Papers by Mahmoud Ebrahimi
Journal of Applied Sciences, Jan 1, 2010
In this study, deformation behavior of commercial pure aluminum in the equal channel angular pres... more In this study, deformation behavior of commercial pure aluminum in the equal channel angular pressing (ECAP) is analyzed successfully using three dimensional (3D) simulation by finite element methods (FEM). The investigations include the effects of various die channel angles, low-friction and high-friction conditions and the application of the back punch pressure. It is observed that the level of the strain and homogeneity of strain distribution increase with decreasing the die channel angle or increasing the friction coefficient or imposing the back punch pressure in the outlet channel. Also, increasing the magnitude of the strain causes high punch pressure and higher press pressure for processing should be applied.
Equal channel angular pressing is both a novel and an industrialized process among severe plastic... more Equal channel angular pressing is both a novel and an industrialized process among severe plastic deformation methods to fabricate ultra-fine-grained metals and alloys. Verification of a three-dimensional finite element model which compares various strength coefficients and strain-hardening exponents (virtual materials) defined for the plastic deformation behavior of materials was performed with experimental tests. The virtual process numerically analyzed the effects of the strain behavior and pressing force. The results show that strength coefficient enhancement leads to decreased effective strain value, heterogeneous strain distribution and higher pressing force, and an increment in the strain-hardening exponent results in lower pressing force. However, this parameter does not have an obvious effect on the effective strain magnitude and strain dispersal uniformity. Furthermore, the highest imposed effective strain, the best strain distribution homogeneity and the lowest required punch load were achieved for the deformed material with the lowest strength coefficient and highest strain-hardening exponent.
Materials & Design, 2014
Ultrafine grained materials have experienced a rapid development during the last two decades. Con... more Ultrafine grained materials have experienced a rapid development during the last two decades. Constrained groove pressing (CGP) process is one of the severe plastic deformation methods to fabricate ultrafine grain sheet materials. In this research, wear behavior of brass sheet subjected to CGP process was investigated. Generally it is shown that CGP process enhances the wear resistance of the material and this behavior is improved by increasing pass number. Also, the effect of initial pass and lower applied normal load on the wear resistance is more profound than subsequent passes and higher applied normal load, respectively. In addition, the influence of normal load is more profound than pass number at the increment of friction force. Although CGP process results in reduction at the specific wear rate, the influence of the first pass is much higher than the subsequent ones. Furthermore, lower specific wear rate is occurred at the higher applied normal load. The scanning electron microscopy analyses indicated that the wear mechanism is transferred from adhesion, delamination, abrasion and oxidation for the annealed condition to abrasion and adhesion for the third pass CGP sample. Also, it is found that there is a reverse relationship between specific wear rate and hardness.
International Journal of Minerals, Metallurgy, and Materials, 2014
Progress in Natural Science: Materials International, 2012
Equal Channel Angular Pressing (ECAP) is currently one of the most popular methods for fabricatin... more Equal Channel Angular Pressing (ECAP) is currently one of the most popular methods for fabricating Ultra-Fine Grained (UFG) materials. In this work, ECAP process has been performed on commercial pure aluminum up to 8 passes by route A. After verification of FEM work, the influences of four die channel angles, three outer corner angles and pass number up to 8 have been analyzed to investigate strain distribution behavior of ECAPed material. Two methods for quantifying the strain homogeneity namely inhomogeneity index (C i ) and standard deviation (S.D.) are compared. It is shown that C i is not a good candidate for examining the strain distribution uniformity. Moreover, it is suggested that designing of ECAP die geometry to achieve optimum strain distribution homogeneity is more suitable than the optimum effective strain magnitude. The best strain distribution uniformity in the transverse plane is obtained with F¼ 601 and C ¼ 151 and for the bulk of the sample, F¼ 1201 and C ¼151 or 601, gives the highest strain dispersal uniformity.
