Brazing is a highly versatile joining technique that can be used to join many different types of metals, and can even be used to permanently bond engineered-ceramic materials, such as alumina, to a variety of metals. It is being done everyday in industry.
Alumina, which consists of aluminum-oxide powder granules imbedded in a glassy matrix binder system of calcium-oxide and silicon-dioxide (among others), can be joined to ceramic or metal structures primarily by two different methods, as shown in Fig. 1
The first method involves the use of the standard multi-step moly-manganese surface-metallization process to prepare the alumina-ceramic substrate for subsequent brazing, whereas the second method involves the use of a special “active” brazing filler metal (BFM) which allows you to braze directly to the alumina without having to go through extensive metallization of the alumina prior to the brazing process.
This article will look only at the second method listed above, namely the direct brazing of materials to alumina by the use of so-called “active” brazing filler metals (BFMs).
First of all, it is very important to remember that for any brazing process to work effectively, including active-metal-brazing (AMB), the brazing filler metal (BFM) must be able to form a strong, permanent bond with the base materials (alumina, metals, etc.) being joined. This can be difficult to achieve with ceramics, such as alumina, since we know that the presence of oxides in the brazing joint is supposed to prevent brazing from occurring! So, how then does one deal with all the oxides that are present in a material such as alumina (aluminum-oxide)? Extensive testing over many years has shown that the addition of certain “oxygen-getters” such as titanium (up to about 5% maximum) to the BFM works well to achieve this.
When a BFM contains titanium additions to assist in the brazing process, the BFM is commonly called an “active brazing alloy” (ABA). They are commercially available from a number of sources.
Because titanium is a very strong “getter” of oxygen (and there is obviously a lot of oxygen present in oxide-form within the sintered matrix of ceramic materials such as alumina), this joining process will depend very much on the ability of titanium in the BFM to react – at brazing temperature – with these oxides that are present in the matrix of the ceramic structure.
Before proceeding further, however, please remember – titanium is a strong oxygen-getter, and thus will react with any oxygen that it can as it is heated from room temperature up to brazing temperature. Therefore, if the titanium starts “reacting” too early, such as with free oxygen or water-vapor in the furnace atmosphere, or with metal-oxides on the metal surfaces during heat-up (such as when the metals are not properly cleaned prior to brazing), then the so called “brazing” (joining/bonding) of alumina-to-metal may be completely prevented from happening.
Thus, to keep the titanium addition as “active” as possible during this high-temperature joining process, it is wise to use a good vacuum brazing furnace, since a good, clean, tight vacuum furnace is probably the best way to exclude as much oxygen as possible from the furnace throughout the entire brazing cycle. Vacuum levels on the order of 10-5 to 10-6 Torr are often used, and the vacuum furnace leak-up rate should be less than 5-microns (millitorr) per hour. Additionally, the vacuum furnace should be very clean, with no significant contamination on the inner cold-walls of the chamber due to condensation of outgassing materials from previous furnace cycles. By carefully controlling each of these items, you can help to insure that the titanium content of the BFM will be protected, and will only react, as desired, at high temperature with the oxides within the glassy matrix of the ceramic.
Next, be sure that the surface of the alumina being joined is very smooth, with a minimum of surface cracks. The surface preparation techniques used on the alumina can have a profound affect on the strength of the joint resulting from the joining process. So it is important to be sure the alumina surface is prepared correctly. When machining is employed on such surfaces, tiny surface cracks can result, and these can have a negative effect during the brazing process. So, to handle this issue, it is wise that any machined alumina surfaces be sintered, or gently lapped, after the machining is done, to remove these cracks as much as possible. This is shown in the process chart in Fig. 1, and is based on testing done back in the mid-1980’s, as shown in Fig. 2, below.
An ABA filler metal can be applied to the substrate in a variety of ways, including paste, preforms, cladding, etc., and then heated up to brazing temperature.
By the way, some people have questioned the use of ABA filler metals in paste form, thinking that typical brazing pastes have too much oxygen present in their binder systems to work properly for active-metal brazing. So, can an active-brazing paste actually be made for use in 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, the titanium is usually added as a titanium-hydride powder. As stated by Dr. Jacobson and Dr. Humpston in chapter 7 of their book “Principles of Brazing” (ASM, 2005):
“….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.” (p. 242)
Therefore, yes, an active-brazing BFM in paste form can be used. Obviously, it would involve a lot of trial-and-error testing to develop, on one’s own, an effective ABA-paste, so, as an alternative, some of the BFM suppliers out there have already done all that leg-work and development, and can offer an ABA-paste “off-the-shelf” fairly quickly.
Now let’s look a bit further at what reactions occur up at the brazing temperature when the titanium starts “reacting”:
First of all, let me state that the bonding of the filler metal to the alumina is not technically “brazing” per se, since brazing typically involves the alloying of the two materials being joined, and the BFM does not like to bond to, or flow over, oxides during that process. So then, what is actually occurring? Look at the drawing in Fig. 3.
The titanium in the ABA apparently reacts quickly at the “brazing-temp” with the oxides of silicon (and also those of calcium?) that are present in the glassy-phase of the matrix-binder that holds together the particles of aluminum-oxide. Remember that the alumina is actually a sintered product in which aluminum-oxide particles are bonded, under pressure, with this glassy matrix-binder between those alumina particles. Apparently the bonding process in ABA-brazing involves the formation of ceramic “spinel-like” structures by the reaction of titanium with these glassy phases, rather than by any actual “wetting” of the aluminum-oxide particles. The formation of these reaction phases with the titanium apparently results in a bonding-interface strength sufficient to meet demanding service requirements in a variety of end-use applications.
IMPORTANT NOTE: Be very careful when considering this process, because although the bonding may be very successful, the difference in the thermal expansion between the ceramic and the metal to which it is being joined may cause pre-mature cracking to develop in the brazed joint upon cooling, or during subsequent use in service. It is very important to try to match the expansion characteristics of the ceramics to the metals, so that huge stresses in the joints are not built up. Thus, it is often a common practice to braze the ceramic first to a low-expansion metal such as Kovar (the chemistry of which is shown in Fig. 1), and then later use this bonded metallic layer as the base to which to join other metal components later on.
Dan Kay – Tel: (860) 651-5595 – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 40-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Dan can be reached via e-mail at email@example.com, and his website can be visited at: http://www.kaybrazing.com/
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