Flexible Roll Forming Research Articles

Flexible roll forming of surface developable profiles from Dual Phase steel

Flexible Roll Forming (FRF) can roll-form variable cross-sectional profiles for Electric Vehicle (EV) production however, a major limitation exists due to flange wrinkling while forming high-strength steels. Flange wrinkling can be eliminated by reducing the required level of membrane deformation in the longitudinal direction. Although reducing the severity of the profile's transitions minimises the strains, the overall complexity of the parts is also lowered. Origami-based developable profiles can be created from curved creased folding without membrane stretching or compression. In FRF, such types of profiles can be formed by combining a variation in width and depth over the length of the part. This study presents, for the first time, the analyses of forming a developable shape in a FRF operation. Firstly, analytical equations are applied to calculate the strains and forming stability of each pass which is followed by experimental FRF trials on two high-strength Dual Phase steels. Finally, Finite Element Analysis is used to investigate the forming behaviour of the two types of developable profiles. The experimental results show that the forming of one type of developable profile improves the shape, while the numerical analyses showed that an additional top-hat forming is required for the second profile type.

Manufacturing constrained shape optimisation of variable width flat web formed channels

The trend in automotive manufacturing towards lower volumes and an increased number of car variants combined with the need for forming higher strength metals to reduce weight has led to the implementation of alternative and flexible manufacturing methods. These have new manufacturing constraints compared to conventional stamping that change the part shapes that can be formed. This requires new methods for part shape optimisation. This study proposes a novel parametrisation for shape design that allows: 1) implementation of a gradient-based optimisation approach; and 2) taking manufacturing constraints into account. Our novel parameterisation can describe most long automotive structural parts using only a small number of design variables. The parts are described using multiple series of straight and curved connected profiles. We have uniquely conducted a detailed sensitivity analysis on the profiles to determine analytical solutions for the first order derivatives of the design variables with respect to the surface area/mass of a generic part. The profiles are also used to determine the final manufacturing strains in a part based on ideal forming. These ideal manufacturing strains can be compared to manufacturing process strain limits to determine the potential manufacturability of the part. The proposed parametrisation is applied to optimise a variable width channel formed by flexible roll forming. The channel is optimised to maximise the stiffness while maintaining both mass and manufacturability. In detail, the effectiveness and the general applicability of the established parametrisation technique and shape optimisation platform are demonstrated using three case studies of a flexible roll formed automotive S-rail channel part subjected to compression and bending loads. Furthermore, the manufacturability of the optimised structure is demonstrated by a forming model of the flexible roll forming process, where the model has been previously validated against experimental data. These examples show that the presented parametrisation and the associated shape optimiser can be successfully applied to increase part stiffness while reducing weight and maintaining manufacturability. The range of problems analysed demonstrates the flexibility and capability of the newly developed optimisation platform.

Numerical simulation research on UDF flexible roll forming of multi-specification thin-walled circular tubes

Numerical simulation research on UDF flexible roll forming of multi-specification thin-walled circular tubes

Part quality improvement in flexible roll forming of a combined variable width and depth component

Flexible Roll Forming (FRF) is an advanced manufacturing process that can be used to form long and complex part families from hard to form materials for the automotive and truck industry. However, depending on the part complexity and material strength shape defects such as end flare, springback and twist can occur. In this work a high strength automotive part (Sled runner) is produced from 3 high strength steels; the complex component combines shape variations in width and depth. The optimum forming sequence is identified to produce the part without any flange wrinkles. In addition to this, a new over forming technique is introduced to overcome end flare and springback. The experimental results are evaluated and discussed with numerical analysis and suggest that the presented forming technology enables the manufacture of high strength components in accordance with automotive specifications and quality standards.

Development of a reversible top-hat forming approach for reducing flange wrinkling in flexible roll forming

Flexible roll forming (FRF) addresses the need for flexible manufacturing and light-weighting in electric vehicle production, but its widespread implementation is prevented by severe wrinkling in the flange. Wrinkling in sheet metal forming depends on the tool support and material deformation. In FRF the tool effects on wrinkling are complex, given that the sheet undergoes unsupported deformation when it enters and exits the forming roll and there is no fundamental understanding of material deformation in the critical forming regions where wrinkling occurs. This prevents an effective tool and process design. This work develops a new model to analyse the stability of the flange by combining plastic bifurcation analysis with thin-shell assumptions. The model defines a stress ratio parameter that can be used to determine when in the process, deformation becomes unstable considering the tool support conditions and material deformation. Based on this a new approach for wrinkling reduction is developed that includes the forming and bending back of a top-hat shape to provide additional flange stiffness when wrinkling is likely to initiate. The top-hat shape is optimised using a series of linear perturbation ‘buckle’ analyses to provide the highest possible stiffness increase in the flange. This is followed by experimental manufacturing trials to test and prove the new concept for wrinkling reduction and to validate the model. It is shown that the new top-hat forming approach can eliminate wrinkling in the flange even when flexible roll forming Ultra High Strength Steels. The analytical model results suggest that this is due to the stiffening effect of the top-hat and the reduction of the effective flange length which decreases the stress ratio. The new model in combination with the new forming approach to eliminate flange wrinkling represents a vital step towards a more effective process and tool design that will enable the widespread application of FRF in automotive manufacturing.

