Warm Deformation Research Articles

Accounting of Deformation Heating During Upsetting of AMg6 Alloy

The AMg6 alloy, which belongs to the Al–Mg–Mn system, has high corrosion resistance in various environments, good weldability, and good mechanical properties. During analytical and experimental studies, it was established that the AMg6 alloy, when deformed in the temperature range of 130–175 °C, has high plastic properties and can withstand large degrees of deformation without destruction and crack formation. At the same time, its microstructure retains the texture of deformation, and the hardness of the alloy increases, which indicates its deformation hardening. The article presents the results of numerical and laboratory experiments on upsetting of 20 mm diameter workpieces from a heating temperature of 20, 130, 260 and 390 °C. Using numerical experiments, the dependences of deformation heating on the upsetting rate and the initial temperature of the workpiece were obtained. Deformation heating should be taken into account when choosing heating before deformation since it can be critical in terms of overburning and loss of plastic properties and corrosion resistance of finished products. The deformation behavior of the AMg6 alloy at a heating temperature of the workpiece up to 130–175 °C, revealed in this study, indicates the prospects for conducting additional research on the study of changes in the microstructure and mechanical properties of this alloy during warm deformation.

Realizing the outstanding strength-toughness combination of API X65 steel by constructing lamellar grain structure

The development of pipeline steels with exceptional cryogenic toughness is deemed imperative for their applications in various engineering fields. In this work, API X65 steel with lamellar grain (LG) structure is constructed through warm deformation techniques. Comparative analysis with raw API X65 steel revealed notable enhancements in both yield strength (YS) and ultimate strength (UTS), exhibiting increments of 315 MPa and 164 MPa, respectively, while retaining substantial fracture toughness (KIc = 251.14 MPa m0.5). Remarkably, the LG steel exhibits a distinct absence of a ductile-brittle transition behavior over a wide temperature range, maintaining an exceptionally high impact energy from room temperature (RT) to liquid nitrogen temperature (LNT), and the Charpy impact energy exceeding 330 J at LNT, nearly 16 times higher than the raw API X65 steel. This performance can be primarily attributed to the superior plastic deformation capability, coupled with the delamination toughening mechanism. The resultant tortuous path of crack propagation effectively facilitates the absorption of substantial energy, decreases notch tip sensitivity depending on the temperature, stress triaxiality, and strain rate, thereby fostering excellent cryogenic toughness.

A unified constitutive model and microstructure evolution of a solution treated 7075 aluminum alloys under warm forming condition

The warm flow characteristics and microstructural evolution features in a solution-treated 7075 aluminum alloy are revealed. An elastic-visco-plastic constitutive model is developed to capture the flow stress behavior of 7075 aluminum alloy under warm deformation. The variations of microstructure features, including grain and subgrain size, are also analyzed under warm deformation, and are further incorporated in the proposed unified constitutive model. The results indicate that the dynamic recovery (DRV), related to dislocation annihilation and formation/growth of sub-grains, is the domain softening mechanism. The DRV behavior is enhanced at higher temperatures or lower strain rates. Warm external temperature field assists the plastic deformation by means of activating and accelerating the dynamic softening, while the reduced strain rate also gives sufficient time for the softening. Through experimental validation, the proposed constitutive model successfully captures the grain/subgrain features and flow behavior of the studied 7075 aluminum alloy during warm deformation. The predicted result is in a great agreement with the experimental stress-strain data, demonstrating the excellent predictive performance of the model. Moreover, the proposed model also successfully predicts the average grain/subgrain size.

