Active braze alloys

Xiong, H., Li, X., Mao, W Cheng, Y. (2003). Wetting behavior of Co based active brazing alloys on SiC and the interfacial reactions. Materials Letters, 57,3417-3421. doi 10.1016/S0167-577X(03)00087-9. 

About 2.47 million kg of silver are lost each year to the domestic biosphere, mostly (82%) as a result of human activities. As discussed later, the photography industry accounts for about 47% of all silver discharged into the environment from anthropogenic sources. In 1990, about 50% of the refined silver consumed domestically was used to manufacture photographic products 25% in electrical and electronic products 10% in electroplated ware, sterlingware, and jewelry 5% in brazing alloys and 10% in other products and processes. 

Rapid fluid flow cannot be achieved with active metal brazes because of the need to form solid wettable reaction product layers for their liquid fronts to advance. Equations (10.1) to (10.2) relating liquid flow rates to the opposed effects of surface energy imbalances and of viscous drag are not relevant. Actual penetration rates are so slow, usually of the order of 1 pm.s, that the usual practice is to place the active metal braze alloy within the joints rather than expecting it to fill them, and, as explained already, gap width is not the dominant consideration when designing ceramic-metal joints. 

The compositions of useful braze alloys have been carefully devised to ensure satisfactory wetting of the proposed component materials, but other adjustments have been made to achieve desirable mechanical characteristics of the joints. This adjustment has been made for virtually all commercial braze families and has been particularly important in the development of active metal brazes for ceramic components. 

The prime requirement of an active metal braze is that it should be able to change the chemistry of the ceramic surface to make it wettable, usually by forming hypostoichiometric TiC, TiN or TiO. This necessitates using alloys with high Ti activities, but alloys with high Ti concentrations are seldom suitable as brazes. Thus, Cu alloyed with 5 or 10 wt.% of Ti wets many ceramics well but... 

Braze alloys for ALOs-based ceramics might be Ag-Cu, Au-Ni, or Ag-Cu-Zn, but these alloys generally do not wet ceramics. The oxidation potentials of Cu and Ag are less than that of Al, so they do not react with the ceramic. If a small percentage of an active metal, e.g., Ti, is added, then the high oxidation potential of the Ti causes it to undergo a redox reaction with the ceramic (AI2O3). 

Active-brazed joint between graphite and a TZM molybdennni alloy (see Tables 44-60 and Figs. 101-134). 

The process flow for active metal braze substrates usually involves coating the braze alloy on the ceramic substrate of interest in a paste form, or as a metal foil. The copper foil is then placed on top of the braze alloy, and the whole assembly is heated in an inert atmosphere. The braze alloy melts and forms a strong bond with the copper and substrate. In many cases, the braze alloy and copper are patterned before bonding to eliminate the need for... 

Alternatively, a complete coahng of braze alloy and copper foil can be placed on the ceramic. In this case, the copper is then etched much like a DEC substrate. However, in this case, an etching or abrasion process must be used to remove the unwanted braze alloy and activated substrate layer. 

Titanium Basis of the active metals process. May be used as powder or foil prior to braze alloying. Original work by Kelley and Bondley of General Electric. Titanium-bearing brazes wiU wet and flow over ceramic in vacuum, almost as well as solder over cojjier. Frequently applied as the hydride, which dissociates at <800°C, providing nascent hydrogen which tends to scour the... 

Flux A flux that is fluid and chemically active at brazing or soldering temperature should be used when necessary to ehminate oxidation of the filler metal and the surfaces to be joined, and to promote free flow of brazing alloy or solder. 

A simpler (one-step) and more economical joining process is direct brazing in a furnace under a vacuum or inert gas atmosphere through the use of active filler metals [Mizuhara Cl al., 1989]. An active element such as the commonly used Ti in the filler metal forms a true alloy with the base metal. The difference in the thermal expansion coefficients between the ceramic membrane and the metal housing can lead to high stress at the... 

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