Abstract
Nickel-iron Alloy 718 is widely used for fabricating parts by superplastic deformation. Refinement of grains down to a nanostructure (NS) size improves the alloy’s processing properties. Thermomechanical treatment has been carried out to form a NS state in bulk alloy by multiple isothermal forging (MIF) at gradually decreasing temperatures. Investigation of superplastic properties and processing behavior of Alloy 718 has been performed. The alloy with a grain size of 80 nm displays superplasticity (SP) at a temperature which is lower than for a conventional fine grained alloy by about 350°C. The values of the relative elongation and the strain rate sensitivity coefficient m are 350% and 0.37, respectively. The experimental data on the influence of grain size on solid-state weldability in the range of SP have been obtained. The application of the effect of low temperature SP yields lower temperatures of superplastic forming (SPF) and pressure welding (PW) as compared with conventional SP of fine-grained material. The experiment of the combined process of SPF and PW by counter-forming of two polished sheets, demonstrates its low temperature processing feasibility using NS specimens. The SPF processing of NS sheets in a cylindrical die has been investigated. It has revealed that macro-deformation is uniform in cross and longitudinal sections. Mechanical properties of Alloy 718 in NS condition and after strengthening heat treatment have been discussed.
Highlights
Realization of the highly efficient technology of complex geometry parts by low temperature superplastic forming (SPF) becomes possible due to ultrafine grained and NS material which provides high plasticity and low flow stress at the processing temperature [1]
NS condition provides widening of temperature-strain interval of SP [1] and decrease of flow stress which is important for hard-to-deform alloys
transmission electron microscope (TEM) studies showed that, within the γ phase grains, there were disk-like γ′′ phase precipitates which were uniformly distributed
Summary
Realization of the highly efficient technology of complex geometry parts by low temperature SPF becomes possible due to ultrafine grained and NS material which provides high plasticity and low flow stress at the processing temperature [1]. NS condition provides widening of temperature-strain interval of SP [1] and decrease of flow stress which is important for hard-to-deform alloys. It is well known [1,2] that SP, for NS material, creates unique potentialities for realizing SPF or/and PW with less power. The investigations for using low temperature SP for production of complex geometry parts from nickel based alloys are urgent. The aim of the present work is to study the superplastic properties of Alloy 718 in NS condition and their influence on its formability, pressure weldability and mechanical properties
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