rus
Issue #5/2025
E.V.Bobruk, M.E.Klimov
REALIZATION OF THE EFFECT OF LOW-TEMPERATURE SUPERPLASTICITY IN ULTRA-FINE-GRAINED Al–Zn–Mg ALLOY UNDER MULTIAXIAL DEFORMATION CONDITIONS
REALIZATION OF THE EFFECT OF LOW-TEMPERATURE SUPERPLASTICITY IN ULTRA-FINE-GRAINED Al–Zn–Mg ALLOY UNDER MULTIAXIAL DEFORMATION CONDITIONS
The presented paper on optimal temperature-velocity deformation modes at lowered temperatures, using finite element method modeling and physical experiments, developed the technology for multiaxial deformation of nanostructured Al–Zn–Mg alloy.
INTRODUCTION
Due to their high specific strength, aluminium alloys are gradually replacing steels. However, forming complex parts and/or structural elements with well-developed geometry from aluminium is not an easy task, especially when forming operations are carried out at room temperature, e.g. by cold forming sheets [1–4]. Traditionally, to improve technological plasticity of Al alloys, the products are produced at elevated temperatures (above 0.5 Тпл) [5]. The most widespread methods used on an industrial scale include hot volume and sheet forging, as well as moulding under superplasticity (SP) conditions. Hot or isothermal forging, as well as SP forming or forging at elevated temperatures provides high technological ductility of most industrial aluminium alloys of such alloying systems as Al–Mg–Si, Al–Zn–Mg, Al–Cu–Mg, etc. The high ductility of the majority of industrial aluminium alloys of such alloying systems as Al–Mg–Si, Al–Zn–Mg, Al–Cu–Mg and others. However, the above-mentioned traditional methods are low-productive and energy-consuming.
This problem can be solved by using aluminium alloys with improved complex of properties, such as high strength and technological plasticity (elongations over 230–300%) at reduced temperatures (up to 0.5 Тпл).
It is known that ultrafine-grained (UFG) alloys, including those based on Al, allow to increase the complex of mechanical characteristics, including increased plasticity at lower temperatures, due to formation of more homogeneous microstructure and regulated distribution of the second phase [6, 7]. For example, in the process of severe plastic deformation (SPD) in the model alloy Al-30Zn a homogeneous UFG two-phase structure with a grain size of about 360 nm is formed [8]. During SPD, the solid solution decomposes and Zn-phase is formed in the triple junctions and in the body of aluminium grains, some boundaries of these grains are covered with Zn segregations. UFG Al-30Zn alloy demonstrates unusually high ductility. Maximum elongations of 235% (at room temperature) and 265% (at 100 °C) were obtained at strain rates of 10–4 s–1. Demonstration of high ductility is associated not only with presence of the UFG structure of the Al-30Zn alloy, but also with presence of zinc segregations along the grain boundaries, contributing to active grain boundary slip and high strain rate sensitivity of the material [8].
The discovered features of mechanical behaviour of UFG aluminium alloys open new possibilities for preparing of perspective high-strength aluminium-based structural materials. In recent studies [7, 9], it was found that some Al-Zn-Mg alloys heavily SPD hardened showed significant elongations (up to 700%) under uniaxial tensile strain in the temperature range of 0.47–0.60 Тпл and uniaxial tensile strain under low-temperature SP (LTSP) conditions retained high strength of the alloys, at 80–90% of the initial strength, before deformation. The high-strength state after deformation under LTSP conditions can be achieved not only due to ultrafine grains preservation, but also due to other strengthening mechanisms that determine the density defect, regulated formation, distribution, size and chemical composition of secondary phase particles, and presence of nanoscale clusters and grain boundary segregations [7, 9, 10].
It is known that in forming operations used to obtain finished thin-walled products with developed geometry, such methods as forming, stamping or drawing are used, in the process of implementation then multiaxial deformation is realised. In this regard, the actual scientific task today is to establish possibility of realisation of biaxial deformation in NS high-strength alloys of 7000 series under LTSP conditions and applicability of these conditions for obtaining real parts or products.
The aim of this work is to make or develop a technological approach that demonstrates feasibility of a forming operation using biaxial deformation at reduced temperatures to prepare the high-strength nano-structured Al–Zn–Mg alloy product. Finite element modelling and physical experiments were used to achieve this task.
