Andreas Volek

Diffusion Brazing of Single Crystalline Nickel Base Superalloys Using Boron Free Nickel Base Braze Alloys

Brazing is a well established repair technique for high temperature components in both industrial gas turbines and aero engines. Conventional nickel base braze alloys contain boron or silicon as melting point depressing elements. The major benefit of boron and silicon compared to other melting point depressants is its large effect on the melting point and its high diffusion coefficient in nickel base superalloys. However these elements promote precipitation of undesired brittle phases during the brazing process. To avoid these phases, transient liquid phase bonding in combination with boron and silicon free brazing alloys will be examined in this work. The influence of the brazing temperature on solidification and diffusion behaviour during transient liquid phase bonding for a single crystalline first generation and a second generation superalloy will be reported. Our experiments show that isothermal solidification without precipitation of brittle phases in the braze joint or the base material can be achieved. The brazed joint consists of fine γ/γ´ microstructure. EBSD measurements demonstrated that the single crystalline orientation of the base material was maintained throughout the joint. Electron probe micro analysis is used to characterize the diffusion behaviour. Solidification velocity will be compared with the theory of transient liquid phase bonding established by Tuah-Poku [1].

Powder Injection Moulding (PIM) of Mesophase Carbon with Water-Based Binders

Since there are no net-shape techniques for complex parts made of mesophase carbon available yet, this work focuses on a powder injection moulding (PIM) approach. The single biggest problem to overcome here is the overlap of debindering and pyrolysis/sintering of mesocarbon when using conventional binders, causing high porosity and cracks. Water-based binders with agar as gelling agent can avoid this problem effectively by removing the binder in an optimized drying step. The subsequent sintering can then be carried out at high heating rates, leading to better densification and good mechanical properties. Furthermore, the dependency of the rheological properties of the water-based feedstock on its water content is investigated.

Influence of Pressure and Heating Rate on the Mechanical Properties of Mesophase Based Sintered Parts

This work focuses on the influence of different sintering parameters on the mechanical properties of pressed and sintered mesophase based parts. In a first approach we analyzed the influence of the heating rate on the sintering process and in subsequent experiments we applied inert gas pressure up to 1000 bar.