جستجوی مقالات مرتبط با کلیدواژه « تغییر شکل پلاستیک شدید » در نشریات گروه « مکانیک »

In the present study, the AA 2024/AA 7072/AA 2024 multilayer composite was fabricated through an accumulative roll bonding process at 250°C. The resultant mechanical properties were evaluated after different deformation cycles at ambient temperature. It was observed that all layers were uniform and continuous during the first cycle of accumulative roll bonding. Still, after the 4th ARB cycle, AA 7072 layers were gradually necked and separated into small fragments with continuous deformation up to the 6th cycle due to lower formability compared with AA 2024 layers. After the 7th deformation cycle of ARB processing, AA 7072 fragments were distributed non-uniformly in the AA 2024 matrix. In addition, fractography analysis was conducted using scanning electron microscopy (SEM) to observe the microstructure evolution and the fracture mechanism. Also, the mechanical properties were evaluated by tensile testing and micro-hardness measurements. It was observed that hardness and tensile strength improve with increasing accumulative roll bonding cycles. Maximum tensile strength of about 389MPa was obtained after 7 cycles of accumulative roll bonding.

In present study, an improved severe plastic deformation process named improved tube cyclic expansion extrusion process has been introduced. The idea of this process is taken from the conventional tube cyclic expansion extrusion process, and in this novel process, it is tried to solve some important problems of the conventional process. Improved tube cyclic expansion extrusion process is capable of severe plastic deforming and improving microstructure and mechanical properties of tubular components. Also, this process can be considered for producing relatively long tubes. For this purpose, the improved tube cyclic expansion extrusion process was successfully performed on AZ91 magnesium alloy tubes, up to two passes. Then, the microstructure evolution and the mechanical properties improvement were scrutinized. The results showed that the microstructure and mechanical properties were improved considerably. In this way, after two passes of this process, an ultrafine grained (UFG) microstructure was formed, and the values of ultimate strength (UTS), hardness (Hv) and ductility (EL%) became 3.6, 1.83 and 1.8 times higher, respectively. Also, the comparison of the results of the improved tube cyclic expansion extrusion process with those of the conventional tube cyclic expansion extrusion process indicated that ultimate strength and hardness of the improved process were near to those of the conventional process, but the value of elongation to failure of the improved process is considerably higher than the value of the conventional process. This can be considered as one of the important advantages of the improved process over the conventional process.

In this study, Tube cyclic Extrusion Compression (TCEC) process was performed on copper pipes at ambient and 200°C (dynamic recrystallization activation threshold temperature) and the effect of process temperature on microstructure and mechanical properties were investigated. The results showed that by applying TCEC at ambient temperature, an equiaxed crystalline structure with an average grain size of 370 nm was obtained while by increasing the forming temperature to 200°C, the equiaxed grains with average size of 720 nm resulted. Applying the TCEC process at ambient temperature on a copper tube, increased its yield strength from 116 to 242 MPa and its average hardness from 69.2 to 101.4 Vickers while at the forming temperature of 200°C, the yield and ultimate strength were about 217 and 225 MPa, respectively, and the average hardness value was about 86.8 Vickers. Therefore, performing TCEC process at 200 °C on copper reduced the strength by 10% and increased the elongation to fracture by 21% compared to the cold forming state.

One of the main methods for strengthening and increasing the hardness of the materials is the use of the severe plastic deformation (SPD) processes. Equal channel angular pressing (ECAP) method is one of the SPD methods. Two challenging issues including (1) the very high required punching force, and (2) the deflection of the punch restrict the application of the method in industry. In this paper, a new design of ECAP process is proposed, which in addition to reducing the punching force, eliminates the deflection problem. The main purpose of this paper is to investigate the effect of the process parameters including process temperature (100- 200-300) ° C, lubrication (dry and graphite) and the number of ECAP passes (1-2-3) on the hardness of 7075 aluminum material and determining their optimal levels. The Taguchi design is used for design of experiment and the S/N ratio is used to find the optimal levels of the parameters and to maximize the hardness of the material. Based on the analysis of the variance, the process temperature and the number of ECAP passes have the greatest effect on the hardness of the post ECAP samples. However, the lubrication condition has not a significant effect on the hardness of the post ECAP samples in all temperatures.

