Aluminium Flame Brazing – Basics Part 4

BASICS

Alternatives

Fluxing Prior to Assembly The procedure described above is the most common and reliable method to ensure good quality brazed joints, but is not the only one. For instance with a tube-to-tube joint, with the filler metal ring in place, the flux can be applied to the lower portion of the straight tube prior to insertion in the expanded tube. In this case, a light flux loading around the circumference of the joint after assembly is still required to cover the filler metal ring.

Feeding the Joint with Filler Metal

Another alternative to using a preform ring is to feed the filler metal into the joint with filler alloy wire or rod during heating, after the flux is molten. This method is frequently adopted by brazers who have had experience with choride flux flame brazing or who have welding backgrounds. To the novice brazer, this method is very difficult because the window between when the flux becomes molten and when the flux dries out is very small, when brazing in air. The window of opportunity is in the order of a few seconds and skilled brazers are able to work within this window. By placing the preform ring at the joint prior to heating ensures that the flux will not dry out before the filler begins to melt.

The table below lists the chemical composition and melting point range of common alloys suitable for NOCOLOK ^® flux flame brazing.

Alloys suitable for flame brazing

As shown in the table, alloys that are considered difficult or impossible to braze by furnace methods can be flame brazed. In furnace brazing, Mg diffuses to the surface and reacts with the surface oxide to form MgO and spinels of MgO:Al₂O₃, which have reduced solubility in NOCOLOK ^® flux. Furthermore, Mg and/or MgO can react with the flux, rendering it less effective. The rule of thumb for furnace brazing is to limit the total amount to less than 0.5  wt %‑Mg.

In flame brazing, higher Mg levels 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. Up to 1  wt %‑Mg can be brazed with relative ease, while greater than 1  wt %‑Mg may be possible under some circumstances (increased flux loadings, extremely fast heating rates).

Precaution

Mg containing alloys have lower melting points than those alloys containing very low or no Mg (see Table above). The most common high strength machineable alloys for example are AA6061 and AA6063 which typically contain 1 wt‑% and 0.5  wt %, respectively. Note that both of these alloys have a solidus of 616 °C. This means that these alloys, when overheated are prone to incipient grain boundary melting. The effect of overheating manifests itself as degradation of the microstructure and a roughening of the skin, commonly known as ‘orange peel’ effect. These effects are illustrated in the photomicrographs below.

Cross-Section of As-Extruded Machineable Alloys

Cross-Section of Overheated Machineable Alloys

This effect is worse in the outer skin since this area sees a higher temperature (Ts surface temperature) than deeper within the component (Tj joint temperature) as illustrated below. Metallurgical degradation of this type can cause problems with the longevity and suitability of machined surfaces such as threads in fittings.