Soldering and brazing titanium and titanium alloys

Before soldering and brazing titanium and titanium alloys the thin oxide layer (passive layer) usually present on as-delivered parts should be removed. The following pickling treatment has proved effective for this:

  • pre-clean with acetone,
  • rinse with water,
  • pickle with 35% HNO3 + 5% HF + balance H20,
  • rinse with water,
  • dry with warm air.

Particularly thick oxide or scale layers must be removed by mechanical methods such as sand blasting, grinding or brushing, followed by pickling as described above. Cleaning should be carried out directly prior to soldering or brazing and should finish off with thorough rinsing and drying in warm air.

Soldering is best carried out with lead-tin-based solders at working temperatures between 200 and 300 °C after the contact surfaces have been electrolytically or dip-coated with copper or silver to improve wettability and adhesion.

For direct application with the flame or the soldering iron aluminum-tin and tin-zinc solders can also be used.

At the higher temperatures used in brazing the formation of intermetallic phases should be prevented by selection of an appropriate brazing filler metal.

Titanium forms brittle intermetallic phases with almost all metals in the fusion zone. The only exception is silver, which is why this metal is used for intermediate layers and as the main component of a number of filler metals for brazing titanium. In addition, silver-base alloys display excellent flowing and wetting properties in conjunction with titanium base metals, allowing the brazing of intricately shaped components.

Despite their good wetting properties the silver-base alloys as well as the aluminum-base alloys have poor corrosion and strength properties and are therefore only used for uncritical joints.

The commercially available titanium-base filler metals Ti-Cu-Ni (15% Cu, 15% Ni) produce joints which have virtually the same mechanical and chemical properties as the base metal. However they have the disadvantage of limited flowing properties and a relatively high working temperature of 950°C. The advanced Ti-Cu-Ni filler metal containing 20% Cu and 20% Ni displays greatly improved flowing properties.

Reduced working temperatures and good mechanical properties are possible using titanium-zirconium-base alloys Ti-Zr-Cu-Ni. However due to inferior corrosion performance their applications are limited to joints where the main requirement is strength.

The copper-titanium alloy CuTi30 can also be used to make high-strength brazed joints but the disadvantages here are high joint preparation requirements, occasional brittle phases in the joint and damage to the base metal.

Heat sources for soldering and brazing include acetylene torches, high-frequency induction coils, infrared heaters, inert gas electric arcs with graphite or tungsten electrodes, in individual cases resistance heating using a spot welding machine or heating in muffle furnaces in an argon atmosphere as well as high-vacuum furnaces, especially for brazing.

Argon with a degree of purity of at least 99.99% should be used as protective gas.

In addition, the moisture content of the argon is important. This is often not stated in the analysis. Only argon with a very low moisture content should be used. The dew point should be below ­50°C if possible.

If a vacuum or inert gas atmosphere is not used, then fluxes are needed to dissolve the oxide layer and prevent further gas pickup. For silver alloys, fluxes consisting of mixtures of alkali chlorides and fluorides with small additions of AgCl and CuCl2 are suitable. The AgCl decomposes and the silver protects the titanium surface.

Most fluxes are protected by patent, as for example for brazing

  • 25 bis 35 % LiF
  • 10 bis 37,5 % KCl und
  • 37,5 bis 55 % HKF2

and for soldering

  • 25 % NaCl,
  • 20 % KCl,
  • 5 % LiF,
  • 6 % AgCl,
  • 22 % ZnCl2 und
  • 22 % NH4Cl.

Notwithstanding the information given above, the soldering and brazing of titanium and titanium alloys require great care and a certain degree of experience.

The main difficulties are caused by the brittle intermetallic phases that form when the filler metals react with the titanium and severely impair the strength and ductility of the joint. In addition, the flux may also not provide 100% protection against pickup of atmospheric gases. Preliminary tests should be carried out on corresponding samples to confirm the suitability of the fluxes and filler metals for the selected soldering or brazing method.

More details on soldering and brazing titanium and titanium alloys are given in the following publications:

  1. J. Breme, V. Wadewitz, U. Fink "Investigations on Samples Joined with Different Brazing Materials" Proceedings of the Fifth International Conference on Titanium, 1984 Published by Deutsche Gesellschaft für Metallkunde, Vol. 2, p. 869/76
  2. E. Lugscheider, L. Martinez "Brazing of TiAl6V4 with Al-, Ag-, Au- and Ti-base Filler Metals" Proceedings of the 18. International AWS-WRC Brazing and Soldering Conference; March, 23.-27. 1987, Chicago/USA
  3. B. Norris "The Development of Titanium Brazing Technologies" Proceedings of the Sixth International Conference on Titanium, 1988 Published by les éditions de physique, Vol. 3, p. 1209/13
  4. B. Wielage, D. Ashoff "Beitrag zur Festigkeitssteigerung von hochtemperaturgelöteten Titanwerkstoffen" Mat.-wiss. u. Werkstofftech. 20 (1989), S. 125/32
  5. E. Lugscheider, L. Martinez "Löten - Stoffschlüssiges Fügen moderner Werkstoffe am Beispiel von Titanlegierungen" VI-Berichte Nr. 734 (1989), S. 359/70
  6. M.W. Ko, A. Suzumura, T. Onzawa "Brazing of Titanium Using Low Melting Point Ti-Base Filler Metals" Proceedings of the 1990 International Conference on Titanium Products and Applications, Vol. 2, p. 592/601 Published by Titanium Development Association, Dayton, Ohio

Deutsche Titan, Nov. 2000

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