Densification and microstructural evolution during co-sintering of Ni-base superalloy powders

Abstract

This work presents the effect of co-sintering on the densification and microstructural evolution in the two-layer stepwise graded composite of INCONEL 718 and INCONEL 625 superalloys. A pressure-less co-sintering method in conjunction with a powder layering technique was used. The sintering was carried out in solid state and liquid phase in temperature ranging from 1260 °C through 1300 °C for 60 minutes in a low pressure of an argon atmosphere. Nonisothermal sintering behavior was also examined by dilatometric analysis. Similarly, the sintering response of the individual layers was characterized. The results reveal an enhanced densification rate during co-sintering of the composite layers. Sintering at low temperatures (T<1270 °C) led to formation of a narrow dense region at the interface while the sintered layers were porous. In contrast, sintering at higher temperatures resulted in significant densification of the layers, although formation of a pore band at the interface was realized. It is suggested that the strain rate incompatibility between the layers during sintering induces mismatch stresses at the interface region. The resulting strain energy influences the densification of the boundary region by exerting a pressure on the sintering neck and affecting the material flow during liquid phase sintering. The induced densification of the bilayer configuration was experimentally measured and was shown to be in good agreement with the strain rate incompatibility.