Deformation Behavior Of Sheet Research Articles

Subregional flexible-die control in magnetorheological pressure forming

Magnetorheological pressure forming (MRPF) has shown great flexibility in process control by adjusting the mechanical property of MR fluid. However, the regulation of flexible-die property at present is holistic. In this paper, MRPF based on subregional flexible-die control was proposed, which makes the MR fluid show different force transfer characteristics to meet the requirements of flexible-die performance in different areas. Two kinds of non-uniform magnetic fields were established using the combination of coil and iron core groups based on the measurement and finite element simulations. The MRPF experiments of the Al1060-O sheet using the holistic and subregional flexible-die control methods were conducted. The effects of subregional flexible-die control on loading curve, strain distribution, configuration, wall thickness thinning rate, and fracture characteristics were analyzed. Compared with the case that the specimen ellipticity changes little under the central-dominated magnetic field (CMF), the ellipticity gradually decreases under the fringing-dominated magnetic field (FMF) with the increase of the current. The fracture modes can be divided into three types, and the fracture position transfers from the center of the specimen to the circle of R10mm and then to the die fillet area. The influence mechanism of subregional flexible-die control on sheet deformation behavior and fracture mode is further clarified. The non-uniform distribution of cavity pressure and friction conditions caused by the magnetorheological effect is the main factor affecting the stress states and final deformation behavior of sheet metal. This study provides a new path for intelligent control in sheet metal forming of complex thin-walled parts.

Investigation on the Crack and Thinning Behavior of Aluminum Alloy 5052 Sheet in Stretch Flanging Process

Stretch flanging is the important sheet metal-forming process which is widely used in automobile and aerospace sectors. The formability of sheet metal depends on various parameters such as material properties, geometry of the tool setup and process parameters. In the present work, effects of different die radius and sheet width on deformation behavior of sheet are studied by FEM simulation and experiments. The predicted FEM results are presented in the form of edge crack location and its propagation, crack length, forming load and strain distribution in sheet along the die profile radius. This study indicates that crack length increases with increase in the sheet width, while the crack length decreases with increase in the die radius. It is found that the crack propagation in stretch flanging process is affected by the strain distribution in sheet and this distribution of strain depends on many parameters. The crack initiates during deformation of the sheet at the die corner edge and propagates toward the center of the sheet along the die profile radius. Simulation results are compared with the experimental one in terms of crack length and variation in sheet thickness. FE simulation results are found in very good agreement with experimental results. Fractography study is also presented in terms of size, shape of the dimples along with their distribution on the fractured surface.

Mechanism of magnetic field condition on deformation behavior of sheet metal using a property-adjustable flexible-die

To provide intelligent control options during sheet metal forming process, magnetorheological (MR) fluid has been applied as a property-adjustable flexible-die. However, the mechanism of the magnetic field effect on the sheet deformation behavior is not clear yet. In this paper, to clarify the deformation mechanism, the bulging behavior of thin-walled Al1060-O sheet was systematically investigated through experiment and numerical simulation. Special attention was paid to the effect of magnetic field on loading curve, bulge dome height, and thickness strain distribution. Results showed that the maximum bulging force increased obviously with the increase of magnetic flux density. The bulge dome height and thickness strain were influenced by magnetic field condition and magnetic particle content synthetically. Appropriate magnetic field condition and magnetic particle content can improve formability. However, excessive magnetic flux intensity and magnetic particle volume fraction, for example, B = 0.20 T and φ = 46%, would lead to premature rupture. Finite element analysis was adopted to assist in explaining the bulging behavior. Detailed analysis indicated that the flow stress variations caused by magnetic field affect the stress states, velocity distribution of MR fluid, and sheet/MR fluid contact condition, further leading to different stress and strain distribution of sheet metal. Finally it causes the great difference in specimen geometry and wall thickness distribution.

