Springback Angle Research Articles

Electroplastic Effect during Tension and Bending in Duplex Stainless Steel.

The deformation behavior of duplex stainless steel under tension and bending, accompanied by a pulsed current and when heated by an external source, is investigated. The stress-strain curves are compared at the same temperatures. The contribution to the decrease in flow stresses is greater when using a multi-pulse current at the same temperature, compared to external heating. This confirms the presence of an electroplastic effect. An increase in the strain rate by an order of magnitude reduces the contribution of the electroplastic effect from single pulses to the reduction in flow stresses by 20%. An increase in the strain rate by an order of magnitude reduces the contribution of the electroplastic effect from single pulses to the reduction in flow stresses by 20%. However, in the case of a multi-pulse current, the strain rate effect is not observed. Introducing a multi-pulse current during bending reduces the bending strength by a factor of two and the springback angle to 6.5.

Reduction of springback of Ti6Al4V alloy by high-density and instantaneous pulsed current

Ti6Al4V alloy has widely been used in the construction of aircraft parts, such as skins and wall plates owing to its low density, high specific strength, and excellent corrosion resistance. However, its large springback challenges the forming process. In this work, the effects of high-density instantaneous pulsed currents on the springback of Ti6Al4V alloy sheets were investigated by experimental and microscopic analysis. The results suggested a decrease in springback angles of workpiece as a function of discharge voltage. At a voltage reaching 6 kV, the springback was eliminated. For distinguished thermal and electrical effects, the maximum temperature of the specimen and thermal bending was measured at 6 kV. The data revealed a much smaller springback of electropulsing treated (EPT) workpiece when compared to heat-bending workpiece. Thu, the thermal effect was not the only reason leading to reduced springback. The mechanical properties tests of the specimens indicated a slight decrease in the strength of EPT-6kV while showing a significant increase in elongation than the as-received specimen. The microstructural evolution depicted recrystallization of EPT-6kV specimen with phase transformation, declined dislocation density, refined grain, decreased texture strength, and raised low Taylor factor grain content, leading to enhanced deformability. The temperature rise during EPT reduced the flow stress and facilitated the springback reduction. The applied current also promoted α→β phase transition, providing additional stress drop, reduction in dislocation density, and release of stress by recrystallization grain growth, further promoting the springback elimination. Overall, these findings look promising for reducing or eliminating the springback of Ti6Al4V titanium alloys.

Effect of pre-strain on microstructure and stamping ability of TA2 titanium alloy and H260 low-alloy steel

Applying pre-strain can not only change the microstructure and the mechanical property of metal material but also affect sheet metal stamping ability. In this paper, taking H260 low-alloy steel and TA2 titanium alloy as experimental materials, pre-strain ranging from 0 to 0.16 was applied using the one-step and the multi-step method respectively. Microstructure observation, tensile test, bending test, and cup bulging test were conducted on the pre-strained samples. Results show that in the pre-strain loading stage, the slipping and twinning deformation occurs in the TA2 alloy sample while dislocation sliding mainly happens in H260 low-alloy. With pre-strain increasing, the product of ultimate tensile strength and elongation rate gradually decreases, and the true ultimate tensile stress and accumulating elongation change rarely. The “product of yielding strength and elongation rate” can be enhanced by applying a pre-strain of 0.04 on the condition that plasticity is still at a high level. In the pre-strain range of 0 - 0.16, a bending test with a 30.8 °-opening angle can be completed on H260 steel. The springback angle changes from negative to positive with an increase in pre-strain. The bending ability of TA2 titanium alloy is very poor, for a crack can easily occur even in the original sample. The cupping value decreases gradually with an increase in pre-strain. The cupping value of TA2 titanium alloy is higher than that of H260 low alloy steel for the constant pre-strain. A quite large difference exists between the two experimental materials. Pre-strain loading pass has no significant effect on the microstructure evolution, mechanical properties, and stamping properties.

