Influence of the grain size of alloy and superalloy additives on green tapes and preforms for gas turbine repair brazing

摘要

Blades and vanes of gas turbines are thermally, chemically and mechanically stressed during operation. The subsequent material removal from the surfaces results initially in a reduction of the efficiency and finally in the failure of the turbine. These turbine damages can be repaired by brazing processes with polymer bonded flexible green tapes or binder-free pre-sintered preforms consisting of brazing filler metal and Ni-based superalloy powders. As the shrinkage and the porosity of the repair products are relatively high after brazing, packing density and homogeneity of the green tapes as well as preforms can be improved by the variation of grain size and proportion of brazing filler metals and superalloy powders. The influence of the grain size distribution on the porosity of green tapes made of BNi-9 and the superalloy Mar M 247 is analyzed and the microstructure of brazed coating is investigated. Moreover, the melting behavior of the tapes in dependence of brazing temperature and time is analyzed by thermal analysis and in melting experiments at brazing process condition.