Superplastic Conditions Research Articles

Relationship between Transport Phenomena and Characteristics of the Cluster Structure

Using a statistical approach, we have estimated the size of water clusters and the number of free molecules participating in transport phenomena. The sizes of clusters and the number of molecules in them, as well as the number of free water molecules and their mean free paths, have been determined using experimental data on the temperature dependence of viscosity and density of water. It has been concluded that a temperature of 36.6°C is a special point in the range of 0–100°C. At temperatures higher than this value, the binding energy of a molecule with a cluster decreases abruptly, while the concentration of free molecules sharply increases. Quantitative data have been compared with the parameters of the superplastic state of titanium. As an example, we consider metabolism in functioning of erythrocytes of blood in an adult human.

Development of an Experimental Technique and Evaluate Limit of Plastic Deformation of Titanium Alloy OT4-0 under Superplastic Conditions

The method and unit for experimental research of membrane formation from titanium alloys under conditions of superplastic is developed, which allows to determine necessary parameters of deformation. The unit is designed to deform titanium sheets in isothermal conditions by applying a uniform distributed load. When membrane is formed by a gas medium, a biaxial tension is realized. This type of test is used to assess the plastic properties of metals. In this work, the limiting deformations of the sheet titanium alloy OT4-0 have been evaluated in the formation of the membrane. Installed during the process of superplastic deformation the connection between the external parameters of deformation (force, temperature) with the depth of formation of the membrane in time and strain distribution along its generatrix.

Formation and Structure Features of the Weld Joints Made by Friction Stir Welding

The microstructure of the weld core and the heat-affected zone, as well as the zone affected by thermal strain located between them, formed during friction stir welding (FSW) of aluminum alloys, copper alloys, 12Kh18N10T steel, and OT4-1, VT-1 alloys is investigated. The core of the welded joints at FSW is formed in the structural superplasticity mode, and its structure has a shear-banded structure of the “onion pattern” type containing a large number of lamellae forming the shear bands under the influence of the movable welding tool. Attainment of the superplastic state during formation of the weld core is ensured by the gradual flow of various plastic deformation mechanisms in the mode of simple, collective, and anomalous dynamic recrystallization initiated by the dynamic recovery and polygonization processes with transition to post-dynamic recrystallization by the Bailey–Hirsch and Kahn–Burgers–Taylor mechanisms. The superplastic state is also maintained owing to twinning recrystallization (copper, steel, Ti and its alloys) and as consequence of α → γ or α → β phase transformations for a group of metals and alloys possessing multiple forms (high-alloy steel, titanium alloy).

SUPERPLASTIC DEFORMATION OF THE AK4-1 ALLOY WITH A SURFACE LAYER MELTED BY ELECTRON PULSE BEAM

Specimens of the aluminum alloy AK4-1, previously irradiated by a pulsed electron beam, are deformed under the conditions of superplasticity. The features of structural changes in the course of superplastic deformation of pre-irradiated specimens are studied. It is shown that the deformation processes occurring in the main part of the specimen and in the layer, melted by a pulsed electron beam, have fundamental differences.

The Influence of the Friction Stir Welding Parameters on the Formation of Welded Joint of Aluminum and Copper Alloys

In the research, welded joints of aluminum and copper alloys were obtained by friction stir welding (FSW). The microstructure, mechanical properties and phase components of welded joints obtained by FSW were studied using various methods of metal physical analysis including X-ray diffraction, mechanical tests for tearing and bending, etc. Welded joints of copper and aluminum parts were formed in a solid state by transferring metals into superplastic state (SPS), forming in the zone weld nugget (WN) a characteristic shear-banded layered structure with alternating layers. The mixed structure, with characteristic inclusions of Cu and Al, was formed on the aluminum side from the side of the WN.

Mathematical Modeling of Process of the Friction Stir Welding

The mathematical model of temperature dynamics in the process of friction stir welding (FSW) is developed. The model allows to take into account changes in the input power and the temperature distribution in the sample during the FSW conversion of the welded material to the superplastic state and at the melting point. Temperature measurements were made at various points in the sample, and the power consumption of the motor of the milling machine was measured. This model is also used to estimate the linear velocity of the FSW, which is in satisfactory agreement with the experimental data. The model allows to establish the connection of the power consumption with the linear velocity of the FSW.