RESEARCH METHODS
Al–Zn–Mg alloy (Al-4.8%Zn-1.2%Mg-0.14%Zr) (weight %) was chosen as the material to study. Billets in the form of discs with a diameter of 20 mm and a thickness of 1.4 mm were subjected to high-pressure torsion (HPT) treatment up to 6 GPa with a strain rate of 1 rpm at room temperature (Fig.1) to form an NS state in them. The total number of revolutions of the moving striker is 10.
The microstructure was analysed by transmission electron microscopy using a JEOL JEM-2100 electron microscope at an accelerating voltage of 200 kV with a LaB6 cathode. The studied objects were thin foils transparent to the electron beam. To prepare the foils, plates with dimensions of 5 × 3 × 0.6 mm3 were cut out. The plates were mechanically thinned on abrasive paper to a thickness of 0.10–0.15 mm. The objects for fine structure studies were produced by jet polishing of thin foils on a Tenupol-5 machine in a solution of 20% HNO3 and 80% methanol at a temperature of 25 °C and a voltage of 15–20 V.
The temperature-rate conditions for uniaxial and biaxial deformation were chosen on the basis of previously obtained results of studies of optimal temperature-velocity conditions for the manifestation of the effect of low-temperature superplasticity by nanostructured Al-Zn-Mg alloy [7, 9]. Tensile tests of Al–Zn–Mg alloy specimens were carried out at temperatures of 120, 150 °C with rates of 10–2 and 10–3 s–1 on an Instron 5982 universal testing machine with Bluehill 3 software. The dimensions of the working part of the specimens were 3.0 × 0.2 × 5.0 mm3.
Finite element modelling (FEM) and biaxial tensile physical experiment were carried out at strain rates of 10–3 and 10–2 c–1 and temperatures of 120 and 150 C, at which the NS Al–Zn–Mg alloy samples exhibited increased ductility under uniaxial tensile loading.
FEM modelling of the solid-phase drawing process was carried out using the software package "Deform" (SFTS PC License. Key #9190/Ufa, Russia). In view of the specimen symmetry was divided into cells: 0.03 mm grid with advancement of the leading edge for mathematical description of the material flow.
To determine suitability of the material by cold sheet forging, extrusion tests were carried out using the Erickson method in accordance with GOST 10510-80.
RESULTS
During HPT treatment at room temperature, a homogeneous structure with an average grain size of 130 nm is formed in Al–Zn–Mg alloys. Decomposition of supersaturated solid solution during HPT leads to nucleation of particles of the MgZn2 second phase inside Al grains and Zn segregations on the Al grain boundaries (Fig.2).
Formation of such a state in Al–Zn–Mg alloy with a regulated distribution of MgZn2 phases both in the body and at the grain boundaries allowed to reach a strength limit of 800 MPa at room temperature and a microhardness level of 230HV, and also allowed to realise the effect of superplasticity at temperatures of 120 and 150 °C and strain rates of 10–2 and 10–3 s–1 (Fig.3).
Ductility (formability) life of Al–Zn–Mg alloy in the NS state has been evaluated in order to be able to fabricate a thin-walled product from sheet material. For mathematical modelling of the biaxial deformation process by the finite element method, the true stress-strain curves obtained in the LTSP mode under uniaxial tension according to the single curve hypothesis (a single stress-strain relationship is valid for all types of stress states (uniaxial, biaxial and triaxial) under conditions of simple loading) were used as input data [11].
For the curves recorded in SP mode, it is assumed that deformation up to 100...150% is uniform, which allows to construct true stress-strain diagrams under constant volume conditions.
The true strain ε– was determined from engineering strain ε [11]:
dε– = dl / l
ε– = ln (1 + ε).
The modelling showed that the most intense deformation (up to 2) occurs at the deformation stage of about 90% (Fig.4), increasing temperature leads to decreasing in the value to 1. At the same time, a decrease in strain rate from 10–2 to 10–3 s–1 favours an increase in strain intensity to 1.5 (Fig.4c). The maximum strain intensity at which the specimen will not fracture (Fig.4a–c) was determined during calculation, increasing the values of strain intensity up to 4.2 at 120 °C leads to formation of the first crack. Increasing the temperature to 150 °C reduces strain intensity value to 3 (minimum value), decreasing the rate at the same temperature significantly increases plastic strain intensity to 6 (Fig.4c–f).
The most uniform thickness of the specimen was achieved by biaxial deformation performed at 150 оC at a rate of 10–2 s–1 (Fig.4b).