Improvement of springback prediction accuracy applying a new constitute model considering damage and nonlinear elastic unloading-reloading behaviors

Nonlinear elastic unloading-reloading behaviours which were closely related to the springback prediction have been observed in high-strength steels forming process. So, the constitute model considering nonlinear elastic unloading-reloading behaviors must be established. There has been the most classic model Quasi-Plastic-Elastic (QPE) model which can well describes nonlinear elastic behaviour to improve springback calculation accurately. But it does not take into account the effects of continuum damage mechanism on the stress distribution and hysteresis loop. Thus, the QPE model was modified and coupled with Lemaitre damage model to simulate the springback. The proposed model was planted in finite element code ABAQUS with user defined subroutine UMAT. To identify the model parameters and verify the model, the uniaxial unloading-reloading experiments were carried out with X80 high strength pipeline steel. Finally, the proposed model was applied to the simulation of three-point bending forming. The results show that the springback angle error predicted by the QPE-Damage model is within 5% compared with that without considering damage, and the accuracy of springback prediction is significantly improved.

Improvement of process control in sheet metal forming by considering the gradual properties of the initial sheet metal

In scientific studies, sheet metal is usually considered as a two-dimensional body. Thus, it is accepted that material properties are in most cases regarded homogeneous in thickness direction. However, a gradation of certain properties becomes apparent when going beyond the standard characterization methods for sheet metals, which can for example, influence the springback behavior and the thinning of the sheet after forming. Thus, the aim of this work is to further improve the prediction accuracy of springback after forming in simulations, by implementing several inhomogeneous properties over the sheet thickness in an existing material model. For this purpose, the entire procedure from the identification of the inhomogeneous properties for describing the gradation to the implementation in a numerical model and its validation by comparing experimental and simulated bending operations is carried out on a DC04 cold-forming steel in order to prove its influence on the springback behavior. It is shown that including graded material properties in simulations does indeed have an impact on the prediction quality of springback and that the information about inhomogeneous properties can be provided by existing characterization methods with a high local resolution like electron backscatter diffraction or X-ray stress analysis. In a further step, it was possible to validate the improvement in numerical accuracy by comparing the prediction of the springback angle from both the existing and the extended model with experimental bending results. Both the initial model as well as the model supplemented with the 3D properties provide a good prediction accuracy in the solution heat treated material state. For the predeformed material, however, the initial numerical model predicts a springback angle of about 13°, which deviates remarkably from the experimentally obtained mean value of about 17°. The extended model delivers a significantly improved accuracy in springback prediction in relation to the initial prediction (deviation of 4°) with a minor deviation of only about 0.8°, which proves the importance of considering the gradation of material properties in thickness direction for an overall higher dimensional accuracy of sheet metal products.

An experimental investigation of formability of inconel sheet plate for different die angles and rolling directions in press brake bending

The formability of Inconel materials is important to be used in engineering applications such as in fields of aircraft and maritime. The aim of study is to investigate bending characteristic and formability of Inconel 625 material having a property of corrosion resistance and high strength. In the paper, spring-back phenomena of Inconel 625 sheets are focused on experimentally. The 4 specimens for each different die angle are prepared to be bent. The press brake is used for forming Inconel 625 sheets. The die angle is altered from 90˚ to 150˚. The different rolling direction such as 0˚ and 90˚ is chosen to investigate the effect of grain orientation on spring-back of Inconel sheets. The bending radius is constant and set as 2 mm for all bending tests. The spring-back angles and amounts are measured. Results show that as the bending angle is increased, the spring-back amount in units of angle is decreased averagely from 3.35˚ to 2.58˚ for 0˚ rolling direction and maximum spring-back angle is obtained at a die angle of 120˚ for rolling direction of 90˚. Finally, Erichsen cupping test is also applied to determine the deformability of Inconel sheets. It is demonstrated that cup height value has been found as 17.20 mm.

Laser-Assisted Robotic Roller Forming of Ultrahigh-Strength Steel QP1180 with High Precision.