In the present study, in addition to reviewing the severe plastic deformation (SPD) processes and their mechanisms in the microstructural evolution, previous researches on the corrosion behavior of various metallic materials that have undergone SPD are investigated. Grain size, as an important metallurgical parameter, affects a broad range of mechanical, chemical, physical and electrochemical properties of metallic materials. As mostly reported, the reduction in grain size is in line with a considerable improvement in the mentioned properties. In recent years, SPD methods have been nominated as an ideal way to reach nano- and ultrafine-grains. Among the mentioned properties, the corrosion performance of ultrafine-grained materials produced by SPD methods is still controversial for researchers. The purpose of this study is to review the SPD methods, the mechanism of grain refinement during them, and their effects on the corrosion behavior of various metals and engineering alloys such as steels, aluminum, titanium, copper, and magnesium.

The fine-grained metals and alloys produced in various ways have good mechanical properties such as high strength, high toughness and proper formability and have certain applications. In recent years, studies of different methods of producing and improving the mechanical properties of specimens have been the subject of much research. Recently, the processing of this type of material using severe plastic deformation has received considerable attention from researchers. In this study, in order to modify the conventional ECAP method, The copper specimen was subjected to severe plastic deformation by means of a new die named combined die. In order to compare the mechanical properties of the produced specimens with this method and ECAP, the experimental tests were performed. By investigation the hardness values at different points from the cross-section of the specimens as well as the determination of strength and grain size in the created microstructures, it was found that after performing the present method, a finer grain structure with a higher degree of hardness, yield strength and Ultimate tensile Strength could be obtained compared to the ECAP method

In this study, commercial pure copper was subjected to a combined forming process. For this purpose, a twist extrusion die with an outlet channel narrowed was used. In this technique, by applying radial and longitudinal strains, different shear sliding systems were activated which leads to a change in mechanical and physical properties of the sample. The results showed that the microstructure of the sample was changed via passing through the twist extrusion die and a very fine gain size was obtained.The grain size was further decreased by passing through the outlet channel and applying direct extrusion, and more homogeneous grains were achieved. The compressive yielding strength was increased from 115 to 167 MPa and the average hardness value was raised from 80 to 127 Vickers by applying the twist extrusion process to the copper. After applying the direct extrusion process to the sample, these values reached 192 MPa and 140 Vickers, respectively. The results also showed that by applying twist and direct extrusion to the copper, its surface electrical resistance increased about 7 %, compared with the reference sample.

In present research, the severe plastic deformation process of hydrostatic tube cyclic extrusion compression was applied through two passes on the aluminum alloy series 5052 tube, and after that the changes of microstructure and mechanical properties of tubes were studied. Hydrostatic tube cyclic extrusion compression process is able to improve the microstructure and the mechanical properties of tubular pieces. Also, this process has the potential to produce relatively long tubes. In this process, owing to the use of pressurized fluid between the tube and die, the friction force is eliminated in these regions. This facilitates the production of longer pieces. Also, in this process, higher hydrostatic compressive stresses are applied on the material causing the delay in the crack formation and propagation. Thus, higher strains can be applied on the material. After two passes of this process, some microstructural and mechanical properties were improved significantly. For instance, the yield strength and the hardness became 1.7, 2.6 and 2.1 times higher, respectively. Also, a loss of ductility of 16℅ was observed. The microstructure analysis revealed that after two passes of the process, the microstructure was changed from a coarse grain microstructure with an average grain size of about 360 μm to an ultrafine cellsubgrain microstructure with average size of about 635 nm.

