Joining: Soldering and brazing: Prerequisites

In order that the surface can be wetted with molten solder, the oxide layer on aluminium has to be removed. As thermal decomposition is not feasible, owing to the high melting point of aluminium oxide around 2050°C, it needs to be removed chemically (fluxes, gases) or mechanically, by brushing, scraping or grinding.

Before soldering, the area of the joint must be clean with a bare metal surface and the parts to be joined have to be fixed. The joint has to be designed in such a way that there is a good flow of solder into the solder gap and that the flux is allowed to escape.

If the work is carried out properly, the joints will be filled completely with molten solder and all flux residues will be forced out of the joint; these have to be removed after soldering otherwise they will attack the metal surface. In general, fluxes are removed using dilute nitric acid and then rinsing is carried out with hot water.

Flux

In Brazing, fluxes based on chlorides and hygroscopic fluorides as well as on lithium compounds are used, which are effective above 500°C (also in the case of AlMg3). However, in view of their high corrosiveness the residues need to be thoroughly washed off. Also, the discharged air causes significant pollution of the environment. The residues of non-chloridic, non-hygroscopic fluxes, in contrast, are non-corrosive. The efficacy of such fluxes starts from 570°C and is rated slightly inferior than that of the previously mentioned group. Ecological damage is also greatly reduced.

Solder

Due to the smelting temperature of aluminium and its alloys and the necessary corrosion resistance of the brazed joint, heavy metal brazing solders are not a viable option. Rather, solders with a lower percentage of heavy metals and a minimum of 70 % aluminium are employed.

As standard solder a nearly eutectic brazing alloy consisting of aluminium and 13 % silicon is used. A lower melting point may be achieved by alloying copper and zinc, albeit at the expense of corrosion resistance.

The vast majority of soft solders, by contrast, is based on tin and/or lead. The solder is delivered in various shapes, for instance as solder wire as rods or rings, granular solders, paste or as soldering foil.

One uses different solders for brazing and soldering. The solder is used in various forms: for example, solder wire as rods or rings, as cast soldering rods or solder in granular form.

Standards

Brazing solders and fluxes for brazing are standardised in exactly the same way as soft solders and their fluxes.