Lehrstuhl Werkstoffkunde und Technologie der Metalle
UniversitÄt Erlangen-NÜrnberg
Prof. Dr.-Ing. Robert F. Singer / Prof. Dr.-Ing. habil. Carolin KÖrner

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Hochtemperaturwerkstoffe

Directionally solidified (DS) and single crystal (SC) superalloys

Important applications for DS and SC nickel-base superalloys are aircraft turbines and combined-cycle power generation systems. Research at WTM mainly concentrates on new production processes with higher productivity, the castability of directionally solidified alloys and repair technologies for thermomechanically stressed turbine blades.

1. Advanced casting processes for DS and SC turbine blades (M. Lamm)

In cooperation with Doncasters Precision Castings/DPC a new casting process using liquid metal cooling (LMC) is being developed which allows production of large DS and SC turbine blades for stationary gas turbines. The principle of this process is shown in Fig. 1: A ceramic mould filled with molten superalloy is lowered from a hot zone at a temperature of about 1500°C through a dynamic baffle and immersed into a liquid metal cooling bath. Investigations consider properties of the dynamic baffle, fluid flow in the cooling bath, optimized mould materials etc. for the production of components of industrial size and shape. The advantages of the LMC-process are e.g. higher thermal gradients and withdrawal rates. This leads to an improved microstructure such as decreased dendrite arm spacing and microporosity.

To confirm the effects of the improved microstructure samples are heat treated, mechanically tested and compared with samples from an industry standard casting method. Fig. 2 shows a rhenium intensity map measured by electron probe microanalysis (EPMA) from an as-cast LMC sample. The blue colour indicates low amounts of rhenium whereas the pink colour indicates high amounts of rhenium. As one can see clearly rhenium enriches in the dendrite core. Thus a heat treatment must be applied to remove the segregation pattern for homogeneous mechanical properties.

Fig. 1	Fig. 2

2. Castability of DS superalloys (Y. Zhou)

The aim of this work is to investigate the grain boundary cracking phenomena during casting for certain DS nickel-base superalloys, and to develop new alloy compositions and microstructures which are less susceptible to cracking. It was found that grain boundary cohesion in the final stages of solidification is of particular importance in order to reduce hot cracking susceptibility and that grain boundary cohesion is affected by remaining liquid geometry. The microstructure of remaining liquid is dominated by the alloy compositions and casting conditions. At low solidification rates continuous films of liquid remain between two coalescing grain (see Fig. 3a). Thermal tensions due to shrinkage during cooling promote the formation of hot cracks at this point. High solidification rate leads to finely dispersed and discontinuous remaining liquid (see Fig. 3b ). As continuous films of remaining liquid are avoided, dendrite tips can touch each other at the end of solidification and the strength of the solidifying skeleton is increased.

Fig. 3a	Fig. 3b

3. Diffusion brazing of single-crystalline turbine blades (P. Heinz)

The aim of this research activity is to develop a new braze material which is applicable for repair brazing of thermomechanically degenerated single crystalline turbine blades made from nickel-base superalloys. Efficient repair technologies applicable for such materials are not yet provided but are mandatory to reduce life cycle costs of gas turbine power plants. The developed braze alloy should be free of Boron which is commonly used as melting point depressant but on the other side leads to the formation of undesired brittle phases. Furthermore the braze alloy composition should be adapted to the compositions of the single crystalline superalloys PWA 1483 und PWA 1484. An attempt will be made to use an electron beam for partial heating which will further improve the epitaxial solidification in the fusion line. Fig. 4 shows a brazed crack and the achieved single crystalline microstructure.

Fig. 4

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Letzte Änderung am 15.09.2010