

Wet Fluxing

Note that splashing will occur inside the fluxing cabinet and cause an accumulation of dried flux on the walls. Therefore the cabinet is washed with water periodically to remove this accumulated flux. The frequency of this maintenance operation is up to the manufacturer, but could be anywhere from once per shift to once per month.

The theoretical amount of flux required to dissolve a 100 Å oxide film is about 0.02 g/m2
(1 Å = 10-10 m = 0,1 nm). For a 400 Å film, still only 0.08 g/m2 flux is required. These do not take into account losses to moisture, oxygen or poisoning of the flux by Mg alloy additions.

In practice however, the recommended loading for fluxing is 5 g/m2, uniformly distributed on all active brazing surfaces. This is more than 250 times the theoretical amount required for oxide dissolution. To visualize what 5 g/m2 flux loading might look like, think of a very dusty car. As the heat exchange manufacturer gains experience with his products, he may find that a little more is required for consistent brazing or that he can get away with a little less flux.

Too little flux will result in poor filler metal flow, poor joint formation, higher reject rates, and inconsistent brazing. In other words, the process becomes very sensitive.

Too much flux will not affect the brazing results. However there will be pooling of flux which can drip on the muffle floor, the surface of the brazed product will be gray and there will be visible signs of flux residue. Furthermore, flux will accumulate on fixtures more rapidly which then requires more frequent maintenance. More importantly yet, using too much flux will increase the process costs.

In some cases, heat exchanger manufacturers use higher than recommended flux loadings to mask furnace atmosphere deficiencies. This should be viewed as a short-term solution and the furnace problems should be addressed.

See also: How to evaluate flux load?

A new coating technology for the functionalization of semi-finished aluminium products for heat exchanger (HEX) applications has been developed by Erbslöh Aluminium GmbH with assistance from Solvay Fluor. In contrast to binder-based flux coatings, it is now possible to apply various kinds of NOCOLOK® fluxes free of any adhesives for the Controlled Atmosphere Brazing (CAB) process to aluminium surfaces, gaining economic and technological advantages. Today’s application examples include coatings on extruded condenser and evaporator tubes, and internal brazing of B-type tubes by integration in tube mills. Other applications, e.g. for selective local coatings, are conceivable.
Several aluminium substrates were coated with different NOCOLOK® fluxes/flux materials and examined in pre and post-brazed condition. As the results show, the new technology has a considerable potential in substituting or even replacing common pre-fluxing processes for CAB in HEX manufacturing.
Controlled Gas Plasma Deposition (CGPD) represents an innovative method for pre-fluxing of semi-finished products. It allows the application of pure flux by means of a plasma source with comparable adhesion properties as achieved with binder-based coatings. Due to the absence of any binder or other redundant chemicals, CGPD is not limited to any drying or hardening time, opening the way for high speed flux application. This comes hand in hand with the higher environmental friendliness of a solvent and binder-free process.

Illustration of the CGPD process

Benefits of brazed aluminium HEXs in Micro Multiport (MMP) design are:
■ Cost reduction
■ Improved performance with downsizing potential
■ Weight reduction
■ Lower refrigerant charge
■ Better corrosion performance
■ Recycling advantages
Improvements in CAB:
■ Use of pre-coated components
■ Process, quality, cost
■ Binder-free CGPD coating for easy use and high speed applications