Flow Stress Behavior Research Articles

Comparative Study on Hot Metal Flow Behaviour of Virgin and Rejuvenated Heat Treatment Creep Exhausted P91 Steel

This article reports on the comparative study of the hot deformation behaviour of virgin (steel A) and rejuvenated heat treatment creep-exhausted (steel B) P91 steels. Hot uniaxial compression tests were conducted on the two steels at a deformation temperature range of 900–1050 °C and a strain rate range of 0.01–10 s−1 to a total strain of 0.6 using Gleeble® 3500 equipment. The results showed that the flow stress largely depends on the deformation conditions. The flow stress for the two steels increased with an increase in strain rate at a given deformation temperature and vice versa. The flow stress–strain curves exhibited dynamic recovery as the softening mechanism. The material constants determined using Arrhenius constitutive equations were: the stress exponent, which was 5.76 for steel A and 6.67 for steel B; and the apparent activation energy, which was: 473.1 kJ mol−1 for steel A and 564.5 kJmol−1 for steel B. From these results, steel A exhibited better workability than steel B. Statistical parameters analyses showed that the flow stress for the two steels had a good correlation between the experimental and predicted data. Pearson’s correlation coefficient (R) was 0.97 for steel A and 0.98 for steel B. The average absolute relative error (AARE) values were 7.62% for steel A and 6.54% for steel B. This study shows that the Arrhenius equations can effectively describe the flow stress behaviour of P91 steel, and this method is applicable for industrial metalworking process.

Optimization of hot rolling parameters of CRNO steel with the aid of hot compression test and deformation map

Abstract Cold-rolled non-oriented (CRNO) electrical steels find a wide variety of applications in the core of electrical machines due to low core loss and high magnetic permeability. Stringent market conditions not only require CRNO steel with superior magnetic properties but also demand excellent surface conditions. CRNO steel is cold rolled to 0.5 mm in reversing mill. High hot rolled input thickness (>2.6 mm) increases the number of passes during cold rolling and adversely affects the mill productivity. It also results in surface defects such as buckling and coil break. The flow stress of this steel varies differently compared to conventional rolled steel. Thus, it becomes difficult to optimize the reduction schedule and hence safe hot rolling practice is adopted to restrict roll force within permissible limit resulting in higher thickness. A hot compression test was carried out in a Gleeble–3500 to evaluate the flow stress behaviour of this steel and a deformation map was developed to optimize the hot rolling window. The input from the hot compression test and deformation map was used to develop a mill setup model to accurately predict the roll force and optimize the reduction schedule of CRNO steel in the finishing stands of HSM. The final thickness of hot-rolled coils during industrial trials with an optimized reduction schedule was found to be in the range of 2.4–2.6 mm compared to 2.7–3.0 mm during conventional rolling. These coils were further cold rolled and finished in 4–5 passes compared to 6–7 passes with conventional rolling. Reduction in the number of passes has resulted in increased productivity during cold rolling as well as improved surface finish.

Thermal deformation behavior of Mg–3Sn–1Mn alloy based on constitutive relation model and artificial neural network

The flow stress behavior of Mg–3Sn–1Mn alloy during thermal compression deformation was systematically studied. The thermal compression simulation experiment was carried out at different deformation temperatures and different strain rates in the range of 523–673 K and 0.001-1s−1, respectively. It is found that at low temperatures and high strain rates, a large number of twins were generated at the initial stage of thermal deformation, causing an increase in the corresponding flow stress, which makes the traditional constitutive relation model insensitive to predicting the thermal deformation behavior of Magnesium (Mg) alloys with twinning effect. To better evaluate the rheological behavior of Mg alloys, an artificial neural network (ANN) model based on a feedforward and back-propagation algorithm was developed to predict the thermal deformation behavior of Mg–3Sn–1Mn alloy affected by twinning phenomena. The inputs of the model were deformation temperature, strain rate and strain, the output was the flow stress. The comparative evaluation of the obtained results using statistical standard R2 and relative error R¯. The correlation coefficients predicted by the constitutive model were 0.964 and 0.869 at low and high stress levels, respectively. And the correlation coefficient of the neural network predictive model was 0.992. The result shows that the trained ANN is more accurate than the traditional constitutive relation model in predicting the thermal deformation behavior with the twinning effect.

