Push-bending Process Research Articles

An improved FEM-DEM coupling simulation for granular-media-based thin-wall elbow tube push-bending process

The granular-media-based push-bending process has been developed to manufacture thin-wall elbow tube with t/D≤0.01 (the ratio of wall thickness to outer diameter) and R/D≤1.5 (the ratio of bending radius to outer diameter). In the process, a tubular blank is filled with granular media and then pushed into a die to form an elbow shape. To investigate the process, a FEM-DEM coupling model has been developed, in which FEM is used to simulate bending deformation of tubular blank, and DEM is used to calculate contact forces between spherical particles in granular media. In this work, an improved numerical formulation is proposed in order to reach mechanical equilibrium quickly and accelerate the convergence of DEM simulation, when the new contacts are no longer created and the old contacts are no longer deleted in granular media. Using the proposed numerical formulation, the improved FEM-DEM coupling simulation for granular-media-based thin-wall elbow tube push-bending process is less time-consuming than before under the same simulation condition.

Differential flow velocities control method for push-bending of the thin-walled tube with a 0.9D bending radius by differential lubrication

The smaller the relative bending radius of the bent tube, the more difficult the tube blank is to be bent. In this paper, the push-bending process for the aluminum tube with a relative bending radius of 0.9D was analyzed through simulation. There was tangential tensile stress concentration occurring at the front endpoint of tube intrados, which increased the risk of cracking. That is because the bending tube’s extrados moves less angular distance than its intrados by the uniform lubrication method, causing excessive deformation of the cross-section. Therefore, a differential lubrication method using the lubricant with a larger friction coefficient at intrados and the lubricant with a smaller friction coefficient at extrados was proposed to increase the angular velocity of the tube’s extrados. It was found that the differential lubrication method helped avoid excessive thinning at the front endpoint of tube intrados through simulations. Furthermore, push-bending experiments of 5A02 aluminum alloy with a relative bending radius of 0.9D were carried out to investigate this effect. The experimental results showed that the differential lubrication method effectively avoided cracking, which agreed with the simulation results.

Warping and springback reduction in bending of U-profiles through partial heating over the cross-section

Bending of profiles is challenging due to the high stiffness and possible deformations in the process. Especially bending of profiles with asymmetric cross-section regarding the force initiation axis leads to unwanted warping of the bent profile. This warping results from the position difference between the force initiation axis and the shear center, implying torsion moments on the profile. To prevent profile warping the use of shape-bound tools or a change of the force initiation axis position are common practices, though these methods reduce the flexibility of the process. A new method to prevent profile warping during bending is the use of partial, cross-sectional heating. Due to thermal softening in one profile area a quasi-symmetric bending case is achieved, which changes the position of the stress-free fiber and thus reduces the torsion moment. In this work, the warping and springback behavior of partially heated U-profiles consisting of S500MC steel after a three-roll push bending process is investigated using experimental methods. Through partial heating of one profile area to up to 600°C, a warping reduction of 83 % and a springback reduction of 69 % were achieved.

Process parameter optimization for thin-walled tube push-bending using response surface methodology

In this paper, the response surface methodology (RSM) and finite element (FE) simulation were applied to optimize the push-bending process parameters of the thin-walled tube with polyurethane mandrel. The objective of the present work is to predict the optimal set of process parameters including the length to diameter ratio of the mandrel (L/D), the friction coefficient between die and tube ( $${\mu }_{1}$$ ), the friction coefficient between polyurethane and tube ( $${\mu }_{2}$$ ), and Poisson’s ratio of polyurethane ( $$\upsilon$$ ) to obtain qualified bent tubes. Three empirical models were developed to describe the relationship between process parameters and quality parameters of the bent tubes. In addition, the significant factors affecting the forming quality were analyzed using analysis of variance (ANOVA) of each model. Response surfaces were constructed to study the effect of each process parameter on the quality of the bent tubes. Finally, the process optimization window with the maximum thinning rate ( $$\varphi$$ ) less than 20%, the maximum thickening rate ( $$\psi$$ ) less than 17%, and the maximum cross-section ovality ( $$\xi$$ ) less than 5% of the bent tube was established. Qualified bent tubes with diameter of 144 mm, wall thickness of 2 mm, and bending radius of 280 mm were formed experimentally by following the established process window.

