مجله مهندسی مکانیک امیرکبیر
سال پنجاه و دوم شماره 3 (خرداد 1399)

Forming limit diagram (FLD) is a useful tool for investigation of sheet's formability for designing industrial products. Experimentally extracting FLD requires exact experimental tests, and is time consuming, and expensive. Therefore, several studies have been carried out on the usage of theoretical methods and finite element software for determining these diagram. In this study, FLD for AA3105 aluminum alloy sheet were obtained by simulating the Nakazima and modified Marciniak test in ABAQUS software. In order to numericaly determine FLD of AA3105, Hill yield criterion, Hosford yield criterion and GTN damage model based on Hosford criterion and Voce and power law hardening rules were investigated. Due to the lack of the Hosford yield criterion and the GTN damage model based on Hosford criterion in the ABAQUS software, VUMAT subroutines has been developed and used to determine the behavior of the AA3105 aluminum alloy. The results showed that the predicted FLD based on the Hill criterion, shows large deviation from experimental results. The usage of the Hosford criterion and GTN damage model for aluminum alloys showed a better correlation with experimental results. Also, due to the presence of voids in metals, the GTN damage model which is based on the void volume fraction has a greater physical justification than other yield criterion. Furthermore, by comparing numerical FLDs that obtained from Nakazima and modified Marciniak tests, it was concluded that the limit strains in modified Marciniak test is lower than the Nakazima test.

Shear forming is a kind of spinning process which can be used for manufacturing of components that have a high weight to strength ratio. this process can benefichially used in the conic or semi conic shaped specimens as it done in this paper. In this paper , the main parameters for manufacturing of a constant thickness conic shaped specimen have been extracted, using a pre-form design. These parameters include mandrel rotational speed , roller feed rate, tailstock pressure and friction ratio . To this regard, the optimized efficient parameters were extracted and the obtained data has been compared and validated with the experimen . Morever , a new cup shape preform has been used. With comparison of the results, the optimized efficient forming parameters of the offered preform have been derived . It was observed that using preform will cause in better cone part specifications due to it's close dimensional and geometrical specifications . It was shown that the proposed method can beneficially lead to better results in the shear spinning process .

Forming limit diagram is one of the most applicable methods for prediction of plastic instability in sheet metal forming that temperature and strain rate is among factors effecting on the FLD. In this paper, the effects of temperature and strain rate at quasi-static condition on the true stress-true strain curve and forming limit curve of AA3104 aluminum are analytically investigated using Marciniak-Kuckzynski method and Johnson-cook model. The stress-strain response of numerical results validation using the Ludwik model is performed with experimental results. Theoretical results show a good agreement with experimental results. Then, according to the stress-strain curves obtained based on the Ludwik equation, Johnson-Cook coefficients are calculated for AA3104 and the true stress-true strain respond and forming limit diagram are produced for different temperatures 50°C، 300°C and 400°C and several t strain rates 〖10〗^(-5)، 〖10〗^(-4)and 〖10〗^(-3) s^(-1). The stress-strain curve decreases with increasing temperature and increases with increasing strain rate. Also the FLD increases with increasing temperature and decreases with increasing strain rate. The results exhibit a positive sensitivity of temperature on the limit strain due to the thermal softening and the negative strain rate sensitivity on the FLD AA3104 due to the behavior of crystallographic structure of the material

