The role of palladium in silver-copper braze alloys.

The research began with a survey of methods used in the brazing of different metals taking 1965 as a 'benchmark' year. The survey took into account types of heating, filler alloys available, design considerations and situations where this method of joining had proved a satisfactory engineering technique. Following visits to a number of organisations the position of brazing was again reviewed after an interval of c. 25 years. The major changes noted related to improvements in vacuum technology as a result of which aluminium is now brazed on a tonnage basis, hydrogen and other gas atmosphere furnaces have largely been superseded for brazing the high nickel alloys and stainless steels, and satisfactory methods have been adopted for titanium, cermets and ceramics to metals. Cleaner and more easily applied filler alloys have also given impetus to many more applications for the technique. The surveys revealed that one problem which had remained unanswered related to cracking of the low expansion alloys, used in glass-to-metal seals, when high-temperature brazed with filler alloys based on the silver-copper eutectic. Cracks formed as a result of stress which could arise during the brazing cycle but the mechanism was uncertain and a research programme was therefore formulated to establish reasons for the crack formation. Using a 42% nickel-iron alloy with silver-copper eutectic and three available modifications of this alloy containing nominally 5%, 10% and 25% palladium, a static beam loading technique was devised to assess stress cracking. Specimens were examined by Auger/XPS Spectroscopy, Optical and Scanning Electron Microscopes. In the case of the silver-copper eutectic braze, molten alloy preferentially dissolved grain boundary nickel atoms leaving vacancies which were replaced by copper. Applied stresses caused propogation of cracks initiated at the weakened boundaries and the mechanism was considered to be a form of stress corrosion cracking. The solidified brazing alloy contained iron and nickel, with the ratio of Fe to Ni increasing as Pd increased. Stress corrosion pits and tunnelling noted with the Pd-free alloy were not found in the 5 or higher % Pd alloys. It was concluded that penetration of grain boundaries in the parent metal reduced as the Pd was increased due to uptake of Fe by the molten brazing alloy