Longitudinal strain and wrinkling analysis of variable depth flexible roll forming

Flexible roll forming (FRF) enables the forming of high-strength sheet metal materials into complex shapes relevant to the transportation industry until recently, however, has been limited to the forming of variable-width profiles where the web section is flat with a curved flange. Recent developments in 3D flexible roll forming now enable forming of variable depth components that have a curved web with flat flanges; these shapes are better suited for automotive applications. However, wrinkling in the flange due to excessive compressive longitudinal stress remains a major challenge. Deformation strain and stress analysis has been applied in previous studies to estimate the wrinkling tendency, but current analytical equations are limited to variable-width shapes with limited accuracy, particularly regarding the level and distribution of longitudinal strain. In the current study, a new analytical model is developed to analyse the distribution and level of longitudinal edge strain for a variable depth component shape. The analytical predictions are compared with experimental measurements for key conditions. This shows that the new analytical equations developed in this study provide higher accuracy compared to existing models and enable for the first time the prediction of the distribution of longitudinal strain in the flange of a flexible roll forming component. For some of the tested conditions, flange wrinkling was observed and there the model correlation was poor given the theory assumes a wrinkle-free flange. The strain deviation observed here is associated with the shape deviation due to wrinkling. The analytical model predicts the strains that are required to form a perfect part shape. Wrinkling reduces the compressive longitudinal strains that are formed into the flange, and this leads to a poor correlation between the experimental and the predicted strain values if the wrinkling severity is high. The model brings some insights into the wrinkling behaviour in variable depth flexible roll forming by predicting the variation of the peak strain and overall distribution with key geometric parameters. The new analytical model for longitudinal edge strain presented in this study will enable a more rapid process design and optimisation before time-consuming FEA analysis is performed. It, therefore, presents a vital step towards a wider spread in the application of flexible roll forming in the automotive and transport industries.

Prototyping of straight section components using incremental shape rolling

Flexible Roll Forming (FRF) allows the forming of components with a variable cross section along the length of the component. However, the process has only limited application in the automotive industry due to wrinkling in the flange which currently prevents the forming of high strength steels and limits the part shape complexity. This paper presents a new forming technology, Incremental Shape Rolling (ISR), where a pre-cut blank is clamped between two dies, and then a single forming roll is used to incrementally form the material to the desired shape. The new process is similar to some Incremental Sheet Forming (ISF) approaches but with the difference that Incremental Shape Rolling (ISR) allows the manufacture of longitudinal components from high strength metal sheets. In this work, a numerical model of the ISR of a straight section is developed. Experimental prototyping trials are performed and are used to validate the numerical model which is then applied to analyse the new forming process. The results show that in ISR, tensile residual strains are developed in the flange. Flange wrinkling is observed and directly linked to the number of forming passes that are used in the process.

A novel incremental sheet metal forming process for long and open section profiles

Complex components from High Strength Steel (HSS) can be formed with Flexible Roll Forming (FRF). However, flange wrinkling limits the part shape complexity that can be achieved and hence limits its application in the automotive industry. A novel forming technology is introduced in this study, Incremental Shape Rolling (ISR), where a pre-cut blank is clamped between a top and bottom die and then a single forming tool incrementally forms the material according to the bottom die’s geometry. It is believed that ISR can significantly reduce the wrinkling issue compared to FRF. In the contrast to the incremental sheet forming process, ISR allows the manufacture of long and open sections from HSS sheets. In this work, the experimental prototyping trials of the ISR process are performed using a 5-axis milling machine and then are used to validate the FEA model which is developed to simulate and analyse the deformation behaviour of the ISR process. The results show a clear development of transverse tensile strain along the flange which can overcome the wrinkling issue when the ISR process is applied to complex profiles.