Effect of punch profile on deformation behaviour of AA5052 sheet in stretch flanging process

Stretch flanging is a type of bending process widely used in automobile and aerospace industries. Forming of the stretch flange is mainly affected by three important parameters: materials of the sheet, the geometry of tools and different process parameters. This work focuses on the effect of punch profile on deformation behavior of AA5052 alloy sheet to form the stretch flange. Six punches of different geometry i.e. cylindrical, two stepped, three stepped, six stepped, conical and hemispherical are used. Results are presented in the form of edge crack in the sheet at edge corner and its propagation towards center, forming load comparison for different punch profile and distribution of radial and circumferential strain in the sheet. It is observed that the punch profile has a considerable effect on the deformation behavior of the sheet. Circumferential strain, radial strain and load requirement to form the flange are found to be minimum in hemispherical punch profile as compared to other punch profiles. Experiments are performed to validate the FE simulation results and results are found in very good agreement in terms of edge crack length. Fractography study shows uniform and large number of small size dimples at the fractured surface for hemispherical and conical punch profile.

Prediction of metal sheet forming based on a geometrical model approach

The panel production of small batch sizes for the hull of large ships requires a stable and flexible forming process, which is momentarily manually controlled by a system operator. The manual forming press control includes the metal sheet handling above the forming tool for defining the contact point and engagement depth of the sword and subjective monitoring of the forming degree by using the light gap check method. For objectifying the process monitoring and reducing the dependency on the experience of the system operator an automated solution is needed. Within the automated process control the metal sheet deformation behavior has to be predicted in real-time during the forming process. To achieve this, the deformation prognosis for the ship panel’s production is handled inside the described work. Based on a state of art analysis a geometrical approach to describe the metal sheet deformation behavior is developed for the multi-step forming process by three-point-bending. The related geometrical parameters are predicted using a new type of prediction method by means of an artificial neural network. This prediction method requires the network definition and extensive experimental investigations for training the artificial neural network.

Investigation of dynamic deformation behaviour of large-size sheet metal parts under local Lorentz force

Electromagnetic incremental forming (EMIF) is an effective manufacture method for large-size sheet parts of aluminium alloy due to the advantages of improving material formability and increasing the part dimensions. However, the dynamic deformation behaviour of sheets under local Lorentz force is so complex that it is difficult to achieve precise forming control of large-size sheet parts of aluminium alloy in the EMIF process. To address this challenge, a numerical simulation model was established and validated by experiments in this study. Then the forming height, plastic strain, and velocity propagation of a sheet were analysed. In addition, the Lorentz force action stage and inertia effect stage were distinguished quantitatively, and on this basis, the effects of Lorentz force and the inertia effect on deformation were analysed. Finally, the effects of discharge voltage and discharge position on the deformation were revealed. The results showed that there are differences between the distribution rules of forming height and plastic strain. The velocity peak first appears at the sheet area corresponding to the radius centre of coil, then the accelerating downward velocity peak propagates to the sheet centre, while the downward velocity spreads outside. The Lorentz force is the leading factor of the forming peak height, while the inertia effect is decisive on the forming area. The forming peak height and forming area, especially at the inertia effect stage, can be effectively controlled by changing the discharge voltage and the discharge position, thus guiding the adoption of the process parameters according to the requirement of the deformation.

The Influence of Alternating Low-Cycle Bending Loads on Sheet Properties Having an Hcp Crystal Lattice

The process of cyclic bending was investigated using thin sheets of the magnesium alloy AZ31 and α-titanium. These materials possess an hcp crystal lattice with different c/a ratios. It turned out that the latter have a substantial influence on the sheet deformation behavior. Even for small deformations (up to 2% strain), a large influence on the yield stress was present for both materials. In addition, cyclic bending contributes to the activation of prismatic slip, which is accompanied by twinning and detwinning. The changes in sheet anisotropy following cyclic bending were determined using texture measurements. Specifically, the AZ31 alloy sheets exhibited a considerable change in anisotropy of the mechanical properties with an increasing number of bending cycles. The anisotropy in the yield stress increases from 15% in the initial condition to 40% after three cycles. For the α-titanium sheet, the change in anisotropy was approx. 26% less. In general, the largest changes in properties occurred already in the first bending cycle and a stabilization took place upon further cycling.

