Papers by Keyword: High Temperature Brazing

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: Continuous process brazing in shielding-gas furnaces is tailor-made for manufacturing mass production components by means of brazing technology. Which brazing tasks can actually be carried out by a shielding-gas furnace, depend on many ancillary conditions. In particular these are, besides the component size and joint geometry, the base materials and brazing filler metals as well as the material specific process parameters which are to be maintained in a continuous furnace in order that a process assured brazing can be guaranteed. In this context, the activation of the component's surfaces play a central role for wetting with the braze melt. Within the scope of this contribution, the variables and ancillary conditions concerning this matter are presented and discussed using the brazing process on stainless steels as an example.

Abstract: Joining of SiC ceramic to graphite is important from both technical and economical points of view. High temperature brazing of recrystallized SiC ceramic to high strength graphite has been realized using Ni-51Cr (consisting of Ni + 51wt% Cr powders) powders as filler. The obtained maximum three-point bending strength of joints is 32.3MPa, which is equal to 80.8% of the strength of the graphite. Microstructure and phase analysis reveals that interdiffusions and chemical reactions take place in the weld zone. A reaction layer and an interlayer form in the interfacial area. The reaction layer, of which the thickness is about 60-100μm, is contacted with the SiC ceramic. The interlayer with the thickness of about 200μm exists between the graphite and the reaction layer. The reaction layer is mainly composed of Ni2Si, while the interlayer is mainly composed of Cr23C6 and Ni2Si.

Abstract: Joining of ceramics is of importance from both technical and economical points of view. Brazing is a widely used process to join ceramics. In order to increase the working temperature and weld strength of joints, a high temperature brazing process using Ni-Cr-SiC powders (consisting of Ni, Cr and SiC powders) as filler to join recrystallized SiC ceramic has been investigated. The obtained optimized technological parameters are joining temperature of 1360°C, holding time of 5min and filler mass of 280mg. Under these conditions the maximum relative bending strength of joints, 70.5%, is achieved. Microstructure and phase analysis reveals that interdiffusions and chemical reactions take place in the weld zone. A reaction layer, of which the major phase is Ni2Si, exists between the welding base material SiC ceramic and the filler reaction product layer, called as interlayer, of which the major phase is Cr23C6.

Abstract: In this paper, the disadvantages of the current CBN (Cubic Boron Nitride) grinding wheels were firstly introduced briefly, for indicating that it was very urgent and important to develop new kinds of grinding wheels with excellent performance to replace the conventional wheels. Then high temperature brazing experiments of monolayer CBN wheels with Ag-Cu-Ti filler alloy were carried out. The result shows that the filler alloy has good wetting capability towards CBN grits. The results of scanning electron microscope (SEM) and energy dispersion spectrometer (EDS), as well X-ray diffraction (XRD) analysis show that, just because during brazing titanium atoms in filler alloys segregated preferentially to the surface of the CBN to form Ti-nitride or Ti-boride layer by reaction between titanium atoms and nitride and boron atoms at elevated temperature, strong chemical joining was formed in the interface between CBN grits and filler alloys. Finally, the contrastive grinding experiments were performed between the monolayer brazed CBN grinding wheels and the electroplated ones. The results show that the brazed wheels have more excellent performance than the latter.