Active metal brazes alloys

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... 

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... 

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). 

Brazing of ceramics is not without difficulty. In contrast to glasses, many metals and alloys bead up on ceramic substrates, that is, the molten metal does not wet the ceramic. To promote wetting, the surface must either first be metallized, or a braze containing a chemically reactive or active metal, often titanium, must be used. Titanium reacts with the ceramic and generally facilitates wetting via the formation of a more metallically bonded reaction product at the interface. 

The possibility of galvanic corrosion must be considered when filler metals are selected for brazing titanium-base metals. While titanium is an active metal, its activity tends to decrease in an oxidizing environment, because its stirface undergoes anodic polarization similar to that of aluminum. Thus, in many environments, titanium becomes more chemically inactive than most structural alloys. The corrosion resistance of titanium is generally not affected by contact with structural steels, but other metals, such as copper, corrode rapidly in contact with titanium xmder oxidizing conditions. Thus, filler metals must be chosen carefully to avoid preferential corrosion of the brazed joint. 

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... 

In designing an air braze filler metal, it is critical that there is some degree of mutual solubility between the metal and oxide constituent(s) in the liquid state. Experimental evidence suggests that the likely indicators for this behavior are measurable oxygen solubility (i.e., oxygen activity) in the liquid metal constituent, compatible melting points between the metal and oxide to be alloyed, and multiple valence states in the cation species of the oxide constituent that allow it to serve as an efficient oxygen buffer in the noble metal. Phase formation and phase equilibria in the filler metal system must be appropriate for the application of interest. For... 

Other successful examples of ceramic-to-metal packages include alumina to Ti-45%Nb alloy package brazed with TiNi-50 active filler metal, for cochlear implant application alumina to pure niobium case bonded with TiNi-50 filler metal, also for cochlear implant and alumina to a metal assembly brazed with a modified active filler metal, for artificial retina packaging. 

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