Showing papers on "Superplasticity published in 2021"

High strain rate superplasticity via friction stir processing (FSP): A review

Abstract: High strain rate superplasticity by the friction stir processing (FSP), an adaptation of the friction stir welding (FSW), is summarized in this overview article. As a common severe plastic deformation (SPD) processing technique, the microstructures prepared by FSP are characterized by fine grain sizes, being homogeneous with fragmented and dispersed particles, and having a high proportion of high-angle grain boundaries. These attributes are beneficial to the superplastic forming operations at high strain rates and low temperatures. In this monograph, the principles of superplasticity are reviewed, where the importance of grain boundary sliding (GBS), strain rate sensitivity index, grain refinement, and deformation temperature on the remarkable enhancement of ductility is emphasized. Afterwards, FSP is introduced and the effects of the main processing parameters on the heat input and grain size are critically discussed. Finally, the recent progress in the application of FSP for processing of superplastic materials is thoroughly overviewed and the influence of thermal stability against grain growth, addition of alloying elements to form pinning particles, external cooling for obtaining ultrafine grained (UFG) microstructure, FSP process variables such as tool rotation rate and traverse speed, and multi-pass FSP is summarized. Accordingly, this overview presents the opportunities that FSP can offer for controlling the superplastic behavior of materials.

Developing superplasticity in magnesium alloys with the help of friction stir processing and its variants – A review

Abstract: Friction stir processing (FSP), an adaption of the solid-state joining process friction stir welding (FSW), is now a widely recognized severe plastic deformation (SPD) technique. It induces microstructural refinement in the metallic materials which enhances their formability and other mechanical properties. Dynamic recrystallization occurs during the stirring phase which leads to reduction in the grain size and texture modification. Breaking up of the intermetallics and precipitates with their homogeneous distribution in the matrix is also accompanied. This further improves the material's ability to attain maximum ductility during plastic deformation at higher temperatures, resulting in very large uniform elongations (>200%) termed as ‘superplasticity’. Optimization of FSP parameters activates superplastic behaviour in different magnesium alloys at low temperatures and high strain rates. It has become the focal point of the recent researches owing to its huge potential in the light-weight structural applications. In addition to the essential aspects of superplasticity, this article highlights the major explorations in the area of superplasticity of magnesium alloys using FSP method and it's recently developed variants.

Hot tensile deformation behavior of extruded LAZ532 alloy with heterostructure

Abstract: Hot tensile test was performed at deformation conditions of 150– °C 300 °C and 8.33 × 10−5 s−1~1.67 × 10−3 s−1, respectively, to systematically study the microstructure development and deformation mechanism of extruded Mg–5Li–3Al–2Zn (LAZ532) alloy with a heterostructure of both the fibrous extrusion zones (FEZs) and non-fibrous extrusion zones (non-FEZs) coexisted. The results showed that the peak stresses decreased gradually, while the fracture strains increased gradually with the decrease of strain rates or the increase of deformation temperatures, and the alloy exhibited superplastic characteristics at deformation conditions of 300 °C and 8.33 × 10−5 s−1~1.67 × 10−3 s−1. By microstructure observation, the alloy showed that in the initial deformation stage, the deformation of the grains in the FEZs was prior to that in the non-FEZs. In the middle deformation stage, the deformation of the grains in the FEZs was dominated by intragranular slip and accompanied with grain boundary slip (GBS), while the deformation of the grains in the non-FEZs was dominated by GBS and accompanied with intragranular slip. In the later deformation stage, the continuous dynamic recrystallization (CDRX) generated in the coarse lamellar grains in the FEZs due to dislocation pile-up, in contrast, the discontinuous dynamic recrystallization (DDRX) generated at the grain boundaries in the non-FEZs. Moreover, based on theoretical calculation and result analysis, the activation energy was about 110.0 kJ/mol, and the hot tensile deformation mechanism was each other alternating and coordinated deformation mechanism among GBS, intragranular slip and dynamic recrystallization (DRX).