In this paper, the severe plastic deformation by twist extrusion (TE) process is investigated. In this process, the workpiece is forced through a twist channel, which develops shear strains in the material. These shear strains result in a fine-grained structure in the material. For investigating the material deformation, the TE process was simulated in the Abaqus finite element (FE) software. The effects of backpressure and friction between the sample and channel were investigated. In the FE model, a sample of AA6063 aluminum alloy was formed in a twist channel with an angle of 45 degrees and a length of 16 mm. The FE simulation results were validated by conducting experimental tests. When the backpressure of 50 MPa was used, the sample was distorted, which improved by increasing the backpressure to 70 MPa. The highest strains occurred in the corners of the sample, which was about twice of the plastic strains obtained at the side edges of the sample. Furthermore, the calculated plastic strains in the central areas of the sample were very low (0.042) in comparison to the strains developed in the corners and side edges of the sample.

In this study, the effect of multidirectional forging (MDF) was studied on the microstructure and mechanical properties of Ti-modified SiP/ZA22 composite containing 4 and 8 wt. % Si. The forging process was performed at 100 °C by two and five passes. Based on the obtained results, Ti modification refined the coarse primary dendrites, and reduced the size of primary Si (SiP) particle as well as grains. Applying MDF also gradually eliminated the dendritic structure, promoted fine distribution of SiP particles, second phases, and porosities in the microstructure. According to the image analysis results, the average size of SiP particles in as-cast composite reduced from 25 and 30 μm to about 6 and 7 μm, respectively in 5-pass MDFed composites containing 4 and 8 wt. % Si. The mechanical properties results also showed work softening during the MDF where after two-pass MDF the hardness and tensile strength of the base sample reduced by 30 and 25%, while its elongation and toughness improved by 120 and 325%, respectively. In MDFed composites, the presence of SiP particles maintains the hardness and strength. According to the results, in the case of 2-pass MDFed composite containing 4 wt. % Si the hardness and tensile strength reduced by 18 and 2%, respectively, but the elongation and toughness increased by 25 and 175%, respectively.

In present study, hydrostatic tube cyclic extrusion compression process is introduced as a novel severe plastic deformation process for grain refining and improving mechanical properties of tubular components. Also, this process has the potential to produce relatively long and large tubes. In this process, because of the utilization of pressurized hydraulic fluid between the tube and die, there is nearly no contact friction. This leads to a significant reduction in pressing load. In this research, after applying hydrostatic tube cyclic extrusion compression process on pure copper tube, the microstructure evolution and the mechanical properties improvement were examined. The results denoted that this process was successfully performed on pure copper tube. In this way, the microstructure and mechanical properties were improved significantly. For example, after this process, the ultimate strength of pure copper, the yield strength and the value of hardness became 1.57, 1.85 and 1.86 times higher, respectively, and a low loss of ductility was achieved. Also, after this process, an ultrafine cellular microstructure with average size of about 990 nm were observed. While, the average value of grain size for the unprocessed tube was about 40 μm. The stages of the formation of the observed microstructure are as follows: the creation of a high density of dislocations, the dislocations coalescence with each other and the formation of tangled structures, the formation of ordered arrangements of dislocations and low angle boundaries, the formation of dislocation cells to diminish strain energy, the creation of new dislocations and their movement to boundaries.

In this study, the feasibility of aluminum-copper bimetallic bar production was investigated. The aim was to achieve a strong interlayer bond so that the mechanical properties of the produced bar be better than embedded metals. For this purpose, an equal channel angular pressing process (up to a maximum of four passes) together with two steps of direct extrusion process (before and after ECAP process) was considered. The effect of stored strain in each step on the strength of the bond and some other mechanical properties was investigated. In bimetallic samples that were subjected to two steps of direct extrusion, the compressive yield strength and bond shear strength were 263 and 9.4 MPa, respectively. Applying and repeating the ECAP process increased these values so that for the bimetallic samples under two steps of direct extrusion plus 4 passes of ECAP, compressive yield strength and bond shear strength increased by about 38% and 150%, respectively. Comparing the hardness of the primary and final samples showed a significant increase in the processed samples. The results were confirmed using the microstructure analysis of copper and aluminum and observation of the bonding layer in each of the bimetallic samples.

