Brazing of Aluminium Alloys withHigher Magnesium Content using Non-Corrosive Fluxes – Part 3

Technical Information by Leszek Orman, Hans-Walter Swidersky and Daniel Lauzon

Abstract

For just as long as aluminium has been used for brazing heat exchangers, there has been a trend to down-gauging components for weight savings. The most common alloying element to achieve higher strength alloys for the purpose of down-gauging is magnesium. While magnesium additions are helpful in achieving stronger alloys, the consequence is a decrease in brazeability. This article discusses the mechanism of brazing deterioration with the addition of magnesium and proposes the use of caesium compounds as a way of combating these effects.

We split the article in five parts:

1. Introduction
2. Effects of Mg on the Brazing Process
3. Mechanism of Magnesium Interaction with the Brazing Process
4. Caesium Fluoroaluminates
5. NOCOLOK^® Cs Flux

Mechanism of Magnesium Interaction with the Brazing Process

According to M. Yamaguchi et al [5], when magnesium diffuses to the surface during brazing, a chemical reaction takes place with the flux resulting in the generation of KMgF₃.

The authors suggest the following equations to explain some of the chemical interactions between magnesium and K_1-3AlF_4-6 flux:

• 3 MgO + 2 KAlF₄  →  MgF₂ + 2 KMgF₃ + Al₂O₃  (a)
• 3 MgO + 2 KAlF₄  →  2 MgF₂ + K₂MgF₄ + Al₂O₃  (b)
• 3 MgO + 2 K₃AlF₆ > →  3 K₂MgF₄ + Al₂O₃  (3)

By performing XRD (X-ray Diffraction) phase identification on products brazed with Mg containing alloys, A. Gray et al [6] confirmed the presence of K₂MgF₄, spinel oxide (Al₂MgO₄) and possibly KMgF₃. These magnesium containing compounds have a characteristic needle like morphology as shown in Fig. 5.

Fig. 5: Morphology of magnesium containing compounds as seen by Scanning Electron Microscope [6].

H. Johansson et al [7] also determined that at temperatures above 425°C the magnesium diffusion to the surface is very rapid resulting in the formation of magnesium oxide (MgO) and spinel oxides (Al₂MgO₄). These oxides have very low solubility in NOCOLOK® Flux. Subsequently these magnesium oxides react with the flux resulting in the formation of magnesium fluoride (MgF₂) and potassium magnesium fluorides (KMgF₃, K₂MgF₄, see equations a), b), and c)). These reactions change the flux chemical composition causing its melting range to rise. The melting point of these magnesium fluorides is very high, which in turn drives the melting point of the flux upwards, thereby decreasing the activity of the flux. The above factors also cause a decrease in the flowing characteristics of the flux thus lowering its overall effectiveness. Therefore the desired key point to limit the flux poisoning effect would be to reduce the formation of magnesium oxides and potassium magnesium fluorides.

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References:

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