Microstructural evolution and superplastic behavior of a fine-grained Mg−Gd−Y−Ag alloy processed by simple shear extrusion

Abstract: Two magnesium alloys with the nominal compositions of Mg−6Gd−3Y (GW63) and Mg−6Gd−3Y−1Ag (GW63−1Ag) were hot extruded and then processed by 6 passes of the simple shear extrusion (SSE) process at 553 K. Both SEM and EBSD studies confirmed the mean grain size reduction of the extruded base GW63 alloy from 10.1 to 2.9 μm by adding 1 wt% Ag. This decrease in the grain size was due to the formation of the Ag-containing precipitates with network-type morphology at the grain boundaries of the extruded GW63−1Ag alloy. Further grain refinement was achieved after SSE through the occurrence of DRX in both Ag-free and Ag-containing alloys. However, the GW63−1Ag alloy showed a higher fraction of DRX grains and HAGBs after SSE processing compared to the Ag-free alloy. The enhanced DRX behavior was attributed to the smaller initial grain size of the Ag-containing alloy in the as-extruded condition and the role of Ag addition in suppressing the solute drag effect of the segregated Gd and Y elements at the grain boundaries. The superplastic behavior of the alloys was assessed via the shear punch testing (SPT) method performed in the temperature ranges of 623–723 K for the extruded specimens and 573–673 K for the SSE processed alloys. It was revealed that none of the extruded alloys exhibited superplastic flow, because of their large grain sizes. The GW63 alloy processed by 6 passes of SSE displayed a maximum m-value of 0.38 at 648 K, while the high strain rate sensitivity (SRS) index of 0.5 was measured at 623 K for the Ag-containing alloy after SSE. This was accompanied with a superplastic deformation behavior of the latter alloy, for which an activation energy of 105 kJ mol−1 that is close to that of the grain boundary diffusion of pure Mg was obtained. Therefore, grain boundary sliding (GBS) associated with grain boundary diffusion was introduced as the dominant deformation mechanism during the superplastic flow of the Ag-containing alloy processed by SSE.

Superplastic behavior of fine-grained extruded ZK61 Mg alloy

Abstract: The superplasticity of a fine-grained extruded ZK61 magnesium (Mg) alloy was investigated by tensile testing at various elevated temperatures and strain rates. The results demonstrated the strain rate sensitivity (m-value) ~0.51 and ~0.4 and stress exponent (n-value) ~1.96 and ~2.5 at temperatures 673 K and 623 K, respectively. The numerical simulation revealed that the activation energy (Q-value) was in the range of 92.9–146.7 kJ/mol. Therefore, the high elongation to fracture (FE) ~400% and ~334% were achieved at temperatures 673 K and 623 K under tensile loading at a strain rate of 0.001 s−1. The microstructure was thermally stable at elevated temperatures and attributed to MgZn2 phase particles. Based on m-value, n-value, FE, average Q-value ~114 kJ/mol, and thermally stable microstructure, it was found that the governing deformation mechanism was the intra-granular slip. These all feathers assisted in understanding the superplastic behavior of the ZK61 Mg alloy.

Superplasticity and mechanical properties of Al–Mg–Si alloy doped with eutectic-forming Ni and Fe, and dispersoid-forming Sc and Zr elements

Abstract: The development of Al–Mg–Si-based alloys with advanced superplasticity and improved mechanical properties is an important task for further applications of superplastic blow-forming technology in the production of complex-shape parts. The current study focuses on the influence of eutectic forming Ni and Fe and dispersoid forming Sc and Zr alloying elements on the microstructural evolution, superplastic behavior, and tensile properties at room temperature for the Al-1.2%Mg-0.7%Si-1.0%Cu (AA6013-type) alloy. The X-Ray diffraction, scanning, and transmission electron microscopy were used for microstructural characterization. The bimodal particle size distribution was observed after the thermomechanical treatment of the alloy studied. The high density of the L12 coherent dispersoids with a mean size of 10 ± 1 nm providing a significant Zener pinning effect was formed during homogenization annealing. The near-equiaxed coarse particles with a size in a range of 0.5–5.0 μm belonged pre-dominantly to the Mg2Si and Al9FeNi phases, which led to an important particle-stimulated nucleation (PSN) effect. The alloys studied exhibited a superplastic behavior in temperature and strain rate limits of 440–520 °C and 1 × 10−3 s−1 - 1 × 10−2 s−1, respectively, with an elongation-to-failure range of 350–480%. The age-hardening heat treatment provided a yield strength of 370 ± 4 MPa, an ultimate tensile strength of 415 ± 2 MPa, and elongation of 6 ± 1%.