Progress in Natural Science: Materials International, 2014
In this paper, a new severe plastic deformation method called equal channel forward extrusion (EC... more In this paper, a new severe plastic deformation method called equal channel forward extrusion (ECFE) process has been proposed and investigated by experimental and numerical approaches on the commercial pure copper billets. The experimental results indicated that the magnitudes of yield strength, ultimate tensile strength and Vickers micro-hardness have been markedly improved from 114 MPa, 204 MPa and 68 HV as the annealed condition to 269 MPa, 285 MPa and 126 HV after the fourth pass of ECFE process, respectively. In addition, scanning electron microscopy observation of the samples showed that the average grain size of the as-received state which is about 22 μm has been reduced to 1.4 μm after the final pass. The numerical investigation suggested that although one pass ECFE process fabricates material with the mean effective strain magnitude of about 1, the level of imposed effective plastic strain gradually diminishes from the circumference to the center of the deformed billet.
Materials Science and Engineering: A, 2010
Equal channel angular pressing (ECAP) was successfully performed on commercial pure aluminum, alu... more Equal channel angular pressing (ECAP) was successfully performed on commercial pure aluminum, aluminum 6061 alloy and commercial pure copper by route B C . Tensile and fatigue (under constant stress) tests shows a significant enhancement in mechanical properties consisting of hardness, yield strength, ultimate tensile strength and endurance limit, but the ductility of the alloys reduced with employing this process. Two fatigue design factors K l and K h has been suggested to co-related fatigue data from coarse grained (CG) to ultra-fine grained (UFG) materials.
Materials Science and Engineering: A, 2010
In this paper, the influences of die parameters (die channel angle and channel displacement) and ... more In this paper, the influences of die parameters (die channel angle and channel displacement) and material properties (strength coefficient and strain hardening exponent) are investigated in equal channel angular pressing with parallel channels (ECAP-PC) using three dimensional (3D) finite element simulation (FEM). It has been shown that die parameters play an important role in the magnitude and homogeneity of effective strain. In general, die channel angle has more influence on the magnitude rather than homogeneity of effective strain, decreasing the die channel angle results in a higher magnitude of effective strain imposed on the sample and a higher pressing force. But on the other hand, channel displacement has more influence on the homogeneity of effective strain, increasing the die channel's displacement results in a more homogenous effective strain on the sample cross-section. Simulation on various strength coefficient and strain hardening exponents showed that the magnitude of the strain hardening exponent does not have much effect on the pressing force. On the other hand, the level of pressing force increases when the magnitude of the strength coefficient is increased.
Applied Energy, 2014
The main results of the aerothermodynamic hypersonic characterization of the intermediate experim... more The main results of the aerothermodynamic hypersonic characterization of the intermediate experimental vehicle, by means of both computational fluid dynamics simulations and wind-tunnel measurements, have been reported and analyzed in this work, developed in the frame of ESA Future Launcher Preparatory Program. A test campaign has been carried out at the von Kármán Institute Free Piston Longshot wind tunnel in a cold hypersonic regime. The numerical simulations, rebuilding the von Kármán Institute wind-tunnel conditions, have been performed by the Italian Aerospace Research Center, the goal being to support the procedure of extrapolation-to-flight of the measurements and the general aerothermal characterization. A detailed comparison of all measured and predicted hypersonic relevant phenomena and aerothermodynamic parameters, such as surface pressure and heat flux, is reported in the paper, together with a detailed description of the configuration, freestream conditions, and model attitude effects. The reproducibility of flight conditions in the von Kármán Institute Longshot test campaign is qualitatively very good, both in terms of flow features and surface properties as indicated by wind-tunnel numerical simulations. The comparisons of numerical and experimental results indicate a good agreement in terms of external flow structure, i.e., bow shock, shock layer, expansion at the junction on windward side, shock-wave boundary-layer interaction in the flap area, and multiple-shock interaction above the flap. Sideslip, angle-of-attack, and flapdeflection effects are correctly reproduced by computational fluid dynamics in terms of pressure coefficient and heat flux.
