A study of the transient liquid phase bonding process applied to a Ag/Cu/Ag sandwich joint

Transient liquid phase (TLP) bonding is a process currently used for joining heat resistant alloys, for example nickel- and cobalt-based superalloys. It involves the formation of a liquid layer between two adjoining pieces and the formation of a solid bond as the liquid disappears during annealing at a suitable constant temperature. In the present study, a model Ag/Cu/Ag sandwich joint associated with a simple eutectic phase diagram was used to study the different stages of this process. The results confirm that the TLP bonding is a diffusional process occurring in clearly distinctive stages. The two most important stages are the widening and homogenization of the previously dissolved liquid interlayer, and the subsequent solidification and shrinking of the interlayer. Whereas the former stage involves diffusional processes both in the liquid phase and in the adjoining solids, the latter is controlled mainly by the diffusion in the solid phase. A modeling approach has been explored which shows that in most eutectic systems there exists an optimal bonding temperature corresponding to the shortest time needed for complete solidification. The results of a study on a Ag/Ag-20 wt pct Cu/Ag sandwich joint provide evidence that the use of an alloy close to the eutectic composition as an interlayer material shortens the TLP process substantially.