Microstructure and superplasticity of Mg-2Gd-xZn alloys processed by equal channel angular pressing

Abstract: Microstructure, mechanical properties and superplastic behavior of Mg–2Gd–xZn (x = 0, 1, 2 and 3 wt%) alloys were investigated after extrusion and equal channel angular pressing (ECAP). After only 2 passes of ECAP, a homogenous fine-grained microstructure with a grain size of 2.33 μm and a high fraction of high-angle grain boundaries of 84% were formed in the Mg–2Gd–3Zn (GZ23) alloy, while 4 ECAP passes were necessary to create such a structure in the other alloys. This was attributed to the higher solute drag effect in the other alloys, retarding dynamic recrystallization (DRX). Although DRX occurred more easily in the GZ23 alloy, the final DRX grain size was slightly coarser compared to the other alloys. Shear punch testing (SPT) showed that grain refinement during ECAP leads to a slight increase in the shear yield strength of all studied materials after 2 ECAP passes, which was mostly balanced by texture softening caused by the shear texture component and grain growth after 4 ECAP passes. Contrary to the other alloys, the GZ23 alloy exhibited superplastic behavior after a lower number of ECAP passes. In addition, the superplastic temperature for GZ23 was 648 K, which was lower than the 673 K observed for the other alloys. The m-values of ~0.45–0.5 and activation energies of 98–114 kJ/mol suggested grain boundary sliding (GBS) controlled by grain boundary diffusion as the dominant deformation mechanism in the superplastic regime. This was confirmed by microstructural observations.

Superplastic behavior of a severely deformed Mg–6Gd−3Y−0.5Ag alloy

Abstract: An extruded Mg–6Gd−3Y−0.5Ag magnesium alloy was processed by the simple shear extrusion (SSE) technique at 553 K for 1, 2, 4 and 6 passes to refine the microstructure. The electron back scattered diffraction (EBSD) analysis was used to investigate the microstructural evolutions of the alloy after the SSE processing. The grain orientation spread (GOS) maps revealed that by increasing the SSE passes, the fraction of the fine dynamically recrystallized (DRXed) grains increased accordingly from 3% after 1 pass of SSE to about 81% after 6 passes. The fraction of low angle grain boundaries (LAGBs) was relatively high in the early stages of the SSE processing, due to occurrence of dynamic recovery (DRV), but it began to drop significantly after 4 and 6 SSE passes, as the DRX proceeded. Therefore, continuous dynamic recrystallization (CDRX) was identified as the governing recrystallization mechanism during SSE at 553 K. The shear punch testing (SPT) was carried out at different temperatures and under various shear strain rates to assess the superplastic behavior of the alloys. It was found that only the alloy processed by 6 SSE passes exhibited superplastic flow, for which a maximum strain rate sensitivity (SRS) index of 0.45 and an average activation energy of 112 kJ mol−1 were obtained. Accordingly, grain boundary sliding (GBS) associated with the diffusion of grain boundaries was suggested to be the prevalent deformation mechanism during the superplastic flow of the 6-pass SSEed condition. Moreover, the SPT results revealed that as the SSE pass number increases the maximum SRS indices increase and shift to the lower temperatures indicating the rise in the contribution of the GBS mechanism. The kernel average misorientation (KAM) maps delineated that after SPT at 623 K the fine equi-axed DRXed grains were almost strain-free without any change in their shape for the 6-pass SSEed condition, while the one after 2 SSE passes was comprised of extremely deformed grains along the SPT loading direction.

