Sil Flux Brazing

NOCOLOK® Sil Flux brazing is a technique, which eliminates the need for clad brazing sheet or conventional Al-Si filler metal. Sil flux brazing uses filler metal generated in-situ to effect brazing. The mechanism for creating this filler metal in-situ is described below:

  1. One of the surfaces to be joined is coated with a mixture of NOCOLOK® Flux and metallic Si powder. The coated assembly is then heated in the same fashion as in conventional furnace or flame brazing techniques.
  2. As the temperature rises, the flux melts at 565°C, dissolving the oxides on both the Al substrates and the Si particles.
  3. The bare Al surface is now in contact with metallic Si, and in the absence of oxides, allows solid-state inter-diffusion of Al and Si. Very quickly the composition near a Si particle reaches that of the Al-Si eutectic (Al-12.6% Si).
  4. As the temperature increases beyond the eutectic reaction temperature of 577°C, the formation of a liquid pool is established. The formation of the liquid leads to rapid dissolution of the remaining Si through liquid diffusion. The pool of liquid continues to grow, consuming Al, until all of the Si is consumed in the melt. In the presence of a joint, the liquid pool is drawn to the joint by capillary action.
  5. Upon cooling, the liquid layer solidifies to form a metallurgical bond between the components.

Sil Flux Process

/3 Kommentare/von

Post-Braze Flux Residue Properties

The general appearance of NOCOLOK® brazed parts can range from relatively bright to light grey depending on the flux loading and furnace dew-point. When either is increased excessively over recommended levels, the appearance moves towards the grey colour. The flux residue usually can not be seen by the naked eye, however, it is visible under a microscope at 50x magnification. Higher magnification SEM views of the flux residue are shown in the pictures below.

SEM photomicrographs of NOCOLOK flux residue

SEM photomicrographs of NOCOLOK flux residue

The pictures on the left are typical of a tunnel furnace brazed surface, needle-like in structure possibly including the odd flat platelet. The pictures identified M-70323 are typical of a furnace atmosphere containing higher than recommended levels of O2 specifically during the cooling cycle in a batch furnace. The morphology is almost 100% flat platelets. This surface has been reported to have better corrosion resistance in service.

Other flux residue properties are as follows:

a) Residue Thickness

Typically 1–2 microns. This can vary depending on flux coating weight prior to brazing.

b) Hardness

The residue hardness is about 4 on the Mohs scale.

c) Adhesion

No measurable loss has been found in circulation tests with freon or glycol type coolants using recommended flux loading. However there are reports that some detachment may occur where higher than recommended flux loading is used, particularly where molten flux pooled in downside areas.

d) Wettability

The post-braze flux residue has a hydrophilic (wetting) surface, however that wettability decreases with time.

e) Corrosion Resistance

The presence of flux residue on the part surface mildly increases corrosion resistance under normal conditions.

f) Solubility

Solubility of flux residue is influenced by the method of measurement. A typical value are between 1.2 and 3.0 g/l with Al, F and K ion concentrations approaching the stoichiometry of the compound KAlF4 .

g) Post-Braze Odour

There is a slight odour from minute amounts of H2S immediately after brazing. It disappears within a short time. If objectionable, the odour may be eliminated by rinsing the part with water.

h) Post-Treatment

The flux residue provides a good base for coatings. However, thicker residues resulting from higher than recommended flux loading can result in the poor or non-adhesion of wet or dry powder paint coatings.

NOCOLOK® Flux residue is not easily removed from the surface of brazed parts. Mechanical abrasion, such as wire brushing or grit blasting, can be used to clean off heavier flux residues from „robust“ joints. No practical chemical cleaning solution has been found. Boric acid and nitric acid solutions at higher temperature will partually remove the residues, however the times required (~ 1 hour) and the dangerous fuming with nitric acid preclude their use. Basically, the best procedure is to flux the product properly so that there is no visible after-braze residues and therefore no flux removal required.

/2 Kommentare/von