It is known that the most dangerous and stressful is the first stage of drawing [11]. The modelling results show that the maximum equivalent stresses occur at the 60–70% deformation stage (Fig.5) and do not exceed the stress value in uniaxial tension (Fig.3). Increasing temperature to 150 °C and strain rate to 10–2 s–1 (c, d) leads to a significant decrease in equivalent stresses (Fig.6).
To validate the FEM modelling results, physical Eriksen draw experiments were carried out at temperatures of 120 and 150 °C at rates of 10–2 and 10–3 s–1.
The experimental results are in good agreement with the modelling results. Estimation of the equivalent strain realised in the conducted biaxial tensile tests was performed by the deformed discs thickness located at the section 20–45° relative to the vertical axis of the dome. The equivalent strain was determined as follows:
ε = ln (t0/t),
where t0 is the initial thickness, t is the final thickness of the specimen.
According to the results of the study, graphs of dependence of the maximum load (Fmax) and the value of the true equivalent strain on the temperature-rate conditions of biaxial tensile tests of experimental nanostructured samples of the alloy of the Al–Zn–Mg system were plotted (Fig.7). The graphs also show the values obtained from evaluation of the material flow during the modelling process; using FEM modelling and physical experiment, it was found that deeper drawing occurs at a temperature of 150 °C.
DISCUSSION
The obtained results will provide development of approaches in ultrafine-grained Al alloys production in the form of semi-finished and finished products with an increased level of properties. Application of forming operations under LTSP conditions will be aimed at increasing energy efficiency and productivity of obtaining high-strength products of complex shape from aluminium alloys, which fully meets modern world development trends in promising designs of land, water and air transport.
CONCLUSIONS
Modelling of the flow process of nanostructured Al–Zn–Mg alloy carried out on the basis of the results of mechanical tests at temperatures of 120 and 150 °C and strain rates of 10–2 10–3 s–1 allowed us to establish a number of practical parameters of multiaxial deformation under LTSP conditions. In the process of modelling, equivalent stresses, accumulated plastic deformation were estimated.
The multiaxial tensile parameters of nanostructured Al–Zn–Mg alloy specimens have been determined by the results of modelling and experimental analysis:
maximum value of Fmax (about 390 N) and maximum equivalent strain (less than 2) is demonstrated at a deformation temperature of 150 °C and a strain rate of 10–3 s–1;
failure of the specimens may occur when the true equivalent strain reaches a value of more than 2.75.
ACKNOWLEDGEMENTS
The research was supported by the Ministry of Science and Higher Education of the Russian Federation under State Assignment No. FEUE-2023-0007 (UUST). The experiments were carried out at the Nanotech Collective Use Centre of the Ufa University of Science and Technology.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Due to their high specific strength, aluminium alloys are gradually replacing steels. However, forming complex parts and/or structural elements with well-developed geometry from aluminium is not an easy task, especially when forming operations are carried out at room temperature, e.g. by cold forming sheets [1–4]. Traditionally, to improve technological plasticity of Al alloys, the products are produced at elevated temperatures (above 0.5 Тпл) [5]. The most widespread methods used on an industrial scale include hot volume and sheet forging, as well as moulding under superplasticity (SP) conditions. Hot or isothermal forging, as well as SP forming or forging at elevated temperatures provides high technological ductility of most industrial aluminium alloys of such alloying systems as Al–Mg–Si, Al–Zn–Mg, Al–Cu–Mg, etc. The high ductility of the majority of industrial aluminium alloys of such alloying systems as Al–Mg–Si, Al–Zn–Mg, Al–Cu–Mg and others. However, the above-mentioned traditional methods are low-productive and energy-consuming.
This problem can be solved by using aluminium alloys with improved complex of properties, such as high strength and technological plasticity (elongations over 230–300%) at reduced temperatures (up to 0.5 Тпл).
It is known that ultrafine-grained (UFG) alloys, including those based on Al, allow to increase the complex of mechanical characteristics, including increased plasticity at lower temperatures, due to formation of more homogeneous microstructure and regulated distribution of the second phase [6, 7]. For example, in the process of severe plastic deformation (SPD) in the model alloy Al-30Zn a homogeneous UFG two-phase structure with a grain size of about 360 nm is formed [8]. During SPD, the solid solution decomposes and Zn-phase is formed in the triple junctions and in the body of aluminium grains, some boundaries of these grains are covered with Zn segregations. UFG Al-30Zn alloy demonstrates unusually high ductility. Maximum elongations of 235% (at room temperature) and 265% (at 100 °C) were obtained at strain rates of 10–4 s–1. Demonstration of high ductility is associated not only with presence of the UFG structure of the Al-30Zn alloy, but also with presence of zinc segregations along the grain boundaries, contributing to active grain boundary slip and high strain rate sensitivity of the material [8].
