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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):
“… it 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.”
So, 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 about this.