mohammadjavad mirnia

The size effect in microscale occurs due to the presence of a limited number of grains in deformation area and affects the mechanical behavior of the material. Based on this, describing the material behavior in the micro scale requires new relations and theories. In this article, the fracture behavior of 304 stainless steel thin sheet was tested by performing a uniaxial tensile test on the specimens with different thicknesses and grain sizes in the micro scale. The results showed that the fracture strain of different specimens in the micro scale decreases with decreasing thickness or increasing the grain size of the sheet. In order to consider the interactive effect of sheet thickness and grain size, the aspect ratio parameter was defined as the ratio of thickness to the average grain size of the sheet. Based on this, Ayada phenomenological ductile fracture criterion was modified by considering the size effect in the micro scale. Predicting the fracture locus using the modified criterion is in accordance with the fracture strain of different specimens in the micro scale, and this criterion can predict the fracture strain of different specimens with a maximum error of 4.4%. Based on the obtained results, it was found that the proposed fracture criterion can be used to predict fracture in the micro scale processes.

In this article, two different deformation strategies of single point incremental forming were investigated for producing a circular flange from AA6061 aluminum sheet with a thickness of 1mm. The effect of parameters such as step increment, tool diameter, initial height of the flange free edge, and deformation strategy on dimensional accuracy and thickness distribution was investigated Results revealed that employing the 2S tool path strategy, which involves a multi-stage forming process, significantly enhanced dimensional accuracy in flanging for a 5mm free edge compared to the 1S tool path strategy, a single-stage process. Additionally, the initial height of the flange free edge that could be formed was greater in the multi-stage 2S strategy. The thickness distribution of the 1S strategy exhibited a 1.02% improvement over the 2S strategy for a 5mm free edge, whereas for a 10mm free edge, the 2S strategy showed a 2.15% improvement compared to the 1S strategy. It was also observed that with increasing tool diameter and step increment, dimensional accuracy increased and part thickness decreased with increasing tool diameter and step increment.

In this paper, introducing Electrical Resistance Hot Spinning (ERHS), heat transfer between the component of circuit is investigated. To this aim, at first the equipment and configuration of the process are described by which mechanical force of roller toll and resistance heating simultaneously deformed pure titanium blank to a conical shape in spinning process. Then by study in electrical resistance of the component and heat transfer between them, the amount of heat generated in blank which raises its temperature to recrystallization is estimated and achieved. For measuring the electrical resistance of the components this parameter calculated by geometrical and physical characters of each component. Furthermore, investigation of heat transfer between them and to environment because of the spinning of blank and moving the roller tool simultaneously is too complicated, but neglecting some of nonsignificant parameters this aim was achievable and the highest temperature of the blank the time needed was estimable. The results of the study which investigated in transient and steady state and verified by experimental examination showed that radiation in transient and convection for unrotated parts are neglectable.

Incremental forming of metal sheets is one of the new methods of forming, in which the local exertion of forming forces and the absence of a matrix enhances the forming limit of the sheet and extends the flexibility of process in producing of complex geometries. In this research, the forming limit of aluminum/copper bilayer sheets in the single-point incremental forming of different geometries was studied. Considering the instability mechanism of the sheet, the Xue-Wierzbicki damage criterion was used in the form of the VUMAT subroutine of Abaqus in the numerical prediction of the growth and damage initiation of bilayer sheets. Experimental tests showed that the type of geometry has influences on the forming height limit due to the different induced stress and strain states on the sheet. The prediction of the numerical model of the forming height limit on average for different geometries with difference of 8% compared to the experimental tests, which indicates the validity of the numerical model. Accordingly, using the numerical model, the effect of changes in equivalent plastic strain and triaxial stress as crucial variables on the distribution of surface strains and damage was analyzed. Also, cyclic and nonlinear loading in this process was shown by plotting the strain path for different geometries.

