ABSTRACT
Successful brazing of any Aluminium alloy requires prior removal of the native surface oxide film,
which is usually done by employing a flux. Magnesium additions in Aluminium-alloys, although
helpful in achieving stronger material, lead to a decrease in brazeability. During the brazing cycle
Magnesium deteriorates oxide removal and Al-alloys with a Mg-level only up to 0.5 % can be safely
brazed with the standard non-corrosive brazing flux. The present work is focused on physico-
chemical aspects of the brazing of higher Mg-content Aluminium alloys, namely heat-treatable AA-
6082 (0.7-1.2 % Mg; 0.9-1.3 % Si; 0.5 % Fe; 0.5-1.0 % Mn; 0.25 Cr; 0.20 Zn; 0.1 Ti) to AA-1050
substrates with a near-eutectic Al-Si filler metal using non-corrosive fluxes.
I. Introduction
Although being state-of-the-art and widely used as enabling technology in manufacturing
various products, joining of Aluminium is still needed further steps and developments. Amongst the
available joining techniques suitable for joining Aluminium alloys, like fusion welding, diffusion,
friction stir bonding, joint rolling, explosion bonding, soldering and brazing, the last method
(brazing) attracts special attention. This is because of this technique allows for a sound bonding to
be readily accomplished even if components intended to be joined are not very precisely placed
within the assembly, which is an extremely important (practical) issue, given the apparent
complexity of the brazed assembly design.
Currently, a NOCOLOK-type process (Controlled Atmosphere Brazing, or CAB technology)
is employed for brazing components of the heat-exchangers, and typical wrought, non-heat treatable
(e.g. 3xxx series) Aluminium alloys used in the component design [1].
It is well known that Mg-additions in Al-alloys are helpful in achieving stronger materials.
However, the consequence of such alloying is a decrease in brazeability. During brazing cycle
Magnesium negatively influences the oxide removal and it is generally accepted that Magnesium-
level only up to about 0.5 % can be safely brazed with the standard non-corrosive flux.
It is also to be mentioned that as an alternative to the brazing in CAB furnace, it is possible to
performed brazing of higher Mg-content Aluminium alloys in air just by using commercially
available corrosive (Chlorine-containing) fluxes. However, it seems that this is not always a
particularly viable option because of a thorough post-brazing cleaning is required. In order to prevent from possible corrosion degradation of the product assembly any flux residues must be
removed.
The present investigation is a part of the ongoing activities at Mat-Tech BV aiming at the
development of technology suitable for brazing of Aluminium alloys (including those having higher
Mg-content) using Al-Si based filler metals and non-corrosive fluxes.
II. Brazing alloys, Fluxes and Atmospheres – General considerations
The standard brazing process involves joining of components with a brazing alloy, typically
(near-)eutectic Al-Si filler metals [2]. The eutectic isotherm in the binary Al-Si system lies at 577 °C
and the eutectic alloy composition is 12.6 wt. % of Silicon. The binary Al-Si phase diagram is given
in Fig. 1 [3].
Fig. 1: The binary Al-Si phase diagram
In general, the choice of brazing filler depends upon the Aluminium alloys being used,
brazing process and the joint design, including clearance between the parts (or brazing gap).
Successive brazing of any Aluminium alloy requires prior removal of the native surface
oxide film, which is usually done by employing a flux. The flux must be capable of displacing the
oxide film barrier during brazing and allows the filler metal to flow freely and must prevent the
alloy surface from re-oxidizing.
When brazing Aluminium, two families of fluxes are commercially available: corrosive and
non-corrosive. Corrosive fluxes (e.g. FIRINIT 200 [4], etc.) are water-soluble and usually
hygroscopic, containing both chloride and fluoride salts, and residues can be washed off the part
after brazing, and the resulting joint has a clean appearance.
The key benefit of non-corrosive flux is the elimination of post-brazing washing and the
potential for corrosion from the corrosive (hydroscopic) flux residues. Fluoride-based non-corrosive
fluxes of the KF-AlF 3 system (Fig. 2 [5]) are used to displace the surface oxide film on Aluminium
alloy during brazing process.
A commonly used non-corrosive flux of the general formula K 13 AlF 46 is known under the
trademark name NOCOLOK Flux with a melting range 565 – 572 °C [1], which is below the
eutectic isotherm in the binary Al-Si system (Fig. 1). As already explained, there is, however, a
limit to the amount of Magnesium in Aluminium alloy (of about 0.5 %) that can be tolerated for the
standard NOCOLOK Flux brazing process.
When during brazing Magnesium diffuses rapidly to the Aluminium alloy surface, the
Magnesium oxide (MgO) as well as spinel (MgAl 2 O 4 ) formation takes place. The oxide phases have
very low solubility in the K 13 AlF 46 (i.e. in the ordinary NOCOLOK Flux), and their reaction with
the flux constituents results in the formation of Magnesium Fluoride (MgF 2 ) and Potassium
Magnesium Fluorides, KMgF 3 and K 2 MgF 4 [6]. The described processes can be represented by the
following reaction equations:
3 MgO + 2 KAlF 4 = MgF 2 + 2 KMgF 3 + Al 2 O 3
3 MgO + 2 KAlF 4 = 2 MgF 2 + K 2 MgF 4 + Al 2 O 3
3 MgO + 2 K 3 AlF 6 = 3 K 2 MgF 4 + Al 2 O 3
These reactions change chemical composition of the flux causing its liquidus temperature to
rise, thereby decreasing activity as well as flowing characteristics of the flux, and hence, its overall
effectiveness. Therefore, the desired key point to limit the “flux poisoning effect” is to reduce the
formation of Magnesium Oxides and Potassium Magnesium Fluorides.