Hot Deformation Behavior of Ni-Modified Graphene/2024 Al Matrix Composites

The 0.5 wt.% Ni-modified graphene/2024 Al matrix composites were formed by hot compression at the temperature(T) was 300-450 °C, the strain rate() was 0.001-1 s−1. The influence of T and on the flow stress(σ) behavior of the composites was analyzed. Moreover, the metamorphic activation energy of the composites was deduced from the stress-strain data, and the Arrhenius constitutive equation relation of the composites was constructed. According to the dynamic model, the hot processing maps of composites were calculated and analyzed. The Arrhenius constitutive equation of the material is established and the Q is 222.22951 KJ/mol by the peak stress. According to the hot processing maps, the hot instability region of the hot deformation in the experimental parameter interval is determined. Furthermore, the optimal machining interval parameters of the material are proposed to be the of 0.011-0.368 s−1, the hot working T of 420 - 450 °C.

High Temperature Deformation Behavior of Near-β Titanium Alloy Ti-3Al-6Cr-5V-5Mo at α + β and β Phase Fields

Most near-β titanium alloy structural components should be plastically deformed at high temperatures. Inappropriate high-temperature deformed processes can lead to macro-defects and abnormally coarse grains. Ti-3Al-6Cr-5V-5Mo alloy is a near-β titanium alloy with the potential application. The available information on the high-temperature deformation behavior of the alloy is limited. To provide guidance for the actual hot working of the alloy, the flow stress behavior and processing map at α + β phase field and β phase field were studied, respectively. Based on the experimental data obtained from hot compressing simulations at the range of temperature from 700 °C to 820 °C and at the range of strain rate from 0.001 s−1 to 10 s−1, the constitutive models, as well as the processing map, were obtained. For the constitutive models at the α + β phase field and β phase field, the correlated coefficients between actual stress and predicted stress are 0.986 and 0.983, and the predictive mean relative errors are 2.7% and 4.1%. The verification of constitutive models demonstrates that constitutive equations can predict flow stress well. An instability region in the range of temperature from 700 °C to 780 °C and the range of strain rates from 0.08 s−1 to 10 s−1, as well as a suitable region for thermomechanical processing in the range of temperature from 790 °C to 800 °C and the range of strain rates from 0.001 s−1 to 0.007 s−1, was predicted by the processing map and confirmed by the hot-deformed microstructural verification. After the deformation at 790 °C/0.001 s−1, the maximum number of dynamic recrystallization grains and the minimum average grain size of 17 μm were obtained, which is consistent with the high power-dissipation coefficient region predicted by the processing map.

Promotion of thermomechanical processing of 2-GPa low-alloyed ultrahigh-strength steel and physically based modelling of the deformation behaviour

A low-alloyed ultrahigh-strength steel comprising CrNiMoWMnV was designed based on thermodynamic calculations and by controlling the microalloying elements to promote various strengthening mechanisms upon processing. The hot deformation behaviour and mechanism were correlated with the processing parameters, that is, strain rate and temperature. The fine features of the deformed microstructures were analysed using electron backscatter diffraction (EBSD) and MATLAB software, combined with the MTEX texture and crystallographic analysis toolbox. The flow stress behaviour at high temperatures was modelled using the dislocation density-based Bergström's model, which could be applied up to the peak strain. However, the diffusional transformation (i.e. recrystallisation)-based Kolmogorov–Johnson–Mehl–Avrami model has been applied to fit the flow stress over a wide deformation strain. The effective grain size (EGS) of martensite and prior austenite grain size (PAGS) were correlated with the deformation temperature and strain rate. Because the PAGS was significantly refined from 16 μm in the initial microstructure to 6 μm after processing at 850 °C/0.01 s−1, the corresponding martensite EGSs were 1.38 and 1.01 μm, respectively. Therefore, these fine-controlled characteristics of the processed microstructures at high temperatures help to enhance the mechanical properties, such as the strength and toughness, of the designed ultrahigh-strength steel.

Hot Workability of the Multi-Size SiC Particle-Reinforced 6013 Aluminum Matrix Composites.