Laser-assisted forming provides a perfect solution that overcomes the formability of low-ductility materials. In this study, laser-assisted robotic roller forming (LRRF) was applied to bend ultrahigh-strength steel sheet (a quenching and partitioning steel with a strength grade of 1180 MPa), and the effects of laser power density on the bending forces, springback, and bending radius of the final parts were investigated. The results show that LRRF is capable of reducing bending forces by 43%, and a compact profile with high precision (i.e., a springback angle smaller than 1° and a radius-to-thickness ratio of ~1.2) was finally achieved at a laser power density of 10 J/mm2. A higher forming temperature, at which a significant decrease in strength is observed, is responsible for the decrease of forming forces with a laser power density of higher than 7.5 J/mm2; another reason could be the heating-to-austenitization temperature and subsequent forming at a temperature above martensitic-transformation temperature. Forming takes place at a higher temperature with lower stresses, and unloading occurs at a relatively lower temperature with the recovery of Young's modulus; both facilitate the reduction of springback angles. In addition, the sharp bending radius is considered to be attributed to localized deformation and large plastic strains at the heating area.

Investigation of the spring-back phenomenon in two-dimensional bending of continuous glass fiber-reinforced polyamide composite sheets

Thermoplastic composite sheets have great potential for use in industrial applications owing to their fast-forming properties in the mold. One of the important matters of forming is the tendency of the part to recover elastically after it comes out of the mold. This behavior affects the dimensional accuracy of the final part. In this study, V-bending experiments were carried out to understand the spring-back phenomenon of continuous glass fiber-reinforced polyamide-6 composite sheets. Unidirectional and cross-ply composite sheets were used in the experiments and bending angle, die radius, pressure, and dwell-time were investigated systematically. After bending, the spring-back angles of the parts were measured and the effects of the inspected parameters were compared. Also, variations in part thickness caused by spring-back were evaluated. Finally, the deformations in the specimens resulting from the residual stress were visualized with a scanning electron microscope. When the results were compared, it was determined that the bending conditions where the spring-back defect could be minimized were 6 mm radius, 90° angle, 120 s dwell-time, and 3 MPa pressure. Under these situations and in sheets bent 90°, the amount of spring-back was determined as 22° for unidirectional composites and 15° for cross-ply composites.

Springbek optimizacija za CNC mašinu za savijanje cevi zasnovanu na veštačkim neuronskim mrežama (ANN)

Predicting the springback angle has become the major production problem among tube benders. Springback is where the tube on a mandrel-less rotary draw bending tends to bounce back after being bent when the clamps are released. Accurately predicting the springback angle is crucial for effective tube bending. Machine learning (ML), a popular prediction approach, was applied to functions such as prediction or function approximation, pattern classification, clustering, and forecasting. To achieve this, the springback angle values from 27 experiments were collected and used as input into artificial neural networks (ANNs) in one area of ML. This research was conducted to study the optimization of the springback angle when bending ASTM A-210 Gr. A1 seamless tube with an outside diameter of 44.45 mm, using the 4 input factors Wall Thickness, Bending Radius, Dwell Time, and Bending Angle. The results showed that all factors significantly influence the springback angle in the tube bending process; different prediction methods were analyzed by comparing the results using different activation functions. The results showed that the optimal neural network architecture is 4-98-1; these results were achieved using the Sigmoid function, giving the lowest mean squared error (MSE) = 0.001892. The resulting coefficient of determination (R2 ) = 99.42%, the ReLU function R2 = 98.99%, the TanH function R2 = 98.53%, and the Identity function, which was 79.53%. It was also found that the best prediction of the springback angle using the best regression equation, with R2 = 82.32%, was better than the prediction using the 65 neurons with the Identity function R2 = 79.53%, a 2.79% difference in favor of the regression equation.

