Résumé des résultats (Abstract)
(Anglais)
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Material Testing:
In 1999, the alloy 'A' (Ti-23Al-25Nb-0.35Si) was selected for detailed evaluation as well as for the manufacturing of final components.
In 2000, this alloy, both in the 'VGS' and in the forged condition, was examined to evaluate the corresponding material properties and the allowable temperature range for the components and to define the optimum-processing route. The specimens machined from pure VGS extrusions showed some kind of notch sensitivity behaviour under creep conditions, whereas those machined from pure forgings did not or at least less than the VGS ones.
In the year 2001, the alloy 'A' in VGS, forged and VGS+forged condition was further evaluated with respect to tensile and creep properties. The goal was to investigate the creep behaviour and notch sensitivity at 650°C, 600°C & 550°C for both types of structures. The results achieved indicated, that the forging of VGS extrusions might be the best production process. The specimens machined from forged VGS exhibited fewer tendencies for notch sensitivity than those of pure VGS did. Hence it was decided to use this process, which combines both forming types, i.e. VGS followed by forging, in order to make use of the advantages in mechanical properties offered by both of these structures.
In 2002, the material evaluation was completed. Material data sheets were produced and the material data were summarised in corresponding graphs.
Conclusions: Based on the indicated results, one can point out the following: The VGS+forging is the optimum manufacturing route. The notch sensitivity under creep conditions and the oxidation behaviour limit the application temperature to below 550°C.
Machinability trials & production of pilot components
The results of the machining trials by turning & milling were not very encouraging. The surface finish is poor and the feed path length as a measure of machinability is about fifteen times shorter than for conventional Ti-6-4 alloys. The short life of the machining tools is preventing from the economical production of an impeller by milling from a full blank.
Grinding trials of a turbine foot showed, that it is possible to machine this material by grinding. The surface and the geometry are all right. No cracks could be detected in the grinded surface. In comparison to common material, an additional cycle of rough grinding is needed, which extends the total cycle time by 10%. However the orthorhombic material requires specific cutting disks. These disks unfortunately have a detrimental effect on the diamond trim grinding wheels i.e. they shorten the life of the trim wheels by a factor of 10 thus having a significant influence on the machining costs.
The originally planned production of ABB impellers was cancelled; Turbomeca impellers will be produced instead. The contribution of ABB was changed to the production of blades from forged blanks. Due to limited size of the available test material, a gas turbine blade was machined instead of a turbocharger blade. Geometry and surface are satisfactory, however the root part of this blade is easier to machine than for a turbocharger blade. Conclusions: The production of blades by milling and grinding (root part) is possible; the machining is difficult and needs further optimisation regarding tools and parameters to achieve an economic production.
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