The size and distribution of ceramic particles in aluminum matrix composites have been reported to remarkably influence their properties. For a single ceramic particle, the particle size is too small and prone to agglomeration, which makes the mechanical properties of the composites worse. When the ceramic particle size is too large, the particles and alloy at the interface are not firmly bonded, and the effect of dispersion distribution is not achieved, which will also reduce the mechanical properties of the composites. The multi-size ceramic particles are expected to improve this situation, while their effect on hot workability is less studied. In this study, the hot deformation behavior, constitutive model, processing map and SEM microstructure were investigated to evaluate the hot workability of multi-size SiC particle-reinforced 6013 aluminum matrix composites. The results showed that the increased deformation temperature and decreased strain rate could decrease flow stresses. The flow stress behaviors of the composites can be described by the sine-hyperbolic Arrhenius equation with the deformation activation energy of Q = 205.863 kJ/mol. The constitutive equation of the composites is ε ˙=3.11592×1013sinh0.024909σ4.12413exp-205863RT. Then, the hot processing map of the SiCp/6013 composites was constructed and verified by SEM observations. The rheological instability zone was in the region of a high strain rate. The optimal processing zone for composites was 450~500 °C and 0.03~0.25 s-1. In addition, the strain level was found to increase both the Q value and the area of the instability zone.

Hot workability characteristics of two A335 P92 steels for power plant application: A comparative study

The article reports on the workability of two P92 steels having a chromium content of 8.29 and 9.48 wt%. Constitutive equations were used to calculate material parameters describing the hot deformation flow stress. Hot deformation tests were conducted using the Gleeble® 3500 thermomechanical facility. Test conditions were: temperature of 850-1000°C and strain rate of 0.1-10s-1 to a strain of 0.5. The flow stress curve results show that dynamic recovery was the only softening mechanism. A comparative study of the two steels revealed that Cr content had a marginal significance on the flow stress behaviour. Constitutive analysis results of the material parameters were: a stress exponent of 9.0 (P92-A), and 11.0 (P92-B), while the activation energy was 369 kJmol-1 (P92-A), and 472 kJmol-1 (P92-B). A brief explanation of the material parameter results is in this article. A flow stress model was developed to predict the flow stress behaviour of the two P92 steels investigated. The results show that the model accurately predicts the flow stress at all the deformation conditions applied. The statistical parameters showed a good correlation between the predicted and the experimental data. Therefore, this model can be used to develop metal forming schedules for industrial applications.

Effect of rolling deformation and passes on microstructure and mechanical properties of 7075 aluminum alloy

The research on Al–Zn–Mg–Cu alloys mainly focuses on the effects of solution aging and strain rate on the microstructure and properties of the alloy, as well as the flow stress behavior of the alloy. There are few studies on the effects of hot rolling deformation and reduction pass on the microstructure and properties of aluminum alloy. The dislocation density of alloy is affected by changing the rolling deformation, and post-heat treatment improves fine grains segregation and break coarse second phases. The alloys comprehensive mechanical properties are improved by controlling the recrystallization fraction and size. Different shape variable rolling and different passes, optical microscope (OM), field emission scanning electron microscope (SEM) with energy dispersive spectrometers (EDS), X-ray diffractometer (XRD), microhardness tester, and other methods were used to characterize the samples, and simulation of the rolling process analyzed by Deform-3D software. The effects of hot rolling deformation and deformation pass on microstructure and properties of aluminum alloy were also studied. The results show that dynamic recrystallization was beneficial to grain refinement when deformation increases to 40%. The coarse second phase was broken and refined into chains and partially spheroidized to dissolve in the matrix under large deformation. Dislocation density decreased after solution treatment when deformation was less than 80%, and increased after large deformation was 80%. The equivalent strain obtained by numerical simulations increases with the increase of deformation. The maximum equivalent strain was on both sides of the rolling surface, and this area was the most prone to crack in rolling. When the increase of the shape variable, the recrystallization volume fraction increased, and the grain size decreased.

