How to get flawless brazing with vacuum furnaces

How to get flawless brazing with vacuum furnaces

Vacuum furnaces allow high quality production at a low unit cost, eliminating excessive production waste and wastage of raw materials. Regardless of the industry in question (automotive, aerospace, energy, etc.), vacuum provides the ideal brazing conditions for the production of heat exchangers made from aluminium alloy.

In this article I'm not going only to explain why you should braze under vacuum, but I'll give useful tips on vacuum furnace cleaning and I’ll show you how to avoid two potentially hazardous risks.

As you know, brazing is a delicate process owing to the difficulty in separating the surface oxide film from the base material (3003, 3105, 6951, etc.). So why vacuum furnaces are being considered the best choice for brazing aluminium alloy components?

8 reasons to braze under vacuum

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

Let’s take a look now on how to clean you vacuum furnace before brazing and after that I’ll show you how to avoid some quality pitfalls.

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Tips for the best cleaning cycle before brazing

I will never tire of repeating this: vacuum processes need a clean environment. The cutting oil residue (used for the production of plates, turbulators, separator elements, etc.) hinders the maintenance of ideal brazing conditions as the oil evaporated from the parts contaminates the furnace and diffusion pumps. This creates an obstacle to continuous brazing and therefore results in defects.

Before loading the heat exchanger components into the furnace for vacuum brazing, I suggest the following cleaning cycle:

Plated components (4004, etc.)

  • steam at 80° C
  • ultrasound cleaning
  • drying

Not plated components

  • steam degreasing
  • acid or alkaline solution
  • rinsing in water
  • nitric acid bath
  • rinsing in water
  • drying

Excess of magnesium: Beware of the risk!

The furnace for brazing aluminium alloy components needs magnesium. However, excessive formation of magnesium oxide (residues left by the brazing process) prevents the furnace from brazing properly and can cause two potentially hazardous situations.

  1. The formation of deposits of magnesium oxide (MgO) and magnesium (Mg) condensed on cold surfaces can trigger a short circuit between the resistor and the chamber’s reflective thermal shielding. The oxide creates deposits which are firmly attached and hard to remove. The deposit maintains sufficient electrical conductivity to make a short circuit possible. Specific shields can be used to avoid this occurrence thanks to the continuous evolution of materials science. And that’s not all! Nowadays, an alternative to the use of Mg as a getter to improve the vacuum during the process has been found in the cryogenic pump, which removes residual traces of water vapour from the washing of these materials.
  2. Deposits of magnesium (Mg) can trigger a highly dangerous situation during removal. In the presence of air (a situation which occurs when the operator attempts to remove the deposits) magnesium ignites extremely easily and burns forming droplets at 2000° C which cannot be extinguished with water and which can cause serious injury operators.

And what about you? Are you still using magnesium powder or pads for brazing aluminium alloy components of heat exchangers?

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See also

Vacuum brazing: the guide to perfect brazed joints

Striving for perfect joints? Vacuum brazing is a high-end joining technology as it produces higher quality joints and with no residual corrosive flux.

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