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ramin ebrahimi

Shiraz University, Materials Science and Engineering, Faculty Member

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Papers by ramin ebrahimi

Ten years of severe plastic deformation (SPD) in Iran, part II: accumulative roll bonding (ARB)

The present paper is the second part of a previously published overview entitled “ten years of se... more The present paper is the second part of a previously published overview entitled “ten years of severe plastic deformation (SPD) in Iran”. Part I concentrates on the equal channel angular pressing (ECAP). In this part, the focus is on the accumulative roll bonding (ARB) because, currently, Iran is ranked the first in the world by the total number of publications in this field. In the present section, the emphasis is not on the microstructure and ultrafine-grained materials produced by ARB. Instead, its focus is on several aspects of ARB to which small attention has been paid so far. The impact and contribution of Iran to each category is evaluated in comparison to researchers from other countries. The main interest of Iranian researchers in the field of ARB is to fabricate the composite materials, particularly metal matrix composites (MMCs). The Iranian researchers were the first who introduced ARB as an effective method to produce particulate MMCs.

The Relationship between Constant Friction Factor and Coefficient of Friction in Metal Forming Using Finite Element Analysis

Frictional shear stress is usually determined by utilizing the coefficient of friction or the con... more Frictional shear stress is usually determined by utilizing the coefficient of friction or the constant friction factor models. The present study deals with finite element analysis of double cup extrusion process to determine the relationship between constant friction factor (m) and coefficient of friction (µ), since the metal flow in this process is very sensitive to frictional conditions. Therefore, the Finite Element-Code Deform 2D is used which is capable of utilizing both µ and m. According to this analysis a new equation between constant friction factor (m) and coefficient of friction (µ) is suggested. Moreover, in order to evaluate the suggested equation and to compare it with the pervious relationship, finite element analysis of barrel-compression test is carried out. Finite element results indicate that the new equation can accurately predict the relation between m and its equivalent μ value. The importance of converting these factors to each other is specially highlighted t...

Optimization of Specific Die Profiles in Thin Walled Tube Extrusion

In the present work, three extrusion profiles have been investigated objectively, these are a con... more In the present work, three extrusion profiles have been investigated objectively, these are a conical, a cosine which is proposed in this study and a profile designed to impose equal strain increments over the equi-spaced sections. Each of them reduces a portion of the required power for extrusion. Conical profile provides the least frictional surface, cosine profile omits the surfaces of velocity discontinuity and the other profile reduces the power attributed to redundancy during deformation. However, the capability of these profiles in reducing the total power of the process is very different. Results suggest that cosine profile is the best energy-wise, whereas the profile which imposes equal strain increments over the equi-spaced sections provides the best distribution of strain in the product. In addition, a simple exponential equation as a function of die geometry is presented for the case of the profile which imposes equal strain increments over the equi-spaced sections.

Mechanical behavior of TWIP steel in high strain rate torsional tests

Advanced high strength steels (AHSS) have recently attracted great attention because of their sup... more Advanced high strength steels (AHSS) have recently attracted great attention because of their superior mechanical properties. A modern group of these steels, known as twinning induced plasticity (TWIP) steels, shows a unique combination of strength and ductility even at high rates of strain. In order to examine the functionality of such steels in dynamic loading conditions, their mechanical behavior should be characterized by high strain rate experiments. Regarding the precision of its results and characteristics of the loading pulse, the torsional Kolsky bar experiment has well-known advantages over similar high rate experiments. In this study, a high strain rate torsional testing machine was designed and constructed. By using high strain rate torsional experiments in the range of strain rates from 500 to 1700 /s, mechanical behavior of two high manganese steels with different levels of carbon (0.49 and 0.07 wt%) was surveyed. Through the experiments, it was observed that the phenomenon of adiabatic temperature rise considerably decreased the work hardening rate during the dynamic testing of the high-carbon steel, whereas the low-carbon steel showed a definitely higher hardening rate. The influence of pre-straining on the dynamic mechanical behavior of this low-carbon steel was also studied.

