For brazing aluminum and aluminum alloys with hard solders, depending on the heat source, the following methods are used:
- manual soldering with a torch;
- soldering by immersion in flux;
- oven brazing (vacuum, in a protective atmosphere)
More complex and expensive methods for brazing aluminum are also known., which are of limited use :
- TIG soldering;
- MIG soldering;
- plasma soldering;
- laser soldering;
- induction soldering;
- electrical soldering resistance.
Soldering with a torch (picture 1) usually used for repair work, small volumes of production, instead of welding. Soldering is done with the same burner., which is used for welding. However, other torch tips and eye protection lenses are used when soldering.. Good joint preparation before soldering is very important.. Clearances between surfaces in the joint usually withstand 0,1 to 0,65 mm.
Figure 1 - Brazing with a torch
Aluminum becomes softer at brazing temperatures. Product, which is being soldered, the torch may sag under the influence of its own weight and gas pressure. This is especially true for long horizontal parts.. To avoid this, such products are somehow supported from below..
The soldering temperature is usually judged:
- by flux - it becomes transparent at soldering temperatures;
- on the surface of aluminum - begins to glow with a silvery color;
- on solid solder - softens and begins to melt;
- with the help of special pencils - the pencil trace changes its color when the soldering temperature is reached.
Aluminum brazing has been widely and successfully used for many years., especially in the manufacture of complex structures. This method allows fast and uniform heating and, Moreover, provides very narrow dimensional tolerances.
details cleaned before soldering, assembled and tightly connected to each other together with solder installed at the junction. The whole structure is heated in an oven to about 540 oC, and then immersed in molten flux on 1-2 minutes (drawings 2 and 3). Preheating avoids a drop in the temperature of the flux in the bath.
Figure 2 - Soldering by immersion in flux
Figure 3 – Dip Soldering Manufacturing Process 
When soldering by immersion, the temperature of the molten flux can be controlled with an accuracy of ± 3 ºС. This is significantly better than accuracy., which can be achieved on any other heating equipment. This allows the use of solders with liquidus temperature., which only 5-6 ºС lower than the solidus temperature of the base metal. The method of soldering by immersion in molten flux is best suited for joining parts with different wall thicknesses and sizes. On the other hand, this method requires a large flow of fluxes.
One of the disadvantages of this method is that, that a laborious operation of cleaning the product after soldering is required to remove flux residues. This also imposes certain restrictions on the design of the product., to avoid possible air congestion.
Another drawback of the dipping method is that, that it poses significant environmental problems, When soldering with this method, pairs are emitted., which are highly corrosive, as well as a large amount of wastewater. Therefore, this method is finding less and less use..
Brazing aluminum in a furnace
The method of brazing in the furnace is the second most popular after soldering immersion in flux. Parts are brazed prior to brazing as well as for other brazing methods.. Solder surfaces are treated with flux, set solder, parts are assembled and firmly fixed in relation to each other. Frequently connected parts are designed this way., so that they fix each other.
Figure 4 - Brazing
For brazing aluminum with solid solders, two types of furnaces are used: with batch loading and continuous type. The temperature of the furnace is controlled within ± 3 ºС. The heat output of the furnaces should be large enough, to ensure fast heating to soldering temperature. This is to ensure, to minimize diffusion of silicon into the base metal. After soldering, the product is cooled or, if necessary, quenching.
- TALAT Lecture 4601, European Aluminium Association, 1994.
- EEA Aluminium Automotive Manual – Joining – Brazing, EEE, 2015