Preliminary Study of the Spring-back/Spring-go Phenomenon in the V-Bending Process Using SGCC Steel Thin Material

This research analyses v-bending results using galvanized SGCC (JIS G 3302) sheet steel. The accuracy of the angle and dimensions of the bending results are the significant elements of the bending process that must be achieved. The bending spring-back/spring-go phenomenon has an impact on bending angle accuracy. If proper parameters are chosen, the spring-tub/spring-go angle should be minimized. This study aimed to see how the proposed v-bending process affected the spring-tub/spring-go angle value. An experimental approach was selected in this study. V-die bending opening, V-die punch angle, V-die punch speed, and bending force are all input parameters for the v-bending process. Meanwhile, in this study, galvanized steel with a thickness of 1.2 mm was used. The results of the ANOVA evaluation showed that the v-die punch speed and bending force are two parameters that affect the response of the variable, with a percentage contribution of 31.0% each. The minimum spring-back angle was found in the second sample, while the minimum spring-go angle was found in the fourth sample

Springback prediction model of Ti-6Al-4V tube warm bending based on modified JC model considering variable temperature field

The plastic forming of titanium alloy tubes at room temperature exhibits strong anisotropy, low elongation, and large deformation resistance. Numerical control warm bending assisted forming technology can improve the stability of the titanium alloy tube bending process, increase the bending limit and reduce the springback angle. This technology promotes the wide application of high-precision titanium alloy bent tubes in many high-end fields such as aerospace, shipbuilding, chemical industry, and military industry. However, the titanium alloy tube bending at high temperatures involves complex material rheological behavior and microstructure characteristics. The coupling relationship between the stress-strain properties of the material and the temperature field makes it difficult to predict the springback behavior of titanium alloy tubes after warm bending. In this paper, based on the mechanical behavior of titanium alloy tubes at high temperatures, a constitutive model of titanium alloy suitable for local heating is improved. The theoretical model of springback for warm rotary draw bending of titanium alloy tube is established. The influence of warm bending strategies on the springback of bending is studied. The theoretical model is verified by establishing a thermal-mechanical coupling finite element model for the whole process of warm bending under the state of heat balance. The results show that the bending springback theory based on the modified John-Cook model can accurately simulate the temperature change of titanium alloy tube section, adapt to local heating strategies and provide high prediction accuracy of bending springback.

An improved springback model considering the transverse stress in microforming

To satisfy the higher accuracy requirement in the microforming of thin-walled structures, the springback behavior under miniaturizing dimensions need to be accurately predicted and controlled. However, as the feature size approaches the thickness in microforming, many assumptions in conventional springback model for macroforming are not valid making these models can hardly provide effective springback analysis in microforming. In the present work, the applicability of a springback analytical framework of conventional stretch-bending was first explored via bending tests of 0.1 mm-thick SS316L sheets under different holding conditions with a punch radius from 3 to 5 mm which is much greater than the thickness. The analytical springback angle results agree with the experimental ones with the maximum error of 4.2% for different punch radii, grain sizes and stretching forces conditions, indicating the correctness of the model. The model was further verified in the microforming of parallel channels with a fillet radius of 0.15 mm. Finite element simulations were conducted to obtain the stretching loads for each channel. However, a significant underestimation of the channel height after springback for over 15% was found especially for the middle channel. The transverse stress is revealed to be no longer neglectable due to the strong stretching load and the small die radius during the microforming of parallel channels, which substantially contributes to yielding, hardening and thickness reduction in the stretch-bending area. In addition, the uneven stretching also affects the springback angle and channel height, which is caused by the nonuniform plastic deformation of the adjacent channels. By incorporating the effects of the transverse stress and the non-uniform stretching loads, an improved springback model was developed for microforming of parallel channels in this work. The shift distance of the neutral plane is better estimated by the improved model than by the conventional one. The predictive heights by the improved model are also revealed to agree with the experimental ones during the microforming of parallel channels with a maximum error of less than 5%.

