Peak Flow Stress Research Articles

A study on the dynamic mechanical behavior and microtexture of 6082 aluminum alloy under different direction

The dynamic mechanical behavior and texture of 6082 aluminum alloy along the rolling direction (RD), transverse to the rolling direction (ND), and perpendicular to the rolling direction (TD) were examined within a high strain rate range of 1000 s−1-6000 s−1, using the split Hopkinson pressure bar and X-ray diffraction (XRD). The results indicated that the yield strength and peak flow stress on the RD plane of the material increased as a function of the flow stress, while the flow stresses on the ND and TD planes showed obvious regionalization, with 3900 s−1 being the critical strain rate for regionalization. Analysis of the orientation distribution functions (ODFs) revealed that before dynamic impact, the main textures on the RD, ND, and TD planes were brass {011} <2 1 1>, copper {012} <1 1 1>, and S<123>{634} textures, respectively.

Plastic deformation behavior and dynamic recrystallization of Inconel 625 superalloy fabricated by directed energy deposition

In this study, the hot plastic deformation behavior and dynamic recrystallization (DRX) phenomenon of Inconel 625 superalloy prepared by directed energy deposition (DED) are investigated by carrying out hot compression tests. The true stress–strain curves indicate that with the increment in deformation temperature and decrease in strain rate, the peak stress and flow stress reduce. Compared with wrought Inconel 625 superalloy, Inconel 625 superalloy fabricated by DED shows a higher flow stress, lower peak stress, and lower deformation activation energy under the same hot compression conditions. The fraction of recrystallized grain and recrystallized grain size increases with increasing temperature and decreasing strain rate. At lower deformation temperatures, continuous dynamic recrystallization is the main mechanism of DRX. However, discontinuous dynamic recrystallization becomes the predominant operating mechanism of DRX at high deformation temperature. In addition, the kinetics of DRX in DED Inconel 625 superalloy is much slower than that in forged Inconel 625. The results also show that DED Inconel 625 superalloy has higher thermal stability than that of forged Inconel 625 superalloy, which makes it possible for DED Inconel 625 superalloy to be employed at high temperatures.

On the tensile flow stress response of 304 HCu stainless steel employing a dislocation density based model and electron backscatter diffraction measurements

ABSTRACTTensile flow stress response of 304 HCu stainless steel is investigated using a dislocation density based model and electron backscatter diffraction (EBSD) studies. The model considers two types of dislocations explicitly, i.e. mobile and forest, to model the flow behaviour in the temperature range 300–973 K, at strain rates 3×10 −3 and 3×10 −5 s-1. The flow behaviour indicates the predominance of thermal recovery at higher temperatures, except at 923 K/3 × 10−5 s−1. The dislocation model predicts a rapid increase in both mobile and forest dislocation densities at the early stages of deformation and thereafter reach saturation/steady state. Higher strain rate leads to an increase in dislocation densities with concomitant increase in peak flow stress, indicating significant dislocation multiplication. The dislocation densities decreased with an increasing temperature and is attributed to thermally activated recovery by glide (slip and cross-slip) and climb of dislocations. However, an offset to the thermal recovery is observed at 873–923 K at 3 × 10−5 s−1 wherein the magnitudes of peak flow stress, boundary and forest dislocation densities are of similar magnitude at both the temperatures, thereby signifying the occurrence of matrix hardening by DSA and precipitation. Boundary dislocation densities estimated from EBSD data have shown affinity to that of predicted forest dislocation densities.

High Temperature Plastic Deformation Constitutive Equation of a New Alloy for Piston Ring

Hot compression tests of a new CrMoV alloy for piston ring are conducted by Gleeble-3800 thermal mechanical simulation tester at the temperature of 950, 1000, 1050, 1100 and 1150 °C and strain rate of 0.01, 0.1, 1, 10 s−1. The results show that the true stress strain curves first go up to peak, and then go down to steady state in accord with characteristics of dynamic recrystallization, indicating that the main softening mechanism of this alloy during hot deformation is dynamic recrystallization. The hot deformation behavior of this alloy is characterized by Arrhenius hyperbolic sine relationship with the activation energy and stress index. On the basis of the true stress strain curves, the constitutive equation of the peak flow stress of this alloy is established. With the equation, the flow stress of this alloy can be predicated by numerical simulation for a new type piston ring. The hot deformation behavior of this alloy was characterized by Arrhenius hyperbolic sine relationship, the activation energy and stress index was obtained. Constitutive equation of the peak flow stress of this alloy was established. The results can make contribution to numerical simulation of the manufacturing process for a new type piston ring.