Two points incremental forming is divided into two categories, negative and positive, and the process investigated in this research is Two points incremental forming. Today, in the forming industry, the production of parts with Dimensional accuracy and proper thickness distribution is very important. The aim of this research is to improve the dimensional accuracy and thickness distribution of the sample produced by the negative two-points incremental forming process. For this purpose, thickness distribution and dimensional accuracy of conical parts have been investigated by making changes in process parameters such as tool diameter, step down, type of lubricant and deformation strategy. To do this research, a numerically controlled milling machine, a complete set of negative die, St12 steel sheet with a thickness of 0.9 mm, spherical tool and SAE 10W oil lubricant were used The analysis of the results of the tests shows that due to the increase in the diameter of the tool at a constant step down, the minimum thickness increases from 0.24 to 0.5 mm, which has caused a decrease in thinning and ultimately the possibility of tearing in the sheet. Reducing the diameter of the tool in a constant step down has led to better shaping in the sharp corners of the production parts, and by applying a new shaping strategy and increasing the number of stages from one stage to two stages, the minimum thickness has increased. This state has improved the thickness distribution of the sample and reduced the possibility of tearing in the sample, and according to the investigations carried out in this research, the lubricant did not have a significant effect on the thickness distribution and dimensional accuracy of the production sample, and the use of the lubricant in this research only reduced the surface roughness.

In forging of bimetallic components, products with a high strength to weight ratio can be obtained. The purpose of this research is to forge a bimetallic component made from aluminum and brass alloys as the inner and outer parts, respectively. The forging temperature for the brass and aluminum alloys is considered in the hot work range as 700 °C and 450 °C, respectively. First, the forging of a single component is investigated numerically and experimentally. After validating the finite element model, bimetallic components are forged in single stage. The effect of brass ring thickness and height difference between aluminum core and brass ring is studied on the success of the single-stage hot forging process. The results show that the thickness and height of the brass ring do not have a significant effect on the success of the process in terms of complete covering of the core by the ring. In the following, to solve this problem, non-simple geometries are designed for the core and ring as a preform, then forging of the second stage are numerically investigated. The results show that in order to produce a bimetallic component with a complete covering of the aluminum core by a brass ring, a two-stage forging is needed using a suitable non-simple preform. Finally, the preform approved by the FEM is experimentally produced and hot forging is performed on it. The experimental results confirm the numerical results. SEM images show that an appropriate metallurgical bonding is created at the interface of two metals.

The flaring of the ends of thin-walled tubes is a subset of single-point incremental forming. In this research, the experimental study and statistical analysis of the variables affecting the end diameter of formed tubes were considered. In the present paper, the design of the experiment was done based on the response surface methodology. In order to form the end of the AISI 304 steel tube and perform the statistical analysis, process input variables including tool diameter, tool angular step, tool vertical step, and type of lubricant were selected. Then the effect of input variables on the tube end diameter was analyzed. Also, the tube end diameter function was extracted from the input variables in terms of linear, interactive, and quadratic expressions, and its competency and adequacy were confirmed. The analysis of variance results showed that the expressions of the tool vertical step, tool diameter, and the interactive effect of the product of tool diameter and tool angular step are the most effective expressions on the tube end diameter. Finally, the optimal combination of process input variables to achieve the maximum tube end diameter was determined using the desirability method, and by running the verification test, the correctness of the regression equation to predict the tube end diameter was confirmed.

In recent years, the hydroforming process for manufacturing complex parts such as combustion chamber and gas turbine nozzle baffle, in which the forming process of materials, especially super alloy sheets, has a key role, has attracted the attention of power plant industries. Also, this process is used as a supplementary and sizing process at the end of the manufacturing process to increase the dimensional accuracy of the assembled component. In this research, the hydroforming process of AISI304 stainless steel tube for manufacturing of gas turbine nozzle baffle has been investigated. Before conducting the experimental tests and for a more accurate evaluation, the hydroforming process of the pipe was simulated in three stages: pre-forming the pipe, heat treatment of the pre-formed pipe and finally performing the hydroforming process in the final form, and in this regard, a numerical model was created. Finite element simulation was developed in Abaqus commercial code environment. In the following, the simulation results were compared with the laboratory results and a good agreement was observed between them. Also, the best form change mode to achieve the final form of the baffle is hydroforming the annealed preform tube at 100 bar oil pressure.