A study of incremental sheet forming by using water jet

In this work, a variant of the incremental sheet forming (ISF) process, namely the incremental sheet forming by using water jet (ISF-WJ), was studied. In the investigation, an ISF-WJ prototype machine was designed and developed. Different design concepts of the water jet nozzle were proposed and evaluated to achieve the maximum forming pressure by performing computational fluid dynamic (CFD) simulations. Based on the forming pressure distribution modeled by CFD simulations, finite element (FE) models were developed to study the sheet deformation behavior under the ISF-WJ process condition. Based on the understanding gained from the numerical study, experiments were conducted to validate the ISF-WJ process and the developed prototype machine. The results suggest that ISF-WJ is a feasible process to achieve improved surface finish of thin sheet parts. In addition, this study has found that water jet pressure plays an important role in preventing sheet wrinkling and obtaining an accurate geometry of formed parts.

Investigation on a new hole-flanging approach by incremental sheet forming through a featured tool

One of the major challenges in conventional incremental sheet forming (ISF) is the extreme sheet thinning resulted in an uneven thickness distribution of formed part. This is also the case for incrementally formed parts with hole-flanging features. To overcome this problem, a new ISF based hole-flanging processing method is proposed by developing a new ISF flanging tool. Comparative studies are conducted by performing hole-flanging tests using both ISF conventional ball-nose tool and the new flanging tool to evaluate the sheet deformation behavior and the quality of the final part. Stress distribution and strain variation are investigated by analytical approach and numerical simulation. Experiments have been conducted to validate the analytical model and simulation results, and to further study the fracture behavior. Results show that the new flanging tool generates greater meridional bending than stretching deformation in conventional ISF. The combination of bending-dominated deformation mode with localized deformation of ISF ensures more uniform thickness distribution on hole-flanging part with better resistance to fracture.

Research on the effect of flow stress calculation on aluminum alloy sheet deformation behavior based on warm bulging test

Confirmation of material properties is an important research area for determining metals deformation behavior. In order to research the effect of flow stress calculation on aluminum alloy sheet deformation behavior, warm sheet bulging test was carried out to obtain bulging height - pressure curves with different bulging heights and diameters in this study. Based on the bulging parts profile data measured by three coordinate measuring machine, the least square circle fitting radius were fitted. Existing theoretical models for radius of curvature and thickness were compared and best models were selected to obtain more accurate stress — strain curves by calculating the bulging flow stress. Combination model was used to calculate the bulging height — pressure curves obtained by bulging test and the stress-strain curves with different temperatures and pressure rates were obtained. A monotonous increasing function was resulted which is of great significance in the area of formability and deformation behavior of warm sheet hydroforming.

Strength and Formability Improvement of Al-Cu-Mn Aluminum Alloy Complex Parts by Thermomechanical Treatment with Sheet Hydroforming

Normally, the strength and formability of aluminum alloys can be increased largely by severe plastic deformation and heat treatment. However, many plastic deformation processes are more suitable for making raw material, not for formed parts. In this article, an experimental study of the thermomechanical treatment by using the sheet hydroforming process was developed to improve both mechanical strength and formability for aluminum alloys in forming complex parts. The limiting drawing ratio, thickness, and strain distribution of complex parts formed by sheet hydroforming were investigated to study the formability and sheet-deformation behavior. Based on the optimal formed parts, the tensile strength, microhardness, grain structure, and strengthening precipitates were analyzed to identify the strengthening effect of thermomechanical treatment. The results show that in the solution state, the limiting drawing ratio of cylindrical parts could be increased for 10.9% compared with traditional deep drawing process. The peak values of tensile stress and microhardness of formed parts are 18.0% and 12.5% higher than that in T6 state. This investigation shows that the thermomechanical treatment by sheet hydroforming is a potential method for the products manufacturing of aluminum alloy with high strength and good formability.