Study of Formability of Brass Sheet Metal Under Different Temperature Conditions

In the present work focus devoted to the results obtained from uniaxial tensile test were utilized to analyze flow stress behavior of brass under different orientation, temperature and strain rate conditions and the study of forming limit diagrams for stretch forming of brass sheet material at room temperature and at various elevated temperatures have been estimated experimentally by performing stretch forming operations using warm forming tooling setup (i.e., suitable punch – die and blank holding set-up). After stretch forming the brass sheets metal at different temperature conditions (i.e., 300 K to 773 K) the minor and major strains are measured by using the electron microscope and then forming limit diagrams (FLDs) were constructed. With the help of forming limit diagram (FLDs) formability of brass analyzed. These formability limit diagrams (FLDs) were co-related with mechanical properties such as tensile strength and % elongation, and in-plane anisotropy of the brass sheet material.

A Flow Stress Equation of AA5005 Aluminum Alloy Based on Fields-Backofen Model

Tensile tests on AA5005 alloy were conducted on model MTS-810 tensile test machine during temperature 633-773 K and strain rate 0.0003-0.03 s-1. The flow stress–true strain curves were obtained. In order to analyze the flow stress behavior of aluminum AA5005 alloy, the phenomenological Fields-Backofen equation based on the fitting regression analysis was developed. The flow stress values calculated by the obtained model keep coincidence with experimental values. Eventually, the statistical analysis methods (correlation coefficient (R), average absolute relative error (AARE)) were adopted to examine the credibility of the established model. Results show that the R-value is 0.99592 and the AARE is 3.3128 %, which indicates the high fitting accuracy of the Fields-Backofen equation. Consequently, the Fields-Backofen model can describe the constitutive relationship of AA5005 alloy credibly.

Comparative study on high temperature deformation behavior and processing maps of Mg-4Zn-1RE-0.5Zr alloy with and without in-situ sub-micron sized TiB2 reinforcement

Mg-4Zn-1RE-0.5Zr (ZE41) Mg alloy is extensively used in the aerospace and automobile industries. In order to improve the applicability and performance, this alloy was engineered with in-situ TiB 2 reinforcement to form TiB 2 /ZE41 composite. The high temperature deformation behavior and manufacturability of the newly developed TiB 2 /ZE41 composite and the parent ZE41 Mg alloy were studied via establishing constitutive modeling of flow stress, deformation activation energy and processing map over a temperature range of 250 °C - 450 °C and strain rate range of 0.001 s −1 - 10 s −1 . The predicted flow stress behavior of both materials were found to be well consistent with the experimental values. A significant improvement in activation energy was found in TiB 2 /ZE41 composite (171.54 kJ/mol) as compared to the ZE41 alloy (148.15 kJ/mol) due to the dispersed strengthening of in-situ TiB 2 particles. The processing maps were developed via dynamic material modeling. A wider workability domain and higher peak efficiency (45%) were observed in TiB 2 /ZE41 composite as compared to ZE41 alloy (41%). The Dynamic recrystallization is found to be the dominating deformation mechanism for both materials; however, particle stimulated nucleation was found to be an additional mode of deformation in TiB 2 /ZE41 composite. The twinning and stress induced cracks were observed in both the materials at low temperature and high strain rate. A narrow range of instability zone is found in the present TiB 2 /ZE41 composite among the existing published literature on Mg based composites. The detailed microstructural characterization was carried out in both workability and instability domains to establish the governing deformation mechanisms.

A comparative study on phenomenological and artificial neural network models for high temperature flow behavior prediction in Ti6Al4V alloy

Due to its critical use in lightweight components requiring elevated temperature operation, it is very important to determine and model the high temperature thermomechanical flow behavior of Ti6Al4V. In this study, uniaxial tensile tests were performed at quasi-static strain rates and at temperatures ranging from 500°C to 800°C. The ductile behavior provided at a temperature of 800°C and at a strain rate of 0.001 s -1 can be preferred for forming operations due to the steady state flow behavior. However, stress peaks during deformation at the strain rates of 0.1 s -1 and 0.01 s -1 are indicative of an unsafe zone. For modeling the flow stress behavior, three models including the Artificial Neural Network, Modified Hensel-Spittel and Arrhenius are employed with varying prediction performance as shown by the correlation coefficient (R) and average absolute relative error (AARE) values. Accordingly, the Artificial Neural Network model is claimed to be a more suitable approach for capturing the mechanical behavior of Ti6Al4V within the forming temperature range utilized in this study. • Determination of hot deformation mechanisms and prediction of the high temperature mechanical behavior are vital for hot forming processes. • Modified Hensel-Spittel, Arrhenius and Artificial Neural Network models were used to predict the flow behavior. • The Arrhenius model including the activation energy is more accurate than the modified Hansel-Spittel model. • Neural Network model provides higher accuracy than the other empirical models evaluated in this study.