Theory of Superplasticity and Fatigue of Polycrystalline Materials Based on Nanomechanics of Fracturing and Failure

Fracture nanomechanics is the study of the interconnected process of the growth and birth of cracks and dislocations in the nanoscale. In this paper, it is applied to superplasticity and fatigue of metals and other polycrystalline materials in order to derive the basic equations describing some main features of these phenomena, namely, the fatigue threshold and the enormous neck-free superplastic elongation. It is shown that in most metals and alloys the fatigue threshold is greater than one per cent of the value of fracture toughness. Using the concepts of fracture nanomechanics, we study the superplastic deformation and fracturing of polycrystalline materials under uniaxial extension and calculate the neck-free elongation to failure in terms of strain rate, stress and temperature. Then, we determine the optimum strain rate of the maximum superplastic elongation in terms of temperature, creep index and other material constants. Further, we estimate the critical size o f ultrafine grains necessary to stop the growth of microcracks and open way to the superplastic flow, and find the superplastic deformation of grains, their maximum-possible elongation and the activation energy of superplastic state. Also, we introduce the dimensionless A-number in order to characterize the capability of different materials in yielding the superplastic flow. A t a very high elongation the alloying boundary of grains proves to be broken by a periodical system o f dead fractures of some definite period. It is shown that experimental results of the testing of the Pb-62% Sn eutectic alloy and Zn-22% A l eutectoid alloy at T = 473 K have substantially supported the theory of superplasticity advanced herewith.

Связь явлений переноса с характеристиками кластерной структуры воды

AbstractUsing a statistical approach, we have estimated the size of water clusters and the number of free molecules participating in transport phenomena. The sizes of clusters and the number of molecules in them, as well as the number of free water molecules and their mean free paths, have been determined using experimental data on the temperature dependence of viscosity and density of water. It has been concluded that a temperature of 36.6°C is a special point in the range of 0–100°C. At temperatures higher than this value, the binding energy of a molecule with a cluster decreases abruptly, while the concentration of free molecules sharply increases. Quantitative data have been compared with the parameters of the superplastic state of titanium. As an example, we consider metabolism in functioning of erythrocytes of blood in an adult human.

On structural and phase transitions in aluminum alloys

Mathematical modelling of behavior of group of the industrial aluminum alloys with initial varying grain size structure showing superplastic properties in certain temperature and strain rate ranges is presented. It is known that the structural superplasticity is connected with facilitated by fine-grained structure formation (1…10 microns) at the preliminary stage. However, for realization of superplasticity of “dynamic type” there has to be a replacement of an initial varying grain size structure state of material with another, ready for superplasticity. Therefore the used definition “the dynamic superplasticity” reflects consecutive change of states which happens in material with initial varying grain size structure under the changing temperature-rate conditions: initial varying grain size → equiaxed fine-grained structure (4…7 microns) formed under the temperature-rate conditions of superplasticity → coarse-grained at further increase in strain rate. These changes are caused by simultaneous action of deformation rates and structural (phase) transitions of evolutionary type in open nonequilibrium systems. In particular, for the considered commercial aluminum alloys with initial varying grain size structure such irreversible transition is dynamic recrystallization. The association of deformation process with metal flow that has irreversible structural and phase transition of indistinct type at one of the stages, allows using synergetic approach. The mathematical model formulated from positions of solid mechanics use allow to research nonequilibrium system reaction to behavior of thermodynamic response functions – the specific heat and entropy – and to establish implementations features of the irreversible indistinct phase transitions observed in the conditions of dynamic superplasticity for aluminum alloys.

Effect of surface relief on solid phase joining of heat-resistant nickel superalloys

ABSTRACTPressure welding (PW) is an advanced method for obtaining solid phase joints (SPJs). Using physical and finite element modelling (FEM) it is shown that PW under superplasticity conditions provides SPJs between dissimilar cast single-crystal intermetallic and deformed polycrystalline heat-resistant Ni-based superalloys. Special attention is paid to exclude the stagnant zones in the solid phase joining region using surface relief. The results of physical modelling and FEM are in qualitative agreement. It is revealed that the surface relief can decrease the stagnant zone and increase localisation of deformation in the joining region, which helps to improve the joining quality. A monotonous change in microhardness in the joint zone is shown.