Reduction of Warping in Kinematic L-Profile Bending Using Local Heating

Kinematic bending of profiles allows to manufacture parts with high flexibility concerning the geometry. Still, the production of profiles with asymmetric cross-sections regarding the force application axis using kinematic bending processes offers challenges regarding springback and warping. These geometric deviations can be reduced by partial, cross-sectional heating during the process as it lowers the flow stress locally. In this work, the influence of partial, cross-sectional heating during a three-roll push-bending process on the warping and springback of L-profiles is investigated. Numerical and experimental methods reveal the influence of temperature on warping and springback. A newly developed analytical model predicts the warping and bending moment in the design phase and assists to understand the effect of warping reduction through partial heating during plastic bending. With increasing temperature of the heated profile area, the warping is reduced up to 76% and the springback of the bend profiles is decreased up to 44%. The warping reduction is attributed to a shift in stress free fiber due to the temperature gradient between heated and room temperature areas. The shift of stress-free fiber leads to an adapted shear center position, resulting in an approximated “quasi-symmetric” bending case.

A simplified formulation for predicting wrinkling of thin-wall elbow tube in granular media–based push-bending process

Wrinkling is the key forming defect in thin-wall tube bending process, when the wall thickness-to-diameter ratio is smaller than 0.01. The granular media-based thin-wall elbow push-bending process involves filling a tube with spherical particles and pushing the tube into a die to bend a tubular blank into an elbow shape. If the wall thickness of tube is smaller than the particle size, wrinkling significantly depends on micro contact force distribution in granular media, and the granular media cannot be approximately simulated by FEM. In the present paper, an elbow tube with 0.3 mm wall thickness, 30 mm outer diameter, and 90 mm bending radius is formed by granular media-based push-bending process. The wrinkling is investigated; especially, the micro contact force distribution at the initial buckling location is observed based on a 3D FEM-DEM coupling model. In the coupling model, FEM is used to simulate bending deformation of tubular blank, and DEM is used to simulate micro contact forces of particle flow. Finally, a simplified formulation is proposed for predicting wrinkling of thin-wall elbow tube in granular media-based push-bending process and validated by experiment of an elbow tube with 0.7 mm wall thickness, 70 mm outer diameter, and 105 mm bending radius.

Correction to: A Study on the Optimization of Joint Mandrel Shape for Manufacturing Long Type Elbow Using Push Bending Process

Correction to: A Study on the Optimization of Joint Mandrel Shape for Manufacturing Long Type Elbow Using Push Bending Process

A Study on the Optimization of Joint Mandrel Shape for Manufacturing Long Type Elbow Using Push Bending Process

Pipes are widely applied across many industries such as the automotive, aerospace and air conditioning industries. With the development of industrial technology, various pipe materials and various pipe shapes are produced according to the purpose. Many researchers have been studied pipe bending technology to meet these diverse needs, and among them, the long-type elbow is a typical one that changes the moving direction of fluid or gas among various pipe types. Many researchers work on the pipe segment. In-process push-bending used in pipe bending is a bending process where a cut pipe is inserted into a die. Push-bending is effective for reducing the weight of parts that do not induce material loss, but the process is more complicated than the draw-bending process. In this study, we proposed an analytical model for a push-bending process in which a joint type mandrel was added to a long-type elbow made of C1220 material for weight reduction. The forming analysis is to verify the experiment by comparing the analysis result and the experimental result through the forming analysis software DEFORM to confirm the change of the wrinkle and the thickness of the bent pipe. In addition, we confirmed the wrinkling of the bent pipe and the die non-contact phenomenon depending on the angle and position of the joint mandrel and presented the optimum angle and center of gravity position of the joint mandrel using the experimental design method.

Influences on the extended length and performance of push bending for the GH4169 superalloy tube with small bending radius of 1D

It is extremely difficult to eliminate the defects like wrinkling, which is generated in the bending of tube with small bending radius. In this thesis, the push-bending process of the GH4169 superalloy tube with a large diameter-thickness ratio of 60 and small bending radius of 1D is studied by numerical simulations and experiments, aiming to eliminate wrinkles on the inside of the tube and obtain enough extended length (e) of straight segment as well as minimal changes in the wall thickness. Three influencing factors, such as the inclined angle (α) at the initial end, the reverse thrust (P) of polyurethane blocks, and the lubrication, are investigated in the simulation. According to the simulations, the proposed experiment is successfully implemented on an independent development of push-bending equipment. The results are in good agreement with the simulation. The optimum parameters for the GH4169 superalloy alloy tube with a large diameter-thickness ratio of 60 and small bending radius of 1D are simulated. And then α is optimized to 45° and P is optimized to 48 MPa. And differential lubrication is used for longer extended length of straight segment and more uniform distribution of wall thickness on both sides of the deformed elbow.

