Brazing-beryllium

for delicate applications.

SOLUTIONS with Effective, Practical Advice

How to Apply Beryllium Brazing?

Brazing-beryllium is generally performed under high vacuum or under strictly controlled atmosphere of very pure argon.

The reason is that beryllium reacts strongly with carbon, oxygen and nitrogen at usual brazing temperatures.

Fluxes can also be used, although potential corrosion problems from residues must be taken into account.

Beryllium has one of the highest melting points
(1285 °C) of the light metals. It has exceptional elastic rigidity, the modulus of elasticity of beryllium being about 50% greater than that of steel.

Beryllium is used in aerospace structures because of its low density
(2.7 g/cm3), high stiffness to weight ratio and exceptional dimensional stability over a wide temperature range.

Most of beryllium primary forms are based on powder metallurgy technology.

It has been used for windows in x-ray tubes and x-ray fluorescence (XRF) devices.

In fact it has relatively high transparency to X-rays and other ionizing radiation types. It is also widely used for nuclear applications.

While handling of bulk metal does not pose safety issues, Beryllium and its compounds are carcinogenic, dangerous to human health due to the toxicity (especially by inhalation) of beryllium-containing dusts.

Beryllium particles can cause chronic ulceration if entering in open wounds. Therefore any dust generating machining operation must be strictly controlled.

Brazing-beryllium Principles

Brazing-beryllium is well established because other joining techniques are less successful. Welded joints have low strength and ductility because of a brittle, coarse grained microstructure.

A 38% Aluminum-Beryllium alloy is ductile and weldable, but can be brazed, although the filler metals should be based on zinc or aluminum, having lower brazing temperature than silver-copper alloys.

Bare metal surfaces must be thoroughly degreased, and cleaned with strong acids before Brazing-beryllium, because all traces of refractory oxides formed on its surface must be removed.

Sometimes a light coating of titanium, or silver, copper or aluminum is deposited to promote wetting by various brazing filler metals.

Joint design needs special study to avoid asymmetric stress patterns, due to the low ductility and reactivity of beryllium with many filler metals.

Rapid heating and cooling cycles are preferred, to minimize the formation of intermetallic compounds.

Filler metals for Brazing-beryllium are mainly selected from zinc, aluminum and silver alloys. Aluminum or its alloys with 7.5 or 12.5% Silicon are normally used for providing acceptable strength in the joints up to 175 °C (345 °F). However due to poor flow in capillary joints, the filler has to be preplaced.

The following are the normal temperatures used for Brazing-beryllium

  • Zinc (427 to 454 °C or 801 to 849 °F)
    Note: This temperature is at the minimum conventional limit established for brazing.
  • Aluminum-silicon (645 to 655 °C or 1051 to 1210 °F)
  • Aluminum (645 to 655 °C or 1185 to 1659 °F)
  • Silver-copper (640 to 904 °C or 1185 to 1659 °F)
  • Silver (882 to 954 °C or 1620 to 1749 °F)

Zinc has no undesirable reaction with beryllium and the temperature is so low that no recrystallization occurs in beryllium sheets.

Although the procedures normally used for Brazing-beryllium are known and proved, they should be carefully planned for any new application.

They must be developed while familiarizing with the details, and the results should be tested to verify the suitability of the process for the intended service.

On safety issues see also the following documents:

Safety and Health Topics - Beryllium
http://www.osha.gov/SLTC/beryllium/index.html

Questions on Beryllium
http://www.atsdr.cdc.gov/tfacts4.html

Watch the following Video

Beryllium Unearthed

http://www.youtube.com/watch?v=Ii5GfEahbbE

* * *

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