Use of the superplasticity effect of materials in engineering technology

Annotation. Technological aspects of using the effect of superplasticity in the manufacture of many mechanical engineering products by pressure treatment methods are considered. Superplasticity, which manifests itself under special conditions, allows the formation of complex forms of specific materials with heat and deformation resistance, hardness and brittleness, and atypical physical and technical properties at low costs. The conditions and factors of the manifestation of superplasticity effect for traditional and innovative materials, such as metallic glass and nanoceramics, are revealed. New mold forming technologies that do not require press equipment are considered, and the areas of optimal use of the capabilities of the superplastic state of materials are specified.

Influence of Structural-Phase State on Low-Temperature Superplasticity of Ultrafine-Grained Titanium Alloys

The influence of the structural-phase state on the development of low-temperature superplasticity of titanium alloys with an ultrafine-grained structure and different volume fraction of the β phase was studied. It was shown that the formation of the structure by multi-directional pressing leads to a decrease in the temperature of the beginning of the superplastic flow realization up to 823K regardless of phase composition of the titanium alloys. It is assumed that the non-monotonic character of the temperature dependence of relative elongation to failure of the Ti-4Al-2V and Ti-6Al-4V alloys is related to their transition to a fine-grained state with an increase in the deformation temperature. It was established that the character and volume fraction of the β-phase precipitations have a significant effect on the grain growth and the temperature dependence of the relative elongation of the alloys to failure under conditions of superplasticity.

Studies on Titanium Alloys for Aerospace Application

Since the development of the Ti54M titanium alloy in 2003, its application within the aerospace sector has gradually increased due to the combination of properties such as improved forgeability and machinability, low flow stress at elevated temperatures, and superplastic characteristics. However, for the successful exploitation of Ti54M a comprehensive understanding of its mechanical characteristics, microstructure stability, and superplastic behaviour is required. The superplastic forming of titanium alloys is characterised by high deformation at slow strain rates and high temperatures which influence the material microstructure, and in turn, determine the forming parameters. These mechanisms make the prediction of the material behaviour very challenging, limiting its application within the aerospace industry. Even though Ti54M has been commercially available for over 10 years, further studies of its mechanical and superplastic properties are still required with the aim of assessing its applicability within the aerospace industry as a replacement for other commercial titanium alloys. Therefore, in this work a study of the mechanical and superplastic properties of Ti54M, in comparison with other commercial titanium alloys used in the aerospace industry - i.e. Ti-6AL-4V, and Ti-6-2-4-2 - is presented. The final objective of this study, carried out at the Advanced Forming Research Centre (AFRC, University of Strathclyde, UK), is to obtain material data to calibrate and validate a model capable of estimating the behaviour and grain size evolution of titanium alloys at superplastic conditions.

On Dynamic Superplasticity of Aluminum Alloys with Initial Varying Grain Size Structure

In the submitted paper there is a speech about mathematical modeling of behavior of group of the industrial aluminum alloys with initial varying grain size structure showing superplastic properties in certain temperature and strain rate ranges. Superplasticity of dynamic type are caused by simultaneous action of deformation rates and structural (phase) transitions of evolutionary type in open nonequilibrium systems. The association of deformation process with metal flow, that has irreversible structural and phase transition of indistinct type at one of the stages, allows to use synergetic approach. The experimental study of deformation behavior of group commercial aluminum alloys with initial varying grain size structure in the wide temperature and strain rate ranges (not only in case of superplasticity, but also in the boundary fields of thermoplasticity and high-temperature creep.) has led to creation of the mathematical model formulated from positions of mechanics of solid for simple tensile and compression. At the same time the offered model contains analytically formulated conditions for transition of material to a superplastic state. With using of the theory of elasto-plastic processes of small curvature the developed mathematical model is generalized for combined stress for technological problems of volume forming with use of superplasticity for the purpose receiving a semi-finished product with fine-grained structure.

On the Theory of Isothermal Hot Rolling of an Aluminum Alloy Strip

The two-dimensional problem of isothermal (including superplasticity conditions) rolling of an aluminum sheet (at a low angle of bite) is analytically solved, and the influence of high homologous temperature on the pressure on rolls is estimated. The energy–force and kinematic parameters of rolling are found using a dynamic model, which adequately describes the experimental data obtained for commercial aluminum alloys. Calculation results are presented for AMg5 and D18T alloys.