The formation of the “poisoning” Potassium Magnesium Fluorides can be reduced in the
presence of Cesium Fluoroaluminate compounds. The Cesium-compound commonly used for
Aluminium alloy brazing contains mainly CsAlF 4 . Cesium acts as “a chemical scavenger” for
Magnesium because during the brazing process it reacts with Mg-containing phases leading to the
formation of CsMgF 3 and/or Cs 4 Mg 3 F 10 which melt at lower temperatures that the standard Al-Si
filler metals.
As such these Cs-containing compounds do not significantly interfere with Aluminium
brazing and alloy the flux to retain much of its oxide dissolution and wetting promoting capability.
As a more practical means of obtaining better brazeability of Aluminium alloys with Higher
Magnesium-content is the use of a mixture of standard NOCOLOK Flux and Cesium
Fluoroaluminates , like for example, commercially available NOCOLOK Cs Flux [7].
III. Case Study – Brazing of AA-6082 alloy foam to AA-1050 substrate
In the recent several years, there has been an increased interest in metal foams. These
materials are now penetrating a number of applications where their unique suite of properties makes
them superior to solid materials, such as lightweight structures, packaging and impact protection,
and filtration and catalysis [8]. However, brazing of any metal foams is technologically difficult
because of varying pore size and hence, non-uniform interaction of the thin-wall skeletal structure
with the molten brazing filler. Both liquid filler alloy and flux can penetrate the foam and thin walls
of the foam skeletal structure can partially (locally) melt [9].
Brazing of AA-6082 Aluminium alloy (0.7-1.2 % Mg; 0.9-1.3 % Si; 0.5 % Fe; 0.5-1.0 %
Mn; 0.25 Cr; 0.20 Zn; 0.1 Ti) foam, in particular, encounters an additional obstacle, mainly because
of its relatively high Mg-content.
Near-eutectic AlSi12 alloy was used in the present study as a filler metal. The brazing alloy
microstructure is shown in Fig. 3.
Fig. 3: Microstructure of the (“as-cast”) near-eutectic AlSi12 brazing alloy. (Bright-field optical
image)
Brazing of test AA-6082 alloy coupons to none heat-treatable AA-1050 (wrought) alloy with
the AlSi12 filler metal was performed at 620 °C in flowing Nitrogen using Cs-containing (non-
corrosive) NOCOLOK Cs Flux and a sound brazement was obtained (Fig. 4).
Fig. 4: Morphology of the AA-6082/AA-1050 joint developed during brazing with AlSi12 filler
metal at 620 °C in flowing Nitrogen using Cs-containing NOCOLOK Cs Flux: a) bright-field
optical image and b) Secondary Electron Image taken from the central part of the brazement. (The
eutectic constituent of the brazing seam microstructure is indicated by arrows.)
After further optimization (and fine tuning) of the process parameters, the AA-6082
Aluminium alloy foam was successfully brazed to AA-1050 alloy substrate. General view of the
product brazement is shown in Fig. 5.
Fig. 5: Optical image of the brazed assembly based on AA-6082 Aluminium alloy foam and AA-
1050 Aluminium substrate. Note. The open-cell structure of the form is recognizable.
IV. Concluding Remarks
- A higher Mg-content AA-6082 alloy can be brazed successfully with a near-eutectic AlSi12 filler metal employing a NOCOLOK-type process and using a commercially available NOCOLOK Cs Flux
- Careful control of the flux quality is vital to providing high quality joints.
- No post-brazing washing of the brazed joints is required
- In order to recover an initial “temper” of the heat-treatable 6082 alloy, a post-brazing thermal treatment of the product joints can (in principle) be envisage
References
[1] The NOCOLOK Flux Brazing Process, Solvay Special Chemicals (www.solvay.com)
[2] Specification for Filler metals for Brazing and Braze Welding, 10 th edition, AWS
A5.8M/A5.8:2011-AMD1, American Welding Society, 2011
[3] T. B. Massalski, Binary Alloy Phase Diagrams, ASM, Metal Park, Ohio, 1986
[4] Hygroscopic Fluxes for Aluminium Brazing and Welding, www.firinit.de
[5] W. T. Thompson, D. G. W. Goad, Can. J. Chem., 54 (1976) 3342-3349
[6] J. Garcia, C. Massoulier, Ph. Faille, Brazeability of Aluminium Alloys Containing
Magnesium by CAB Process Using Cesium Flux, Society of Automotive Engineers, 2001
[7] Aluminium Brazing with NOCOLOK – 7 Steps to Successful Aluminium Brazing, Solvay
Fluor (www.solvay-fluor.com)
[8] M. F. Ashby, A. G. Evans, A. G. N.A.Fleck, L. J. Gibson, J. W. Hutchinson, H. N. G.
Wadley, Metal Foams: A Design Guide, Butterworth-Heinemann, Boston, 2000
[9] J. Nowacki et al., Welding Technology Review, 1 (2014) 7-12