Research on hot deformation behavior and constitutive model to predict flow stress of an annealed FeCrCuNi2Mn2 high-entropy alloy

In the present study, the hot deformation and dynamic recrystallization behaviors of an annealed FeCrCuNi2Mn2 high-entropy alloy were investigated through the hot compression tests. Flow curves were evaluated within a certain temperature range (700–1000 °C) and initial strain rate range (0.001–0.1 s−1), and the corresponding microstructures were assessed. Constitutive equations with strain-dependent material constants were established for the calculation of material constants and for modeling of flow stress behavior through the analysis of true stress-strain curves. The critical and peak stress and strain (dynamic recrystallization parameters) were extracted from the work hardening rate against stress curves. These parameters were then used to determine the recrystallized volume fraction under different conditions. The results indicated that with the increase in deformation temperature and decrease in strain rate, dynamic recrystallization parameters decreased. This trend was followed by an increase in recrystallized volume fraction and grain growth, resulting in a decrease in the hardness value of the alloy. Findings further revealed that the flow stress in the annealed alloy was higher compared with that in the as-cast alloy under the same deformation conditions. Microstructural observations and the evaluation of work hardening rate against stress curves showed the typical dynamic recrystallization characteristics as the dominant softening mechanism under different deformation conditions. The activation energy of hot deformation was determined to be 437 kJ/mol. In the present study, a power equation was established between critical stress and strain and the Zener–Hollomon parameter with the exponents of 0.089 and 0.086, respectively. Moreover, the dynamic recrystallization grain size was found to be proportionate to Z−0.11.

Reveal the hot deformation behaviour and microstructure evolution in additively manufactured 316L stainless steel

The novel hybrid manufacturing process incorporating additive manufacturing (AM) with a subsequent hot compression process has been proposed and applied to 316L stainless steel (316L SS). Compared to the conventional wrought 316L SS, the significantly coarser grain size was characterized in the directly AMed specimen, resulting in unacceptable mechanical properties as safety-critical parts. These coarse grains can be refined through the subsequent hot compression process. However, the detailed grain refinement process in these AMed materials has not been exploited. This motivates the study of the grain refinement of AM specimens through dynamic recrystallization (DRX) in hot compression. Hot compression was applied on AMed 316L SS specimens at temperatures from 800 °C to 1000 °C and different strain rates of 0.01 s−1, 0.1 s−1 and 1 s−1 by Gleeble. The results were compared with conventional wrought-annealed 316L SS specimens. To explain the flow stress behaviour, the underlying grain size, orientations, morphologies, and geometrically necessary dislocation (GND) density distribution and evolution were characterized by the electron backscatter diffraction (EBSD). The results suggest that the initial microstructure difference plays a dominant role in the flow stress response, and the DRX behaves very differently in these AMed and wrought-annealed specimens.

The microstructure evolution and deformation mechanism in a casting AM80 magnesium alloy under ultra-high strain rate loading

Ultra-high strain rate impact tests were conducted by Split-Hopkinson pressure bar to investigate the microstructure evolution and impact deformation mechanism of a solution treated casting AM80 Mg alloy at 25, 150 and 250 °C with a strain rate of 5000 s −1 . The microcrack and dynamic recrystallization (DRX) preferentially nucleate at grain boundary (GB) and twin boundary (TB), especially at the intersections between GBs and TBs, and then propagate along twin direction. In contrast, the adiabatic shear bands preferentially occur at high-density twined regions. At 25 °C, the dominated deformation mechanisms are basal slip and twinning. As deformation temperature increases to 150 and 250 °C, the deformation gradually shifts to be dominated by a coordinated mechanism among non-basal slip, twinning and DRX. The flow stress behavior and deformation mechanism indicate that the degree of decrease in flow stress with temperature is associated with the change of deformation mode.