The discovered features of mechanical behaviour of UFG aluminium alloys open new possibilities for preparing of perspective high-strength aluminium-based structural materials. In recent studies [7, 9], it was found that some Al-Zn-Mg alloys heavily SPD hardened showed significant elongations (up to 700%) under uniaxial tensile strain in the temperature range of 0.47–0.60 Тпл and uniaxial tensile strain under low-temperature SP (LTSP) conditions retained high strength of the alloys, at 80–90% of the initial strength, before deformation. The high-strength state after deformation under LTSP conditions can be achieved not only due to ultrafine grains preservation, but also due to other strengthening mechanisms that determine the density defect, regulated formation, distribution, size and chemical composition of secondary phase particles, and presence of nanoscale clusters and grain boundary segregations [7, 9, 10].
It is known that in forming operations used to obtain finished thin-walled products with developed geometry, such methods as forming, stamping or drawing are used, in the process of implementation then multiaxial deformation is realised. In this regard, the actual scientific task today is to establish possibility of realisation of biaxial deformation in NS high-strength alloys of 7000 series under LTSP conditions and applicability of these conditions for obtaining real parts or products.
The aim of this work is to make or develop a technological approach that demonstrates feasibility of a forming operation using biaxial deformation at reduced temperatures to prepare the high-strength nano-structured Al–Zn–Mg alloy product. Finite element modelling and physical experiments were used to achieve this task.
RESEARCH METHODS
Al–Zn–Mg alloy (Al-4.8%Zn-1.2%Mg-0.14%Zr) (weight %) was chosen as the material to study. Billets in the form of discs with a diameter of 20 mm and a thickness of 1.4 mm were subjected to high-pressure torsion (HPT) treatment up to 6 GPa with a strain rate of 1 rpm at room temperature (Fig.1) to form an NS state in them. The total number of revolutions of the moving striker is 10.
The microstructure was analysed by transmission electron microscopy using a JEOL JEM-2100 electron microscope at an accelerating voltage of 200 kV with a LaB6 cathode. The studied objects were thin foils transparent to the electron beam. To prepare the foils, plates with dimensions of 5 × 3 × 0.6 mm3 were cut out. The plates were mechanically thinned on abrasive paper to a thickness of 0.10–0.15 mm. The objects for fine structure studies were produced by jet polishing of thin foils on a Tenupol-5 machine in a solution of 20% HNO3 and 80% methanol at a temperature of 25 °C and a voltage of 15–20 V.
The temperature-rate conditions for uniaxial and biaxial deformation were chosen on the basis of previously obtained results of studies of optimal temperature-velocity conditions for the manifestation of the effect of low-temperature superplasticity by nanostructured Al-Zn-Mg alloy [7, 9]. Tensile tests of Al–Zn–Mg alloy specimens were carried out at temperatures of 120, 150 °C with rates of 10–2 and 10–3 s–1 on an Instron 5982 universal testing machine with Bluehill 3 software. The dimensions of the working part of the specimens were 3.0 × 0.2 × 5.0 mm3.
Finite element modelling (FEM) and biaxial tensile physical experiment were carried out at strain rates of 10–3 and 10–2 c–1 and temperatures of 120 and 150 C, at which the NS Al–Zn–Mg alloy samples exhibited increased ductility under uniaxial tensile loading.
FEM modelling of the solid-phase drawing process was carried out using the software package "Deform" (SFTS PC License. Key #9190/Ufa, Russia). In view of the specimen symmetry was divided into cells: 0.03 mm grid with advancement of the leading edge for mathematical description of the material flow.
To determine suitability of the material by cold sheet forging, extrusion tests were carried out using the Erickson method in accordance with GOST 10510-80.
RESULTS
During HPT treatment at room temperature, a homogeneous structure with an average grain size of 130 nm is formed in Al–Zn–Mg alloys. Decomposition of supersaturated solid solution during HPT leads to nucleation of particles of the MgZn2 second phase inside Al grains and Zn segregations on the Al grain boundaries (Fig.2).