This research is mainly focused on to study microstructure and mechanical properties of AA5051 aluminum alloy deformed by equal-channel angular pressing (ECAP) process at 200˚C and in BC routes and 4 four passes. The ECAP processing was carried out using die with an intersecting channel angle ‘ϕ’= 120° and corner angle ‘Ψ’= 20°. The results of uniaxial tensile test showed that tensile strength was found to be increased from 115MPa for annealed sample to 239MPa after four passes ECAP in route BC that shows that the strength in ECAP samples has increased. In addition, the percentage of elongation also decreased in initially passes and then increased slowly. Microstructure and grain refinement of specimens were investigated by optical microscopy and scanning electron microscopy and fractography were investigated by scanning electron microscopy. The grain size of annealed sample was 123μm and decreased to 18μm after four passes ECAP in route BC. The hardness also increased from 51HV in annealed sample to 90HV the fourth passes.

Fine grain materials exhibit excellent mechanical properties and are widely used in various industries. One way to produce fine grain bar is by using the severe plastic deformation techniques. Cyclic extrusion and expansion of the sample is used as one of the methods of severe plastic deformation for production of fine-grained bars. As the length of piece increases, the friction force increases, so that the required force for shaping operation is increased to such an extent that the process cannot be performed. For solving this problem, the "Cyclic Extrusion and Expansion under Hydrostatic Pressure" is proposed as a new method of severe plastic deformation for production of long-length fine-grained bars. In this method, the forming operation was done by using a pressure oil, so the hydrostatic compressive stresses are applying to the material and improve the mechanical properties. Also, the results of simulation of finite elements of this method show the effect of friction coefficient on the forming force and independence of the forming force from the bar length due to the hydrostatic process. Therefor the process is capable of producing rods of long length and fine structure. Results of pure copper rebar underwent this process showing that the yield strength and final strength increased by 200% and 33%, respectively. Also, the sample hardness increased substantially by 120%, and the distribution of relatively homogeneous hardness in rebar diameter was obtained. The microstructure results showed a fine-grain after the process, with the grain size reduced to 8μm in center and 5μm in outer diameter.

Magnesium and its alloys have received much attention not only in the aerospace and electronics industry, but also in medical applications due to its low density, excellent physical properties, and biocompatibility. However, magnesium and its alloys have low ductility and poor strain hardening ability because of the hexagonal crystal structure with the limited number of slip systems at room temperature. Therefore, it seems necessary to improve their ductility and other mechanical properties via novel technologies. In this research, hydrostatic cyclic expansion extrusion has been used to produce ultrafine-grained magnesium rod. Properties of produced rods have been investigated morphologically and mechanically. The numerical investigation has also been performed to show the effects of hydrostatic pressure on strain distribution. Due to the brittleness of magnesium, the process has been conducted at elevated temperatures. Also, due to the fluid limitation at high temperatures, melted polyethylene has been used as the fluid in the process. The results showed that the yield and ultimate strength increased by 54% and 43% after only one pass of the hydrostatic cyclic expansion extrusion process, respectively. Also, elongation increased by 46%. Furthermore, microhardness has also increased with an average of 57 Hv to 70 Hv. The microstructure result showed that the grains become ultrafine-grained after only one pass of the process. Finite element investigation revealed that high hydrostatic pressure has a good effect on improving the strain distribution and the microstructure. This process seems very appropriate for industrial applications due to its ability to produce long ultrafine-grained rods.