Hot deformation behavior and processing map of Ti-6Al-6V-2Sn titanium alloy

Deformation behavior of Ti-6Al-6V-2Sn alloy possessing an equiaxed α-β preform microstructure was theoretically simulated at a temperature of 800 °C–920 °C and at a strain rate of 0.01–10 s−1 for the purpose of analyzing its processing window and microstructure evolution during hot processing. As temperature increased, flow stress decreased when alloy samples were deformed at a fixed strain rate. Softening behavior can be observed at lower temperatures at all strain rates, which can possibly be explained by the dynamic recrystallization (DRX) of the lamellar α-phase. Alloy microstructure did not change significantly during deformations when the temperature is lower than 830 °C. Thus, the microstructure evolution of Ti-6Al-6V-2Sn alloy could only be quantitatively described under such conditions. For deformations occurring in the range of 830 °C–860 °C, α-polymorph formed spherical clusters, whereas β-polymorph underwent DRX. With regard to temperature above 860 °C, content of the initially major α-phase declined rapidly. Meanwhile, content of the secondary acicular martensite α′-phase rose up, and grains of the β-polymorph became larger. At strain rate equals to 0.1 s−1, flow stress peak was not able to be observed at the initial stage. When it finally became detectible, the value of the flow stress peak first went up and then gradually fell down. Such behavior was attributed to the formation of three different phases at a certain condition (α-Ti, β-Ti and α″-Ti). Optimum hot deformation conditions of Ti-6Al-6V-2Sn determined from the processing map were in the temperature range of 887 °C–920 °C and at 0.01–0.75 s−1 strain rates.

Effect of Hot Extrusion on the Flow Behaviour of a Nickel-Based P/M Superalloy

This research investigated the effect of hot extrusion on the flow behaviour of nickel-based superalloy FGH4096 by hot compression experiments in the temperature range from 1020 to 1110 °C and strain rates ranging from 0.1 to 0.001 s-1. The influence of the hot extrusion on the initial microstructures, work hardening rate, strain rate sensitivity, and activation energy of deformation were discussed. The results show that the extruded microstructure is constituted by the fine dynamic recrystallisation of grains. The true strain-true stress curves show that the as-HIPed and as-HEXed FGH4096 superalloy present double flow stress peaks and discontinuous flow softening. The as-HEXed FGH4096 is easily dynamically softened at high temperatures and high strain rates compared with as-HIPed microstructures. As for the work hardening rate, the as-HEXed FGH4096 exhibits higher θ values than that of as-HIPed. It is beneficial to the homogenous deformation and grain refinement during subsequent turbine disk forging. Comparing to as-HIPed FGH4096, the highest strain rate sensitivity value of as-HEXed is 0.306 at 1110 °C. The isothermal superplastic forging of a P/M turbine disk may be carried out at this temperature. The deformation activation energy value of the as-HIPed FGH4096 is lower which means that dislocation sliding and climbing can be easily initiated in the as-HIPed alloy.

On the nature of a peculiar initial yield behavior in metastable β titanium alloy Ti-5Al-5Mo-5V-3Cr-0.5Fe with different initial microstructures