In recent years, aluminum and magnesium alloys have attracted the attention of researchers and engineers due to higher strength to weight ratio, compared with steels. The main limitation of these alloys is the low formability at room temperature. In order to overcome this limitation, researchers have shown that the formability of aluminum alloys increases at high temperatures. Tube hydroforming is known as a novel metal forming process in which the fluid internal pressure and the axial force are applied simultaneously to the tube. In this study, the formability of 6063 annealed aluminum tube has been investigated in warm tube hydroforming process. The effects of pressure and axial feed on the thickness distribution, bursting pressure and the respecting bulge height at different temperatures have been studied experimentally and numerically. In order to numerically predict the onset of fracture in the process, three criteria, namely equivalent plastic strain acceleration (second derivative), major strain acceleration, and thickness strain acceleration were used. Moreover, a geometrical method was adopted in the simulation to determine the wrinkling. By comparing the results, there was an acceptable accordance between experimental and simulation results. By using experimental tests and finite element simulation, the process windows of the 6063 annealed aluminum tube were obtained at the temperature of 25 °C and 250 °C. According to the process window obtained at 25 °C compared with the one obtained at 250 °C, the pressure interval in the safe zone is about twice but the axial feed interval becomes nearly half.

In the present study, using the ABAQUS 2017 finite element code and using the DFLUX subroutine, a 3D numerical analysis of laser welding in the lap joint of AA6061 and AA5086 aluminum alloys was carried out. The effect of the position of a harder and softer alloy on the upper and lower parts of the weld in two different thicknesses of the two parts was studied on such cases as: thermal distribution, the width of the different welding regions and the residual stress caused by laser welding. In total, and based on the input conditions of the problem, 8 states were prepared for simulation. Based on the results, the A4 sample has the lowest maximum temperature difference between the upper and lower parts in all of these states, which is due to the presence of harder metal with lower thickness in the upper part of the joint. In all cases, regardless of the position of the upper or lower parts, the higher longitudinal residual stresses will occur in the harder part, the AA6061 alloy, and in all states the maximum longitudinal residual stress formed over the yield stress of the harder alloy. Regarding the level of σzz in the workpiece, the best conditions are, respectively, A1 and B1, because they also experience lower residual stresses levels, and the difference in residual stress between the two upper and lower regions of these specimens is lower.

Micromilling is one of the main manufacturing processes of creating miniaturized parts which are highly demanded in many industries nowadays. Like any other machining processes, cutting fluids are used for cooling and lubricating during micromilling while it can be challenging due to small cutting zone. In this study, the effects of different cooling and lubricating systems such as dry cutting, wet condition and MQL systems are investigated on such characteristics as surface quality and wear of the micro tool. In case of the MQL, two methods of single-nozzle and bi-nozzle spraying systems are applied and their effects on such characteristics are compared to each other. Machining tests are carried out using a two-flute micro cutter with diameter of 0.8 mm on a titanium alloy Ti6Al4V. Results show that the MQL is significantly effective on the both cooling and lubricating whereas wet application has no effect on the cooling. Finally, using MQL applications results in lower tool wear and better surface finish compared to those of the dry and wet conditions hence the one with two-nozzle is more advantageous in micro end-milling of this alloy.

Magnetic flux leakage (MFL) technique is a widely used and effective approach for detecting and sizing of the corrosion defects in ferromagnetic pipelines. In general, corrosion defects occur in dense clusters and affect each other. However due to the interaction between the MFL signals, these defects can-not be accurately characterized using the traditional MFL method. In order to discriminate the individual defects and improve sizing performance, tri-axial MFL technique is used. The study is performed using the extensive finite element modeling (FEM) focusing on the spatial distribution of tri-axial MFL components produced by the nearby corrosion defects. This type of defect geometry comprises two pits that are sufficiently close to influence flux distributions in the area between them. Various degrees of closeness are considered by varying the spacing of the two pits. Following the simulations, experimental MFL tests are performed on the steel plates containing nearby pits. The experimental and FEM results indicate that combining the axial, radial and circumferential MFL data can discriminate and characterize the nearby pits. Finally, the experimental and FEM results are compared.