Acta Metallurgica Sinica (English Letters), 2014
A new technique to produce ultra-fine grained tubular specimen has been proposed, and the experim... more A new technique to produce ultra-fine grained tubular specimen has been proposed, and the experiments have been performed using equal channel angular pressing (ECAP) with an angle of 90°between two intersecting channels and also the use of rubber pad as a mandrel during process. Commercial purity copper tubes have been pressed up to three passes through four different fundamental routes (A, B A , B C , and C) directions of which are identified in the text below. The influence of each route on the value, distribution, and homogeneity of hardness has been investigated by applying Vickers micro-hardness measurements at various locations of the tube's transverse planes. Significant enhancement of the hardness is observed after the first pass ECAP. Also, routes C and B C show, respectively, better average hardness magnitude and hardness distribution uniformity. In addition, the results indicate that there is about 50% and 62% reduction of the grain size, compared to the annealed condition, following ECAP process of the copper tube sample after the first and the third pass via route B C .
Acta Metallurgica Sinica (English Letters), 2013
A new technique to equal channel angular pressing of tubular samples has been proposed and invest... more A new technique to equal channel angular pressing of tubular samples has been proposed and investigated through experiments and simulations. Deformation behavior of copper tube sample was numerically analyzed during the first pass of tubular ECAP process. The investigation included the effect of various tube wall thicknesses on the effective strain magnitude and strain distribution uniformity. It is shown that tube wall thickness of 3.5 mm gives the optimum value for strain behavior. In addition, copper tube specimens with 3.5 mm wall thickness have been successfully ECAPed up to four passes with the die channel angle of 90 • using flexible polyurethane rubber pad. Micro-hardness measurements on both annealed and ECAPed tubes show that 33% and 57% increases in hardness value and also, 50% and 70% reductions in the grain size were achieved after the first and fourth passes respectively. Furthermore, tube wall thickness measurements show that the process does not change the dimension of deformed specimens.
Materials Science and Engineering: A, Dec 19, 2011
Equal channel angular pressing as a material processing technique imposes high but in-homogeneous... more Equal channel angular pressing as a material processing technique imposes high but in-homogeneous strain at the cross-section of the workpiece. In this research, cylindrical shaped commercial pure aluminum undergoes ECAP process up to four passes by route A using the conventional and covered tube casing (CTC) methods. The influence of CTC on strain distribution in the region of uniform longitudinal strain field was investigated experimentally and numerically. The Vickers hardness measurement at the cross-section ...
Equal channel angular pressing is one of the attractive methods of severe plastic deformation tec... more Equal channel angular pressing is one of the attractive methods of severe plastic deformation techniques to fabricate
ultrafine grained materials. In this study, eight different equal channel angular pressing configurations including the punch
shape, sample preform and die geometry have been analyzed using finite element method to achieve materials with the
high effective strain magnitude, better strain distribution uniformity and less required pressing force. The results show
that the combination of punch shape and back slant specimen is the most efficient modification as compared to the conventional
equal channel angular pressing process. Furthermore, investigations on various die channel and sample slant
angles indicate that the configuration of punch shape and back slant sample with the die channel angle of 75 and sample
slant angle of 30 is the optimum condition. Finally, experimental work has been performed to validate the results.
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Papers by Mahmoud Ebrahimi
ultrafine grained materials. In this study, eight different equal channel angular pressing configurations including the punch
shape, sample preform and die geometry have been analyzed using finite element method to achieve materials with the
high effective strain magnitude, better strain distribution uniformity and less required pressing force. The results show
that the combination of punch shape and back slant specimen is the most efficient modification as compared to the conventional
equal channel angular pressing process. Furthermore, investigations on various die channel and sample slant
angles indicate that the configuration of punch shape and back slant sample with the die channel angle of 75 and sample
slant angle of 30 is the optimum condition. Finally, experimental work has been performed to validate the results.
ultrafine grained materials. In this study, eight different equal channel angular pressing configurations including the punch
shape, sample preform and die geometry have been analyzed using finite element method to achieve materials with the
high effective strain magnitude, better strain distribution uniformity and less required pressing force. The results show
that the combination of punch shape and back slant specimen is the most efficient modification as compared to the conventional
equal channel angular pressing process. Furthermore, investigations on various die channel and sample slant
angles indicate that the configuration of punch shape and back slant sample with the die channel angle of 75 and sample
slant angle of 30 is the optimum condition. Finally, experimental work has been performed to validate the results.