The compressive yielding phenomenon of titanium alloys is not as focused and sufficiently ascertain as the tensile yielding phenomenon. In the present work, the peculiar compressive yielding behavior and the different dynamic responses of three different initial microstructures (single β, clavate α and lamellar α) were investigated in an attractive metastable β titanium alloy Ti-5553 using electron microscopes/crystallographic calculation/crystal plastic finite element simulation. Results reveal that the distinct compressive yielding behavior, steep peaks of sudden drop in the initial stage (very small true strain ˜0.03) of stress loading have appeared in the compression stress-strain curves except for the lamellar α initial microstructure. Dislocation slip is the essential mechanism of the initial yielding behavior. Interlaced multiple-slip bands formed in the single β initial microstructure during the warm deformation process. A small quantity of single slip bands was observed in the deformed clavate α initial microstructure. The abundant varied nano/ultrafine αs precipitates were nucleated dynamically and dispersedly in all the three deformed initial microstructures. The multiple-slip bands formation and substantial nanoscale αs result in the highest peak of flow stress for single β initial microstructure. The compressive slip bands are formed early in the elastic – plastic deformation stage. As the increasing strain, the sample showed a significant compressive bulge, or eventually forming a strong adiabatic shear band or crack. These results are expected to provide a reference for the study of deformation behavior and mechanical properties of metastable β titanium alloys.

Effects of Mn addition and related Mn-containing dispersoids on the hot deformation behavior of 6082 aluminum alloys

The hot deformation behaviors of 6082 aluminum alloys containing different Mn contents (0.05–1.0 wt%) were systematically investigated by uniaxial compression tests in a temperature range of 400–550 °C and strain rate range of 0.001–1 s-1. Prior to the hot deformation, a low-temperature homogenization (450 °C for 6 h) was carried out on direct-chill cast billets to promote the precipitation of Mn-containing dispersoids. The large numbers of dispersoids in the Mn-containing alloys yielded significantly increased high-temperature flow stresses, compared to that of the base alloy without dispersoids. The material constants and activation energies for hot deformation were determined using the hyperbolic-sine constitutive equation and experimental peak flow stress data. The activation energy increased from 191 kJ/mol for the base alloy to 286 kJ/mol for the alloy with 0.5% of Mn. With further increase in Mn content, the activation energy increased only moderately to 315 kJ/mol for the alloy with 1.0% of Mn. The influences of the Mn content and deformation conditions on the dynamic recovery and recrystallization were quantitatively analyzed. The precipitation of dispersoids in the Mn-containing alloys promoted the retardation of the dynamic recovery and inhibition of the recrystallization owing to their strong pinning effect on the dislocation movement and subgrain migration.

Characterization and Modeling of Hot Deformation Behavior of a Copper-Bearing High-Strength Low-Carbon Steel Microalloyed with Nb

This study investigates hot deformation behavior of a newly developed Cu-bearing high-strength low-carbon steel microalloyed with Nb (Nb-HSLC). A computational method based on experimental data was employed to design the chemical composition of the alloy. Compression tests were carried out in the temperature range of 850-1100 °C as well as strain rates of 0.001-10 s−1 using BAHR Dil 805 A/D thermo-analyzer equipment. The Arrhenius-type constitutive equations were used to model the hot working behavior of the designed steel. Effects of friction and temperature rise during deformation were corrected to obtain the actual stresses. The results showed that the peak flow stress was increased with increasing Zener–Hollomon parameter. The obtained flow curves at strain rates lower than 0.1 s−1 and temperatures above 950 °C represented the typical dynamic recrystallization (DRX) behavior, while the flow curves at temperatures lower than 950 °C at all strain rates were associated with continuous strain hardening. This feature is in good agreement with the precipitation temperature range of Nb(C, N) particles, i.e., 800-1000 °C. Moreover, the flow curves showed the serrations during hot deformation at strain rates of 0.001 and 0.01 s−1, indicating that the dynamic strain aging (DSA) phenomenon occurred at low strain rates. The best fit between “peak stress” and “deformation conditions” was obtained by a hyperbolic sine-type equation (R2 = 0.993). Therefore, the average activation energy was determined as 348 kJ mol−1. The agreement between the achieved model and experimental flow data was verified using the results of additional tests at a strain rate of 5 s−1. The maximum difference between the measured and predicted “peak stresses” was calculated as 5 Mpa.

