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.

M-Cr-Al-Y and in particular Fe-Cr-Al-Y alloys with high aluminium matrix content have a tendency to form thin, stable and tightly adherent alumina scales even at low oxygen partial pressures. This forms the basis of their superior hot gas oxidation, carburization and sulfidation resistance when used at high temperatures. However, the same tendency makes the alloys more difficult to braze because the easily formed and highly stable ceramic surface layers significantly reduce wettability and hence braze flow. Fe-Cr-Al-Y alloys have recently been suggested as promising alloys for use in gas turbine engines as abradable honeycomb gas path seals. This paper reviews the vacuum brazing of honeycombs made from highly alloyed Fe-Cr-Al-Y foils to metal backing members. Most suitable Fe-Cr-Al-Y materials, commercial braze filler alloys and braze cycles are presented. Emphasis is placed on industrial equipment rather than laboratory vacuum furnaces. Brazing under high vacuum conditions in all-metal furnaces is recommended as a brazing procedure for honeycomb made from MI 2100, which is high in aluminium content, to various commonly used carrier structure alloys.

The present article demonstrates the importance of metallography as an indispensable tool for process control as well as for establishment of basic principles for the process development based on a concrete practical example from the field of brazing of gas turbine components. This example serves to identify and prove the effects of differences in the thermal expansion behavior of non-similar materials on the braze joint by means of metallographic examinations. The knowledge acquired is used to realize braze connections of respective material composites.