Effect of texture and mechanical anisotropy on flow behaviour in Ti–6Al–4V alloy under superplastic forming conditions

Abstract: The dependency of anisotropic flow behaviour on crystallographic texture is investigated in Ti–6Al–4V alloy at 750 °C and 900 °C under constant strain rates of 10−2, 10−3 and 2 × 10−4 s−1. The evolution of microstructure and crystallographic texture during these test conditions has been studied using electron backscatter diffraction (EBSD). Anisotropic flow stress behaviour was observed at 750 °C irrespective of the applied strain rate. The maximum flow stress at this temperature was recorded for samples with their lengths perpendicular to the rolling direction (RD), which had //Transverse Direction (TD) ± 20°, Basal TD texture. The presence of a banded microstructure appeared to be the prime reason for the anisotropic tensile behaviours at lower temperatures. However, at the higher temperature of 900 °C isotropic deformation was achieved disregarding sample orientations, i.e., parallel or perpendicular to the RD. Rachinger grain boundary sliding along α-β boundaries, accommodated by intragranular slip, was seen to contribute towards the total strain in samples perpendicular to the RD deformed under 2 × 10−4 s−1 strain rate. As such, Rachinger grain boundary sliding is the dominant deformation mechanism in the direction perpendicular to the RD at 900 °C. On the other hand, although exhibiting isotropic flow behaviour, the same texture is not observed for the samples parallel to the RD at 900 °C under the same strain rate (2 × 10−4 s−1). Thus Rachinger grain boundary sliding is not thought to be the dominating deformation mechanism for this sample orientation and potentially Lifshitz sliding is active. It is concluded that despite not having a strong effect on flow behaviour, microstructural texture determines the mechanism (i.e., Rachinger, Lifshitz) by which the superplastic deformation is driven.

Superplastic deformation mechanisms of an Al–Mg–Li alloy with banded microstructures

Abstract: The superplastic deformation behaviors and the evolution processes of the microstructures of an Al–Mg–Li alloy with initial banded grains were studied by means of SEM, EBSD, TEM and FIB techniques. Furthermore, the contribution of GBS and IDS of true strain from 0.21 to 0.74 was quantitatively calculated. The results showed that during the stretching process, the initial banded grains were transformed into equiaxed grains, accompanied by dynamic recrystallization. Dynamic recrystallization refined the grain size, increased the high-angle grain boundaries and reduced the texture. The true stress-strain curve showed work hardening and strain softening. At the initial stage of superplastic deformation, dislocations accumulated obviously, which counteracted the softening effect caused by dynamic recrystallization. Moreover, in this stage, the IDS was the dominant deformation mechanism, with a maximum contribution of 62.3%. In the strain softening stage, the change of m value showed that GBS was the dominant deformation mechanism, and DC and IDS were accommodation mechanisms.

Effect of Homogenization Treatment Regime on Microstructure, Recrystallization Behavior, Mechanical Properties, and Superplasticity of Al-Cu-Er-Zr Alloy

Abstract: Aluminum-based alloys with advanced processing and service properties are required for the automotive and airspace industries. The current study focuses on the microstructure, recrystallization behavior, and elevated- and room-temperatures tensile properties of the novel Al-Cu-Er-Zr-based alloy pretreated using different homogenization annealing regimes. Aluminum solid solution, Al8Cu4Er phases of crystallization origin, and nanoscale L12-Al3(Er,Zr) precipitates were observed in the studied alloy. The alloy exhibited a non-recrystallized structure after annealing of cold-rolled sheets at 300°C, with yield strength of 300 MPa and ultimate tensile strength of 330 MPa at room temperature. The fine-grained structure of the alloy provided superplasticity with elongation to failure up to 450% in the temperature range of 550°C to 605°C and a strain rate range of 10–3 s–1 to 10–2 s–1.

