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For added strength and machineability, certain alloys contain Mg. Most notably are the 6XXX series alloys (up to 1% Mg) that are used for fittings and machined components and the so-called long life brazing sheet alloys (up to 0.3% Mg in the core). There is a limit to the amount of Mg tolerated in NOCOLOK® Flux brazing. Up to 0.5% Mg can be tolerated in furnace brazing while around 1% Mg is tolerable for flame brazing.
When an Al alloy containing Mg is heated, the Mg diffuses to the surface and reacts with the surface oxide to form MgO and spinels of MgO:Al2O3. The diffusion is time-temperature dependent and is rapid above 425°C. These spinel oxides have reduced solubility in the molten flux. Furthermore, Mg and/or MgO can react with the flux forming compounds such as MgF2, KMgF3 and K2MgF4. All of these serve to poison the flux and significantly reduce its effectiveness.
In flame brazing, higher Mg concentrations can be tolerated since the faster heating rates do not allow the diffusing Mg enough time to appreciably decrease the beneficial effects of the flux. Flame brazing components containing > 1% Mg may be possible under some circumstances with increased flux loadings and very fast heating rates (<20 second braze cycle).
It should be noted that when one speaks of the brazing tolerance to Mg, it is the total sum of the Mg concentrations in both components:

[Mg] component 1 + [Mg] component 2 = [Mg] total

The figure below shows the effect of Mg on fillet size and geometry:

0.1% Mg

0.4% Mg

If the user is experiencing difficulties brazing and suspects elevated Mg levels as the cause, there are a couple of ways to be sure. First, check with the supplier of the alloys or perform a chemical composition analysis on the suspect alloys. This is the most certain way. Secondly, look for a golden hue on the brazed product. This is an indication that Mg alloys are being used and the color is a result of the increased oxide thickness. Furthermore, there may be a very light, almost fluffy residue on the brazed component that can literally be blown off by mouth. These visual indicators can most certainly be traced back to poor brazing results due to the presence of Mg.

Improving brazeability
There are a few ways in which the brazeability of Mg containing alloys can be improved:

  1. Increasing the flux loading. A substantial improvement is gained when increasing the flux loading up to 10 g/m2 or more in furnace brazing. In cases where there is just one component containing Mg such as in a fitting, extra flux can be brushed around the area of the joint.
  2. Increasing the heating rate. Slow heating rates allow more Mg to diffuse to the surface thereby hindering brazeability. For furnace brazing Mg containing alloys, the fastest possible heating rates achievable without sacrificing temperature uniformity will increase the tolerance to Mg.
  3. Combining increased flux loadings and faster heating rates.
  4. Maintaining proper gap tolerances and joint designs.
  5. Increasing the nitrogen flow rate to minimize furnace atmosphere contaminants that also compete to reduce brazeability.

Tip: NOCOLOK® Cs Flux

Better results are reported when using cesium containing fluxes for aluminum alloys containing Mg up to 0.6 – 0.8% Mg. Fewer leaks are observed when compared with standard flux and less porosity is noted in the joint areas. Furthermore, standard flux loads and braze cycles can be used with Cs containing fluxes.

NOCOLOK® Cs Flux is a flux of the general formula KxCsyAlFz where Cs is chemically bound. It has a melting range of 558°C – 566°C. The maximum Cs content is limited to 2% to keep the cost of the flux down. Increasing the Cs content does not increase brazeability as shown below:

Cesium reacts as a chemical buffer for Mg by forming CsMgF3 and/or Cs4Mg3F10. The flux inhibiting factors of Mg are therefore reduced.

How to measure?

In the case of heat exchangers, the surface area being fluxed must first be determined. For ease of calculation, the louvers on the fin can be ignored. The radius on the fin can also be ignored.

Imagine then the fin pulled out of the heat exchanger and straightened out to form one long strip. Similarly, the surface area of the slots in the header can also be ignored.

Remember that in calculating the surface area of the heat exchanger, there are 2 sides to every tube, 2 sides to every fin and 2 sides to the headers. The total surface area is then expressed in m2: All dimensions are in meters (m) to yield a surface area in square meters.

Header


Assuming it is a cylindrical (condenser) header:

SA (m2) = (2 x 3.14 x radius of header(m)) x length of header (m) x 2 headers

Assuming it is a radiator header:

SA (m2) = length of header (m) x width of header (m) x 2 (sides/header) x 2 (headers)

Tubes

SA(m2) = width of tube(m) x length of tube (m) x 2 (sides/tube) x total number of tubes

Fins

Ignore the louvers in the fins

SA (m2) = width of fin (m) x (fin height (m) x number of fin legs/tube) x 2 ( sides/fin) x total number of fins

Total Surface area in m2 = SA headers + SA tubes + SA fins

To determine the flux loading, a degreased and thoroughly dry heat exchanger is weighed. The heat exchanger is then run through the fluxer, blow-off and dry-off section of the furnace. The heat exchanger is removed just prior to entering the brazing furnace and weighed again.

The flux coating weight is then determined using the following formula:

Weight of unit fluxed and dried (g) – weight of unit un-fluxed (g) x Surface area (m2)

To make sure that the flux loading was determined on a completely dry unit, run it through the dry-off section a second time and re-weigh.

See also: Flux loading

1. Have a clean surface

2. Heat the joint evenly to brazing Temperature

3. Choose the right brazing alloy for the job

4. Select the appropriate means of removing the oxide skin from the faying surfaces of the joint

5. Use a capillary gap of the appropriate size

6. Apply the brazing alloy to the last part of the joint to reach brazing temperature.