Change in elastic properties of eutectic alloys under conditions of superplastic deformation

In this study, experimental data have been obtained which reveal taking place of complex physical processes under conditions of the structural superplasticity not only in the grain boundary, as it is commonly believed, but also in the grain body. It was established for the first time that superplastic deformation of the eutectic alloys is accompanied by significant changes in their Young’s moduli. It was demonstrated that under superplastic deformation the observed changes of the elastic properties of the eutectic Sn–38wt%Pb alloy under study are caused mainly by the decomposition of supersaturated solid solutions on the basis of the alloy components and the relaxation of internal stresses. It was also found that the viscous dislocation–diffusion non-conservative flow is actively developing in the eutectic alloys under conditions of superplasticity, in contrast to the existing ideas about grain boundary sliding as the main mechanism of the matter transport. The experimental results obtained are important for the deeper understanding of the physical nature of the structural superplasticity effect.

Об испытании сплошных цилиндрических образцов при растяжении с кручением

One of the features of the rheological behavior of metals and alloys at high temperature is their sensitivity to the rate of deformation. This paper proposes a method for studying the dependence between stresses and strain rates based on experimental data for solid cylinder specimen deformed in tension with torsion under non-proportional deformation (complex loading). A strain trajectories with two straight lines is considered. The deformation process is described by two components of the strain rate tensor (axial and shear) in the form of functions of time. At each time, we record the axial force and torque corresponding to them. The function of the force parameters (axial force and torque) recorded in the experiment and the preset kinematic parameters (the rate of tension and the torsional velocity), the strain rate sensitivity coefficient, the strain hardening exponent and the approach angle between the stress vectors and the strain rate on the specimen surface under non-proportional deformation is obtained. The paper also considers particular cases of deformation trajectories of the double-link type, when the second link is uniaxial tension, pure torsion or tension combined with torsion. Analytic formulas for calculating the stress vector and its components, and the approach angle that characterizes the direction of the stress vector in relation to the strain rate vector for points on the specimen cylindrical surface are derived. The method proposed is a generalization of the experimental procedure for plotting shear diagrams from the torsion test data for a material sensitive to the rate and level of strains, based on the Fields Backofen equation. The proposed method was used in processing experimental data obtained after testing the Ti-6Al-4V titanium alloy under non-proportional deformation in tension with torsion under conditions of superplasticity. When processing the test data, the compliance of the “specimen – testing machine” system was taken into account.

Mechanical behavior of a Ti‐6Al‐4 V titanium alloy with microstructural evolution modeling under hot and superplastic conditions

AbstractThe present work aims at evaluating and understanding the mechanical behavior of a Ti‐6Al‐4 V alloy under hot and superplastic forming conditions. Tensile tests were performed at temperature range from 650 °C to 950 °C, at strain rates between 10‐2 s‐1 and 10‐4 s‐1. Three equiaxed microstructures, each characterized by a different starting grain size (4.9 μm, 3.0 μm and 0.5 μm), are compared to better understand the microstructural evolutions under hot and superplastic forming conditions and their influence on the mechanical behavior. Hence, an accurate model with microstructural considerations is proposed. The model capabilities consider the grain size evolution that is influenced by the temperature and deformation. The computed flow stresses strongly depend on the strain rate and on the considered initial grain size. Temperature and strain rate conditions may lead to a strain hardening phenomenon in some cases. The comparison between the model response and experiment shows a good agreement for all the tests carried out on Ti‐6Al‐4 V.

Erratum to: Evolution of Dislocation Structure in a Ni3Al Single Crystal Under the Conditions of High-Temperature Superplasticity

The evolution of defective structure in a single crystal of a Ni3Al-base intermetallic with iron and chromium additives under the conditions of superplasticity (δ = 140%) is studied by transmission electron microscopy. The tensile deformation is performed a 1150°C. Planar flaws and recrystallized grains are detected in the fractured zone of the metal. The mechanisms of high-temperature superplasticity of alloys based on Ni3Al intermetallic are discussed.

Diffusion welding through vacuum-deposited porous interlayers

Results of studying the processes of diffusion welding of similar (steel) and dissimilar (steel and bonze) materials through interlayers in the form of thin foil of porous nickel and copper are considered. Thin foils of porous structure were produced by vapour phase condensation on the substrate in electron beam evaporation of elements in vacuum. Application of such foils as interlayers not only promotes formation of defect-free joints, but also leads to lowering of temperature and pressure of diffusion welding, compared to welding without an interlayer. Correlation of the change of parameters of diffusion welding through porous foil and lowering of the temperature of its transition into the superplastic state that is accompanied by the change of characteristics of foil porous structure, suggested that the processes running in the interlayer may promote establishment of physical contact between the surfaces being welded and accelerate the diffusion processes in the joint zone.