The superplasticity improvement of Inconel 718 through grain refinement by large reduction cold rolling and two-stage annealing

Abstract: To improve the superplasticity of Inconel 718, cold rolling with 84% reduction, followed by two-stage annealing, was designed to refine and homogenize the grains. Compared with cold rolling and single-stage annealing, a finer grain size of 1.21 μm was obtained and provided better superplasticity (up to 657% elongation) at 950 °C with a strain rate range of 6 × 10-4 to 1 × 10-2 s-1. Sizeable plastic deformation introduces plenty of crystal defects and storage energy, which contributes to the dense and fine distribution of δ phase in the first stage annealing and enhances the driving force for static recrystallization (SRX) in the subsequent high-temperature annealing. The pre-existing δ phase stimulates the nucleation of SRX and hinders the growth of grains. In addition, the second phase particles form a uniform distribution due to dissolution, thereby controlling the difference in grain size. The sufficient pre-precipitation and subsequent uniform dispersion of δ phase achieve a significant grain refinement effect. This paper is expected to serve as a reference for the industrial production of superplastic Inconel 718 sheets.

Ultralow-temperature superplasticity and its novel mechanism in ultrafine-grained Al alloys

Abstract: The important benefits of ultrafine-grained (UFG) alloys for various applications stem from their enhanced superplastic properties. However, decreasing the temperature of superplasticity and provid...

Initial microstructure influence on Ti–Al–Mo–V alloy's superplastic deformation behavior and deformation mechanisms

Abstract: Superplastic forming is an effective way to manufacture complex-shaped parts of titanium-based alloys. This paper studies the influence of the initial microstructure and its strain-induced evolution on superplastic deformation behavior and the formability of a titanium-based alloy. Two types of Ti–Al–V–Mo alloy samples having a different fraction of recrystallized grains before the start of the superplastic deformation were studied. The deformation behavior, including strain hardening and strain rate sensitivity of the flow stress, was analyzed in a temperature range of 775 °C–900 °C and a strain rate range of 10−5 to 10−2 s−1. Strain-induced changes of the microstructure within the bulk of the samples and on the surface of the pre-polished samples were studied during superplastic deformation with a constant strain rate. The dynamic recrystallization and dynamic grain growth in the volume of the samples and the multiple slip bands on the samples' surface were revealed after superplastic deformation. The grain structure evolution and slip bands localization depended on the samples’ initial microstructure. The results showed that the samples with an increased fraction of recrystallized grains exhibited better superplasticity and higher quality of the formed parts with a more uniform thickness distribution across the section than the samples with a lower initial recrystallized fraction.

Microstructure evolution and mechanical responses of Al–Zn–Mg–Cu alloys during hot deformation process

Abstract: Al–Zn–Mg–Cu alloys can be fabricated by a series of thermo-mechanical processing methods (e.g., hot rolling, forging and extrusion), which is able to serve in aeronautic, automobile, and marine industries because of its excellent physical properties. However, reaching the balance between high strength and favorable ductility to present its high performance is still in progress, during which temperature and strain rate are two very important external variables. More importantly, the core lies in sophisticated microstructure evolution paths involved in hot deformation, which consists of different microstructure mechanisms and behaviors and can be expressed as various mechanical responses. Therefore, a fundamental review of microstructure mechanisms and behaviors, microstructure evolution and relevant mechanical responses of Al–Zn–Mg–Cu alloys during high-temperature deformation is of great significance. In present paper, first, various experimental methods have been introduced. Second, general trends of mechanical properties changing with temperature and strain rate have been summarized. Third, major microstructure mechanisms and behaviors have been discussed. Then, a schematic illustration originating from dislocations’ movement has been depicted, which succeeding microstructure evolution and mechanical responses (including superplasticity) have been reviewed accordingly. Finally, further suggestions of hot deformation of Al–Zn–Mg–Cu alloys have been given.

Effect of calcium on the superplastic behavior of AZ31 magnesium alloy

Abstract: In this short communication, the superplasticity of AZ31-0.5Ca Mg alloy was investigated and was compared to that of the Ca-free counterpart (AZ31 Mg alloy). For this purpose, AZ31 and AZ31-0.5Ca Mg alloy samples were subjected to elevated temperature tensile tests at various strain rates. The results reported in this work showed that the Ca addition lead to a higher superplasticity (~320%) at strain rate and temperature of 0.001 s−1 and 300 °C, respectively, as compared to that observed in the Ca-free AZ31 alloy.