Mechanism of dynamic recrystallization of a FeCrAl alloy during hot compression

FeCrAl alloys have the opportunity to replace current Zr-based cladding materials in light water reactors, and dynamic recrystallization (DRX) plays an important role on microstructural controlling of FeCrAl alloys during hot or warm deformation, such as forging, hot rolling, extrusion. In this research, dynamic recrystallization mechanism of FeCrAl alloy has been systematically investigated by microstructure characterization after isothermal compression. Results show that deformation strain rate and temperature could significantly influence microstructure and DRX mechanism of FeCrAl alloy. When deformed at 0.1s−1, prior grains become long strip with serrated boundaries and are refined by geometric dynamic recrystallization (GDRX). Prior grains are not separated by high angle grain boundaries (HAGB) even deformed at 1200 °C, because strain is too small to impingement of HAGB. Whereas, when deformed at 10s−1 strain rate, there are lots of sub-boundaries with about 2° misorientation within grains, and several strain-free grains are found located at boundaries when deformed at 900 °C or 1000 °C, while the prior grains are divided by HAGB and equiaxed grains are formed at 1100 °C. It was found that mechanism of DRX changes from discontinuous dynamic recrystallization (DDRX) to continuous dynamic recrystallization (CDRX) with increasing temperature from 900 °C to 1100 °C. The phenomenon results from the dislocation movement behaviors changes with increasing temperature. This research will provide meaningful guidance for industrial application of FeCrAl alloys.

Temperature-dependent electroplasticity in the Invar 36 alloy

Electroplasticity of the Invar 36 alloy is investigated based on comparisions between warm and electrically-assisted (EA) uniaxial tensions experiments at different temperatures (100–600 °C) and corresponding microstructural characterizations. Under warm deformation (WD), the flow stress decreases with the increase in temperature, which is directly attributed to dynamic recovery and dynamic recrystallization. With the introduction of high electric current during deformation, it shows a temperature-dependent electroplasticity phenomenon. No difference could be observed between WD and EA samples below a critical temperature of 500 °C, above which, an obvious softening behavior takes place in EA samples. This transition in softening respective to deformation temperature clearly demonstrated that it correlates with the intrinsic current-induced dislocation annihilation and recovery rather than the traditional theory of the Joule heating effect. Our findings shed more light on the utilization of EA softening effect on the development of novel forming routines, especially for materials with limited deformability.

Mechanical Behavior of Al0.5CoCrFeNi HEA During Warm Deformation

Mechanical Behavior of Al0.5CoCrFeNi HEA During Warm Deformation

Elucidating the warm compression of CoCrCuFeNi high entropy alloy: Modeling and microstructural evolution

The warm compression tests were carried out at various temperatures of 400, 550, and 700 °C and strain rates of 0.01, 0.001, and 0.0001 s−1 to unravel the warm deformation behavior of CoCrCuFeNi high entropy alloy (HEA). The microstructural evolutions were characterized using x-ray diffraction (XRD), differential scanning calorimetry (DSC), a field emission gun scanning electron microscope (FESEM) equipped with energy dispersive spectroscopy (EDS), and electron backscattered diffraction (EBSD) detector. These microstructure studies show a dendritic structure containing a bright phase rich in Cu and a darker phase rich in Fe, Cr, and Co. The solidification and melting points of the bright phase are 1136 °C and 904 °C, respectively and the dark phase nucleates at 1365 °C based on DSC results. A Zener-Hollomon parameter was employed to model and predict the warm compression behavior by calculating the activation energy (∼451 kJ/mol), developing a precise constitutive equation (correlation coefficient: R = 0.99), and constructing a processing map for determining the stable temperature and strain rate (strain rates lower than 10−2.75 s−1 and temperatures between 400 and 700 °C) of the plastic flow behavior of the alloy. The FESEM results confirmed the stability and instability domains predicted by the processing map. Moreover, the EBSD analysis indicated that the continuous dynamic recrystallization (CDRX) grains nucleated in the strain rate of 0.0001 s−1 at temperatures higher than 400 °C. Larger grains were formed at 550 and 700 °C due to lower Z parameters in higher temperatures.

