Hot deformation is a key method of processing metallic materials and controlling their final properties through structure-forming processes. The ability to exploit the structural potentiality of both ...traditional alloys and new progressive materials is crucial in terms of sustainable development and economic growth. This reprint focuses not only on conventional technologies (e.g., rolling or forging) but also on modern procedures, such as various types of complex thermomechanical processing and controlled cooling. Most papers are based on the application of advanced hot deformation simulators and structural analysis methods, as well as computer simulations of bulk-forming processes.
The steady-state behavior of dynamic recrystallization (DRX) was studied in commercially pure copper and the austenitic steel alloy 800H. Investigations on the flow stress behavior during strain-rate ...and temperature-change tests in the steady-state regime regarding the grain size sensitivity of the flow stress were analyzed. The results confirmed the predicted connection of DRX grain size and deformation-induced subgrain size. Furthermore, the grain size distribution during steady-state DRX was evaluated and found to remain constant. A continuity equation of the growing and shrinking grain distributions is proposed which allows the steady-state flow stress to be calculated.
This study focuses on deformation characteristics of superalloy IN718 by formulation of a new flow stress model and detailed evaluation of intrinsic workability through the generation of ...three-dimensional (3D) processing maps with the support of optical microstructural observations. Based on thermomechanical simulation tests using a Gleeble-1500 machine, the flow stress model for superalloy IN718 was built and the flow stress throughout the entire deformation process was described by a peak stress only depending on Zener–Hollomon parameter and strain. The developed model exhibited the strain softening due to dynamic recrystallisation (DRX). The intrinsic workability was further investigated by constructing 3D processing maps. The 3D processing maps described the variations of the efficiency of power dissipation and flow instability domains as a function of strain rate, temperature and strain, from which the favourite deformation conditions for thermomechanical processing of IN718 can be established.
The strain rate dependent compressive flow stresses of a Selective-Laser-Melt 316L (SLM-316L) alloy and a commercial (annealed-extruded) 316L (C-316L) alloy were determined, for comparison, between ...1x10-4 and ∼2500 s-1 and between 1x10-4 and ∼2800 s-1, respectively. The Johnson and Cook flow stress material model parameters of both alloys were also determined. The microstructural examinations of the deformed cross-sections of tested specimens (interrupted tests) showed a twinning-induced-plasticity in SLM-316L alloy and a martensitic transformation-induced-plasticity in C-316L alloy. Twin and martensite formations were detected microscopically higher in the dynamically tested specimens until about 0.22 strain, while the twin and martensite formations decreased at increasing strains due to adiabatic heating. The rate sensitivity of SLM-316L was determined slightly higher than that of C-316L within the quasi-static strain rate range (1x10-4 and 1x10-2 s-1), while the rate sensitivities of both alloys were similar in the quasi-static-high strain rate range (1x10-4 and ∼2500-2800 s-1) at low strains. A more rapid decrease in the rate sensitivity of C-316L at increasing strains was found in the quasi-static-high strain rate range. The similar activation volumes of both alloys, corresponding to the dislocation intersections, indicated a similar thermally activated deformation process involvement in both alloys.
Constitutive models were built based on the results of isothermal hot compression tests for a 6061 aluminium alloy at temperatures of 400, 450, 500, and 550 °C and strain rates of 0.1, 1, and 10 s−1, ...which reproduced conditions of the hot rolling forming process for this alloy. The Garofalo-Arrhenius, Johnson-Cook, and Hensel-Spittel material models, modified versions of the latter two, as well as a newly proposed Johnson-Cook model were applied based on the experimental data. The predictive power of the constitutive models was assessed for a wide range of plastic strains, from the start of the plastic region up to a strain value of 1, including strain hardening at the beginning of the flow curve. Comparisons between experiments and models by means of the Pearson correlation coefficient and relative errors, considering different strain ranges, showed that the goodness of the models depends strongly on the considered strain range. Results revealed that the Garofalo-Arrhenius model provided the highest accuracy at any strain range, followed by the Hensel-Spittel models and the newly proposed Johnson-Cook model, which performed more accurately than its commonly employed modified version.
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•Constitutive models were constructed based on hot compression tests for the 6061 aluminium alloy.•Strain hardening and strains up to 1 were accurately represented by certain models.•A new Johnson-Cook model was more accurate than its popular modified version.•Garofalo-Arrhenius was the most accurate model in different strain ranges.
In this study, computational fluid dynamics was used as a powerful tool to study 3D printing and give industrially relevant information in terms of how easy or difficult a material of interest can be ...successfully printed. Gels of selected grains (black rice, job's tear seeds, mung bean, brown rice and buckwheat) were tested as potential 3D printing materials. Simulation was conducted by employing Bird-Carreau model, which yielded good fitting on the gels with shear-thinning characteristics. The simulated required piston pressure for printing differently tested gels was the highest for mung bean gel and reduced in descending order for gels from brown rice, buckwheat, black rice and job's tear seeds. These results are consistent with those of simulated minimum flow stress and those from printing experiments, confirming that simulated piston pressure could be used to evaluate and predict the ease of 3D printing a material.
•Grain gels showed shear-thinning behavior that fits well with Bird-Carreau model.•Highest shear rate near the outlet was noted to be 360.8 and 726.6 s−1.•Extrusion stress and printing tests confirmed validity of simulated pressure.•Simulated piston pressure can be used to predict printing behavior of grain gels.