Superior low-temperature superplasticity in fine-grained ZK60 Mg alloy sheet produced by a combination of repeated upsetting process and sheet extrusion

Abstract: Fine-grained magnesium alloy sheets are potential candidates for superplastic forming applications. In the present study, the ZK60 Mg alloy sheet was obtained by a combination of repeated upsetting (RU) process, as a severe plastic deformation method, and subsequent forward sheet extrusion. Performing extrusion on the specimens processed by 1 and 5 passes of RU resulted in sheets with average grain sizes smaller than 10 μm, and a basal texture component. The sheet produced by 5 passes of RU and subsequent extrusion (denoted as A5RE) showed low-temperature superplasticity with the strain rate sensitivity index (m-value) of 0.57 and 0.62 at 523 K and 573 K, respectively. The microstructural characterization revealed that a fully recrystallized (VDRX = 96%) fine-grained microstructure containing a large volume fraction of high-angle grain boundaries (HAGBs) of 80.5%, together with the presence of thermally stable secondary phase particles, are the main reasons for achieving the superplasticity in the A5RE condition. The m-value greater than 0.5 and activation energy of 106 kJ/mol calculated for this condition indicate the grain boundary sliding (GBS), accommodated by grain boundary diffusion, as the dominant deformation mechanism during superplastic forming.

Vacuum superplastic deformation behavior of a near-α titanium alloy TA32 sheet

Abstract: The superplastic deformation and microstructure characterization of a near α titanium alloy TA32 sheet with a nominal composition of Ti-5.5Al-3.5Sn–3Zr–1Mo-0.5Nb-0.7Ta-0.3Si in vacuum environment were systematically investigated. The maximum superplastic elongation was obtained to be 702% at 940 °C with a strain rate of 3.33 × 10-3 s-1. The superplastic deformation constitutive equation was established and the average apparent activation energy (Q) and strain rate sensitivity (m) were calculated to be 316.3 kJ/mol and 0.46, respectively. The spheroidizing process of α lath was accelerated by dynamic globularization (DG). Active dislocation motion was observed and developed an attractive interaction with Silicides ((TiZr)6Si3). Transformation of low angle grain boundaries (LAGBs,2° 15°) was promoted by dynamic recrystallization (DRX). Significant change in texture intensity of B-type and R-type texture was observed after superplastic deformation, which was attributed to grain rotation. Superplastic deformation of TA32 sheet was mainly controlled by grain boundary sliding (GBS) along with kinds of accommodation processes, including DG, DRX, dislocation motion, grain rotation and grain growth.

Superplastic deformation mechanical behavior and constitutive modelling of a near-α titanium alloy TNW700 sheet

Abstract: TNW700 is a newly developed near-α high temperature titanium alloy and has great application potential in the aerospace industry owing to excellent creep strength and short-term service temperature up to 700 °C. The superplastic tensile deformation behavior and constitutive model of TNW700 alloy in the temperature range of 900–975 °C and strain rate range of 0.0005–0.01s−1 were investigated. Results indicated that TNW700 alloy exhibits significant work hardening behavior, which may be related to dynamic growth of β grains, dislocation evolution, and silicide precipitates. In addition, work hardening coefficient (n) as well as the critical strain between flow hardening and softening increases with decreasing strain rate, and first increases and then decreases with increasing deformation temperature. TNW700 alloy exhibits excellent superplasticity at 900 °C–950 °C corresponding to strain rate sensitivity exponent (m) greater than 0.3, and expresses poor superplasticity at 975 °C with lower m value. The m value decreases monotonously with strain owing to decrease of grain boundary sliding contribution caused by dynamic grain growth of β grains. The deformation activation energy increases with decreasing strain rate due to the change of deformation mechanism. A phenomenological constitutive model considering strain hardening, strain rate hardening and temperature softening was proposed and material constants were calibrated by true stress-strain data. The constitutive model was implemented into Abaqus code by UHARD subroutine to simulate the superplastic forming of cone parts, and thickness distribution and bulging heights under different forming time were used to verify the validity of constitutive model.