Microstructure, Mechanical Properties and Fracture of Austenitic Steel EK-164 After Warm Deformation at 600–900°C

Microstructure, Mechanical Properties and Fracture of Austenitic Steel EK-164 After Warm Deformation at 600–900°C

Study of rotational dynamic recrystallization during warm compression of an Mg-Zn-Nd-Zr alloy

The dynamic recrystallized grains obtained by rotational dynamic recrystallization (RDRX) mechanism are finer and have fewer defects, which can effectively improve the performance of magnesium alloys. However, the impact of temperature and strain on RDRX is currently unclear. In this study, an extruded Mg-Zn-Nd-Zr alloy with trace amounts of rare earth elements and fine grain size was used as the raw material, and a warm deformation temperature interval was selected, which suppressed the large-angle migration, reduced the activity of twinning, and successfully ensured RDRX as the main recrystallization mechanism. The effects of temperature and strain on RDRX of magnesium alloys were systematically investigated, and it was found that the compression temperature and strain jointly affected the formation of RDRX. The RDRXed grains emerged during the dynamic softening stage and abundantly developed during the dynamic equilibrium stage. The percentage of the RDRXed grains increased with the increase of temperature for temperatures below 290 °C and decreased with the increase of temperature for temperatures above 290 °C at low strain levels (0.4). The percentage of RDRX monotonically increased with the increase of compression temperature at high strain (0.7). The size of the recrystallized grains increased with the increase of compression temperature. The formation of the RDRXed grains could effectively release the local strain and form a strain free region without distortion, resulting in strain softening.

Thermomechanics of Al–Cu–Li alloy plastic flow

Flat compression tests of Al–Cu–Li alloys are made. Samples deformed in isothermal conditions at deformation value ε = 55%, in strain rate intervals 10–3–10–1 s–1 and temperatures 400–480°C. The empirical equations connecting thermomechanical parameters of Al–Cu–Li alloys deformation are received. The hightemperature deformation mechanism diagrams (DMD) Al–Cu–Li alloys are plotted. Temperature and strain rate mechanisms action areas of hot deformation and warm deformation are specified. It is shown that the warm deformation area on DMD is equal to the dynamic poligonization area on structural conditions diagram, and the hot deformation area is equal to the partial recrystallization area.

Accelerating bainite transformation by concurrent pearlite formation in a medium Mn steel: Experiments and modelling

Bainite transformation has yet to be utilized and even thoroughly studied in medium Mn steels. Here, we investigate the isothermal bainite transformation in a 10Mn steel at 450 °C experimentally and theoretically, focusing on the effect of dislocations introduced by warm deformation. We show that the bainite transformation in the studied medium Mn steel exhibits extremely sluggish kinetics (on a time scale of days), concurrent with the pearlite formation. The introduced dislocations can significantly accelerate bainite transformation kinetics while also facilitating the pearlite reaction. This is likely the first report on the simultaneous occurrence of these two solid-state reactions in medium Mn steels. With respect to the roles of dislocations in the acceleration of bainite transformation observed in this work, we propose a new ‘carbon depletion mechanism’, in which dislocations-stimulated pearlite formation makes a twofold contribution: facilitating the formation of bainitic ferrite sub-units to further enhance the autocatalytic effect and preventing the carbon enrichment in the remaining austenite. On this basis, a physical model is developed to quantitatively understand the bainite transformation kinetics considering the effect of concurrent pearlite formation, revealing good agreements between model descriptions and experiment results. Our findings, herein, offer fundamental insights into the bainite transformation in medium Mn steels and uncover a previously unidentified role played by introduced dislocations in influencing the kinetics of bainite formation, which may guide its future application in manipulating microstructure for the development of advanced high-strength steels.

In-situ investigation of dynamic precipitation in pre-aged Al-Zn-Mg-Cu alloy AA7075

High strength Al-Zn-Mg-Cu alloys rely for their properties on nano-scaled precipitates formed during artificial ageing. Deformation profoundly affects the evolution of these precipitates. This study investigates the effect of warm deformation on precipitation in a pre-aged AA7075 aluminium alloy. Warm deformation was performed using an electro-thermomechanical testing machine for rapid resistance heating. Transmission electron microscopy showed slightly larger, yet evenly distributed, precipitates in the warm deformed sample compared to the thermally treated condition without deformation. Synchrotron small angle X-ray scattering revealed accelerated growth rates of precipitates due to deformation. At low plastic strains, the growth rate increased linearly with applied strain, while at high plastic strains, a slight deviation from linear behaviour was observed. Temperature and strain rate effects were also examined, with a stronger enhancement effect at low temperature and fast strain rate for a given strain. The deformation enhancement effect disappeared rapidly upon finishing plastic deformation (within 400 s for the studied condition). The enhanced growth rate during and after deformation can be reasonably explained by the excess vacancy concentration, indicating the influence of strain-induced excess vacancies. However, the simple excess vacancy model used fails to explain the observed strain rate effect, and possible explanations for this discrepancy are discussed.

