Platinum Metals Rev., 1963, 7, (1), 8
High Purity Palladium Brazing Alloys
Multi-Stage Jointing in the Manufacture of Thermionic Valves
The possibility of using palladium as a component of brazing alloys for high temperature service was first explored by Rhodes, working in the laboratories of the Mond Nickel Co Limited, who studied the effect of additions of this metal on the pertinent properties of silver-copper alloys.
This early work was directed towards satisfying the demand for high-temperature brazing alloys suitable for the fabrication of turbine assemblies for jet engines. It resulted in the development of a series of ternary silver-copper-palladium alloys containing between 5 and 25 per cent palladium. All these alloys freeze at temperatures higher than the melting point of the silver-copper eutectic (778°C); they have sufficiently narrow melting ranges, they wet readily nickel-based alloys without causing cracking due to inter-granular penetration and they meet many requirements imposed by high service temperatures in a large number of industrial applications.
The manufacture of special-purpose thermionic valves, such as magnetrons and klystrons, represents a relatively recent field of application of palladium-bearing brazing alloys.
Since these devices operate at relatively high temperatures and under conditions of high vacuum, the use of standard cadmium-and/or zinc-bearing silver brazing alloys is ruled out. Silver-copper eutectic has been widely used in this application, but more satisfactory results can often be obtained with the palladium-bearing alloys because of their better mechanical properties at elevated temperatures, improved wetting characteristics on molybdenum, tungsten, and nickel alloys, and reduced risk of failure due to intergranular penetration. Gold-copper and gold-nickel alloys are also used in this field.
The manufacture of thermionic valves may entail fabrication of components comprising several joints that cannot be brazed in a single operation. A range of brazing alloys is therefore required with the melting points spaced at convenient intervals so that complex assemblies can be fabricated by the step-by-step brazing technique. In addition, best results in brazing close-fitting parts are achieved with alloys of the eutectic type - having a single melting point or a narrow melting range - whereas an alloy with a relatively wide melting range may be required to bridge wide joint gaps or to form large fillets.
These requirements are met by a range of palladium-bearing brazing alloys, marketed by Johnson Matthey under the name “Pallabraze”. The range includes six ternary silver-copper-palladium alloys and three binary compositions: 5 percent palladium-silver, 18 percent palladium-copper and 40 percent palladium-nickel, the last of these alloys constituting a useful addition to the range because of its high single melting point of 1237°C. Particulars of these alloys are set out in the table on page 8. In each case the reference number of the alloy is its liquidus temperature.
Importance of High Purity
A feature distinguishing the Pallabraze alloys from similar materials intended for general engineering applications is their extremely high degree of purity. This consideration is of primary importance for several reasons.
First, since the application of brazing fluxes in the fabrication of thermionic valve components is inadmissible, brazing alloys used for this purpose must be virtually free from impurities that might impair their wetting and spreading characteristics; this group of impurities includes metals forming refractory oxides that do not dissociate except at very high temperatures and low pressures and that can only be reduced in reducing atmospheres of purity difficult to obtain in industrial brazing practice.
Secondly, efficient functioning of thermionic valves and other similar devices depends on maintaining a high vacuum; this means that all the materials of construction, including the brazing alloys, must be free from volatile constituents. These include not only metals that have high vapour pressures at temperatures at which thermionic valves operate, but also carbonaceous surface contaminants (grease, oil, lubricants, etc.) that might affect the vacuum as a result of gradual build-up of carbon monoxide in the valve.
These requirements impose the need both for rigorous control of the purity of the raw materials used in the manufacture of brazing alloys for this application and for special precautions at every stage of the manufacturing process.
Based on the extensive experience of Johnson Matthey in the production and application of ultra-pure materials, very low limits of impurities for Pallabraze alloys have been established. These are set out in the table. All the JMC Pallabraze alloys listed in the table on page 8 are made to this specification, and every batch of wire is checked to ensure that these limits are not exceeded.
Although palladium-bearing alloys can, in principle, be used for torch brazing, this method is not applicable in the manufacture of thermionic valves because of the complications caused by the necessity of using a brazing flux. For this reason brazing must be carried out either in vacuum or in a neutral or reducing atmosphere. Depending on the nature of the parent metals, a vacuum of 10-5 mm Hg, or a protective atmosphere with a dew point lower than -50°C, may be required. Hydrogen and cracked ammonia are both suitable as protective atmospheres, provided that their moisture content is sufficiently low.
|Impurity||Parts per Million, Maximum|
The brazing alloys are supplied ready for use in the form of wire. They can, however, become contaminated during any forming operation or in handling, in which case they should be vapour-phase degreased immediately before brazing. Similar steps should, of course, be taken to ensure cleanliness of the work pieces.
Difficulties encountered in fluxless brazing of components made from alloys containing metals which form refractory oxides can sometimes be overcome by raising the brazing temperature. If this expedient fails, recourse may be taken to nickel plating the components before brazing.