A constitutive model to describe high temperature flow behavior of AZ31B magnesium alloy processed by equal-channel angular pressing

Equal channel angular pressing (ECAP) is a well-known process for developing an ultrafine-grained microstructure in metals and alloys. In this study, the hot deformation behavior of an AZ31B Mg alloy has been evaluated as a function of ECAP passes and processing routes. Specimens were subjected to three passes of ECAP using a die angle of 120° using processing routes BC and C. Isothermal hot compression testing at strain rate ranging from 0.01 to 10 s−1 and temperatures ranging from 250 to 400 °C were conducted and flow curves generated. The flow curve data generated were fitted to standard constitutive equations used to describe hot working behavior and the activation energies were determined. It was observed that the activation energies for hot deformation decreased after ECAP and this can be related to the grain refinement due to the ECAP process. Activation energy decreased from 161.6 kJ/mol in the annealed condition (grain size: 29 μm) to 145.3 kJ/mol for route 3BC (grain size: 4.7 μm) and 132.3 kJ/mol for route 3C (grain size: 4.9 μm). Equations for predicting the peak flow stress for various ECAP passes has been generated using the Zener Hollomon parameter and these are in agreement with the experimentally observed values.

Microstructure, mechanical properties and deformation behavior of new V-free low-cost Ti-6Al-xFe-yCr alloys

A variety of low-cost V-free near-β titanium alloys, Ti-6Al-xFe-yCr(x = 1, 2 and y = 9, 12 wt%), were designed and fabricated to research the influence of Fe and Cr content on the phase composition, microstructure and properties of the alloy. The Ti-6Al-xFe-yCr alloy is less costly to prepare than the commonly used Ti-6Al-4V alloy, which exhibited a typical near-β microstructure. When the Fe content was 2 wt% and the Cr content was 12 wt%, the resulting alloy’s microstructure exhibited a great amount of β phase and a small quantity of α phase, which suggested the addition of Fe and Cr was an effective method to restrain α phase formation, thereby making efforts to stabilize β phase. To study the hot workability of the Ti-6Al-2Fe-12Cr titanium alloy, the thermal compress deformation behavior was studied at the strain rate ranging from 0.001 s−1 to 5 s−1, in the temperature variation of 800 °C up to 950 °C. The results showed that the thermal deformation behavior of the Ti-6Al-2Fe-12Cr alloy was susceptible to the deformation temperature and strain rate. The peak flow stress reduced as experimental temperature increment and the strain rate decrement. A constitutive model of the Ti-6Al-2Fe-12Cr alloy during thermal deformation was obtained based on the Arrhenius equation.

Hot deformation behavior of high nitrogen martensitic stainless steels

The effect of deformation temperature and strain rate on hot deformation behavior of high nitrogen martensitic stainless steels was investigated by hot compression tests at temperatures of 900 °C–1150 °C and the strain rates of 0.1–10 s−1. Combining with microstructural evolution, the relationship of flow stress with strain rate and deformation temperature was analyzed. The results indicated that the true stress-true strain curves of the experimental material displayed dynamic recrystallization character under the present deformation conditions. The peak stress and flow stress decreased with increasing deformation temperature and increased with increasing strain rate. The thermal deformation activation energy of the experimental steel was 710 038 J · mol−1, and the constitutive equation of the plastic deformation of high nitrogen martensitic stainless steel was obtained by introducing the Zener-Hollomon parameter.

Developing constitutive equations of flow stress for hot deformation of AZ31 magnesium alloy under compression, torsion, and tension

A comparative study was carried out on AZ31 (Mg-3Al-1Zn) magnesium alloy to understand the effect of deformation mode on the hot flow stress. Accordingly, the peak flow stress resulted from hot compression, hot tension, and hot torsion tests were correlated to the Zener-Hollomon parameter using the Sellars-McTegart approach. Based on the analysis, by applying the lattice self-diffusion activation energy of magnesium (135 kJ/mol) as the deformation activation energy, the hyperbolic sine exponents were determined to be ~ 5. Consequently, by consideration of creep exponent of 5, some reliable semi-empirical constitutive equations were proposed for high-temperature deformation of AZ31 alloy and the effect of deformation mode was quantified.