Analysis of hot tensile behaviour and prediction of thermal stresses during processing of AZ31 magnesium alloy

The use of magnesium alloys is rapidly increasing in automotive and aerospace industries. Accurate constitutive modelling of material at high temperature is a key input for numerical simulation of manufacturing processes. In the present work, hot tensile deformation behaviour of AZ31 alloy was studied by performing uniaxial tensile tests in the temperature range of 250°C–450°C and strain rates ranging from 0.1 to 0.001 s−1. The flow stress behaviour of AZ31 alloy was modelled using two rate-dependent inelastic models, viz. Garafalo law and extended Ludwik law. Various constants in these models were predicted as a function of strain rate and temperature. These rate dependent laws were integrated using Finite Element (FE) based algorithms for thermo-mechanical simulation. Evolution of thermal stresses in solid state quenching and static casting of AZ31 alloys were studied. Results reveal that both material models accurately predict the evolution of thermal stresses for high temperature quenching and casting processes.

Application-Oriented Digital Image Correlation for the High-Speed Deformation and Fracture Analysis of AISI 1045 and Ti6Al4V Materials

In order to achieve realistic simulations of the chip formation, high quality input data regarding the flow stress and damage behavior of the materials are required. The split Hopkinson pressure bar (SHPB) test setup for the characterization of highly dynamic material properties offers a suitable method for generating high strain rates, similar to those in the chip formation zone. However, the strain measurement in SHPB is usually performed by means of strain gauges. This leads to an unreliable evaluation of strain rate and flow stress/shear flow stress in the case of an inhomogeneous material deformation, since this method presents the total strain whilst excluding local deformations. Inhomogeneous deformations are induced deliberately in special shear specimens, as they are also observed in the investigated cylindrical specimens. The present work deals with this issue by providing two additional measurement techniques, which are applied in SHPB tests for cylindrical specimens made of AISI 1045 and Ti6Al4V. To enable a local strain resolution, digital image correlation (DIC) is applied to high-speed images of the deformation process. In order to allow for the detection of shear bands in the specimens, a deep-learning-based approach is presented. The two measurement methods (strain gauges and DIC) are compared and discussed. In particular, the findings regarding the inhomogeneous deformation of Ti6Al4V allow for future improvements in the result quality of SHPB tests. The presented algorithm shows promising predictions for shear band detection and creates the basis for an automated evaluation of shear sample results, as well as an AI-based pre-selection of frames for the DIC evaluation of SHPB tests.

Constitutive modeling for the flow stress behaviors of alloys based on variable order fractional derivatives

During hot working, alloys may experience three kinds of flow stress behaviors, including strain hardening, strain softening, or steady flow, because of the competition of work hardening and thermal softening. Modelling the flow stress behaviors plays an essential role in understanding the mechanical properties of alloys. In this paper, the variable order fractional model is provided to describe the flow stress behaviors of alloys. The variation of the fractional order between 0 and 1 can reflect the mechanical property changing between solids and fluids. By assuming that the fractional order varies linearly with time, the proposed model can describe both the strain softening and strain hardening behaviors of alloys. The model fitting results are compared to the experimental data of A356 alloy for strain softening and Cu-Cr-Mg alloy for strain hardening under different temperatures and strain rates. It is validated that the variable order fractional model can accurately describe the flow stress behaviors of alloys. Furthermore, the rule of the variable order is also discussed to analyze its overall values and the changes before and after the yield point. It is concluded that the variation of the fractional order can intuitively reveal the changes in mechanical properties in the flow stress behaviors of alloys, including both strain softening and strain hardening.

Modelling Dynamic Recrystallization of A356 Aluminum Alloy during Hot Deformation

The flow stress and dynamic recrystallization behavior of A356 aluminum alloy was studied, with a strain rate ranging from 0.001 s−1 to 1 s−1 and temperature ranging from 300 to 500 °C. Both the true stress–strain curves and microstructure examination of A356 aluminum alloy indicated that dynamic recrystallization occurred during the isothermal compression. A physical dynamic recrystallization model based on the Arrhenius equation was developed, and this model can accurately predict the dynamic recrystallization fraction of A356 aluminum alloy during the isothermal compression. Finally, this model was implemented in FEM software Forge, and the microstructure evolution was simulated well.