In this study, aluminum sheets reinforced by carbon nanotubes were fabricated using the Accumulative Roll Bonding (ARB) method. The ARB process was chosen among the severe plastic deformation methods to strengthen metal sheets using carbon nanotubes owing to the enhanced microstructure and mechanical properties of final products. In order to evaluate the mechanical properties of the specimens, tensile tests were carried out and the strength of sheets made by ARB method was compared to single-layer pure aluminum and reinforced composite sheets. Microstructural changes of composite sheets were studied by optical microscopy after each cycle of rolling process. The results showed that spreading of (0. 5 to 1.5) wt% of CNTs increased the ultimate strength of the composite sheets while by aggregating the CNTs more than 1.5 wt% a decreasing trend of the ultimate strength was observed. Furthermore, the composites fabricated from 7 cycle of rolling process had a homogeneous distribution of particles and strong bonding between particles and matrix without having any porosity. Also it was found that the tensile strength of composite sheets also increased as the number of cycles increased.

In this paper, using the theory of Continuum Damage Mechanic and experimental methods, damage evolution is investigated. There are various experimental and theorical methods for measuring the Damage in materials. the experimental methods can be categorized in two type: destructive methods, for example: analysis of chord modulus variation, and the non-destructive methods, for example: microhardness or macrohardness evaluation. In this paper at the first, true stress-true strain diagram of material using the simple tension test and chord modulus variation Using repetitive loading - unloading test is obtained. For measuring plastic strain, grids are etched on the tension sample by using electroetch device. And then by measuring major ellipsoid diameter on the broken sample and comparing with the preceding circle, plastic strain at various points are specified. At specified points of the sample, the microhardness and the macrohardness are measured. Using the data of these three experimental methods, the corresponding damage is evalvated and the corresponding charts are ploted. finally The evaluated damage by these three methods is compared. It’s observed, microhardness and L-U-T test have relatively similar progress, but macrohardness progress had significant difference withn these two methods. It’s also observed, average of three methods is close to microhardness method rather than two other.

In this paper, considering the importance of surface integrity, effect of ultrasonic vibration of the Monel 400 electrode with a frequency of 20 kHz on the output parameters of the aluminum surface properties (surface hardness and wear resistance, thickness of the surface layer, the depth of copper and nickel diffusion to the aluminum surface and surface roughness) by the method of electrical discharge surface alloying has been studied. Based on the results of the SEM images, the surface layer thickness in the combination of ultrasonic vibration with the electrical discharge alloying process is more than the non-vibration mode. Therefore, increasing the thickness of this layer increases the surface wear resistance. Also, the microhardness test results show that surface hardness of the aluminum has increased to 450 vickers after the combination of ultrasonic vibration with the alloying process. According to the results of EDX analysis, the diffusion depth of alloying elements (copper and nickel) is higher in ultrasonic electrical discharge alloying, So that the surface layer thickness is up to 50 microns. Also, based on the results of surface roughness measuring, ultrasonic vibrations reduce the aluminum surface roughness after surface alloying.

Lapping process is one of the most important polishing processes in order to achieve a flat surface. In this paper, effects of parameters such as abrasive particle size, abrasive particles concentration in slurry, and lapping pressure on material removal rate, flatness and surface roughness were investigated in single sided lapping of flat workpieces made of 440c steel. To conduct experiments, Lapmaster 15 lapping machine has been exerted. The most important problem in lapping process is the low material removal rate which cause increase cost and production time. Therefore, in the lapping process, the selection of conditions that in addition to the production of pieces with geometric accuracy and surface roughness needed have a high material removal rate, is very important. In this paper, for the first time, the material removal rate, surface roughness and flatness lapped pieces have been optimized using non-dominated sorting genetic algorithm II (NSGA II) and, the Pareto optimal solutions were obtained. The results show that NSGA II is a useful and powerful tool for optimizing the material removal rate, surface roughness and flatness lapped pieces. Using this optimization algorithm, pieces with surface roughness and flatness requirements can be produced at high material removal rate. As a result, with this optimization algorithm, in addition to creating optimal quality parts, the production cost and time are reduced.

