Presumable Mechanism of the Formation of a Diffusion Joint of Bonded Metals of a High Affinity to Oxygen

Book excerpt: The tests conducted on the diffusion bonding of 1H18N9T and 2H13 steels, alloys E1437B and Armco iron in an oxidized state, as well as the thermodynamic analysis of the interoxide reactions taking place showed that a diffusion joint forms either as the result of the chemical reactions between oxides, or on account of the mutual disintegration of the oxides and production of solid oxidizing solutions in the zone of joining. The proposed hypothesis explains why a joint cannot be obtained during diffusion bonding of austentitic steels and nimonic type of alloys without an interlayer, explains the mechanism of joint development by using an interlayer of material having a great affinity to oxygen, for example nickel or cobalt, and gives a new criterium for selecting metal for the interlayer. In light of this hypothesis it appears that the use of a high vacuum in bonding oxidizing alloys is pointless, since a vacuum or protective atmosphere can only prevent further oxidation of the distribution surface of bonded metals at the time of their joining, so that interoxide reactions could take place. This affords the possibility of simplifying the technology in the majority of the uses of diffusion bonding with the exception of vacuum-tight joints or the joining of oxygen-free metals since this introduction of oxides or a solid solution of oxygen into a joint is inadmissable. In these cases bonded surface should be metallicly pure. (Author).