In-situ measurement of three-dimensional deformation behaviour of sheet and tools during stamping using borescope

An in-situ measurement approach for three-dimensional deformation of sheet and tools during stamping using borescopes was developed. The borescope, consisting of a small CCD camera with a flexible cable, was connected to personal computer and placed inside a small space between the tool cavities to measure the deforming behaviour of the sheet. The deformation behaviour of the sheet and punch, the real springback and wrinkling in shrink flanging were measured. The deformed shape of the sheet measured by the borescope was in good agreement with that measured by the laser displacement sensor. In addition, three-dimensional deforming behaviour was measured by a stereoscopic method using two borescopes. It was found that the borescopes are effective in in-situ measurement of three-dimensional deformation behaviour of the sheet and tools.

二軸バルジ試験法による高強度鋼板の加工硬化特性の測定と材料モデリング

The deformation behavior of high strength steel sheets with a tensile strength of 590 MPa under biaxial tension was investigated for a strain range from initial yielding to fracture. The test material was bent and laser welded to fabricate a tubular specimen with an inner diameter of 44.6mm, a wall thickness of 1.2mm, and an axial length of 200mm. Multiaxial tube expansion tests were performed using a servo-controlled tension-internal pressure testing machine. Many linear stress paths in the first quadrant of stress space were applied to the tubular specimens to measure the forming limit curve (FLC) and forming limit stress curve (FLSC) of the as-received sheet material, in addition to the contours of plastic work and the directions of plastic strain rates from initial yielding to fracture. Results calculated using the Yld2000-2d yield function with exponents of 6 to 8 provided the closest agreement with the measured work contours and directions of plastic strain rates for a reference plastic strain range of 0.002 ＜ε0p＜ 0.20. It was concluded that the multiaxial tube expansion test is effective for measuring the multiaxial deformation behavior of sheet metals for a wide range of plastic strains.

高張力鋼板のロール成形におけるスプリングバックの検討

To produce electric resistance welded pipes using high-strength steel sheets, it is very important to accurately predict the amount of springback in roll forming processes. In this study, the three-dimensional deformation behavior of sheets at a single bending stand consisting of vertical rolls with a circular profile is investigated in detail using the static-implicit finite element program Marc. The calculation results are in good agreement with the experimental results. The amount of springback in roll forming processes is much smaller than that predicted by the theoretical analysis of plane strain bending. It is found that the small springback is due to the reduction in bending moment caused by tensile deformation in the circumferential and longitudinal directions followed by compressive deformation in the circumferential direction in the later stage of roll forming processes.

Anisotropy of Deformation Behavior of AZ31 Magnesium Alloy Sheets

In order to develop magnesium alloy sheets with high formability at room temperature, the anisotropy of deformation behavior of AZ31 magnesium alloy sheets produced by equal channel angular rolling were examined, which were compared with that of the sheets produced by the unidirectional hot rolling. The differences in the deformation behavior of the sheets at the rolling direction (0°), 45° and the transverse direction (90°) were discussed in term of the texture and microstructure. Compared with the as-received specimens, the anisotropy of deformation behavior of AZ31 magnesium alloy sheet produced by equal channel angular rolling was enhanced, which was following with an improved ductility and a large work hardening phenomenon. These could be due to the non-basal texture, which was induced by the continuous shearing deformation during equal channel angular rolling procedure. The fracture mechanism transferred from the cleavage fracture for the unidirectional rolling to the quasi-cleavage fracture for the sheets produced by equal channel angular rolling, which proved that the non-basal texture was in favor of the ductility of magnesium alloy.

