Sharif Digital Repository

In this work, a neural network model is used to calculate flow stress of deforming metal as a function of temperature, strain and strain rate. Then, with the aid of this model and employing a finite element analysis, flow behavior of material and the temperature variations in hot upsetting process are predicted. To examine the model, hot nonisothermal forging of a low carbon steel is performed while force-displacement behavior during hot deformation is recorded. A good agreement is observed between the predicted data and the measured results. © 2006 Elsevier B.V. All rights reserved  

Flow behavior of a high carbon steel under hot deformation condition has been studied and a mathematical model for describing the flow stress under isothermal and constant strain rate as well as under non-isothermal and varied strain rates has been developed. For doing so, Bergstrom dislocation model together with the additivity rule for strain have been employed also, hot compression experiments at various temperatures and strain rates have been utilized to achieve the kinetics of dynamic recovery and recrystallization of the high carbon steel. The comparison between the predicted and experimental results under both isothermal and non-isothermal conditions verifies the validity of the... 

A thermo-mechanical model is developed to predict metal behavior during hot working operations. At first, a neural network model is trained to calculate flow stress of deforming metal as a function of temperature, strain and strain rate and then by coupling the neural network model and a thermo-viscoplastic finite element model, temperature and velocity fields during hot open die forging process are predicted. To examine the model, hot nonisothermal upsetting on a low carbon steel is performed while force-displacement behavior and temperature history during hot working are recorded. A good agreement is observed between the predicted data and the measured results. © 2007 Elsevier B.V. All... 

The hot deformation characteristics of 7075 aluminum alloy (AA7075) are investigated by means of hot compression tests carried out in the temperature range of 200-450 C and strain rate range of 0.0003-1 s-1. Two novel processing maps based on flow localization parameter and enhanced DMM are developed and compared with conventional DMM results. It is observed that processing maps based on flow localization parameter can be used successfully to predict AA7075 thermomechanical behavior. Also, the comparison of the DMM results indicates that the new approach to calculate DMM power dissipation efficiency and instability criteria corresponds better with experimental observations. The occurrence of... 

In order to predict the flow stress curves up to peak under the hot working conditions two crucially modified models are presented. The modification is done by considering the effects of both temperature and strain rate. Besides, the ability of prediction at very low strains is added to them. Furthermore a new mathematical model has been developed to estimate the stress-strain curves beyond the peak via a secant hyperbolic function. The predicted results are found to be in accord with empirical values  

The effect of cooling rate after hot deformation on the microstructure and mechanical properties of a commercial microalloyed forging steel (30MSV6 grade) was investigated using the hot compression test. Hot compression test was performed at 1150 and 925 °C followed by air cooling at different rates. Final microstructures and mechanical properties were evaluated by optical microscopy and shear punch test (SPT), respectively. The results indicated that by increasing the cooling rate, the as-received ferritic-pearlitic microstructure changes to the acicular ferrite, bainite or martensite. It is shown that both yield and ultimate strength increase but the ductility decreases significantly. At a... 

In the present work, hot workability of particulate-reinforced Al6061-20%SiC composite produced by direct hot extrusion technique was studied. Uniaxial hot compression test at various temperatures and strain rates was used and the workability behavior was evaluated from the flow curves and the attendant microstructures. It was shown that the presence of SiC particles in the soft Al6061 matrix deteriorates the hot workability. Bulging of the specimens and flow lines were observed, which indicates the plastic instability during hot working. Microstructure of the composites after hot deformation was found to be heterogeneous, i.e. the reinforcement clusters were observed at the flow lines. The... 

A mathematical model has been proposed to predict flow behavior of steel under hot deformation conditions with regard to the effects of dynamic recovery and dynamic recrystallization. For this purpose, Kocks-Mecking dislocation model together with a first order rate equation have been utilized. To associate temperature and strain rate variations on flow behavior of deforming metal, a thermo-viscoplastic finite element model has been coupled with the proposed model. To verify the modeling results, hot rolling experiments were performed and roll forces at various temperatures and rolling speeds were recorded. A good agreement was found between the predicted and the experimental data. © 2005... 

This paper models the kinetics of dynamic recovery and recrystallization under hot working conditions. For doing so, the first-order kinetics equation is coupled with a thermo-viscoplastic finite element analysis to determine the kinetics of dynamic softening processes at different points of deforming metal. The proposed model can consider the effects of macro parameters, such as temperature and strain rate variations as well as the influences of microparameters as initial grain size on the rate of dynamic softening processes. © 2005 Elsevier B.V. All rights reserved  

The hot working behavior of a low carbon resulfurized free-cutting steel was studied by hot compression tests at temperature range of 1000-1200°C with strain rates of 0.001 to 1s-1. The conventional parameters such as activation energy of deformation and relationships between flow stress/strain and Zener-Hollomon parameter were determined. Both the critical stress and strain for initiation of dynamic recrystallization (DRX) were determined using: (1) strain hardening rate versus stress curve, (2) the natural logarithm of strain hardening rate versus strain curve, and (3) the constitutive equations. In summary, for low carbon resulfurized free - cutting steels, the activation energy of... 

In this study, artificial neural network (ANN) was used to model the hot deformation behavior of 7075 aluminum alloy during compression test, in the strain rate range of 0.0003-1 s-1 and temperature range of 200-450 C. The inputs of the model were temperature, strain rate, and strain, while the output of the model was the flow stress. The feed-forward back-propagation network with two hidden layers was built and successfully trained at different deformation domains by Levenberg-Marquardt training algorithm. Comparative analysis of the results obtained from the hyperbolic sine, the power law constitutive equations, and the ANN shows that the newly developed ANN model has a better performance... 

