Sharif Digital Repository

Two-dimensional finite element analysis together with stream function and neural network models are employed to determine thermo-mechanical behavior during hot strip rolling of AA5083. An appropriate velocity field and stream function is first determined using the rule of volume constancy and upper bound theorem and then temperature field within the metal is predicted by means of a two-dimensional conduction-convection model. In order to consider the effect of flow stress and its dependence on temperature, strain and strain rate, a neural network model is also employed in the analysis. Based on the performed tensile tests, two different neural network models are constructed one for smooth... 

Serrated flow was investigated in superalloy IN738LC, a nickel-base γ′ age-hardened alloy. In this material serrated flow appeared between 350 and 450 °C and strain rate of (8.77 × 10-5 to 8.77 × 10-3) s-1. Activation energy for this process was calculated to be 0.69-0.86 eV which is in good agreement with the values reported for similar alloys. Results show that the diffusion rate of substitutional solute atoms at this temperature range is too low to cause this effect. This suggests that the interaction of solute atoms and moving dislocation is responsible for the observed serrated flow in this alloy. © 2007 Elsevier B.V. All rights reserved  

Low cycle fatigue is one of the most important phenomena affecting the lifetime of jet engine blades. In this paper, the effect of aluminide diffusion coating (CODEP-B) on low cycle fatigue properties of René 80 has been studied at temperature of 871 °C, R = 0 and strain rate of about 2 × 10-3 s-1. Experimental results show that the applied cyclic strain is lower than 0.8%, the presence of aforesaid coating increases the fatigue lifetime. In coated specimens, while the total cyclic strain is lower than 0.8%, the nucleation of the cracks occurs merely in substrate, but in cyclic strain more than 1%, as a result of simultaneous nucleation of cracks in the coating surface, diffusion layer and... 

In this study, strain ageing during and after warm rolling of a carbon steel has been investigated. At the first step, the occurrence of serrated flow was studied by means of tensile tests at different temperatures and strain rates. In the next stage, warm rolling experiments were performed under different rolling conditions and then the samples were aged at the room temperature for a period of 3 months. For both aged and non-aged samples, tensile tests were employed to evaluate their mechanical properties. The results show that static strain ageing is possible to happen in the utilized ageing period and increases the yield stress of the aged steel. However, the samples that experience both... 

High bainite dual phase steel has been subjected to tension test at different temperatures from 25 to 500 °C with strain rate of 4.6 × 10-4 s-1 to investigate the effect of temperature on its mechanical properties. Stress-strain curves of steels showed serration flow at temperature range of 200-350 °C and smooth flow at the other temperatures. In agreement with previous studies on some steels, peaks in the variations of yield strength (YS) and ultimate tensile strength (UTS) and minima in ductility were observed at temperature range of 200-350 °C which are various manifestations of dynamic strain aging (DSA). It has been also found that ferrite volume fraction has no effect on 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... 

A modified analytical model is developed for analysis of 3D elastic stress fields in short fiber composites subjected to an applied axial load. Two sets of exact displacement solutions for the matrix and fiber are derived based on the theory of elasticity. The superposition state of these solutions are then used to obtain the analytical expressions for the 3D stress field components over the entire composite system including the fiber end region, which is modeled by the use of imaginary fiber technique. The main difference with the previous works here is that the stress field is considered to be a function of both radial and axial directions. Such an assumption made it possible to calculate... 

A model has been developed for evaluation of recrystallization kinetics during hot rolling of a low carbon steel. For doing so, the finite difference and the finite element methods have been coupled to consider the interconnections among temperature, strain rate, strain and the kinetics of recrystallization within the steel under non-isothermal hot rolling conditions. To verify the model predictions, hot rolling experiments are carried out and the roll forces and microstructure of the rolled metal are compared with the predicted results. The comparison between the two sets of data shows a good agreement. © 2006 Elsevier B.V. All rights reserved  

The hot deformation behavior of A-286 superalloy has been characterized using hot compression experiments in the temperatures between 1000 and 1100 °C and strain rates varying between 0.001 and 0.1 s-1. In addition, hot workability of this alloy has been analyzed by employing flow-localization parameter. The results show that both kinds of softening mechanism, dynamic recovery and dynamic recrystallization, occur during hot working, where at 1000 °C the main mechanism is dynamic recovery and at higher temperatures and strain rate of 0.001-0.01 s-1 dynamic recrystallization takes place. Calculations demonstrates that this alloy mainly have a good workability for the utilized deformation... 

In this paper, two corotational modeling for elastic-plastic, mixed hardening materials at finite deformations are introduced. In these models, the additive decomposition of the strain rate tensor as well as the multiplicative decomposition of the deformation gradient tensor is used. For this purpose, corotational constitutive equations are derived for elastic-plastic hardening materials with the non-linear Armstrong-Frederick kinematic hardening and isotropic hardening models. As an application of the proposed constitutive modeling, the governing equations are solved numerically for the simple shear problem with different corotational rates and the stress components are plotted versus the... 

