Aluminium brazing and soldering

Methods of aluminium brazing

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Discover the process of aluminium brazing and its applications. Learn about the difference between brazing and soldering and the importance of surface quality.

Brazing and soldering

Traditionally, brazing and soldering of aluminium are distinguished from each other in the following manner [2]:

  • Soldering: T < 450 °C
  • Brazing: T > 450 °C.
  • Brazing is a metal-joining process where a filler metal is heated above its melting temperature but below the melting point of the metals being joined.
  • The liquid filler metal is then drawn into the gap between the closely fitted surfaces of the joint by capillary action.
  • Because of the importance of wetting and capillary effects, the quality of the surfaces to be joined is of utmost importance and the clearance between the parts must be small (generally less than 0.2 mm).

Brazing aluminum

Manual brazing

The method of brazing with a torch (Figure 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.

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Figure 1 – Thr brazing method with a torch

Aluminum becomes softer at brazing temperatures. Product, which is being brazed, 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..

Brazing temperature

The brazibg  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.

Submerged brazing

Technology

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 brazing, 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 и 3). Preheating avoids a drop in the temperature of the flux in the bath.

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Figure 2 – Brazing by immersion in flux [2]

Figure 3 – Dip Brazing Manufacturing Process [2]

When brazing 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 brazing 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.

Disadvantages

One of the disadvantages of this method is that, that a laborious operation of cleaning the product after brazing 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 brazing 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.

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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 brazing temperature. This is to ensure, to minimize diffusion of silicon into the base metal. After soldering, the product is cooled or, if necessary, quenching.

Sheets for brazing

For the manufacture of automotive heat exchangers, aluminum sheets are used with a lining of their aluminum-silicon alloy.. These sheets can be of various types depending on the specific application (figure 5). The principle of furnace brazing technology is shown in Figure 6.

Figure 5 – Various types of aluminum sheets for brazing [3]

Figure 6 – The principle of oven brazing using solder sheets [3]

Sources:

  1. Introduction to Brazing of Aluminium Alloys – TALAT Lecture 4601 / R. Mundt, Hoogovens, Koblenz – 1994.
  2. Brazing / Joining / EEA Aluminium Automotive Manual – EEA, 2015
  3. https://www.uacj.co.jp/english/products/sheeting/