Warm Deformation Behavior and Flow Stress Modeling of AZ31B Magnesium Alloy under Tensile Deformation.

Constitutive equations were recognized for AZ31B magnesium alloy at higher temperatures and strain rates from conventional empirical models like the original Johnson-Cook (JC), modified JC, and modified Zerilli-Armstrong (ZA) models for capturing the material warm deformation behavior. Uniaxial warm tensile tests were performed at temperatures (50 to 250 °C) and strain rates (0.005 to 0.0167 s-1) to probe AZ31 magnesium alloy flow stress values. Depending on the calculated flow stress, constitutive equations were recognized, and these established models were assessed by the coefficient of determination (R2), relative mean square error (RMSE), and average absolute relative error (AARE) metrics. The results demonstrated that the flow stress calculated by the modified JC and ZA models revealed good agreement against the test data. Thus, the outcomes confirmed that the recognized modified JC and modified ZA models could effectively forecast AZ31 magnesium alloy flow behavior by capturing the material deformation behavior accurately.

Effect of forging pretreatment on the microstructures and mechanical properties of the Al-Cu-Li alloy

The 2195 Al-Cu-Li alloy is widely utilized in the aerospace industry due to its lightweight nature and high strength. Consequently, gaining a comprehensive understanding of techniques to enhance its microstructural and mechanical properties is crucial for the consistent production of reliable and high-strength aerospace components. In this investigation, a pristine 2195 industrial ingot was subject to three distinct forging pretreatments: PT1, which involved a direct three upsetting and three stretching (3U3S) process; PT2, which combined double stage homogenization (DH) with 3U3S; and PT3, which involved DH along with six upsetting and six stretching (6U6S). Subsequent warm deformation and T8 aging treatment were applied to the pretreated forgings. The microstructure evolution during the processes and the final mechanical properties in three orthogonal dimensions were examined. The findings revealed that the combination of DH and multi-dimensional forging (MDF) facilitated the dissolution of non-equilibrium phases to a greater extent. Consequently, the main strengthening phase transformed from the T1 + δ` phase observed in PT1 to the denser T1 + little θ` phase observed in PT2 and PT3 after T8 aging treatment. Notably, the tensile strength and yield strength of PT2 and PT3 specimens increased by 20–30 MPa in all three directions. Moreover, the MDF process accelerated the recrystallization behavior from PT1 to PT3, resulting in a gradual reduction in the average grain size of the pretreated samples from 356 µm to 186 µm. Following warm forging and T8 aging, the grains exhibited a finer and equiaxed structure. Additionally, the coarse phase was identified as the primary source of transgranular cracking. The coarse phases within the matrix became more homogeneous and finer from PT1 to PT3, with a gradual decrease in their area fraction. On the fracture surfaces, the aggregation of coarse phases diminished. Consequently, the elongation in the z-direction improved, and the elongation anisotropy and standard deviation decreased in all three directions for PT3 specimens. The findings presented in this study provide valuable insights into the high-performance manufacturing of Al-Cu-Li alloys.

Finite Element Analysis of Dynamic Recrystallization Model and Microstructural Evolution for GCr15 Bearing Steel Warm-Hot Deformation Process.