The deformation behavior of the material in micro-forming processes is different from macro-scale due to the size effect. The size effect in micro-scale appears due to few grains in the deformation region and causes the material behavior to be affected by the thickness and grain size of the sheet. Because of this, conventional constitutive models are not suitable for predicting the material behavior in micro-forming processes. In this paper, a new constitutive model based on the Swift equation and considering size effect in micro-scale is presented to describe the strain-hardening behavior of the stainless steel 304 foil. Comparison of flow stress curves of specimens with different grain sizes showed that the prediction of material flow stress with the new constitutive model is improved compared to the existing model, especially at high strains, so that the average and maximum error of the new model is less than one-third and less than half of the conventional model error, respectively. Finite element simulation of the micro-tensile test was performed using the new constitutive model to investigate the size effect on the deformation behavior of the specimens. The new constitutive model was verified by comparing the results of experimental tests and finite element simulation of sheets with different grain sizes. Also, the results revealed that the estimation of the forming force using the new constitutive model is done with higher accuracy than the conventional and existing model for sheets with different grain sizes and high strain ranges.

In this paper, the effect of current intensity on changes in the microstructure of the hot resistance spinning of commercially pure titanium alloy has been investigated. For this purpose, a set-up for applying resistance heating to the sheet was installed and the configuration of the machine was designed and built in such a way that during the spinning process, resistance heating can be applied simultaneously and titanium sheets can be made into cones. The incomplete part was subjected to the hot resistance spinning process. Then, by conducting experiments in different forming conditions with a change in the intensity of the passing current at a constant thickness and feedrate, different parts were obtained and the microstructure of the material and its granulation, in different forming states and in the part of its various aspects have been studied. The results from this research show that the force applied in spinning in resistance heating at all current intensities has caused the material to become fine. But from a certain current intensity, the material has more time after the completion of recrystallization to remain at higher temperatures, and because of this, there is a greater possibility for the growth of recrystallized grains. Also, the hardness changes in the Vickers scale in the intensity of different currents show that with the increase in the intensity of the current in all parts of the piece, its hardness has increased.

In this paper, fracture prediction accuracy was evaluated by the GTN ductile fracture criterion and the effect of its evolution. To investigate the stress states, three calibration tests, including uniaxial tension, plane strain tension, and In-plane shear tension, were used to calibrate the failure criterion and determine the accuracy of fracture prediction. For simulation of the fracture behavior in Aluminum 6061-T6, the GTN ductile fracture criterion was calibrated using the combined experimental-simulation method. ABAQUS software was used to simulate the forming process, and fracture criteria were implemented to the software by the VUMAT subroutine. The force-displacement values and the fracture displacement in the experimental tests were used to validate the numerical results and evaluate the fracture criterion accuracy. According to the results, calibration using uniaxial tension and In-plane shear tension tests predicts failure with an average error of 6.17%. While the original GTN criterion cannot predict the fracture of the In-plane shear tension test, the error value in the plane strain tension test reaches 24%. A U- bending test was performed to investigate the fracture behavior of Aluminium 6061-T6 sheets and validate the calibrated fracture criterion in a more complex process other than tension tests. The Extended GTN criterion was found to predict the onset of fracture in the U-bending process with an error of 3%.

Today there are generally two types of forming methods used in the manufacture of body and metal components for ships and vessels, each of which has its own advantages and disadvantages. In this paper, the use of pulsating pressure hydroforming method has increased the formability of metal sheets and it has been shown that steel plate can be formed with higher height than conventional methods. For this purpose, a sheet hydroforming die was made in which cylindrical workpieces of different heights were formed. The cylindrical workpiece was chosen as a standard shape for forming. To apply the oil pressure with different modes, a device was developed that was capable of generating pulsating pressure paths such as sinusoidal, triangular, stepped and trapezoidal states. The results showed that if the oil was applied at a constant pressure of 180 bar, the formation depth increased by 56.25% compared to the oil-free state. Also, increasing the oil pressure under the sheet increases the height of the rupture. Using pulsating pressure in the range of 180 to 220 bar with a frequency of 20Hz, the rupture height was increased more than 4 times and 47.82%, respectively, without oil and with 220 bar constant pressure. It was observed that as the pressure increased, the deviation from the thickness distribution increased. However, if the pulsating pressure is used, the deviation from the standard thickness of the workpeice is less than 220 bar. However, the forming depth of the workpiece with pulsating pressure is approximately 50% greater than the constant pressure state of 220 bar.

