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:
- 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
- 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
- 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
- B. Wielage, D. Ashoff "Beitrag zur
Festigkeitssteigerung von hochtemperaturgelöteten Titanwerkstoffen"
Mat.-wiss. u. Werkstofftech. 20 (1989), S. 125/32
- E. Lugscheider, L. Martinez "Löten -
Stoffschlüssiges Fügen moderner Werkstoffe am Beispiel von
Titanlegierungen" VI-Berichte Nr. 734 (1989), S. 359/70
- 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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