Hydrostatic tube cyclic expansion extrusion process is a newly invented severe plastic deformation technique for producing long ultrafine-grained and nanostructured tubes with higher mechanical properties. In the present research, this process was applied through two passes at room temperature on the commercial purity copper. Then, the hardness, tensile properties, fracture surface and microstructure of the samples were evaluated. The main goal of this research was to achieve a material with a simultaneous high strength and desirable ductility. In this process, the utilization of pressurized fluid between the die and the tube leads to first, the desired improvement of mechanical properties due to the effects of hydrostatic compressive stress. Second, the reduction of a required deforming force to eliminating the friction between the die and the tube leads to the facilitation of producing relatively long ultrafine-grained and nanostructured tubes. After two passes of process, a nearly equiaxed and homogeneous ultrafine-grained (UFG) microstructure was observed. The yield strength and ultimate strength increased from 75 MPa and 207 MPa to 310 MPa and 386 MPa, respectively. However, elongation to failure decreased from 55% to 37%. Also, the hardness value of the tube increased significantly from 59 Hv to 143 Hv, and the uniform distribution of hardness was obtained through the thickness of the tube. The fractography evaluations revealed that the predominantly ductile fracture happened in all samples of tensile testing. The hydrostatic tube cyclic expansion extrusion process can be utilized as a practical industrial method for producing relatively long ultrafine-grained tubes.

The focus of this paper is to investigate the possibility of consideration of grains and grain boundaries and their elastic-plastic behavior to predict the stress-strain behavior of ECAPed 7075 Al alloy using a finite element micromechanical approach. For this purpose equal channel angular pressing is performed on the alloy and hardness and tensile tests were performed in the macro mode as well as the micro-indentation test on distinct areas of microstructure. Mathematical relations were obtained for the correlate the hardness and static strength properties of the alloy using the obtained data from hardness and tensile tests. In addition to the mathematical relations, backward simulation of the micro-indentation process has been used in the Abaqus finite element software to convert the hardness in the grain and its boundary to stress-strain curves. The elastic-plastic behavior of the phases has been used in microstructural modeling. Modeling of the strain test has been performed in the finite element software for the microstructures using the microstructural image. The predicted stress-strain behavior from microstructural modeling has been compared with experimental results.

Accumulative roll-Bonding process (ARB) that is a severe plastic deformation (SPD) process is investigated by the authors in order to fabricate ultrafine grained aluminum/brass composite . In this study, aluminum composite is reinforced with brass in seven cycles of ARB at room temperature and without any heat treatment between production cycles. Tensile strength and hardness of the composite increases strongly in the first cycle. These mechanical properties do not change significantly with increasing the number of cycles. After seven cycles of process, tensile strength and hardness of the composite compared to the initial Al sheet respectively increased to 46 and 85 percent. Also elongation of the composite decreases strongly in the first two cycles and slightly increased in the following cycles. This changes in the mechanical properties during the ARB process are due to the strain hardening and cold working mechanism in the primary cycles and the grain refinement in the final cycles. The microstracture results of the composite shows that the average value of grains reached 250 nm and the distribution of brass particles in the composite become more uniform with increasing the ARB cycles.

In this research, semi-constrained groove pressing (SCGP) as one of the severe plastic deformation techniques was investigated to achieve an ultrafine-grained structure in interstitial free steel sheets. The maximum of four semi-constrained groove pressing passes was successfully applied on the samples and the effects of the number of SCGP passes on the microstructure and mechanical properties of the samples were investigated. The microstructural investigations of the deformed specimens indicate that the semi-constrained groove pressing can effectively reduce the grain/crystallite size so that it ranges from about 41 μm in annealed condition to 232 nm after four passes. The results also showed that the strength and hardness of the samples are increased significantly by applying the pressing process. The highest tensile and yield strengths were observed in the two-pass SCGP processed sample, which showed an increase of about 90% and 75%, respectively, compared to the initial sample. The maximum hardness value of 165 Vickers was obtained for a three-pass SCGP processed sample, which is about 68% higher than the annealed sample. Regarding the hardness tests results, the uniformity of deformation increased with increasing the number of SCGP passes. Finite element method was used to simulate the semi-constrained groove pressing, and the strain distribution was obtained for the deformed samples. The finite element simulation results correlated fairly well with the analytical results.