Formation of such a state in Al–Zn–Mg alloy with a regulated distribution of MgZn2 phases both in the body and at the grain boundaries allowed to reach a strength limit of 800 MPa at room temperature and a microhardness level of 230HV, and also allowed to realise the effect of superplasticity at temperatures of 120 and 150 °C and strain rates of 10–2 and 10–3 s–1 (Fig.3).
Ductility (formability) life of Al–Zn–Mg alloy in the NS state has been evaluated in order to be able to fabricate a thin-walled product from sheet material. For mathematical modelling of the biaxial deformation process by the finite element method, the true stress-strain curves obtained in the LTSP mode under uniaxial tension according to the single curve hypothesis (a single stress-strain relationship is valid for all types of stress states (uniaxial, biaxial and triaxial) under conditions of simple loading) were used as input data [11].
For the curves recorded in SP mode, it is assumed that deformation up to 100...150% is uniform, which allows to construct true stress-strain diagrams under constant volume conditions.
The true strain ε– was determined from engineering strain ε [11]:
dε– = dl / l
ε– = ln (1 + ε).
The modelling showed that the most intense deformation (up to 2) occurs at the deformation stage of about 90% (Fig.4), increasing temperature leads to decreasing in the value to 1. At the same time, a decrease in strain rate from 10–2 to 10–3 s–1 favours an increase in strain intensity to 1.5 (Fig.4c). The maximum strain intensity at which the specimen will not fracture (Fig.4a–c) was determined during calculation, increasing the values of strain intensity up to 4.2 at 120 °C leads to formation of the first crack. Increasing the temperature to 150 °C reduces strain intensity value to 3 (minimum value), decreasing the rate at the same temperature significantly increases plastic strain intensity to 6 (Fig.4c–f).
The most uniform thickness of the specimen was achieved by biaxial deformation performed at 150 оC at a rate of 10–2 s–1 (Fig.4b).
It is known that the most dangerous and stressful is the first stage of drawing [11]. The modelling results show that the maximum equivalent stresses occur at the 60–70% deformation stage (Fig.5) and do not exceed the stress value in uniaxial tension (Fig.3). Increasing temperature to 150 °C and strain rate to 10–2 s–1 (c, d) leads to a significant decrease in equivalent stresses (Fig.6).
To validate the FEM modelling results, physical Eriksen draw experiments were carried out at temperatures of 120 and 150 °C at rates of 10–2 and 10–3 s–1.
The experimental results are in good agreement with the modelling results. Estimation of the equivalent strain realised in the conducted biaxial tensile tests was performed by the deformed discs thickness located at the section 20–45° relative to the vertical axis of the dome. The equivalent strain was determined as follows:
ε = ln (t0/t),
where t0 is the initial thickness, t is the final thickness of the specimen.
According to the results of the study, graphs of dependence of the maximum load (Fmax) and the value of the true equivalent strain on the temperature-rate conditions of biaxial tensile tests of experimental nanostructured samples of the alloy of the Al–Zn–Mg system were plotted (Fig.7). The graphs also show the values obtained from evaluation of the material flow during the modelling process; using FEM modelling and physical experiment, it was found that deeper drawing occurs at a temperature of 150 °C.
DISCUSSION
The obtained results will provide development of approaches in ultrafine-grained Al alloys production in the form of semi-finished and finished products with an increased level of properties. Application of forming operations under LTSP conditions will be aimed at increasing energy efficiency and productivity of obtaining high-strength products of complex shape from aluminium alloys, which fully meets modern world development trends in promising designs of land, water and air transport.
CONCLUSIONS
Modelling of the flow process of nanostructured Al–Zn–Mg alloy carried out on the basis of the results of mechanical tests at temperatures of 120 and 150 °C and strain rates of 10–2 10–3 s–1 allowed us to establish a number of practical parameters of multiaxial deformation under LTSP conditions. In the process of modelling, equivalent stresses, accumulated plastic deformation were estimated.
The multiaxial tensile parameters of nanostructured Al–Zn–Mg alloy specimens have been determined by the results of modelling and experimental analysis:
maximum value of Fmax (about 390 N) and maximum equivalent strain (less than 2) is demonstrated at a deformation temperature of 150 °C and a strain rate of 10–3 s–1;
failure of the specimens may occur when the true equivalent strain reaches a value of more than 2.75.
ACKNOWLEDGEMENTS
The research was supported by the Ministry of Science and Higher Education of the Russian Federation under State Assignment No. FEUE-2023-0007 (UUST). The experiments were carried out at the Nanotech Collective Use Centre of the Ufa University of Science and Technology.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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