Warm deformation is a plastic-forming process that differs from traditional cold and hot forming techniques. At the macro level, it can effectively reduce the problem of high deformation resistance in cold deformation and improve the surface decarburization issues during the hot deformation process. Microscopically, it has significant advantages in controlling product structure, refining grain size, and enhancing product mechanical properties. The Gleeble-1500D thermal-mechanical physical simulation system was used to conduct isothermal compression tests on GCr15 bearing steel. The tests were conducted at temperatures of 600-1050 °C and strain rates of 0.01-5 s-1. Based on the experimental data, the critical strain model and dynamic recrystallization model for the warm-hot forming of GCr15 bearing steel were established in this paper. The model accuracy is evaluated using statistical indicators such as the correlation coefficient (R). The dynamic recrystallization model exhibits high predictive accuracy, as indicated by an R-value of 0.986. The established dynamic recrystallization model for GCr15 bearing steel was integrated into the Forge® 3.2 numerical simulation software through secondary program development to simulate the compression process of GCr15 warm-hot forming. The dynamic recrystallization fraction was analyzed in various deformation regions. The grain size of the severe deformation zone, small deformation zone, and difficult deformation zone was compared based on simulated compression specimens under the conditions of 1050 °C and 0.1 s-1 with the corresponding grain size obtained with measurement based on metallographic photos; the relative error between the two is 5.75%. This verifies the accuracy of the established dynamic recrystallization and critical strain models for warm-hot deformation of GCr15 bearing steel. These models provide a theoretical basis for the finite element method analysis and microstructure control of the warm-hot forming process in bearing races.

Warm Aging of Pre-aged AA6013 Sheet and Its Relevance to Room Temperature and Warm Forming Applications—Experimental and Modeling Analyses

AbstractWarm forming has been shown to be an effective way to increase the formability of various aluminum sheet alloys. If applied to precipitation hardenable alloys, aging and/or coarsening may occur during heating and/or forming processes. As such, there is a potential to leverage the warm forming process to artificially age precipitation hardenable alloys. In this study, the aging characteristics of a pre-aged Al-Mg-Si-Cu alloy (designated AA6013-PA and comprising a 4 h, 100 °C pre-aging cycle following solutionization) were examined using: (i) room temperature tensile and (ii) bendability experiments, as well as (iii) elevated temperature tensile and formability experiments in the temperature range of 220-280 °C. In the pre-aged condition, the alloy exhibited very high work hardening, elongation and bendability. It was found that T6-like properties can be obtained with short duration secondary aging of the AA6013-PA starting temper (410 s at 235 °C), followed by a paint bake cycle (30 min at 177 °C). In such heavily aged conditions, the bendability is found to be moderate and consistent with a T6 temper. Bendability of the pre-aged alloy is shown to be enhanced by reducing the extent of aging, which may be beneficial for applications requiring energy absorption during crash, for example. In the pre-aged condition, AA6013 displayed a 55% improvement in room temperature formability, relative to the T6 condition, for near plane-strain loading using a Nakazima punch, although neither temper exhibited formability improvements through warm forming. The aging kinetics of AA6013-PA were determined from mechanical test data and used to calibrate an aging kinetics model. A strong obstacle model was used to relate precipitation state to yield strength, for artificial aging in the range 220-280 °C. The incremental strength increase due to exposure to an automotive paint cure cycle was also modeled and found to yield accurate results. Warm deformation, as well as room temperature deformation, were shown to enhance the paint bake response of the alloy, reducing the time to the peak aged condition by up to 14% for artificial aging at 177 °C.

Dynamic restoration and texture evolution of pure tantalum during warm deformation

Microstructural evolution of pure tantalum during compression at temperatures and strain rates of 600–1100 °C/0.001–1 s−1 has been investigated. The results show that the restoration is mainly achieved by dynamic recovery at 600–800 °C/0.001 s−1 or 1100 °C/0.01–1 s−1, while continuous dynamic recrystallization (CDRX) has been obviously activated at a higher temperature and a lower strain rate of 1100 °C/0.001 s−1. The formation of both subgrains and recrystallized grains has been significantly promoted with temperature rising from 800 to 1100 °C at 0.001 s−1 or strain rate decreasing from 0.01 to 0.001 s−1 at 1100 °C, thus resulting in the obviously increased strain rate sensitivity (m). The deformed microstructures possess a mix of 〈111〉 and 〈100〉 textures. The <111> texture is strengthened when a large fraction of deformed grains develops at minor strains, lower temperatures and higher strain rates. The 〈100〉 texture shows an explosive growth via <100> subgrains bulging into regions with high-density low angle boundaries (LABs) in <111> grains when subgrain fraction has substantially increased after 60% height reduction at 1100 °C/0.001 s−1.