Temperature and velocity distributions during hot strip rolling of a low-alloy steel are determined using a finite element method together with a neural network model. The finite element method is utilized to solve the governing equations of heat conduction and plastic deformation; at the same time a neural network model is employed for assessing flow stress of the metal being deformed. In this way, the effects of temperature, strain, and strain rate on flow stress could be included in the finite element analysis. In order to examine validity of the mathematical model, laboratory hot rolling experiments are carried out where the surface temperature and roll force are recorded. Comparison... 

A mathematical model is proposed to predict the flow stress behaviour of aluminium alloys under hot rolling conditions. To do so, a dislocation model for evaluating flow stress during deformation is coupled with a finite element analysis to access metal behaviour under non-isothermal and variable strain rate conditions. Then, with the aid of the proposed model, a hot strip rolling process was simulated. In order to verify modelling results, flow stress behaviour of an aluminium alloy is studied employing hot compression tests in various temperatures and strain rates and the model was examined on this material. Non-isothermal hot rolling experiments were carried out and good agreement was... 

This paper presents the densification and microstructure of bilayer structures made from 316L and 17-4PH stainless steels powders during sintering. The requirements for such objects could be magnetic properties at one area of the part and non-magnetic properties at another area of the object. A pressureless solid state sintering method in conjunction with a powder layering technique was used. The sintering was carried out at temperatures ranging from 1100 to 1340 °C for 120 min in hydrogen and vacuum atmospheres. Non-isothermal sintering behavior was also examined by dilatometric analysis. It was found that the strain rate of 17-4PH stainless steel powder is higher than that of 316L during... 

In this study, hot rolling behavior of a low alloy steel in the dual-phase region is studied. The effects of various process parameters such as initial temperature, soaking time, rolling speed and the cooling conditions are investigated. Then, mechanical testing and microstructural studies are performed and the effects of process parameters are studied and finally the optimum rolling program is determined based on the achieved results. The results show that rolling speed significantly alters the final microstructures and mechanical properties. Higher rolling speed results higher volume fraction of ferrite phase. In addition, the optimum dual-phase microstructure for this steel can be... 

A smeared crack approach has been proposed to model the static and dynamic behavior of mass concrete in three-dimensional space. The proposed model simulates the tensile fracture on the mass concrete and contains pre-softening behavior, softening initiation, fracture energy conservation and strain rate effects under dynamic loads. It was found that the proposed model gives excellent results and crack profiles comparing with the available data under static loads. Morrow Point dam was analyzed including dam-reservoir interaction effects to consider its nonlinear seismic behavior. It was found that the resulted crack profiles are in good agreement with the contour of maximum principal stresses... 

In this study, the prediction of flow stress in 304 stainless steel using artificial neural networks (ANN) has been investigated. Experimental data earlier deduced-by [S. Venugopal et al., Optimization of cold and warm workability in 304 stainless steel using instability maps, Metall. Trans. A 27A (1996) 126-199]-were collected to obtain training and test data. Temperature, strain-rate and strain were used as input layer, while the output was flow stress. The back propagation learning algorithm with three different variants and logistic sigmoid transfer function were used in the network. The results of this investigation shows that the R2 values for the test and training data set are about... 

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 rheological behaviour of powder injection moulding feedstock comprising of 316L stainless steel and 3 wt-%TiC powders was studied using a capillary rheometer. The flowability and the sensitivity of viscosity to shear rate and temperature of the feedstock were investigated and compared with those of the binder system and the 316L SS PIM feedstock. The general rheological indexes were examined through relevant equations and the influence of TiC addition on the mouldability of the 316L SS feedstock was determined. It was found that all the feedstocks are basically pseudoplastic but the values of flow behaviour index n are influenced by the TiC addition, the solid volume fraction and the... 

In this study, the kinetics of dynamic recrystallization is predicted under hot rolling conditions employing a first-order rate equation. The proposed model considers the effects of temperature and strain rate variations on recrystallization kinetics by simultaneously modelling heat transfer and plastic deformation phenomena. To do so, a two-dimensional finite element method is coupled with the transformation equation to calculate the temperature distribution and the strain rate field as well as the kinetics of dynamic recrystallization within the metal, concurrently, The model developed was examined for a low carbon steel. A series of hot compression tests were carried out at different... 

A mathematical model is proposed for evaluating flow behaviour under hot deformation conditions. The effects of dynamic recovery and recrystallisation as well as temperature and strain rate variations are considered in the model by means of Bergstrom's approach and the additivity rule for strain. To verify the model, hot compression tests for three grades of steel together with upsetting experiments are carried out. Comparison between experimental and theoretical results confirms the reliability of the model. © 2003 IoM Communications Ltd. Published by Maney for the Institute of Materials, Minerals and Mining