Research on the Springback Behavior of 316LN Stainless Steel in Micro-Scale Bending Processes

Hydrogen fuel cells have been used worldwide due to their high energy density and zero emissions. The metallic bipolar plate is the crucial component and has a significant effect on a cell’s efficiency. However, the springback behavior of the metallic bipolar plate will greatly influence its forming accuracy in the micro-scale sheet metal forming process. Therefore, accurate calculation of the springback angle of the micro-scale metallic bipolar plate is urgent but difficult given the state of existing elastoplastic theory. In this paper, a constitutive model that simultaneously considers grain size effect and strain gradient is proposed to analyze micro-scale bending behavior and calculate springback angles. The specialized micro-scale four-point bending tool was designed to better calculate the springback angle and simplify the calculation step. A pure micro-bending experiment on a 316LN stainless steel sheet with a thickness of 0.1 mm was conducted to verify the constitutive model’s accuracy.

Late outcomes of the frozen elephant trunk technique for acute and chronic aortic dissection: the angle change of the FROZENIX by "spring-back" force.

To evaluate the outcomes of total arch replacement using the frozen elephant trunk technique with a FROZENIX® J Graft for patients with either acute or chronic aortic dissection, and to evaluate the late-phase outcomes. Between January 2015 and December 2020, we used the frozen elephant trunk technique in 47 patients with acute aortic dissection and 12 patients with chronic aortic dissection. The primary endpoints were 30-day mortality, late aorta-related death and late aortic events. The secondary endpoints included early surgical complications and any aortic events (e.g. stent graft-induced new entry, frozen elephant trunk angle change). In the acute group, there were no aorta-related deaths, although 13 patients (27.7%) experienced an aortic event; stent graft-induced new entry occurred in 6 patients (12.8%). In the chronic group, 1 patient (9.1%) experienced aorta-related death, and 9 (81.8%) experienced an aortic event; stent graft-induced new entry occurred in 4 patients (36.4%). During the late phase after surgery, there was a significant increase in the frozen elephant trunk angle in both groups. In the AAD group, both the FET angle and spring-back angle were significantly enlarged in the late phase. There were no significant differences between groups in the degree of angle change, the overall survival, or aortic event-free survival. Total arch replacement using the frozen elephant trunk technique affords good early-stage results for both acute and chronic aortic dissection. During follow-up, careful monitoring for aortic events and appropriate therapeutic interventions are required. If surgeons are to use this device, they must have a thorough understanding of its spring back force and other features.

An Analysis of Spring-back and Spring-go on Variation of V-Dies Bending Angle Using Galvanized SGCC Steel Sheet

This study discusses the phenomenon of spring-back and spring-go in the bending kinematic shaping process using V-bending dies and SGCC galvanized steel sheet (JIS G 3302) material. During the bending process, the zinc layer on the surface of the galvanized steel must not be damaged. The zinc coating on galvanized sheet steel will affect the material forming process (metal forming process). This study employed an experimental design with four input variables: die opening L (mm), punch angle (punch engel), punch speed (punch speed), and bending force (force bending). The input factors for the experiment are 35 mm opening, 40o punch angle, 30 mm/minute punch speed, and 7500 N bending force. In the die opening setting conditions, the maximum spring-back angle is 35 mm, the punch angle is 40 °, the punch speed is 30 mm/minute, and the bending force is 6500 N. The average minimum and maximum spring-back angles in this condition are 0.71o and 4.08o, respectively. The bending force is 7000 N, the punch angle is 50 °, the punch speed is 40 mm/minute, and the minimum spring-go angle is 30o. In die opening parameters, the maximum spring-go angle achieved is 35o, the punch angle is 60o, the punch speed is 50 mm/minute, and the bending force is 700 N. The average minimum and maximum spring-go angles are 1.90o and 6.23o, respectively.