Dislocation motion in body centered cubic (bcc) metals displays a number of specific features that result in a strong temperature dependence of the flow stress, and in shear deformation asymmetries ...relative to the loading direction as well as crystal orientation. Here we develop a generalized dislocation mobility law in bcc metals, and demonstrate its use in discrete Dislocation Dynamics (DD) simulations of plastic flow in tungsten (W) micro pillars. We present the theoretical background for dislocation mobility as a motivating basis for the developed law. Analytical theory, molecular dynamics (MD) simulations, and experimental data are used to construct a general phenomenological description. The usefulness of the mobility law is demonstrated through its application to modeling the plastic deformation of W micro pillars. The model is consistent with experimental observations of temperature and orientation dependence of the flow stress and the corresponding dislocation microstructure.
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•Mechanisms of the two-slope cyclic hardening behavior of Inconel 617 alloy were revealed.•Dependence of DSA and γ′ precipitate on strain amplitude and its interaction were elucidated.•Evolution of ...dislocation configurations at high and low strain amplitudes were characterized.•Number of crack initiation sites was markedly increased with the increasing strain amplitude.
Effects of strain amplitude ranging from 0.2 % to 1.0 % on the low-cycle fatigue properties of Inconel 617 alloy at 700 °C were studied. Much attention was paid to reveal the physical mechanisms of the cyclic deformation, γ′ phase precipitate and cracking behaviors based on the methods of flow stress partitioning and microstructure evolution characterization. Results showed that the material demonstrated a two-slope cyclic hardening behavior with different hardening rates. The strain amplitude of 0.4 % was found to be the threshold value in terms of both the precipitation behavior and dynamic strain aging (DSA). The diameter of γ′ phase precipitate was decreased from 28 nm to 12 nm when the strain amplitude increased from 0.2 % to 0.4 % and became disappeared at the larger strain amplitudes. The DSA activity characterized by the DSA strain range and maximum stress drop got significantly intensified with the increasing strain amplitude, about five times larger at 1.0 % than that at 0.4 %. Further, the mutual interaction between precipitation behavior and DSA was thoroughly analyzed. The number of crack initiation sites was increased with the increasing strain amplitude, and the dominant damage mechanism responsible for the crack propagation behavior was elucidated.
•Hot deformation behavior and constitutive description of flow stress for HEAs are summarized.•Effects of deformation conditions, phases, and dynamic precipitation are discussed.•Necklace DRX and the ...effects of processing parameters on the DRX grain size are discussed.•Constitutive modeling techniques for the prediction of flow stress of HEAs are critically discussed.•Future prospects in the field of hot deformation and constitutive modeling of HEAs are listed.
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This review article summarizes the hot deformation behavior of high entropy alloys (HEAs) and the corresponding constitutive description of flow stress. The potential of hot working for grain refinement via dynamic recrystallization (DRX), reduction of casting defects, and enhancement of mechanical properties of HEAs is explained. The necklace formation, work hardening analysis for identification of the occurrence and initiation of DRX, and the effects of processing parameters on dynamically recrystallized grain size are discussed. The effects of deformation conditions (represented by the Zener-Hollomon parameter), alloying elements, dynamic precipitation, and the presence of phases on the hot deformation behavior and restoration processes of DRX and dynamic recovery (DRV) are overviewed. The application of processing maps for the characterization of the onset of flow instability, cracking, flow softening, and DRX during hot forming of HEAs is presented. Regarding the constitutive modeling of flow stress for characterization of material flow (at different deformation temperatures, strain rates, and strain), the utilization of the threshold stress (due to the presence of phases or their precipitation during high-temperature deformation), and temperature-dependent Young’s modulus, as well as correlating the obtained values of deformation activation energy and stress exponent with the expected ones from the creep theories are taken into account. Afterward, the available methods and equations for modeling and prediction of flow curves during thermomechanical processing are assessed, where the strain-compensated Arrhenius model, artificial neural network (ANN) model, Zerilli-Armstrong model, Johnson-Cook model, Hensel-Spittel model, and dislocation density-based multiscale constitutive model are presented. Finally, some suggestions for future research works are proposed.
The mechanisms of hot plastic deformation and dynamic recrystallization (DRX) in a powder metallurgical superalloy were investigated by compression tests in the deformation temperature range of ...1020–1140 °C, and the constant strain rate range of 0.001–1.0s−1. The results showed that the stress–strain curves can indicate the intrinsic relationship between the flow stress and thermo-mechanical behavior, which can be used to reflect the internal micro-structural evolution law. All of curves can be divided into three types of DRX flow curves: single peak, cyclic behavior and steady-state flow stress curve. Based on researching of basic constitutive relation, the peak stress can be expressed as follows:σp=0.00013Z0.177. The mechanisms of DRX depended on the operating deformation mechanisms which changed with temperature and strain rate. The inflection point of work-hardening rate curve indicates the onset of DRX. The inflection points are obvious at a low strain rate and a high temperature, while they are not existence at a high strain rate and a low temperature. The strain-rate sensitivity, and thus the deformation mechanisms, are different in the respective temperature regimes as found for the steady-state flow stress and the peak stress, and reflected by the temperature dependence of the stress exponent. Continuous dynamic recrystallization (CDRX) takes place at low deformation temperature. CDRX can lead to a partial softening in hot deformation. A great number of the dislocations are trapped by subgrain boundaries, leading to an increase in grain boundary misorientation and gradual transformation into high-angle grain boundaries (HABs). Discontinuous dynamic recrystallization (DDRX) takes place at high deformation temperature. A great number of new grains formed on the completely dynamic recrystallization microstructure.
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•All of stress–strain curves can be divided into three types of DRX flow curves.•Onset of DRX is more obvious with the decrease of strain rate and the increase of temperature.•CDRX takes place at low deformation temperature because of the generation of HABs.•DDRX takes place at high deformation temperature.
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