The stress-strain state and the microstructure in disk-shaft solid-phase bonds of dissimilar nickel-based alloys

Abstract

Finite-element calculations have been carried out to determine the strain distribution formed during welding by a combination of pressure and shear in samples imitating the constituent parts of a “disk-shaft” type gas-turbine engine bimetallic component made of heterophase nickel-based superalloys. A physical simulation of this process has also been carried out. Computer simulations were accomplished using a two-dimensional axysymmetric model by means of DEFORM-2D package. EP975 and EK79 alloys with ultrafine-grained (UFG) structure were chosen as the materials for the disk and shaft, respectively. The initial strain rate was 10-5 s-1, and the welding temperature 1100°C. The behavior of materials was described by experimental curves obtained for EP975 and EK79 alloys under a uniaxial compression at the welding temperature. According to known experimental data, shear strain near the welded surfaces improves the solid-phase bond quality and strength. Additionally, the solid-phase bond quality is enhanced by grain refinement, which leads to an increase of the area of grain boundaries and to accelerated grain boundary diffusion. The results of computer simulations have revealed that in order to increase the area of regions with large shear strain values one has to increase the displacement of the shaft relative to the disk. From physical simulation, it has been concluded that a high quality solid-phase bond formation is promoted by a small shear strain of EK79 and EP975 alloys with UFG structure under superplasticity conditions. Basing on the studies carried out, a possibility of obtaining a "disk-shaft" type part from EP975 and EK79 nickel alloys with a preliminarily processed UFG structure using solid-phase bonding by means of pressure and shear welding has been shown.