aluminum brazing 

Low-temperature aluminum brazing

Aluminum brazing: hard and soft

For aluminum and aluminum alloys using various soldering methods. Soldering happens:

In English:

  • brazing и
  • soldering, respectively.
  • Solids include solders with a high melting point (liquidus higher 450 ° C).
  • Soft solders melt below the temperature 450 ° C.


Figure – Repair of aluminum pipe by soft soldering [2]

Soft solders for aluminum

Since soldering is carried out at a temperature below 450 ° C, the, naturally, In this case, do not apply brazing materials - aluminum-based solders. Previously, most solders for the soldering of aluminum containing zinc, lead, cadmium and lead. Currently, cadmium and lead are considered harmful to humans and the environment. Therefore, modern soft solder for soldering of aluminum – It is based on tin and zinc alloys.

Tin-zinc alloys

For brazing aluminum to aluminum and aluminum to copper, tin-zinc alloys have been specially developed:

  • 91 % tin / 9 % zinc – eutectic alloy having a melting point 199 ° C
  • 85 % Sn / 15 % Zn – melting range from 199 to 260 ° C
  • 80 % Sn / 20 % Zn – melting range from 199 to 288 ° C
  • 70 % Sn / 30 % Zn – melting range from 199 to 316 ° C
  • 60 % Sn / 40 % Zn – melting range from 199 to 343 ° C

Эvtekticheskie and эvtekticheskie solders

Eutectic solders are widely used for furnace brazing and other automated systems aluminum brazing. This minimizes the heat applied to thin-walled articles by rapid melting and solidification at a temperature 199 ° C.

Interval solder solidification, when he is in the semi-semi-solid state, It allows you to perform additional operations on the goods, until the solder has completely hardened.

Increased content of zinc contributes to a better wetting of solder, but with an increase in the zinc content, the temperature of complete solidification of the solder (liquidus) increases significantly.

Features soft soldering

Brazing soft aluminum solder different from that of other solder metals. The oxide film on aluminum - dense and refractory - requires active fluxes, It is specifically designed for aluminum. the brazing temperature must also be controlled more tightly.

For aluminum, the corrosion resistance is much more dependent on the composition of the solder, than for copper, brass and iron alloys. All soft solders brazed joints have a lower corrosion resistance, than after brazing joints or welding.

The high thermal conductivity of aluminum requires rapid heating, to provide the desired temperature in the seam.

Brazing deformable aluminum alloys

Almost all aluminum alloys way or another can be subjected to soldering. However, their chemical composition strongly influences the ease of soldering, solder type, used soldering method solder and the ability to withstand a variety of products in the load operation.

Relative ability to soldering - soldering – the main wrought aluminum alloys are as follows:

  • perfectly soldered: 1100 (AD), 1200 (AD), 1235 (≈АД1), 1350 (AD0E), 3003 (AMc):
  • well soldered: 3004 (D12), 5357, 6061 (AD33), 6101, 7072, 8112;
  • medium soldered: 2011, 2014, 2017 (D1), 2117 (D18), 2018, 2024 (D16), 5050, 7005 (1915);
  • poorly soldered: 5052 (AMg2.5), 5056 (≈АМг5), 5083 (AMg4.5), 5086 (AMg4), 5154 (≈АМг3), 7075 (≈В95).

alloys, which contain more 1 % magnesium, It can not be satisfactory soldering using an organic flux, and alloys with more than 2,5 % magnesium - with active fluxes. alloys, which contain more 5 % magnesium, can not be with any solder flux.

When brazing aluminum alloys, containing more 0,5 % magnesium, molten tin solders penetrate between the metal grains. Zinc also is able to penetrate at grain boundaries between the grains of aluminum-magnesium alloys, but when the magnesium content of more than 0,7 %. This intergranular penetration is exacerbated by stress, external or internal.

Aluminum alloys, alloyed with magnesium and silicon, less prone to intergranular penetration, than binary aluminum-magnesium alloys.

Aluminum alloys, containing copper or zinc as the main alloying elements, also typically contain a sufficient amount of the other elements. Most of these alloys susceptible to intergranular penetration of solder and they are usually not soldered.

Thermally strengthened alloys typically have a thicker oxide film than that, that occurs naturally. This film impedes soldering. For such alloys typically used prior to soldering the chemical surface preparation.

Brazing of aluminum casting alloys

Most aluminum casting alloys have a high content of alloying elements, which increases the likelihood, These elements will be dissolved in the solder, and the solder will penetrate the grain boundaries. Therefore, the casting aluminum alloy soft solders badly soldered.

Moreover, characteristic of the cast alloys surface roughness, smallest cavities or porosity contribute to the retention of flux and make removal of flux after soldering is very difficult.

Three cast aluminum alloy 443.0, 443.2 and 356 relatively well and easily soldered with soft solders. slightly worse, but still acceptable soldered alloys 213.0, 710.0 and 711.0.

Sources:

  1. Aluminum and Aluminum Alloys, ASM International, 1996
  2. EEA Aluminium Automotive manual – Joining – Brazing, EEE, 2015