Meshless analysis of backward extrusion by natural element method

In this paper, a computational technique is presented based on the natural element method (NEM) f... more In this paper, a computational technique is presented based on the natural element method (NEM) for large plastic deformation simulation of the metal forming problems. NEM is a numerical technique in the field of computational mechanics and can be considered as a meshless method. The selected process is backward extrusion for circular shape hollow components from round billets. The punch stroke value is divided into sub-steps, whereas a new set of nodes become active at the end of each sub-step of deformation. Solutions are obtained for different area reductions under friction condition. Hollman-Ludwik law is selected to explain the material behavior after the yielding point. The experiments are carried out with fully annealed commercial aluminum alloy billets at room temperature, using various punch sizes. A set of die and punches are designed and constructed for experimental works. The validity of the proposed method is verified by comparing the results from deformed geometry, contours of equivalent strain and forming loads with those obtained from finite element simulation and experimental measurements. It is concluded that the results obtained by NEM are in good agreement with those from FEM and experiments and therefore, the meshless natural element method is capable of handling large plastic deformation.

An analytical approach for radial-forward extrusion process

An analytical approach is used to obtain the extrusion pressure and the strain imposed through ra... more An analytical approach is used to obtain the extrusion pressure and the strain imposed through radialforward extrusion. For deformation analysis, the deformation volume is divided into three different regions. By considering a cylindrical coordinates, the components of the velocity field and strain rates are obtained. The effective strain achieved in each deformation region is obtained by multiplying the effective strain rates to the deformation time determined for each region. The total strain obtained from these results is large and comparable to the strain achieved by severe plastic deformation methods. Based on the upper-bound theory, an analytical equation is derived for extrusion pressure by determining the internal power and the power dissipated on all frictional and velocity discontinuity surfaces. The validity of the theoretical results is confirmed by the experiments in term of the hardness measurements and the extrusion force.

Upper-bound analysis of equal channel angular extrusion using linear and rotational velocity fields

An upper-bound approach based on the rotational and linear velocity fields is used to analyze the... more An upper-bound approach based on the rotational and linear velocity fields is used to analyze the equal channel angular extrusion process. Using this model, plastic deformation zone (PDZ) is divided into three rotational velocity fields. The equations and center axis of each velocity discontinuity surface are derived. The total power loss is optimized numerically with respect to the boundary points

Mathematical Modeling of Single Peak Dynamic Recrystallization Flow Stress Curves in Metallic Alloys

Recrystallization, 2012

Deformation of metals and alloys at temperatures above 0.5 T m is a complex process in which mech... more Deformation of metals and alloys at temperatures above 0.5 T m is a complex process in which mechanical working interacts with various metallurgical processes such as dynamic restoration including recovery and recrystallization, and phase transformation for polymorphous materials. The understanding of these processes, however, enables the behavior of the metals and alloys. Recent developments have been described in several review papers. These reviews were in agreement that metals and alloys having relatively low values of stacking fault energy (SFE) could recrystallize dynamically, whereas those of high SFE including bcc metals and alloys which behave in a manner similar to fcc metals of high SFE recovered dynamically only during high temperature deformation. So that, according to microstructural evolutions, material response can principally be divided into two categories in hot deformation: dynamic recovery (DRV) type and dynamic recrystallization (DRX) type. The process of recrystallization of plastically deformed metals and alloys is of central importance in the processing of metallic alloys for two main reasons. The first is to soften and restore the ductility of material hardened by low temperature deformation that occurring below about 50% of the absolute melting temperature which leads to lower forces. The second is to control the microstructure and mechanical properties of the final product. The analysis of metal forming process such as hot rolling, forging and extrusion has been dependent on various parameters including constitutive relation which contributes to stressstrain curves at high temperatures, shape of workpiece and product, shapes of tools, friction, temperature, forming speed, etc. In such parameters, the constitutive equation is one of the most important factors which have an effect on solution accuracy. A number of research groups have attempted to develop constitutive equations of materials and suggested their own formulations by putting the experimentally measured data into one single equation. Misaka and Yoshimoto proposed a model which gives the flow stress of carbon steels as a function of the strain, strain rate, temperature and carbon content. Shida's model takes account of the flow stress behavior of the steels in austenite, ferrite and in the www.intechopen.com Recrystallization 208 two-phase regions. Voce suggested an approximate equation of stress-stain curve considering the dynamic recrystallization. Finally, Ebrahimi et al. obtained a mathematical model according to the phenomenological representation of the shape of the stress-strain curves that consists an additional constant. Due to the importance of flow stress estimation of metals and alloys at high temperatures, the required forces for the deformation processes, dimensional accuracy of final products and simulation of processes; it is necessary to provide a model in order to eliminate the limitations of previous models to some extent. The contexts of this section could generally be classified into two main parts, at the beginning, some previous mathematical models and basic concepts in relation with dominate processes during hot deformation of metals and alloys, such as dynamic recovery (DRV) and dynamic recrystallization (DRX) will be reviewed and finally, by concluding from existing mathematical models related to prediction of single peak flow stress curves, a new model will be introducd. With regards to the importance of macroscopic data from mechanical tests as compared to microscopic ones from metallurgical investigation, due to its less time and cost consuming nature, mathematical and macroscopic aspects of DRX process are considered in this model.

