Nanojoining with Ni Nanoparticles for Turbine Applications

Abstract

Thermal joining can lead to high thermal stresses, undesired structural changes, and the associated loss of properties. In the turbine industry, monocrystalline materials are often used to take advantage of their high creep resistance and heat resistance. For process-related reasons, components are mechanically machined, and the contours usually have slightly work-hardened areas due to the mechanical processing. Downstream thermal processes at temperatures above 1100°C can lead to recrystallization (Rx) at these areas, so that the properties are negatively affected. Usually, the joining temperatures for high-temperature brazing are in the range of 1200°C, both in new installations and in the case of repairs. It is therefore desirable to reduce the joining temperature without changing the choice of filler material, which can lead to susceptibility to corrosion and oxidation. According to investigations of the last years, nanojoining with nanoparticles offers great potential. The joining temperature can be lowered due to the ''surface effect.'' A considerable reduction in the size of the particles leads to a significant increase in surface atoms and thus in the specific surface area. The connection of the materials occurs predominantly due to sintering processes. After the joining process, the properties of a bulk material are available again. Mechanical properties comparable to those of brazing have already been achieved with silver nanoparticles (Hausner in WWA 56, 2015). Up to now, publications on the topic of nanojoining have largely referred to silver nanoparticles/ silver sintering. Due to the temperature application range, silver filler material cannot be used in gas turbines. Therefore, the first results of nickel nanoparticles for joining of the nickel-based superalloy PWA 1483 using induction heating are described in this paper. During joining, the parameters brazing temperature, holding time and the surface treatment of the base materials were varied. It becomes clear that the microstructure of the joint is dependent on temperature and holding time. Moreover, if the temperature is too low and holding time too short, only insufficiently sintering occurs, which leads to sample failure during the metallographic preparation. On the other hand, samples with a tensile shear strength of up to 165 MPa can be achieved with convenient joining conditions.