Critical strains and necking phenomena for different steel sheet specimens under uniaxial loading

When defining ultimate loads or failure criteria for shells, vessels and containment liners, for instance, reference will be made to critical strains of the material under consideration. In general the critical strains are derived from uniaxial tensile test results obtained with rods. The actual deformation behaviour of sheets (plate and shell type structures), however, differs considerably from the uniaxial rod behaviour. From various tests – along with theoretical investigations – it was found that for membrane or even bending stressed sheets higher ultimate strains are reached. The typical strain behaviour caused by increasing load starts with uniform strain distribution over the specimen length and finally ends with localized necking. As “critical strains” the well known uniform elongation strain and the ultimate strain are defined. For sheets in addition the quasi uniform elongation strain has been introduced in this paper. Whereas for rods under tension localized necking is directly following the uniform straining, tests with sheets show a so-called diffuse necking behaviour before localized necking starts. As a consequence the deformation capacity of steel sheets turns out to be significantly higher than relying on the data from standard tension tests with rods.

Effect of Loading Location of Viscous Medium on Sheet Deformation in Viscous Pressure Bulge (VPB) Test

Viscous pressure forming (VPF) uses a highly viscous but flowable material as pressure-carrying medium (PCM). Due to the relative low flowability of viscous medium compared with fluid, nonuniform pressure distribution in viscous medium can be used to control and regulate the deformation sequence of the workpiece through controlling the loading mode of viscous medium. In the present study, viscous pressure bulge (VPB) tests with three kinds of loading location of viscous medium (central zone, corner zone and the whole deformation zone) are conducted and the influences of loading location of viscous medium on sheet deformation behavior are investigated via numerical simulations and experiments. It is found that changing the loading location of viscous medium can greatly affect the deformation behavior of sheet metal. When the viscous medium is injected from the die corner zone, a local high pressure formed at the corner zone of sheet metal and a higher limiting dome height and strains are obtained.

Hole expansion simulation of high strength steel sheet

The deformation behavior of high-strength steel sheet during hole expansion is investigated both experimentally and analytically to clarify the effects of the material model (anisotropic yield function) on the finite element simulation of a hole expansion. The test material used in the hole expansion test is a dual-phase steel alloy with a tensile strength of 780 MPa. The elastic-plastic deformation behavior of the test material is precisely measured using biaxial tensile tests with cruciform specimens to determine the appropriate anisotropic yield function for the test material. Moreover, forming simulations of the hole expansion using selected yield functions are carried out. The Yld2000-2d function [Barlat, F., Brem, J.C., Yoon, J.W., Chung, K., Dick, R.E., Lege, D.J., Pourboghrat, F., Choi, S.H., and Chu, E., Int. J. Plasticity, 19 (2003), 1297-1319.] with an exponent of 4 has given the closest agreement with the experimentally measured contours of plastic work and the directions of the plastic strain rates. It was also found that the Yld2000-2d function with an exponent of 4 has given the closest agreement with the observed thickness distribution along the expanded hole edge. Consequently, anisotropic yield functions significantly affect the predictive accuracy of the deformation behavior of a sheet during a hole expansion

Deformation behaviour of sheets of three aerospace Al-alloys

Room temperature microstructures, tensile properties, void formation and surface roughening due to deformation, forming limit diagrams (FLDs) and strain distributions in punch stretching in sheets of Al–Cu–Mg, Al–Mg and Al–Mn alloys are described. The interdependence between the metallurgical variables and the mechanical properties is shown to be complicated.

Finite Element Analysis of the Effect of Processing Conditions on Thermoforming

A numerical algorithm is developed to simulate the deformation behaviors of a sheet from the simple axisymmetric to the complex three dimensional geometries. This algorithm employs finite element method with the total Lagrangian approach. It is possible to obtain thickness, stress and strain distribution of each loading step in thermoforming process using this algorithm. The change of various processing conditions such as temperature, plug assists are very crucial to the final deformation behaviors of sheet, and these effects are discussed in this paper.