Macro-micro dynamic behaviors and fracture modes of roll bonded 7A52/7A01/7B52 aluminum laminates in high velocity deformation

Damage tolerance improvement in the lamination of aluminum alloy plates and composites has been reported in many studies. In the present study the macro-micro dynamic deformation behavior and related mechanisms of 7A52/7A01/7B52 laminated plates processed by hot roll bonding of 7A52, 7A01 and 7B52 plate at high strain rates and quasi-static compression has been investigated. The microstructure of the laminated plates was examined with backscatter diffraction (EBSD) techniques, scanning and transmission electron microscopies. The results showed that with increased strain rate, obvious strain rate hardening was observed in the single layer specimens. The peak flow stress of the multilayer samples was slightly higher than that of the 7A52 monolayer samples and much lower than that of the 7B52 monolayer samples at the same strain rate. Beyond the peak stress state, the strain hardening was replaced by thermal softening in the 7A52 layer, leading to low resistance of deformation and high tendency to facilitate deformation-induced adiabatic shear bands (ASBs) that consist of dynamic recrystallized grains. ASBs in laminated samples were deflected and bifurcated at the interface of 7A52 layer. In addition, bifurcated ASBs converged at the interface between the 7A52 and 7A01 layers. The high deformation resistance observed in the laminate under dynamic loading was a consequence of the high capacity for strain hardening in the 7A01 layer. This hardening effectively overcame the influences of thermal softening and dynamic recovery during dynamic loading. This study provides an understanding of the laminate's microstructure evolution during dynamic deformation and its relevance to the fracture modes of a multilayer structure under dynamic loading.

Vacuum induction melting and solidification of TiAl-based alloy in graphite crucibles

The effect of vacuum induction melting and solidification in graphite crucibles on chemical composition, microstructure and mechanical properties of Ti-46.6Al-5Nb-0.2B-0.2C (at.%) alloy was studied. The induction melting and solidification under a vacuum pressure of 1 × 104 Pa (low vacuum) has no significant effect on the content of the alloying elements such as Ti, Al, Nb and B but leads to an increase of carbon content to 0.49 at.%. A vacuum pressure of 6.8 Pa (medium vacuum) leads to an increase of C content to 0.68 at.% and decrease of Al content to 45.5 at.% due to its evaporation loss on the expense of increasing Ti and Nb. The medium vacuum results in lower cooling rates, coarser columnar grain structure and finer interlamellar spacing compared to those of the samples prepared under the low vacuum. The Vickers microhardness and hardness of the samples prepared under the medium vacuum is higher than that of the samples prepared under the low vacuum. The melting and solidification under the medium vacuum leads to an increase of compression yield and peak flow stresses at temperatures of 850 and 900 °C compared to those of the as-solidified samples prepared under the low vacuum.

Experimental Investigation and Constitutive Modeling for Hot Deformation Behavior of 2219 Al-Alloy Microalloyed With Silver

2219Al and 2219Al + 0.1 wt % Ag alloys were processed by casting route. The hot compression tests were carried out at constant true strain rates and temperatures in the range of 10−3 to 101 s−1 and 300–500 °C, respectively. Flow stress of the alloy decreases with the addition of silver. The flow stress of both alloys increases with the decrease in deformation temperature and the increase in strain rates. Constitutive models correlating the peak flow stress with deformation temperature and strain rates for the two alloys were developed using hyperbolic–sine relationship. The activation energy for hot deformation of 2219 Al alloy decreases with the addition of silver. Comparison of the predicted and experimental values of peak flow stress reveals that 92% of the data could be predicted within a deviation error of ±10% indicating good predictive capability for the developed constitutive relationships.

Numerical simulation of hot stamping by partition heating based on advanced constitutive modelling of 22MnB5 behaviour

Hot stamping of 22MnB5 sheets by partition heating may represent an effective method to produce parts of the car body-in-white with improved collision performances thanks to proper tailored microstructural characteristics. In accordance with the heating and forming thermo-mechanical features of hot stamping using partition heating, uniaxial tensile tests at varying temperature and strain rate were carried out on a Gleeble™ 3500 thermo-mechanical simulator applying different heating temperatures ranging from 700 °C to 900 °C. The effect of the heating temperature, deformation temperature, and strain rate on the flow stress was analysed, showing that the peak flow stress increased at increasing volume fraction of austenite. An improved unified viscoplastic constitutive model taking into account the austenitization degree was developed and the related material constants identified making use of a genetic algorithm-based optimization tool. The established constitutive model was implemented into the FE-based software Abaqus™ to simulate the hot stamping process of a cup-shaped part using partition heating. The comparison between the numerical and experimental outcomes in terms of thickness distribution proved the accuracy of the 22MnB5 constitutive model taking into account the austenitization degree.

