Abstract:

A theoretical model for the joint formation was developed for the diffusion brazing. The phenomena of dissolution and solidification were included into the model. A thermodynamic justification for the isothermal soldering occurrence in meta-stable conditions was developed. It involved the application of the criterion of higher temperature of the solid / liquid (s/l) interface. The dissolution of the filler metal in the substrate was described by the solute concentration within the dissolution zone (liquid film) situated at the substrate surface. The selection of the  parameter was justified by the Thermocalc calculation of the Ni-Al phase diagram for meta-stable equilibrium. According to the model assumptions, the solidification was accompanied by undercooled peritectic reactions resulting in formation of the intermetallic phases. The average Al – solute concentration measured across a given Al3Ni2/Al3Ni/Al3Ni2 joint confirmed that the solute concentration was conserved within the joint sub-layers. The Ni-Al phase diagram for meta-stable equilibrium referred to the solidification was also calculated by means of the Thermocalc Software. It allowed to locate the solidification path, s/l interface path and redistribution path onto the mentioned diagram. Superposition of both calculated phase diagrams was also given to show that the joint formation occurred cyclically under the meta-stable conditions. The measured  parameter was introduced into the proposed model as an initial condition in order to solve the formulated differential equation describing solute redistribution after solidification. Isothermal formation of the Ni/Al/Ni joints has been performed at different temperatures. The following  temperatures have been applied: 700, 750, 800, 850, 900, 950, 1000 0C. The solidification was arrested and the actual morphologies frozen. It allowed to make a measurement of the Al-solute concentration across each joint by means of the  EDAX micro-analyzer to estimate its average concentration, .  Regardless the  - temperature, the solidification path was always the same.

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