Meta-stable conditions of diffusion brazing

Abstract

A thermodynamic justification for the joint formation is developed on the basis of the diffusion brazing of the Ni/Al/Ni system. The phenomena of dissolution and solidification were included into the description. The first solid/solid transformation is also discussed. Mainly, a description for the isothermal brazing occurrence in the meta-stable conditions is developed. It involves the application of the criterion of higher temperature of the solid/liquid (s/l) interface. The dissolution of the filler metal in the substrate is described by the N0 - solute concentration within the dissolution zone (liquid film) distinguished at the substrate surface. The selection of the N0-parameter by the dissolution is justified by the Thermocalc calculation of the Ni-Al phase diagram for meta-stable equilibrium. According to the model assumptions, the solidification is accompanied by partitioning or by undercooled peritectic reaction resulting in formation of the intermetallic phase. The average Al - solute concentration measured across some Al3Ni2/Al3Ni/Al3Ni2 joints confirms that the N0 - solute concentration is conserved within the analyzed joint sub-layers. The Ni-Al phase diagram for meta-stable equilibrium referred to the solidification is also calculated by means of the Thermocalc Software. It allows locating the solidification path, s/l interface path and redistribution path onto the mentioned diagram. Superposition of both calculated phase diagrams is also given to show that the joint formation occurs cyclically under the meta-stable conditions.