Coupling Computational Homogenization with Crystal Plasticity Modelling for Predicting the Warm Deformation Behaviour of AA2060-T8 Al-Li Alloy.

This study aimed to propose a new approach for predicting the warm deformation behaviour of AA2060-T8 sheets by coupling computational homogenization (CH) with crystal plasticity (CP) modeling. Firstly, to reveal the warm deformation behaviour of the AA2060-T8 sheet, isothermal warm tensile testing was accomplished using a Gleeble-3800 thermomechanical simulator at the temperatures and strain rates that varied from 373 to 573 K and 0.001 to 0.1 s-1. Then, a novel crystal plasticity model was proposed for describing the grains' behaviour and reflecting the crystals' actual deformation mechanism under warm forming conditions. Afterward, to clarify the in-grain deformation and link the mechanical behaviour of AA2060-T8 with its microstructural state, RVE elements were created to represent the microstructure of AA2060-T8, where several finite elements discretized every grain. A remarkable accordance was observed between the predicted results and their experimental counterparts for all testing conditions. This signifies that coupling CH with CP modelling can successfully determine the warm deformation behaviour of AA2060-T8 (polycrystalline metals) under different working conditions.

Microstructure evolution and crack formation mechanism of high-nitrogen bearing steel subjected to warm deformation

In this work, the microstructure evolution and crack formation mechanism of the X30CrMoN15-1 high-nitrogen steel under warm deformation (700 ℃ and 800 ℃) have been investigated. The microstructure observation indicates that the warm deformation (both at 700 °C and 800 ℃) facilitates the ferrite transition and the carbonitrides precipitation, which leads to the reduction of retained austenite by 40% in the specimen deformed 700 °C and 18% for the specimen deformed 800 °C. In addition, cracks are likely to generate under the low strain rates of 0.005 s−1. In the specimen deformed at 700 °C, the lamellar carbonitrides are precipitated along the interface between ferrite and M/A islands, while the spherical carbonitrides are precipitated inside the ferrite. The volume expansion generated by ferrite transformation and the growth of recrystallized grain are hindered by carbonitrides, resulting in the significant stress concentration and even crack initiation. For the specimen deformed at 800 ℃, the spherical carbonitrides are mostly precipitated from the recrystallized ferrite grain boundary. The significant dislocation aggregation around the austenite /ferrite interface and strength mismatch between ferrite and austenite together result in the interfacial debonding and even cracks propagation at phase boundary.

Strain-rate effects on the recrystallization of molybdenum-based MZ17 alloy

Warm compression tests were performed for the Mo-based alloy MZ17 (Mo-1.7% wt.-ZrO2). Macroscopic deformations to about 67% reduction in height were imposed to cylindrical specimens over a wide range of strain rates (from 10−1 to 10 s−1) at 1000 and 1100 °C to investigate the effects of both, the strain rate and deformation temperature on the recrystallization of the alloy. The compression tests were performed under high vacuum (< 5.0 × 10−4 mbar) using a deformation dilatometer followed by fast cooling. Stress-strain curves with very similar shapes can be noticed for all testing conditions. As-deformed microstructures were imaged at the center of the samples where plastic flow is more uniformly distributed using electron backscatter diffraction (EBSD) to distinguish the recovered domains from the recrystallized grains. Electron channeling contrast imaging (ECCI) was used to visualize the dislocation structures. At all tested temperatures and strain rates, only partial recrystallization was detected. The recrystallized volume fraction increases with increasing the strain rate. There is microstructural evidence of three restoration mechanisms acting during warm deformation; i.e., dynamic recovery, dynamic recrystallization and particle stimulated nucleation (PSN), the latter to a minor extent.