An upper-bound analysis of the tube extrusion process

Journal of Materials Processing Technology, 2008

In this paper a new upper-bound approach is used to analyze the tube extrusion process. A kinemat... more In this paper a new upper-bound approach is used to analyze the tube extrusion process. A kinematically admissible velocity field is developed to evaluate the internal power and the power dissipated on frictional and velocity discontinuity surfaces. The total power is optimized with respect to the die angle. The optimum die angle and the critical die angle at which a dead zone is formed are determined. The effect of constant friction factor and reduction in area on the optimum die angle is predicted. In addition, the role of die angle on the relative extrusion pressure is investigated. Comparison of the experimental and theoretical load-displacement curves shows a good agreement.

Effect of thermomechanical parameters on dynamically recrystallized grain size of AZ91 magnesium alloy

Journal of Alloys and Compounds, 2011

Dynamic recrystallization of the AZ91 alloy was studied by conducting hot compression tests at te... more Dynamic recrystallization of the AZ91 alloy was studied by conducting hot compression tests at temperature range of 325-400 • C and strain rate of 0.001-1 s -1 . The influence of the hot deformation variables on flow stress as well as recrystallized grain size was investigated. The results showed that by decreasing temperature and increasing strain rate, flow stress increases while dynamically recrystallized grain size decreases. A power-law relation developed between the characteristic peak strain and Zener-Hollomon parameter and the exponent was determined as 0.17. Besides, the linear regression between the Zener-Hollomon parameter and dynamically recrystallized grain size developed another power law equation, with a stress exponent equivalent to -0.13.

A Modified Model to Estimate Single Peak Flow Stress Curves of Ti-IF Steel

ISIJ International, 2012

In this study a modified model, using a non linear estimation of strain hardening rate vs. strain... more In this study a modified model, using a non linear estimation of strain hardening rate vs. strain up to the critical stress, has been developed to predict single peak dynamic recrystallization (DRX) flow stress curves. The critical stress and strain for initiation of dynamic recrystallization were determined using: (1) the strain hardening rate versus stress curve, (2) the constitutive equation. In order to evaluate the accuracy of the compound model, the mean error was calculated. The results indicate that the proposed model give a good estimate of the flow stress curves.

Optimization of Hot Workability in Ti-IF Steel by Using the Processing Map

Processing map for hot working of Ti-IF steel has been developed in the temperature range of 750 ... more Processing map for hot working of Ti-IF steel has been developed in the temperature range of 750 to 1100 °C and strain rate of 0.01 to 100 s -1 . This map in the austenite region exhibits a single domain with a peak efficiency of 45% occurring at 1025 °C and strain rate of 0.02 s -1 . The domain extends over the temperature range of 1000 to 1100 °C and strain rate range of 0.01 to 1 s -1 . The true stress-strain curves and microstructural observations shows the occurrence of dynamic recrystallization in this domain. In two phase regions, where austenite and ferrite are present together, flow localization occurs in the form of bands with a fine grained structure as a result of dynamic recrystallization in the bands. These deformation bands are formed at 45° with respect to axial direction of compression. The processing map in ferrite region exhibits a domain with a peak efficiency of 38% occurring at 825 °C and strain rate of 0.02 s -1 , so this domain extends over the temperature range of 800 to 850 °C and strain rate range of 0.01 to 0.5 s -1 . The true stress-strain curves and microstructural results confirm the occurrence of partially dynamic recrystallization in this domain.

Optimization of Die Profile for Cold Forward Extrusion Using an Improved Slab Method Analysis

One of the most important design parameters in extrusion process is the shape of die profile. In ... more One of the most important design parameters in extrusion process is the shape of die profile. In the present research work, an optimum extrusion die profile has been obtained through implementation of slab analysis in a computational algorithm. Moreover, extrusion process through both optimum conical and curved die has been performed experimentally and also by finite element method. It has been demonstrated that material work hardening characteristics and friction condition have remarkable effects on the optimum streamlined die profile. Also, results prove that the streamlined die profile designed based on the developed approach, is superior to the conventional conical dies from both metallurgical and manufacturing perspectives. Consequently, the proposed method can be regarded as an efficient and reliable tool for designing streamlined die profiles. Hence, this technique can be used to produce desirable conditions in both process and product quality in terms of extrusion force, deformation homogeneity and die wear.