A modified thin-walled tube push-bending process with polyurethane mandrel

Thin-walled (D/t > 30; D, initial outer diameter; t, initial wall thickness) bent tubes with large diameter (D > 100 mm) and small bending radius (R/D < 3; R, centerline radius) are difficult to form integrally using traditional bending methods. In this paper, using a novel loading method, a modified push-bending (MPB) process with polyurethane as mandrel was developed. A stainless steel bent tube with extreme geometrical specification (D = 144 mm, D/t = 72, and R/D = 1.94) was formed integrally by MPB. Based on analytical and finite element (FE) method, the internal pressure, i.e., contact pressure between polyurethane and tube (CPPT) were investigated. The results show that the CPPT decreases linearly from the back end to the front end of polyurethane rod and increases with smaller μ1 (the coefficient of friction between polyurethane and tube) and larger L (the center axis length of polyurethane rod), and the increase of CPPT is helpful to decrease the ovality of the bent tube.

Push-bending method development of thin-walled tube with relative bending radius of 1 using sectional elastomers as mandrel

It is important for the thin-walled tube with relative bending radius of 1 to be bent with mandrel in order to avoid the defects such as wrinkling and cross-section distortion. In this paper, a novel push-bending method using sectional elastomers as mandrel was proposed to form the 5A02 thin-walled aluminum alloy tube with relative bending radius of 1 and 90° bending angle. Cylindrical polyurethane rubber (CPR) with shore hardness of 80A was chosen as the sectional elastomers. The influence of diameter (d), thickness (t), total length (L), and arrangement of CPR filler on the push-bending deformation of tube was first explored by means of simulations and experiments. According to the simulation results, 5A02 thin-walled aluminum alloy tube with relative bending radius of 1 was successfully fabricated with the sectional CPR as mandrel in the self-developed equipment of push-bending. The experimental results were in good agreement with the simulation results. The optimum parameters of CPR filler in push-bending process were obtained as follows: 37 mm diameter, a piece of CPR with 20 mm thickness contacted with the punch, and other 12 pieces of CPR with 10 mm thickness.

A new strategy for describing the characteristics of bending line in flexible push bending

Purpose When manufacturing an arc-shaped tube product using push bending process, the transition zone and outfeed zone will inevitably occur. Transition zone and outfeed zone are caused by the kinematical motion of mobile tools. The existence of transition zone and outfeed zone will lead to a big deviation between the forming product and desired shape. To improve the forming quality of arc-shaped products in push bending, the transition zone and outfeed zone are investigated in this paper. Design/methodology/approach A piecewise function is used to describe the bending characteristics along bending line, in which a series of vibration parameters are extracted and considered as control values. Findings The new strategy is helpful for finding the relationship between tools motion and curvature distribution and improving the bending lines design procedure in flexible push bending. Originality/value The new strategy is helpful for finding the relationship between tools motion and curvature distribution and improving the bending lines design procedure in flexible push bending.

3D FEM-DEM coupling analysis for granular-media-based thin-wall elbow tube push-bending process

The granular-media-based thin-wall elbow push-bending process involves filling a tube with granular media and pushing the tube into a die to bend a tubular blank into an elbow shape. By means of the mechanical characteristics of granular filler, an elbow tube with t/D < 0.01 (the ratio of wall thickness to outer diameter) and R/D < 1.5 (the ratio of bending radius to outer diameter) can be formed. To investigate the interaction between thin-wall elbow and granular filler, A 3D FEM-DEM coupling numerical model is developed, which takes into account both the deformation behavior of tubular blank (continuum, finite element method FEM) and mechanical characteristics of granular filler (discrete media, discrete element method DEM). By means of the coupling model, the key forming parameters of an elbow tube such as forming force, wall thickness distribution, wrinkling are simulated and compared to experimental results.

Tubular blank design to fabricate an elbow tube by a push-bending process

The elbow push-bending process involves pushing a tube between a die and a mandrel to bend a tubular blank into an elbow shape. In this study, the yield was improved by minimizing the cutting loss at the ends of the elbow tube. This was achieved by developing an optimized method for the design of the tubular blank. The design method is based on the deformation behaviors during the push-bending process and was formulated to minimize the cutting loss. An analysis was done to develop design rules for determining the intrados and extrados lengths and the inclination angles at both ends. Given a radius of curvature and the target dimensions of an elbow tube, it is possible to provide the optimum dimensions of the tubular blank to be bent in the push-bending process. The proposed method was successfully implemented in an actual production line and showed improved dimensional accuracies with reduced cutting losses.