Today, the use of aluminum alloy metal sheets in the shipbuilding industry is expanded due to its high strength to weight ratio and good corrosion resistance. Due to the low formability of this alloy at room temperature, the deep hydrodynamic drawing process of AA6061 aluminum alloy double-stepped part under different heat treatment cycles has been experimentally investigated in this study. Firstly, the maximum forming depth before sheet rupture was obtained at different oil chamber pressures under the initial T6 heat treatment and 12bar was selected as the best sheet forming pressure. The results show that although by choosing the appropriate process parameters, these sheets can be formed with the least defects such as cracking and wrinkling, but the formability depth is very limited in the initial T6 state. Therefore, the effect of solution, aging and annealing heat treatments on the depth of forming, hardness and thickness distribution of hydroformed parts was investigated. Solution heat treatment at the sutiable temperature and time can improve the formability of the parts in the hydroforming process by 91.7%, while the hardness of the part is reduced by 36.2% compared to the initial T6 state. Hydroforming after the aging process reduces the formability by 16.7%, due to the increase in part hardness by 11.9% compared to the initial T6 state. With annealing heat treatment, the forming depth is improved by 116.7% compared to the initial T6, which results in a 69% reduction in the hardness of the workpiece.

Severe plastic deformation processes have been developed for the fabrication of material with proper mechanical properties over the past decade. The Extrusion process with Equal Channel Angular Pressing as a new process is able to apply a severe plastic deformation due to two continuous shear regions with higher strain rates and more homogeneous strain distribution compared to some of the SPD processes like ECAP. In this study, a new process of severe plastic deformation is introduced and fabricating an high strength AA1050 specimen was investigated experimentally and numerically which led to presenting an effective parameter of this process. It has been shown with using this new process, it is possible to fabricate samples with a higher strain rate and more homogeneous distribution compare with conventional ECAP and extrusion, and the limitation of the high length sample was significantly reduced than the conventional ECAP which friction with the die wall prevents it from continuing. It has been shown that the fabricated sample of one pass of this process has a 50% increasing in yield strength than the annealing sample, which indicates the high efficiency of this process.

Welded sheets are made by butt connecting of the base plates in various welding methods. These sheets are called tailor welded blanks, welded sheets (TWBs). Due to their many benefits, they have found many uses in various parts of the industry. In this Research, the formability and effect of weld line of TWBs of St12 and St14 in thicknesses of 1 mm and 1.5 mm in single point incremental forming in the form of a truncated pyramid has been investigated. At first, the maximum depth that can be shaped in single point incremental forming of a truncated pyramid from each of the base plates for different values of wall angle was obtained and the critical wall angle for each of them was determined. Then, the incremental forming of the truncated pyramid was done using welded sheets at the critical wall angle of the base plates. In this regard, TWBs were considered with different layouts and combinations of base plates. To investigate the effect of weld line on forming depth, each combination was prepared in two weld directions of 0 and 45⁰ relative to the rolling direction. The experiment results showed that the fracture depth of TWBs with 45⁰ was reduced by an average of 14% compared to TWBs in the rolling direction. Furthermore, the average depth of the TWBs at the average critical wall angle of the base plates decreases to about 25% compared to the base plates. Therefore, the TWBs should be formed at their critical wall angles.

Incremental forming is considered as one of the rapid prototyping methods and has a high degree of flexibility and cost-effectiveness at low production volume. Meanwhile, the lack of technical knowledge has challenged the use of this method in the industry. One of the things that can help the actual usage of this process is the suitable process window; a window used to determine maximum tearing depth of the sheet with respect to the material, thickness and wall angle. In this study, firstly, the formability of low-carbon steel sheet, St12, with the thicknesses of 1.25 and 1.50 mm in single point incremental forming of a truncated pyramid with different constant wall angles has been investigated experimentally. Then, it is compared with the formability of the truncated pyramid with variable wall angles under two different wall geometries. Based on the experimental results, the process windows are presented in terms of the maximum depth and wall angle and compared to each other under different circumstances. The results showed that the critical wall angle for St12 sheet in incremental forming of a truncated pyramid with a fixed wall angle differs from the pyramid with variable wall angle, but doesn't depend on the size of the pyramid base. The critical wall angle for the fixed and variable wall angle pyramids was obtained 67⁰ and 75⁰, respectively. For a pyramid with a fixed wall angle, the thickness distribution of the wall is almost constant, while for a pyramid with a variable wall, it varies along the path.

