Brazing of Aluminium Alloys with Higher Magnesium-content

I. Introduction
Successful brazing of any Al 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 allow the filler metal to flow freely and must prevent the alloy surface from re-oxidizing. Mg-additions in Al-alloys, although helpful in achieving stronger alloys, lead to a decrease in brazeability. During the brazing cycle Mg deteriorates oxide removal and a Mg-level only up to 0.5 % can be safely brazed with the standard brazing flux [1]. The present work is focused on brazing of higher Mg-content 6xxx series Al alloys, namely heat-treatable AA-6082 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), with a near-eutectic Al-Si filler metal using non-corrosive fluxes.

 

 

II. Brazing Alloys
The standard brazing process involves joining of components with a brazing alloy, typically Al-Si filler metals as shown in Table 1. [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 (fig. 1). The choice of brazing filler depends upon the Al alloys being used, brazing process and the joint design, including clearance of the parts.

 

Table 1: Composition (wt. %) and brazing temperature range of filler metals commonly used for joining Aluminium alloys [2]

 

 

The use of Brazing Fluxes and Atmospheres
Two kinds of fluxes are available: corrosive and non-corrosive. Corrosive fluxes (e.g. FIRINIT 200 [5], 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 benefit of non-corrosive flux is the elimination of both post-brazing washing and corrosion from the corrosive (hydroscopic) flux residues. Fluoride-based non-corrosive fluxes of the KF-AlF3 system (fig. 2) [4] are used to displace the surface oxide film on Al alloy during brazing process.

 

A commonly used non-corrosive flux of the general formula K1 3AlF4 6 is known as NOCOLOK®Flux with a melting range 565 – 572˚C [1], which is below the eutectic isotherm in the binary Al-Si system (fig. 1) [3]. There is, however, a limit to the amount of Mg in Al alloy (of about 0.5%) that can be tolerated for NOCOLOK® Flux brazing.

 

Fig. 2: The KF-AlF3 phase diagram [4]

 

Fig. 3: Microstructure of the (“as-cast”) near-eutectic BAlSi-4 (A94047) brazing alloy (Bright-field optical image).

 

A Typical Example of Aluminium Brazing
Near-eutectic BAlSi-4 (A94047) alloy was used as a filler metal (fig. 3). Brazing of test 6082 alloy coupons to non heat-treatable AA-1005 alloy with BAlSi-4 (A94047) filler metal was performed at 620˚C in flowing N2 using Cs-containing (non-corrosive) NOCOLOK® Cs Flux and a sound brazement was obtained (fig. 4).

 

Fig. 4: Morphology of the AA-6082/AA-1005 joint developed during brazing with BAlSi-4 (A94047) 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).

 

III. Concluding Remarks
A higher Mg-content AA-6082 alloy can be brazed successfully with a near-eutectic BAlSi4 (A94047) 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 quality joints. No post-brazing washing of the brazed joints is required.

 

To recover an initial temper of the heat-treatable 6082 alloy, a post-brazing thermal treatment (ageing) of the product joints can be considered.

 

IV. References
[1] The NOCOLOK® Flux Brazing Process, Solvay Special Chemicals (www.solvay.com)
[2] Specification for Filler metals for Brazing and Braze Welding, 10th 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] W.T. Thompson and D.G.W. Goad, Can. J. Chem., 54 (1976) 3342-3349
[5] Hygroscopic Fluxes for Aluminium Brazing and Welding, www.firinit.de
[6] J. Garcia, C. Massoulier and 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

 

V. Authors
A. Das. A. Kodentsov. L. Krassenburg. H. Schoonderwaldt, N. van Veen and M. Biglari

 

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