Mechanical Properties of Cast & Wrought Hybrid Disks
Résumé
This paper summarizes five years of joint efforts by Aubert & Duval, Ecole de Technologie Supérieure, and ISAEENSMA/Institut Pprime in developing microstructure graded turbine disks. It is mainly focused on the characterization of the mechanical properties of cast & wrought hybrid disks made of UDIMETTM 720Li and the newly developed AD730TM alloys. In such disks, a coarse grain structure (grain size greater than 100 μm) has been introduced in the rim sections, where time dependent damage processes (creep, dwell-fatigue crack growth) are the main life limiting factors, while bore sections keep a fine grain structure (grain size between 5 to 15 μm). Smooth disks with a various diameters and height were processed and investigated. Tensile properties at room temperature, 550 °C, and 700 °C, creep properties at 700 °C/750 MPa, 770 °C/540 MPa, and 850 °C/300 MPa together with low cycle fatigue properties at 550 °C were investigated as a function of radial position. Moreover, the effect of an aging heat treatment after the dual microstructure heat treatment (DMHT) has also been investigated.
From this study, it is shown that tensile and low cycle fatigue (LCF) properties, as well as creep properties at 770 °C/540 MPa and 850 °C/300 MPa, are mainly controlled by the grain size, whatever the y′ precipitation state. Moreover, a y′-subsolvus solution heat treatment is recommended after the DMHT to homogenize the intragranular microstructure through the disks and optimize tensile and LCF properties in the bore sections. Finally, creep and tensile properties in the grain size transition areas are shown to be highly dependent to the fraction of coarse grains.