Rinat V. Safiullin

Advanced superplastic forming and diffusion bonding of titanium alloy

Abstract This paper reports on investigations of the solid state formability and weldability of Ti–6Al–4V titanium alloy sheet with submicrocrystalline structure (grain size 0·5 and 0·8 μm) under superplastic conditions at temperatures of 700 and 800°C. It has been established that the submicrocrystalline condition of the sheet material reduces the temperature of superplastic forming (SPF) by 200°C and of diffusion bonding (DB) by 100°C, compared to the conventional superplastic forming Ti–6Al–4V material with microcrystalline structure (grain size 3 μm). Unlike commercially available sheet, the submicron grained sheet is characterised by isotropic formability. Submicrocrystalline processed Ti–6Al–4V sheet can be successfully used in SPF/DB technology. The temperature range of superplastic forming can be between 700 and 800°C, and diffusion bonding at 800°C.

Mathematical modelling of the superplastic forming of a long rectangular sheet

Abstract Standard finite element software (ANSYS FEM-code) is used to model the superplastic forming of a superplastic sheet alloy into a rectangular die. As distinct from other known approaches the boundary value problem is stated in the present study in terms of the theory of creep. The results of finite element calculations are found to be in a good agreement with corresponding analytical solutions and experimental data for titanium sheet alloy Ti–6Al–4V. The approach suggested can be used in practice for estimating the current geometry and the thickness of the dome and the time intervals both for the constant gas pressure forming and for the constant strain rate forming.

On the model of solid state joint formation under superplastic forming conditions

The peculiarities of solid state joint (SSJ) formation under conditions of superplastic forming (SPF) were investigated for the titanium alloy VT6S (Ti-6Al-4V). The influence of annealing and SPF on the change of state of the alloy surface was considered. A significant role of grain boundary sliding (GBS) in the formation of both surface microrelief and SSJ was established. It was shown that SSJ formation under SPF conditions is primarily a deformation process. Corresponding schemes of the appearance of surface microrelief and SSJ formation are proposed.

Improved Mechanical Properties of Ti2AlNb-Based Intermetallic Alloys and Composites

Mechanical properties of a Ti2AlNb-based intermetallic alloy both at room and elevated temperatures were considerably improved due to formation of a homogeneous microstructure with the average grain size of about 300 nm. At room temperature, elongations up to 25% were obtained and the ultimate strength reached 1400 MPa. The alloy exhibited superplastic behavior in the temperature range of 850-1000°C. The maximum elongation of 930% and steady state flow stress 50 of about 125 MPa were obtained at 900°C and strain rate of 4.210-3 s-1. The nanostructured material was used for production of intermetallic sheets and multilayer composite plates consisting of alternating layers of orthorhombic intermetallic and commercial high temperature titanium alloy. Ti2AlNb-based sheets and composites exhibited improved mechanical properties.

The use of nanostructured materials and nanotechnologies for the elaboration of hollow structures

The principles of fabricating nanostructured bulk and sheet materials by the methods of multiple isothermal forging and warm rolling are formulated. The first method, which is based on dynamic recrystallization, allows one to obtain bulk materials with a uniform nanostructure. The second method makes it possible to transform the latter into a sheet semi-product re taining this nanostructure. Using the example of titanium alloy, it is demonstrated that, when reducing the grain size down to the nanostructure level in a sheet material, technological operations on it—such as pressure welding and superplastic forming—can be performed at significantly lower temperatures than with the use of sheet materials with conventional fine-grained structures. This opens up new possibilities for fabricating hollow structures by a progressive method combining pressure welding and superplastic forming. Using the example of hollow blades, the development of nanotechnologies and nanomaterials for use in industry is demonstrated.

Constitutive Relations for Superplastic Flow Modeling from Two Axial Loading Experiments

A critical analysis of approaches to find constitutive relations from the tests on uniaxial and two axial loading is presented. Then we report on the methodology of fitting parameters of constitutive relations for superplastic forming based on the results of cone cup and elongated wedge cup tests. Optimal cone and wedge angles are estimated analytically and also from the results of finite element simulations. Our methodology is based on comparison of the experimental results and the results of numerical simulations of the corresponding tests in frame of the commercial software such as ANSYS10.0 (ED). We carry out numerical simulations of the cone cup and the wedge cup tests for different sets of parameters of the chosen constitutive relation and different values of friction coefficient in order to reproduce the experimental results. The parameters corresponding to the best fit of the experimental data are then recommended for the use in computer simulations of the superplastic forming of commercial products.

Microstructure and Properties of Nanostructured Alloy 718

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.

Superplastic Properties and Superplastic Forming/Diffusion Bonding of γ-TiAl+α2-Ti3Al Sheet Materials

The as-cast and hot worked microstructures of the newly developed β-solidifying ingot-metallurgy Ti-45Al-X (Nb,Mo,B) alloy and its superplastic properties in the hot worked condition have been studied. The obtained experimental findings were used for research of superplastic forming and diffusion bonding of sheet products, which were cut out of hot worked preform by spark cutting. It was shown that superplastic forming might be successfully applied to the obtained fine-grained sheet materials. Relatively low bonding temperatures and pressures were found to be sufficient to achieve sound joints in the sheet material.

Highly Superplastic Ti-6AI-4V sheet for Superplastic Forming and Diffusion Bonding

O ne approac h to development of metal s wit h improved behavior is pr oduct o f shee t material with very fine grain sizes. These materials have an average grain size of less th an lum and show enhanced physical and mechanic al properti es such as 2 to 3 times increased strength and improved fatigue resistanc e. They also show superplastic behavior at tempe ratures much below th e temperature range typical for materials with micronsized grains; thi s can lead to a decrease in processing tool costs and material sa ving due to reduced co n ta mina t io n . The low temperature superplasticity can be used for structure prod uction by superplastic forming/diffusion bonding (SPF/DB) at much lower temperatures (600-700°C) as compared to th at used presently (8S0-9S0°C). It is very important for SPF/D B th at shee t preforms do not have mechanical anisotropy. The submicrocrystalline or even a nan ocrystalline structure in bulk mat erial can be produced by severe plastic deformation by the use of methods such as equal cha nne l angular extrusion or multi step isotherma l forging. In the present programme a Ti-6AI -4V shee t has been developed with a submicrocrystalline structure and isot ropic prope rties are presented. Room temperature mechanic al properti es and superplastic behav ior of this shee t were also investigated and are shown.

Promises of Low-Temperature Superplasticity for the Enhanced Production of Hollow Titanium Components

Application of the conventional superplasticity (SP) allows producing the unique hollow structures. One remarkable example is the hollow titanium blade of the air engine fan produced by Rolls-Royce. However, high temperature titanium alloys processing (~ 927 °С) limits wide industrial application of the conventional SP. The solution of the mentioned issue can be found through the application of low-temperature SP. Ti-6Al-4V alloy with ultrafine grain structure at the temperature range of 600 800 °С has enough ductility resources for the superplastic forming (SPF) of the parts with the complicated shape. The formation of pores in Ti-6Al-4V alloy at uniaxial and biaxial tension at the temperature 600 °С is not observed. The effect of low-temperature SP also allows lowering pressure welding (PW) temperature essentially. Herewith, there is a possibility to produce the hollow parts by the combination of SPF and PW. The main goal is the optimization of the technological scheme and processing temperature. The use of the low-temperature SP provides high quality of hollow components such as blades.