There is a lot of interest today in joining engineered-ceramic materials (such as alumina) to themselves or to metals. Active metal brazing (AMB) has shown itself to be an interesting method to accomplish this, instead of the standard moly-manganese process of ceramic surface preparation prior to brazing (a process that has been around for many years).
For any brazing process to work effectively,
including AMB, the brazing filler metal (BFM) must be able to form a strong,
permanent bond with the base materials (alumina, metals, etc.) being joined.
Whenever any alumina-based materials are involved, the use of titanium
additions (up to about 5% maximum) to the BFM as an “activator” is what helps
enable any so-called “active-brazing” process to be effectively
accomplished. Titanium is a strong “getter” of oxygen, and there is obviously lots
of oxygen present in oxide form within sintered ceramic materials such as
alumina. It is with these oxides that the titanium is supposed to react for the
bonding process to occur. If, however, other sources of oxygen present
themselves to the titanium before the titanium is able to react with the oxides
in the ceramic, then the so-called “brazing” (joining/bonding) of alumina to metal
may be prevented from happening altogether.
What are these other sources of oxygen that can
hurt the joining process? Often the furnace atmosphere will be a major source
of external oxygen. Standard brazing paste binder-system, if used, can also be
an oxygen source since braze-binders used in making standard brazing pastes
contain large amounts of oxygen in the water products or other oxygen-bearing
products used in making them.
It is therefore essential that the atmosphere in
which the active brazing takes place be free of oxygen. Thus, vacuum-brazing
furnaces are used rather than standard atmosphere furnaces. The BFM powder
itself should be pure and as free of oxygen as possible. Additionally,
commercially available standard brazing-paste binder systems cannot be used.
Can an active-brazing paste actually be made for
active brazing alloy (ABA) brazing? Surprisingly,
the answer is “yes.”
For an ABA
paste to work, in which titanium is to be the active “getter” in the alloy system,
it is probably wise to use titanium-hydride powder as the form of titanium for
making a BFM paste that has a chance of working. As stated by Dr. Jacobson and
Dr. Humpston in chapter 7 of their book “Principles of Brazing” (ASM, 2005, p. 242):
is possible to convert the stock brazes into active brazes by adding
titanium-hydride powder without greatly altering the rheological properties of
the paste. Titanium hydride decomposes into metallic titanium at about 930°F
(500°C), so the active metal (Ti) is effectively protected against degradation
until the process atmosphere has been established.”
yes, an active-brazing paste can be made. Obviously, it may involve a lot of
trial-and-error testing to develop an effective paste for your specific needs. As an alternative, some of the BFM suppliers out there
have already done all that legwork and development and can offer you an ABA paste fairly quickly.
One such supplier is Wesgo (California),
a company specializing in this field that already replied to my earlier blog posting
Active Brazing Alloys (ABA)
By Dan Kay
Dan Kay operates his own brazing consulting practice in Connecticut (since 1996) and has been involved in brazing for almost 45 years. He received his BS in Metallurgical Engineering from Rensselaer Polytechnic Institute in 1966 and his MBA from Michigan State University in 1982.
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