TWI Industrial Member Report Summary 222/1983
By I A Bucklow
The report describes work that is part of a long-term programme to examine the possibility of joining steels at metallurgically low temperatures. The objective of the present research was to explore techniques for assisting the removal of interface voids, following previous work which indicated that the persistence of voids was the controlling factor in the mechanical behaviour of joints made in steel below the A1 temperature. The present work was confined to a plain 0.4% steel, diffusion bonded chiefly at 700°C for 30min. and to two methods of reducing the initial void-content of an interface. These methods were either: to apply a sputter-coated layer of metal to the bonding surfaces, or to disrupt the interface mechanically at the start of bonding. Although tensile properties of joints were either reduced or unaffected by metal coatings, impact properties of joints made with 5um coatings of Ni were, in the best case, increased from the control-specimen values of 20% of parent metal up to 75%. Similar coatings of pure Fe, and thicker coatings of Ni, were not beneficial. Disruption of the interface by slow oscillation at the start of bonding appeared to be beneficial to the tensile strength of the interface. Surface analysis demonstrated that there was a considerable enrichment of P, S and N at the bond interface, and indicated that they had diffused to the interface during bonding. Carbon and oxygen were also found at the interface; it was postulated that oxygen could inhibit the dispersal of voids. It is suggested that thin nickel coatings might act as partial diffusion barriers, and that surface disruption might assist void dispersal by a smearing effect. Other diffusion barrier coatings, and other ways of surface disruption, are suggested.