Turbine blades in jet engines are exposed to high levels of mechanical and thermal stress. The materials used must be highly heat-resistant and possess a high level of creep and rupture strength. Nickel-based forging alloys, those high-temperature properties are based on the mechanisms of mixed crystal, separation and carbide hardening, are especially suited to this.
The profiles of turbine blade roots are manufactured with surface peripheral plunge grinding. The complex geometries and difficult machinability of nickel — based alloys result in high demands on the grinding tools and process design. Galvanically bonded cBN grinding wheels and ceramically bonded corundum tools are used for grinding. The special requirements on tool selection and the choice of parameters is described in the following.
The components are manufactured in a creep grinding process with an overall depth of cut of up to 10 mm. Due to the poor heat conductivity and ductile nature of the material, high demands are placed on chip removal and process heat. These requirements take on particular importance in tool design and the selection of cooling lubricant conditions. For this reason, grinding wheels with large chip spaces are utilised that can be set by means of the specification and dressing conditions. The geometries of the turbine blade roots partially exhibit convex radii that are subject to increased temperature influences. When selecting the marginal conditions, avoiding thermally induced external zone damage is thus a decisive criterion.
In the case of conventional grinding tools, coarse-grained discs with high porosities are used to meet the requirements. The grain size is around 60 US mesh with open structures of 13 to 15. The high initial effective surface roughness of the grinding wheel associated with the technology limits the obtainable surface quality. I practical applications however, qualities of Rz = 1.5 to 2.0 qm are usually sufficient. The dressing parameters and grinding parameters can be adjusted in relation to the target surface roughness. By working almost exclusively in the CD — grinding method, an optimally conditioned grinding wheel is constantly available during the process. This makes it possible to reach high material removal rates, which can be set via workpiece speed due to the defined depth of cut. With increasing material removal rates, the grinding power goes up. This results to a large extent in thermal energy in the workpiece, thereby increasing the danger of thermal damage. The heat action time, which decreases with rising workpiece speeds, is advantageous, as it counteracts this. Material removal rates of around Q’w = 15 mm3/mms with cutting speeds up to 45 m/s are customary.
Galvanically bonded cBN grinding wheels are distinguished by their high grain projection and thus comparatively large chip space in comparison to ceramically bonded tools. A further advantage is the favourable heat conductivity of cBN grains and of the metallic bond. In comparison to ceramically bonded corundum grinding wheels, the diameter is quasi-constant throughout the service life; the contact area is thus constant. A disadvantage of this single-layered coating, besides the relatively expensive tools, the sensitivity to clogging and wear, whereby the selection of process parameters receives enlarged significance. Moreover, a high amount of accuracy in concentricity must be guaranteed by means of an exact clamping, which is of lesser importance in the case of dressable tools. Certain geometries require an angular arrangement of the grinding wheel and can only be realised by means of a 5-axis process. This makes CD grinding impossible.