Microstructure and Texture Inhomogeneity after Large Non-Monotonic Simple Shear Strains: Achievements of Tensile Properties

Metals, 2018

In this study, for the first time, the effect of large non-monotonic simple shear strains on the ... more In this study, for the first time, the effect of large non-monotonic simple shear strains on the uniformity of the tensile properties of pure Cu specimens was studied and justified by means of microstructural and textural investigations. A process called simple shear extrusion, which consists of two forward and two reversed simple shear straining stages on two different slip planes, was designed in order to impose non-monotonic simple shear strains. Although the mechanism of grain refinement is continuous dynamic recrystallization, an exceptional microstructural behavior and texture were observed due to the complicated straining path results from two different slip planes and two pairs of shear directions on two different axes in a cycle of the process. The geometry of the process imposes a distribution of strain results in the inhomogeneous microstructure and texture throughout the plane perpendicular to the slip plane. Although it is expected that the yield strength in the periphe...

Effect of geometric parameters on deformation behavior of simple shear extrusion

IOP Conference Series: Materials Science and Engineering, 2014

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Microstructure quantification and correlation with flow stress of ultrafine grained commercially pure Al fabricated by equal channel angular pressing (ECAP)

Materials Characterization, 2008

Commercial purity Al was severely deformed by equal channel angular pressing (ECAP) up to eight p... more Commercial purity Al was severely deformed by equal channel angular pressing (ECAP) up to eight passes using route BC. The deformation microstructure was characterized quantitatively by electron-backscattered diffraction (EBSD) and transmission electron microscopy (TEM). The microstructural homogeneity was investigated by EBSD at various locations from center to surface of the samples on a longitudinal section parallel to the pressing direction.

The effect of homogenization on microstructure and hot ductility behaviour of AZ91 magnesium alloy

Metallic Materials, 2010

Grade AZ91 is a casting magnesium alloy for ambient temperature applications. Because of good cas... more Grade AZ91 is a casting magnesium alloy for ambient temperature applications. Because of good castability, formability and also for economical considerations, this alloy has vast applications in automotive industries. Microstructure of this alloy after casting includes α solid solution, β precipitates (Mg 17 Al 12) in grain boundaries and α+ β eutectic phase. Morphology, distribution and the amount of precipitates play the crucial role during deformation process and their behaviour can be controlled by selection the appropriate heat treatment procedure. In this study, homogenization heat treatment on AZ91 alloy at 415 °C for 9, 18 and 24 hours was performed and the influence of heat treatment on hot deformation behaviour was investigated. Hot tensile test in the temperature range of 380-440 °C and initial strain rate of 0.001 and 0.1s-1 was carried out and the results revealed that the highest tensile strength and simultaneously the maximum ductility were obtained in the sample held at 415 °C for 24 hours and drawn at 400 °C with initial strain rate of 0.1 s-1. Furthermore, microstructure and the extension of precipitates presence after homogenization heat treatment also were investigated by metallographical examination.

Analysis of the mechanical properties and deformation behavior of nanostructured commercially pure Al processed by equal channel angular pressing (ECAP)

Materials Science and Engineering: A, 2008

In the present paper commercially pure Al was processed by equal channel angular pressing (ECAP) ... more In the present paper commercially pure Al was processed by equal channel angular pressing (ECAP) up to 8 passes using route B C. For ECAP processing a proper die set was designed and constructed. Transmission electron microscope (TEM) and electron backscatter diffraction (EBSD) were used to evaluate the microstructure of the pressed materials. Mechanical properties and the deformation behavior of the ECAP processed material were investigated by the hardness and compression tests. The significant increase in hardness and yield stress after ECAP was discussed by two strengthening mechanisms. Based on these mechanisms, variations of the hardness and yield stress as a function of the pass numbers were related to the calculated dislocation densities and the average boundary spacing. Also it was suggested that the absorption of the dislocations into grain boundaries would be an effective recovery process for the absence of the strain hardening.

Strain composite strips produced by accumulative roll bonding technique

Materials Science and Engineering: A, 2013

Accumulative roll bonding (ARB) is modified for processing strips with selected distribution of a... more Accumulative roll bonding (ARB) is modified for processing strips with selected distribution of accumulated strain and hardness across their thickness, entitled as strain composite strips (SCS). Comparison of SCS and ARB processed strips showed an improvement in ductility of SCS at the cost of a small decrease in strength.