Brazing

Brazing is used to join any combination of similar or dissimilar metals and results in a high-strength joint of good reliability. 

To form a strong joint, the surfaces must be free of any rust, grease or oxide film. 

Brazing alloys are available in a wide variety of forms, including rod, strip, wire, foil and powder. 

The choice of brazing alloy depends upon the materials being joined and the temperature at which the brazed parts are to operate. Brazing brasses are widely used with hand-torch heating for joining ferrous-metal parts. The common composition of brazing brass is shown in Table 12.2. 

Silver Copper Zinc Cadmium Solidus Liquidus 

The joint region in compliant seals can be plastically deformed above room temperatures, which lessens the thermal stresses caused due to CTE mismatch. This category of sealants includes metallic components and hence is electrically conducting. In addition to this, the cell bowing and non-uniformities in the gas distribution are potential problems. Brazing and bonded compliant seals fall in this category and are discussed in Sects. 5.3.2.1 and 5.3.2.2. 

The air brazing technique has been developed to overcome these shortcomings. Air brazing offers the following advantages. 

However, two problems are commonly encountered when using ABAs for pSOFC sealing  

1) The resulting joint is not sufficiently oxidation resistant under the target operating conditions because ABA filler metals are typically Ni- or Cu-based. It has been found that even noble metal ABAs suffer from this problem [72]. 

2) The brazing process causes irreversible chemical reduction and struc-tural/property degradation in the electrochemicaUy active ceramic components used in pSOFC stacks. This is because of the need to carry out brazing under nonoxidizing conditions (i.e., in vacuum, inert gas, or reducing gas) [73]. 

To overcome these issues, an alternative brazing technique has been developed specifically for use in fabricating solid-state electrochemical devices [74]. Referred to as air brazing, the technique employs a molten oxide that is at least partially soluble in a noble metal solvent to promote wetting of the ceramic sealing surface. Due to its ability to dissolve in the noble metal liquid, the oxide compound serves as an in situ oxygen buffer, raises the chemical activity of dissolved oxygen, and enhances 

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 

Although the addition of cadmium has particular advantages, fumes given off can have serious health effects. As a result, safer, cadmium-free alloys are available and should be used wherever practicable. Exposure to alloys containing cadmium must be adequately controlled as covered by COSHH Regulations. 

Aluminium-silicon alloys are used for the brazing of aluminium and aluminium alloys. The composition of a commonly used aluminium brazing alloy is shown in Table 13.4. 

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With suitable bonding techniques, the metal parts can be joined vacuum tight to the ceramic insulator in automated brazing procedures. 

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