Predicting springback radii and angles in air bending of high-strength sheet steel through gaussian process regressions

Predicting springback radii and angles in air bending of high-strength sheet steel through gaussian process regressions

Effects of coating, holding force, stretching height, yield stress, and surface roughness on springback of steel in the V-stretch bending test

Abstract Various types of defects can impact the mechanical or aesthetic properties of stamped sheets, and the phenomenon of springback (SB) is one of them. SB is complicated, and it is hard to simulate it precisely because it is related to many factors such as tooling geometry, material properties, friction, and boundary conditions. In this article, the SB of V-stretch bended samples with uncoated and coated steel is experimentally and numerically investigated. The steel samples are coated with a thin layer of several metals (zinc, tin, nickel, and chromium). This investigation takes into account the influence of various parameters such as the coating thickness, its roughness, the punch depth, the blank holding force (BHF), and the change in the material’s yield strength. A self-developed two-dimensional elastoplastic finite element code was developed using Abaqus/standard. In this work, we also show that the SB angle is smaller for uncoated sheet, higher BHF, higher surface roughness, lower coating thickness, lower stretching height, and lower yield strength.

A Novel Model Developed for Frictional Characteristics Analysis of Axial Symmetric Parts

Friction during contact between metals can be very complex under dynamic conditions. In this study, friction between 304 stainless steel and SKD11 steel with boundary lubrication was studied experimentally using a friction testing machine (MPX-2000). The friction coefficients at different sliding speeds and interface loads were determined, and a new friction coefficient model was established based on the experimental data. The sample surfaces were analyzed using a laser-scanning microscope, and it was found that the friction mechanism under boundary lubrication (where 0.1 < μ < 0.3) was mainly abrasive wear accompanied by slight adhesive wear. The new friction coefficient model developed was applied for a simulation of Axial Symmetric U-Bend parts using finite element methods, and the results were compared with stamping experiments. The prediction errors in the results of thickness and the springback angle showed that the new friction model had a good agreement of the thickness distribution to the experiments with less than 10% error, and the springback angles between the new friction model and the measurements with the errors of 6.86% and 5.13%. The experimental results show that the friction coefficient decreases with the rise of speed when the sliding speed is between 30 mm/s–50 mm/s; the friction coefficient decreases with the increase in interface load. A decreasing trend of friction coefficient gradually slows down when the interface load is between 2.0 MPa–4.0 MPa. This also agrees with the simulations using the new model.

Effects of Process Parameters on Springback of High Strength 21-6-9 Tube Bending Considering Variation of Elastic Modulus

In order to improve forming quality of high strength 21-6-9 tube bending and achieve its precise bending forming, the springback behaviors of high strength 21-6-9 tube in rotary draw bending are investigated. Based on explicit/implicit finite element (FE) software of ABAQUS, the whole rotary draw bending FE model of high strength 21-6-9 tube considering variation of elastic modulus was established, and its reliability was verified. Then, the springback behaviors considering varied elastic modulus of high strength 21-6-9 tube in rotary draw bending under different process parameters were explored and compared that with constant elastic modulus. The results show that the variation tendencies of springback angle are similar with or without considering varied elastic modulus, but augments the value of springback angle with considering varied elastic modulus. Springback angle augments with the increase of the friction coefficient of tube versus mandrel, the friction coefficient of tube versus wiper die or with the decrease of the friction coefficient of tube versus pressure die, while the springback angle firstly declines and then augments with increasing the clearance of tube versus pressure die and the clearance of tube versus wiper die.

DETERMINATION OF PRESS BRAKE BENDING PARAMETERS FOR HARDOX 400 STEEL

In the present study, a specific hydraulic press brake based test setup (HPBTS) was designed and manufactured which was used to determine the bending forces and the punch displacements of Hardox 400 steel depending on the plate thickness, plate length and mechanical properties. The setup was equipped with an image processing system to detect the change of the bending angle during forming. Prior to bending tests, tensile tests were carried out and the identified mechanical properties were compared with the ones reported by the manufacturers. The effect of the difference between the mechanical properties on the bending parameters was investigated. Air bending technique was used for bending tests. Experiments were carried out with varying plate thicknesses, plate lengths, bending angles, forming speeds and channel openings using flat materials supplied from two different manufacturers. The effect of these parameters on bending force, displacement and springback angle was revealed. It was shown that the mechanical properties of Hardox 400 steel vary according to the manufacturer. The difference between the measured mechanical properties and the ones obtained from the manufacturers had a direct influence on the bending properties.