Forming limit diagram (FLD) is one of the useful tools in the assessment of the sheet formability for designing industrial products. Experimental methods have been developed to determine FLDs. Costly and time-consuming experiments have led to several studies on the use of analytical methods and finite element softwares for predicting FLDs. In the present study, the necking and fracture forming limit curves of AA2024 aluminum alloy sheet were experimentally and numerically obtained through the hemispherical stretching test. Different geometries of the initial blank were considered to create different strain paths. The commercial finite element code Abaqus/Explicit was utilized to simulate experimental tests. Using theoretical equations and experimental results, fracture properties of the aluminum sheet in terms of the equivalent plastic strain at fracture, the stress triaxiality and the Lode angle parameter were captured and implemented in the Abaqus software. In order to capture necking forming limit strains, a numerical criterion based on the major strain variation in the necking zone has been considered. The comparison of the results shows that the numerical model can predict the forming and fracture limit strains with the maximum error of about 6%.

Tube hydroforming is a process which is considered to produce integrated and seamless parts in recent years. The numerical prediction of tearing to design the right equipment in this process is important. In this study, the formability of 304 stainless steel tube by free bulge test was experimentally and numerically evaluated to determine the forming limit diagram. The Gurson- Tvergaard- Needleman (GTN) is a micromechanical model to predict ductile fracture of metals. In order to determine the defining parameters of the GTN damage model, the experimental tensile test of the standard sample and the finite element simulation using ABAQUS software was performed. Using this criterion in the ABAQUS software and comparing the force-displacement diagram obtained from the experimental tensile test and the finite element simulation, the parameters of the GTN model was obtained by the inverse method. Then, the geometrical parameters of the die in the free bulge hydroforming process were investigated by the GTN ductile fracture criterion and the forming limit diagram of the 304 stainless steel tube was numerically obtained. The experimental tests were also carried out to verify the results of the finite element simulation. It’s shown an acceptable agreement

Incremental sheet forming has already provided distinct advantages, such as inexpensive tools and the simplicity of the process, over conventional sheet forming processes. However, the method still has some limitations. Among these limitations, severe thinning has significant effects on the performance of the final product. Also, some parts with high wall angles cannot be formed by single stage incremental forming. To overcome these restrictions, multistage incremental forming can be implemented to achieve the desired wall angle, better thickness distribution, and the lower thinning. In this study, a two-stage incremental forming of an aluminum truncated pyramid with a wall angle of 70° was studied experimentally and numerically in order to improve the achievable minimum thickness. By introducing two-stage forming strategies and achieving their defining parameters using finite element simulation, the sheet thinning was compared to the one in the single-stage forming. Experiments were used to validate the finite element analysis. The results revealed that using the two-stage forming strategy, the minimum thickness can be improved twice than the one in the single-stage forming. A good agreement was observed between the thickness distribution obtained by experiments and predicted by the finite element modeling. Finally, the effect of forming strategies on the strain paths was investigated through the finite element simulation and the experimental fracture forming limit diagram.

The single point incremental forming, which is appropriate for low volume production, is one of the simplest varieties of incremental sheet metal forming process. One of the critical issues with single point incremental forming is excessive thinning which affects the strength of the part and confines the applicability of the process to produce only parts with small wall angle. In this paper, multistage single point incremental forming of a truncated cone with 70° wall angle made from an aluminum alloy sheet is studied to alleviate excessive thinning. By proposing a new two-stage forming strategy and obtaining the corresponding parameters using an appropriate algorithm, it is shown that thinning and forming time can be improved through a systematic design of multistage forming. The implementation of the designed two-stage single point incremental forming leads to less thinning in the part when compared to either the two or three-stage single point incremental forming based on a conventional strategy. The bulging at the bottom of the part, which is one of the drawbacks of multistage single point incremental forming, can also be controlled by using the proposed strategy.