A. A. Kruglov

Superplastic forming of a spherical shell out a welded envelope

Abstract Superplastic free forming of edge welded titanium envelopes is investigated theoretically and experimentally. The envelope is transformed into a spherical shell without clamping of its edges so that the final diameter of the spherical shell, D s , produced is less than the initial diameter of the envelope, D 0 . Mathematical modelling of the process under consideration is based on the principal equations of the membrane theory of shells. Experimental justification of the model suggested is carried out using the welded envelopes of various dimensions made from different commercial titanium alloys. Comparison of the theoretical predictions with corresponding experimental data is carried out obtaining good agreement. It is found that the upper limit of the value of D 0 / D s is equal to 1.25, which corresponds to the case of superplastic forming of an isotropic envelope when the influence of the welded joint is negligible. It is shown that superplastic free forming makes it possible to produce spherical shells having more uniform thickness distribution as compared with conventional procedure where the envelope is clamped rigidly along its periphery. Technological operations providing the above-mentioned advantages of superplastic free forming include the suitable choice of D 0 , the correct arrangement of the sheets and an appropriate method of welding. The model developed enables one to calculate the pressure–time cycle, thickness distribution and initial geometry of the envelope.

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.

Micromechanisms of deformation and fracture in a VT6 titanium laminate under impact load

The paper studies the phase composition, microstructure, and mechanisms of plastic deformation and fracture under impact load in a laminate obtained by pressure welding of VT6 titanium alloy sheets. Under impact loading at 20 and -196°C, the material is delaminated into sheet piles with attendant changes in their fracture rate. At fracture surfaces, the initial crystal structure experiences structural phase decomposition which results in dynamic rotations. In fracture and delamination sublayers, the material is fragmented. The effects are more pronounced at Tdef = -196°C.

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.

Pressure welding of Ti-6Al-4V alloy under conditions of low temperature superplasticity

Ключевые слова: сварка давлением, титановый сплав, низкотемпературная сверхпластичность. The results of pressure welding experiments of VT6 titanium alloy in low-temperature superplasticity conditions are presented. It is shown that the lower is welding temperature, the higher should be the value of the pressure applied, regarding the flow stress at this temperature. The quality of the solid bonding of VT6 titanium alloy essentially depends on the strain.

Prospects for the application of nanostructured titanium alloys in mechanical engineering

The results of the application of nanostructured titanium alloys, in particular the VT6 alloy, for the manufacture of hollow structures by superplastic forming and by a method combining it with pressure welding, are presented. Possibilities of the application of nanostructured titanium alloys are shown by the example of the formation of dome-shaped shells, models of a hollow blade of a jet engine fan, and weld-fabricated semi products.

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.

On the formation of pores in VT6 titanium alloy under low temperature superplasticity deformation

Проведены эксперименты по одноосному растяжению и сверхпластической формовке листовых заготовок из ультрамелкозернистого титанового сплава ВТ6 с исходным средним размером зерна 0,2 мкм в температурно-скоростных условиях низкотемпературной сверхпластичности при температурах 600 и 650оС. На установившейся стадии сверхпластической деформации вплоть до 600% на растяжение пор в образцах сплава ВТ6 не обнаружено. Отмечен интенсивный рост зерен α и β-фаз, стимулированный сверхпластической деформацией.

Impact toughness of layered VT6 alloy semiproducts

Layered semiproducts produced by pressure welding of sheet workpieces of a VT6 titanium alloy are studied. Possible methods of achieving isotropic mechanical properties of the semiproducts are discussed. The pores that are present in solid-phase joint zones are found not to influence the impact toughness of the samples in which layers lie perpendicular to a notch. The fracture surface has a ductile character with certain fracture zones.

Resistance welding of components of submicrocrystalline titanium alloy VT6 in the manufacture of multilayered cellular structures

The combination of superplastic forming (SPF) and pressure welding (PW) is used in the manufacture of multilayered cellular structures from materials, capable of superplastic deformation, mostly titanium alloys. The SPF/PW method makes it possible to produce wingshaped and casing sections, consisting of the plating with a filler in the form of stiffeners used in different areas of engineering: aerospace, petrochemical, shipbuilding, and transport, etc. the development of titanium alloys with the submicrocrystalline (SMC) and nanocrystalline (NC) structures, characterized by the unique combination of the physical–mechanical properties, opens new possibilities for the development of the manufacture of multilayered cellular structures. In the manufacture of a multilayered cellular structure, two sheet components produced from a titanium alloy, forming a filler, are bonded together by resistance seam welding (RSW) in accordance with the drawing. The cavity between the components is sealed and a supply pipeline for the working medium (argon) is installed and the entire assembly is placed in the die between the sheet components of the plating. After heating of the die to the temperature of superplastic deformation, the components are shaped by supplying gas into a cavity between them. The superplastic deformation conditions are established as a result of SPF at a specific temperature and deformation rate, ensured by the controlled gas pressure. The effect of pressure from the sheet components of the filler results in the formation of a cell whose shape is determined by the configuration of the welded joints and the distance between the sheet components of the plating. During superplastic deformation, the cells come into contact with each other and with the components of the plating. The gas pressure, after the completion of forming, is increased to the value required for the formation of a solid-phase joint between the cells and the plating and this is followed by holding under pressure leading, in the final analysis, to the formation of a monolithic cellular structure. Another variant of the superplastic deformation/PW is based on forming in the sheet components of the plating. After producing the shape of the die (Figure 1(a)), the sheet components of the filler are subjected to forming (Figure 1(b)), as in the former case. In most cases, the sheet components of the filler are produced by RSW and also laser and electron beam welding. The advantage of RSW is not only the high productivity, economic efficiency, and possibility of producing almost any type of joint but also the formation of the structure of the welded joint which greatly differs from any other welding method. It is well known that the joint produced by RSW consists of several zones: the cast nugget and the sealing belt, i.e. the zone in which bonding takes place in the plastic condition (without melting) and the bonding zone in which the joint reaches only the level of physical contact. It should be mentioned that the plastic properties of these zones and the strength of the joint in the zones change in relation to the distance from the cast nugget. One of the problems in the manufacture of cellular structures is that the forming of the filler may result in the fracture of the deformed component in the vicinity of the welded joint. In addition to this, the wall of the cell in the finished structure may contain stress concentrators as a result of large changes in the cross-section, associated with the presence of the welded joint. To prevent fracture of the deformed component in SPF, it has been proposed to join components of the filler by welded joints with the minimum possible width (US patent 4217397). The results of simulation of the formation of the cell with the welded joint in the form of a non-plastic zone with the thickness equal to that of the sheet components, published in Kaibyshev, showed that the area in the vicinity of the welded joint is characterized by the localization of deformation which may result in the fracture of the deformed sheet component of the filler of the cellular structure. Taking this into account, it has been proposed to join sheet components of the filler by welded joints with the formation of the cast zone with the thickness not exceeding 0.7 of the thickness of the sheet component of the filler (Russian Federation patent 2170636). At the same time, the technical literature does not contain any recommendations for the selection of the conditions of RSW, which take into account the specific