Heat Treatment – Achieving Perfect Brazing using a Vacuum Furnace

Vacuum furnaces enable high-quality production at a low unit cost and, as a result, prevent excess production waste and wastage of raw materials.

Irrespective of the industry in question (energy, aerospace, automotive, etc.), vacuum provides the perfect brazing conditions to produce heat exchangers made from aluminum alloys.

This article not only explains the benefits of brazing under vacuum, but also provides useful tips about cleaning vacuum furnaces and how to avoid two potentially hazardous risks.

Brazing is known to be delicate process because of the difficulty of separating the surface oxide film from the base material (6951, 3105, 3003, etc.). So, why are vacuum furnaces being considered as the best option for brazing aluminum alloy components?

8 Reasons to Braze under Vacuum

The following reasons support the use of brazing under vacuum:

  1. Calibrated use of alloy with magnesium (4045, 4044, 4104, 4004) to prevent excessive Mg sublimation in the vacuum chamber
  2. High final vacuum due to high capacity pumping systems (cryogenic pump + diffusion pumps), which produce a low oxygen atmosphere
  3. Rapid transition of the load temperature during the heating phase from 520 °C (starting temperature for Mg sublimation) to 605/610 °C thanks to high power furnaces, a feature that prevents an oxidized layer (Al2O3) from reforming on the surface of the piece newly reduced to Al
  4. Suitable fixing systems of the individual heat exchanger prevent the piece from collapsing or deforming
  5. Uniform load temperature (± 3 °C at 605 °C) is achieved through accurate control of the output power and by distributing the heating zones
  6. Cleanliness of the components
  7. No emission of contaminated substances into the atmosphere
  8. Controlled cooling of the walls and other elements of the furnace with heated water (80 °C) to prevent atmospheric water vapor from condensing during loading and unloading process

Tips for the Best Cleaning Cycle Before Brazing

A clean environment is important for vacuum processes. The cutting oil residue, used for producing plates, separator elements, turbulators, etc., hinders the maintenance of ideal brazing conditions because the oil that evaporates from the parts contaminates the diffusion pumps and furnace. This creates an obstacle to continuous brazing and, hence, leads to defects.

The following cleaning cycle is suggested before the heat exchanger components are loaded into the furnace for vacuum brazing:

Plated Components (4004, etc.)

  • Steam at 80 °C
  • Ultrasound Cleaning
  • Drying

Not Plated Components

  • Steam degreasing
  • Alkaline or acid solution
  • Rinsing in water
  • Nitric acid bath
  • Rinsing in water
  • Drying

Excess of Magnesium: Beware of the Risk

The furnace requires magnesium for brazing aluminum alloy components. However, the furnace is prevented from brazing properly due to the excessive formation of magnesium oxide (residues left by the brazing process). This can lead to two potentially hazardous situations:

  1. The formation of deposits of magnesium (Mg) and magnesium oxide (MgO) condensed on cold surfaces can activate a short circuit between the chamber’s reflective thermal shielding and the resistor. The deposits created by the oxide are firmly fixed and cannot be removed easily. The deposit maintains adequate electrical conductivity to enable a short circuit. Thanks to the continuous evolution of materials science, this occurrence can be prevented by using specific shields. In addition, an alternative to the use of Mg as a way to improve the vacuum during the process has been identified in the cryogenic pump, which removes residual traces of water vapor from the washing of these materials.
  2. Magnesium (Mg) deposits can trigger dangerous situation during removal. In the presence of air (a situation that occurs when operators try to remove the deposits), magnesium ignites very easily and continues to burn, forming droplets at 2000 °C, which cannot be extinguished with water and can cause serious injury to operators.

This information has been sourced, reviewed and adapted from materials provided by TAV Vacuum Furnaces.

For more information on this source, please visit TAV Vacuum Furnaces.

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