Push-bending Process and Forming Quality of Stainless-steel Tubes

The push-bending process of stainless-steel tubes was investigated by using the experimental method and finite element (FE) analysis based on the commercial software MSC.MARC. The effects of the processing parameters and the lubricating condition on the forming quality of as-obtained ASTM304 tubes were discussed. The results demonstrated that the experiment results were consistent with the simulation both in deformation behaviour and the stress distribution during the push-bending process, which verified the reliability of the established FE-model. The results also show that the maximum residual stress concentrates in the middle of tubes, the residual strain on both the external side increases firstly, then decrease, and increase again with the increase of the location angel. After the push-bending, the interior wall thickness increases while the exterior wall thickness decreases. When the relative bending radius is larger, smaller variation of wall thickness will be obtained. With the increase of the friction coefficient, the wall thickness thinning rate will decrease. With the increase of the pushing speed, the wall thickness thinning rate decrease a little while the thickening rate increases slightly. When push-bending of the large radius tubes, a good lubricating condition can reduce the wall thickening problem at the intrados, which will ensure a better forming quality. Keyword. Bending, Quality control, Simulation

Springback measurement in three roll push bending process of hollow structural sections

Three roll push bending is largely used to manufacture curved hollow structural sections. In this process, when new production batches are launched, long and repetitive trials-and-error procedures are necessary to compensate for the springback that may affect the final geometrical accuracy. Several off-line optimization techniques, both experimental and numerical, are available, but they require costly material characterizations and long computational times. The paper presents a new in-line approach based on inertial measurement techniques, which allows the accurate real-time measurement of the bent geometry during the process. The validation experiments are presented as well as the evaluation of the measurement accuracy.

Flow Stress Analysis and Hot Bending of P11 Alloy Steel

Based on the growing application value of the P11 alloy steel in the nuclear power field, its dynamic recrystallization (DRX) behavior was firstly investigated by means of isothermal hot compression experiments, under the conditions of a testing temperature range between 800 and 950 °C, and a strain rate range between 0.01 and 2/s. Furthermore, optical microscopy and transmission electron microscopy were also employed to analyze the effect of the mechanism of the strain rate on DRX. The results indicated that the grain size could be significantly refined with the increase of strain rate. Also, the recrystallized volume fraction was increased and the dislocation density decreased with the decrease of strain rate, for the same strain values. Subsequently, numerical simulations, under the assistance of experimental results on DRX behavior, were successfully used to study the hot push bending process and simultaneously obtain the processing parameters of the actual work-pieces. Finally, some comparative analyses were performed and discussed in parallel with the deformed actual work-pieces. The EBSD results on the deformed P11 alloy steel were emphasized for exploring the forming properties of this alloy steel.

Experimental and Numerical Investigations of Push Bending of Copper Thin Walled Tubes

In order to realize the successful process of forming the thin walled tubes in different industries, numerous methods have been registered by researchers. The tube push bending technique is a new process which allows forming tubes with lower bending ratio. In this study, the effects of push bending process on thickness variation and stress distribution are investigated. In addition, experimental and analytical studies have been dealt with in order to evaluate different bending radii. Results show close congruity between experiment and simulation.

Prediction of wrinkling in thin-walled tube push-bending process using artificial neural network and finite element method

The tube push bending process is a method that is mainly used to bend thin-walled tubes with a small bending radius. In this paper, artificial neural networks (ANNs) are used to predict the creation of wrinkling in a tube during the bending process. Since training the neural network involves many datasets and all of these data are difficult to generate using experiments, a credible finite element (FE) model is developed. The results obtained from the FE model are validated by conducting experimental tests. The results of the FE simulations are used to train, test, and validate the ANN models. Backpropagation neural networks based on the Levenberg–Marquardt algorithm are constructed using five design parameters including: relative bending radius; relative tube diameter; friction between die and tube; friction between tube and mandrel; and pressure as the network inputs and the maximum wrinkling height (MWH) as the single output. Two different ANN models are trained for two types of tube materials: brass and stainless steel 304. The obtained results show that by using the hybrid method that combines the ANN and FE it is possible to predict the MWH created in the push bending method with a high degree of accuracy.

Numerical Simulation of Push-Bending Process of Aluminum Section Profile

The aluminum profile can be formed into a complicated part such as 360º abnormity section circle parts that is difficult to be manufactured by other techniques. A new bending method was proposed for bending process. The push-bending principle, deformation procedure, curvature spring-back, section distortion and wrinkling are studied numerically. All cross-sections for the profile are provided a rather homogenous deformation degree. The curvature value of component and the distortion of section keep high consistence in push-bending process. The wrinkling tendency is reduced with increasing relative cross section thickness t/H and die radius R. The numerical model represents the observed response in the laboratory tests fairly well.