The static deflection of the machine tool and thus the displacement of the tool due to the high machining forces is the most important factor in reducing the dimensional accuracy of the workpiece. Also, the overlap of the frequency range of operation with the machine natural frequencies causes an undesirable resonance phenomenon. Reducing the strain energy of the column and thus reducing the displacement of the tooltip, and increasing the first natural frequency of the milling machine, given that the frequency range of operating is below the first natural frequency, is a desirable change that can be achieved by optimizing the distribution of the wall thickness of the column and the arrangement of the internal rib stiffeners. In this paper, a method based on sensitivity analysis was presented to optimize the arrangement of rib stiffeners connected to plate and shell structures. In each step of optimization, by establishing a loop of relationship between MATLAB and ABAQUS software and based on the sensitivity analysis performed by the finite element solver, the stiffeners that have the greatest impact in optimizing the objective function are added to the design space. After optimizing the distribution of the wall thickness of the column using the ABAQUS software size optimization module, the method presented in this paper was used to optimize the arrangement of internal rib stiffeners. Ultimately, despite a 0.5% reduction in the weight of the column, the maximum displacement of the machine tool decreased by 6.9% and the first natural frequency increased by 16.5%.

Two specific characteristics of grey cast iron, i.e. good machinability, as well as, high vibration damping, results in widespread applications in industry. In this research the grey cast iron powder which is fabricated via machining was utilized as raw material for producing foams. The porous structures were manufactured by powder metallurgy method were subjected under two major industrial destructive processes, i.e., corrosion and vibration, in a continuous and parallel manner. To demonstrate the degradation potency and comparison of these two destructive factors, changes in the porosity phase orientation as a result of energy absorption was measured. It was found that the amount of energy absorption, which is associated with the most changes in the porosity phase orientation, is depend on porosity volume, the type of destructive processes and the priority of corrosion and vibration. In the case of two vibration and corrosion processes applied successively, the corrosive atmosphere which induced less microstructural changes is the dominant mechanism. If destructive processes are applied in parallel, a sample with a mean value of 42% porosity can absorb the maximum energy, in which vibration is the dominant mechanism for this mode.

Reduce the cost of unscheduled shutdown and enhance the reliability of systems, is one of the important goals for various industries that could be achieved by condition monitoring. Cavitation is a common phenomenon in centrifugal pumps which causes the damage and its true identification in early stage is too important and help to increase the life of pump. In this paper cavitation is identified by use of Artificial Immune Net (AIN) that is modeled on the function of the human immune system. For this purpose, first data collection were done by a laboratory setup and various features extracted form vibration and current signals, in next step, feature selection and dimensions reduction were done by artificial immune method, then with AIN method, the system condition was identified. Finally, for comparing the results of this study with other methods, first feature selection and dimensions reduction were down by the PCA method then fault detection were down by nonlinear supportive vector machine (SVM), multi-layer artificial neural network (MLP), K-means and FCM methods with features of PCA and AIN methods. The results shown that AIN methods have more precise than other methods.

Ultra-precision machining is an advanced method for production of materials with nanoscale surface roughness. It is widely used in the manufacturing of precision components for defense, aerospace, optics, and electronics industries. For this feature, only a few industrial countries have access to this technology. Due to the high precision of this technology, many factors can affect the final surface quality. Machine components, machining conditions, tool geometry and material, environmental condition, workpiece material as well as vibration, are among the factors that are reviewed in this article. Afterwards, the effect of cutting depth on machining mechanism and surface quality is investigated using molecular dynamics investigation. The results revealed that when the ratio of cutting depth to tool edge radius becomes lower than 0.5, the effective rake angle would be bigger than the nominal rake angle. Furthermore, under this condition, the dominant machining mechanism is extrusion, which is different from the micro cutting mechanism. Finally, a series of experiments was conducted to study the impact of the undeformed chip thickness on the chip morphology and surface topography. For this purpose, Field Emission Scanning Electron Microscopy, 2D ultra-precision point autofocus probe as well as white light interferometer were exploited. The results indicated that at the lower relative tool sharpness, chip tearing occurs. Besides, by increasing this parameter to 100nm, silicon nano-ribbons is created.