In this work, an optimum multilayer perceptron neural network is developed to model the correlation between hot working parameters (temperature, strain rate and strain) and flow stress of IN625 alloy. Three variations of standard back propagation algorithm (Broyden, Fletcher, Goldfarb and Shanno quasi-Newton, Levenberg-Marquardt and Bayesian) are applied to train the model. The results show that, in this case, the best performance, minimum error and shortest converging time are achieved by the Levenberg-Marquardt training algorithm. Comparing the predicted values and the experimental values reveals that a well trained network is capable of accurately calculating the flow stress of the alloy... 

Inverse method and artificial neural network were employed in modeling the rheological behavior of the AZ61 Mg alloy. The hot deformation behavior of these alloys was investigated by compression tests in the temperature range 250-350 °C and strain rate range 0.0005-0.1 s-1. Investigation of stress-strain curves and microstructure of the compression specimen illustrate occurrence of dynamic recrystallization. To determining parameters of two suggested constitutive equations global optimization technique, genetic algorithm, was used. The predicted results by inverse method and ANN depicted a good agreement with the experimental data even if the ANN results has shown the best predicted... 

The hot deformation behavior of nickel-base superalloy UDIMET 720 in solution-treated conditions, simulating the forging process of the alloy, was studied using hot compression experiments. Specimens were deformed in the temperature range of 1000 °C to 1175 °C with strain rates of 10-3 to 1 s-1 and total strain of 0.8. Below 1100 °C, all specimens showed flow localization as shear band through the diagonal direction, with more severity at higher strain rates. A uniform deformation was observed when testing between 1100 °C and 1150 °C with dynamic recrystallization as the major flow softening mechanism above 1125 °C. Deformation above γ' solvus temperature was accompanied with grain boundary... 

This paper focuses on the effects of low temperature (subzero) treatments on microstructure and mechanical properties of H13 hot work tool steel. Cryogenic treatment at -72 °C and deep cryogenic treatment at -196 °C were applied and it was found that by applying the subzero treatments, the retained austenite was transformed to martensite. As the temperature was decreased more retained austenite was transformed to martensite and it also led to smaller and more uniform martensite laths distributed in the microstructure. The deep cryogenic treatment also resulted in precipitation of more uniform and very fine carbide particles. The microstructural modification resulted in a significant... 

Hot compression tests were carried out on a Fe-29Ni-17Co alloy in the temperature range of 900 °C to 1200 °C and at strain rates of 0.001-1 s-1. Dynamic recrystallization was found responsible for flow softening during hot compression. The flow behavior was successfully analyzed by the hyperbolic sine equation and the corresponding material constants A, n and α were determined. The value of apparent activation energy was determined as 423 kJ/mol. The peak and steady state strains showed simple power-law dependence on the Zener-Hollomon parameter. The dynamic recrystallization kinetics was analyzed using Avrami equation and the corresponding exponent was determined to be about 2.7. This... 

The hot working behavior of A286 was studied using hot compression tests over temperature range of 950-1100 °C and at strain rates of 0.001-1 s -1. The flow curves of the material over the studied temperatures and strain rates were typical of dynamic recrystallization. However, some points reflected a change in the mechanism of softening with the change of strain rate. The relation between flow stress, deformation temperature and strain rate was examined via power-law, hyperbolic sine and exponential constitutive equations and the hyperbolic sine function was found more appropriate. The peak strain increased with strain rate up to 0.01 s-1 and then unexpectedly decreased at higher strain... 

A comprehensive analysis on flow stress of a wrought Al-Mg alloy is performed to examine the effect of strain. For this study, hot compression tests were carried at different temperatures and strain rates. Corrections of friction and adiabatic heating effects lead to the true stress-true strain curves in the form of dynamic recovery, which reach to a steady-state condition. After correction, constitutive analysis at a constant strain is carried out using hyperbolic-sine equation. The effect of strain on each constitutive parameter is studied to derive a strain-dependent constitutive equation based on hyperbolic-sine equation. Some of constitutive parameters reach to the constant values at... 

The behavior of hot extruded Al-6Mg during hot deformation has been studied using hot compression test. The tests were performed at temperatures of 350, 450, and 550°C and strain rates of 0.001, 0.1, and 1s -1. Due to the effects of friction and adiabatic heating generated during compression, the attained true stress-true strain curves have been corrected. After correction of friction effect, the virtual work hardening has been removed from stress-strain curves. The flow stress increases and reaches a steady state after correction of adiabatic heating effect. Corrected curves and microstructural examinations exhibit the occurrence of dynamic recovery during hot compression of the alloy. The... 

Flow curves and microstructural changes of nanostructured Al6063 alloy produced by mechanical alloying followed by hot extrusion were investigated by means of uniaxial compression test in the temperature range between 300 and 450 °C and strain rate range between 10-2 and 1 s-1. The analysis of flow curves were performed by a hyperbolic sine law equation and the stress exponent and activation energy were determined to be 2.9 and 228 kJ mol-1, respectively. The microstructural constituents for elaboration of deformation mechanisms were observed by TEM and EBSD under FEG-SEM. The microstructural observations determine that dynamic recovery through the formation of subgrains within the grains...