Brazing at TWI

Aluminium

Aluminium can be brazed by torch, induction, dip or furnace brazing, either with flux or controlled atmosphere (including vacuum).

Only certain aluminium alloys can be brazed successfully, including both the non-heat treatable and heat treatable wrought and cast alloys. The Mn (3XXX) and the Mg (6XXX) series are the most commonly brazed. In general, alloys from the Cu (2XXX) and Zn (7XXX) series are not brazed because their alloying additions reduce the melting point to such an extent that there are no commercially available filler metals.

Filler metals

Filler metals for brazing aluminium are based primarily on the Al-13%Si eutectic alloy. Lower melting point filler metals are attained by adding Cu and Zn, however, the corrosion resistance is reduced. Magnesium is added to filler metals for vacuum brazing due to its ability to facilitate the dispersal of surface oxides.

These alloys are supplied in the form of powder, paste, wire or foil. Another method of supply is to use brazing sheet, which is a core of aluminium alloy clad with a lower melting point filler metal. It is possible to use these cladded sheet materials in metal working processes, such as drawing and bending, without removing the cladding.

Joint types

For aluminium brazing, lap joints are preferred over butt joints. With flux brazing, press-fit or tightly fitting joints must be avoided when assembling Al parts to facilitate fluxing and filler metal flow, and to minimise flux entrapment. The exact opposite is required for fluxless brazing where mating parts of the joint are in contact throughout the braze cycle, and the braze alloy is placed in the joint.

Two principal types of fluxes, chloride and fluoride based are used for brazing aluminium. The residues from chloride based fluxes are highly corrosive and must be completely removed. The proprietary Nocolok ^® flux is fluoride based, and its residues do not corrode aluminium, and so can be applied to joint surfaces and left there after brazing.

Brazed aluminium heat exchanger (courtesy of Chart Marston)

Joint Performance

One of the primary problems when brazing Al alloys is that the filler metal flow temperatures are close to the melting point of the base metal. This is further exacerbated by the aluminium not 'glowing' when hot, so temperature assessment relies on flux colour changes or flow of the filler metal.

Control of the time at the brazing temperature is critical to prevent excessive diffusion, resulting in poor strength, and in many cases it is necessary to carry out a post-braze heat treatment to retain the mechanical properties.

Corrosion resistance may be affected by excessive time at the brazing temperature due to the changes in grain structure. This is especially the case with furnace brazing which has long heating times which promotes grain growth and silicon diffusion.