Papers by Author: Joachim Rösler

Abstract: Although Ni-base superalloys meet the gas turbine needs of today, they are used very close to their melting range. Demands for applications at higher temperatures are presently met partly through component cooling and application of thermal barrier coatings. However, this approach can not be sustained indefinitely unless the base metal melting temperature is also significantly increased. Rhenium addition can substantially increase the melting point in Co-base alloys and thereby provide a unique opportunity in the development of new alloys for very high temperatures – e.g. for applications at +100°C metal temperature above present day single crystal Ni-base superalloys. The design considerations behind the Co-Re alloy development are presented in this paper. Selected results from the alloy development studies are also presented.

Abstract: Diffusion brazing is a widely-used technology for the repair of cracks in hot section turbine components, mostly fabricated from single-crystalline Ni-based superalloys. Typically, braze alloys with a composition similar to the base material, enhanced by fast diffusing melting point depressants like B are used. If single-crystalline (SX) components are repaired, an epitaxial healing can be achieved, however, the filling of wide cracks in the range of 100-300 μm is difficult, since the process is completely diffusion controlled which means that wide cracks require very long hold times. If the temperature is lowered before a complete isothermal solidification has been awaited, the poor solubility of B in Ni leads to the precipitation of borides, serving as nucleation sites for stray grains. Thus, especially for the repair of wide cracks, new Ni-Mn-based braze alloys were developed which allow a very fast epitaxial healing. As B is replaced by Mn, the repair process can be significantly shortened since the epitaxial solidification is no longer diffusion controlled but can be enforced by cooling. This is due to the fact that Ni and Mn are almost completely miscible which means that the precipitation of secondary phases during solidification is eliminated. The Ni-Mn-based braze alloys were enhanced by Al, Cr and Ti to provide a sufficient high temperature strength and an appropriate oxidation behavior. Furthermore, heat treatment cycles have been developed producing a γ / γ’-microstructure very similar to the base material. In this work, results from mechanical testing of wide-gap samples which were filled with the new braze alloys are presented and discussed.

Abstract: Nickel-base superalloys like Alloy 625 are widely used in power generation applications and in the oil and gas industry due to their unique properties especially at elevated temperatures. The chip formation process of Alloy 625 is not yet well understood. Therefore, the cutting process of this alloy has been studied in detail by means of orthogonal cutting experiments at conventional cutting speeds and in the high-speed cutting regime. Alloy 625 shows a cutting parameter dependent change in the chip formation process from continuous to segmented chips. Silver has been added to Alloy 625 to improve the machinability. During machining of these modified alloys short breaking chips develop so that cutting processes are eased and can be automated.

Abstract: Single crystal Ni-base superalloys based on the  /  system are widely used in gas turbine applications. To understand the formation of  precipitates, including size distribution and growth, we performed in situ small-angle neutron scattering (SANS) measurements at elevated temperatures and - together with TEM as well as , SEM imaging - studied changes in the precipitates in short and long time scale. In the early stages, a bimodal precipitate size distribution of precipitate is observed, which (depending on the annealing temperature) changes to a cuboidal or nearly spherical morphology with almostmore or less uniform ( unimodal) size distribution. [Note: The term "more or less" is several times repeated in the text. I cannot imagine what it in fact means. Could you change it or explain in a more clear way]

Abstract: The chip formation process of four different titanium alloys has been studied in several cutting experiments. Alloys containing more than 50% of a-phase at room temperature and aged metastable b-alloys form segmented chips independent of the cutting conditions. Solution treated metastable b-alloys show a cutting parameter dependent change from continuous to segmented chip formation. Lanthanum has been added to all four alloys. The microstructure of these alloys consists of a titanium matrix and micrometer-size particles. The presence of grain boundary particles leads to enhanced grain stability at elevated temperatures. In addition, short chips are observed during metal cutting only in case pure metallic rare-earth metal particles are present.

Abstract: In rocket engine combustion chambers, the cooling channels experience extremely high temperatures and environmental attack. Thermal protection can be provided by Thermal Barrier Coatings. Due to the need of good heat conduction, the inner combustion liner is made of copper. The performance of a standard coating system for nickel based substrates is investigated on copper substrates. Thermal cycling experiments are performed on the coated samples. Due to temperature limitations of the copper substrate material, no thermally grown oxide forms at the interface of the thermal barrier coating and the bond coat. Delamination of the coatings occurs at the interface between the substrate and the bond coat due to oxide formation of the copper at uncoated edges. In real service a totally dense coating can probably not be assured which is the reason why this failure mode is of importance. Different parameters are used for thermal cycling to understand the underlying mechanisms of delamination. Furthermore, laser heating experiments account for the high thermal gradient in real service. Pilot tests which led to a delamination of the coating at the substrate interface were performed successfully.