Characterization of semisolid deformation behavior of a high Zr-containing WE magnesium alloy

The effects of temperature and strain rate on the semisolid deformation behavior of a high Zr WE54 magnesium alloys were studied at temperatures of 570, 595 and 620 °C. A flow stress peak was appeared during deformation of the feedstock experimental material at deformation conditions of 570 °C, 0.010 and 0.100 s−1, and 595 °C, 0.100 s−1. The latter results were connected to the occurrence of dynamic recrystallization in solid globules. Also, these conditions were gauged to be able to activate “lubricated flow” mechanism. At 570 °C and strain rate of 0.001 s−1 as well as 595 °C and rates of 0.010 and 0.001 s−1, the flow stress increases and reaches a plateau, where dynamic recovery is considered as the dominant mechanism. However, at 620 °C under all the strain rates, the flow stress drops for a limited strain range and then increases again. The results show that as the liquid removes from the boundaries between the solid parts, the densification of the globules occurs at higher strains, causing an increase in the flow stress. Comparative analysis points out that the feedstock material exhibits a lower flow stress than as-cast alloy associating with a modified morphology of Zr particles after deformation.

High-Temperature Compressive Properties of TiB-Reinforced Ti Alloy via In Situ Synthesis

In this investigation, in situ synthesis of TiB-reinforced Ti-based alloy was carried out by powder metallurgy. The Ti (TiH2)—Al and B powders were ball milled for 40 h to obtain a mixed powder with a nominal composition of Ti-7Al-0.2B (wt. %) and TiH2-7Al-0.2B (wt. %). After milling the mixed powder by using a vacuum hot pressing sintering furnace (1200 °C sintering temperature, 30 MPa pressure, 1 h holding pressure), TiB-reinforced Ti-based alloy was prepared in situ. The compression tests were carried out on a WDW-200 universal testing machine at 550 °C, 600 °C, and 650 °C, and at the strain rates of 1 × 10−4 s−1, 3 × 10−4 s−1, and 5 × 10−4 s−1 to investigate the alloy compression and rheological behavior of the prepared alloy. It was observed that Ti-7Al-0.2B and TiH2-7Al-0.2B alloy formed a TiB phase and α-Ti duplex structure after low-energy ball milling and hot pressed sintering. This is because the in situ synthesis of TiB fiber reinforced phase hinders grain boundary migration and grain growth. Therefore, the TiB phase in Ti-7Al-0.2B alloy is fine, evenly distributed and effective in suppressing grain growth. Alloy materials are heat sensitive and strain-rate sensitive alloys, and the peaks of high-temperature compressive stress decrease with the decrease of the strain rate and the increase of the temperature. At the same temperature and rate conditions, the highest peak flow stress of TiH2-7Al-0.2B is slightly higher than that of Ti-7Al-0.2B. In the ball milling and sintering process of TiH2-7Al-0.2B alloy, the decomposition of H from TiH2 cleanses the surface of Ti particles. This leads to the reduction of the possibility of contamination, and also has a good effect in improving the compression performance.

High temperature deformation of IN718 superalloy: use of basic creep modelling in the study of Nickel and single-phase Ni-based superalloys

A basic model was applied to pure Ni and then to a single-phase superalloy. The high temperature deformation of the superalloy, a solution treated IN718, was investigated by torsion testing in a high-temperature regime (1000–1100 °C) where no precipitation of secondary phases was expected. The material exhibited the classical behaviour of alloys which undergo dynamic recrystallization. The peak-flow stress dependence on strain-rate and temperature was described by a physically-based set of constitutive equations, which took into account both dislocation hardening and solid solution strengthening, and was previously successfully used for the description of creep deformation of Cu, Al alloys and austenitic steels. The model provided an excellent description of the experimental data and for this reason can be considered an excellent basis for further development, which should take into account the precipitation of secondary phases.