Optimising part quality in the flexible roll forming of an automotive component

Optimising part quality in the flexible roll forming of an automotive component

On the prediction of wrinkling in flexible roll forming

Flexible roll forming process is an advanced manufacturing process, for producing complex and lightweight profiles with variable cross-section. One of the major observed defects in this process is the flange wrinkling of the profiles, which has limited its development especially for the automotive industry. In this paper, the deformation mechanics of the flexible roll forming process is revisited, and the methodology for predicting the onset of wrinkling, which had previously been developed based on the wrinkling limit curve in the space of effective strain versus stress triaxiality, is improved. Then, a new criterion based on the wrinkling limit curve is proposed to predict the possibility of wrinkling in finite element analysis. The criterion is applied to analyze the influence of geometric parameters of the variable cross-section profile and material properties of the sheet on the occurrence of wrinkling. The prediction accuracy is evaluated by performing experiments with different profile shapes. Results show that the new proposed criterion can successfully predict the onset of wrinkling in the flexible roll forming process, and its utilization is recommended for predicting wrinkling in the finite element analysis of sheet metal forming processes.

Application of Support Vector Regression and Genetic Algorithm to Reduce Web Warping in Flexible Roll-Forming Process

Abstract In a flexible roll-forming process, a metal blank is incrementally deformed into the desired shape with a variable cross-sectional profile by passing the blank through a series of forming rolls. Because of the combined effects of process and material parameters on the quality of the roll-formed product, the approaches used to optimize the roll-forming process have been largely based on experience and trial-and-error methods. Web warping is one of the major shape defects encountered in flexible roll forming. In this study, an optimization method was developed using support vector regression (SVR) and a genetic algorithm (GA) to reduce web warping in flexible roll forming. An SVR model was developed to predict the web-warping height, and a response surface method was used to investigate the effect of the process parameters. In the development of these predictive models, three process parameters—the forming-roll speed condition, leveling-roll height, and bend angle—were considered as the model inputs, and the web-warping height was used as the response variable. The GA used the web-warping height and the cost of the roll-forming system as the fitness function to optimize the process parameters of the flexible roll-forming process. When the flexible roll-forming process was carried out using the optimized process parameters, the obtained experimental results indicated a reduction in web warping. Hence, the feasibility of the proposed optimization method was confirmed.

Springback and end flare compensation in flexible roll forming

Flexible roll forming enables the forming of complex automotive components from Advanced High Strength Steels (AHSS) and Ultra High Strength Steel (UHSS) but common shape defects such as end flare and springback occur and need to be compensated. In flexible roll forming a pre-cut blank is fed through a set of rolls which are computer numerically controlled (CNC) to follow the 3D contour of the part. The company, dataM Sheet Metal Solutions, have established a new 3D flexible roll forming facility at Deakin University which allows the manufacture of flexible roll formed prototypes.In this study, Deakin’s flexible roll forming facility is employed to implement an approach to overcome springback and end flare defects in a flexible roll formed automotive component of variable width. To do this, the flexible roll forming process is simulated using the commercial software package Copra FEA. Non-uniform springback due to end flare is observed and a variable overbending approach developed and implemented to overcome this defect. Experimental validation is performed on Deakin’s flexible roll forming facility and the results suggest that the flexible overbending approach can be used to control final shape in flexibly roll formed components.

The flower pattern and rolls design for ERW pipes with the different specification in the flexible roll forming process

To produce electrical resistance welding (ERW) pipes with the different specification efficiently and economically, flexible roll forming technology is urgently required. Based on the elastic plasticity deformation theory, combined with the basic parameters of typical roll forming units, the 3D finite element analysis (FEA)models of ERW pipes with seven kinds of sizes were built and the numerical simulations were conducted with the professional roll forming software COPRA. The mechanics law between strip and rolls in conventional roll forming was revealed and the results were obtained that the edge arc segment of rolls plays a major role and the straight segment restricts the deformation of strip in the W-shaped forming process. Considered the mechanics law and forming mechanism, the modular and compact design of flower pattern and rolls in first to fifth stands were conducted; a set of involute-tangent-arc roll profile for flexible roll forming of ERW pipes with seven kinds of sizes in first to fifth stands was proposed and verified with the numerical simulations. Results showed that the final product with the flexible design of flower pattern and rolls satisfied the accuracy requirement and the transverse strain distribution was optimized during the roll forming. The flexible roll forming technology reduces the time and costs for the preparation and replacement of rolls in first to fifth stands when ERW pipes with the different specification are produced, and the total weight of rolls is reduced by about 35%, realizing the lightweight production, which provides a novel method for the flexible roll forming of ERW pipes.

Non-sorting multi-objective optimization of flexible roll forming using artificial neural networks

The main defects due to flexible roll forming (FRF) processes include longitudinal bow and wrinkling. In this study, experimental and numerical analyses were performed using three different blank shapes to characterize the effects of the process parameters on defects in parts fabricated by FRF with and without leveling roll. Owing to the complexity of the FRF process, two algorithms were combined for its optimization. Artificial neural network-based Non-dominated Sorting Genetic Algorithm II (NSGA-II) was used to optimize the effective parameters of the FRF process, such as the sheet thickness, yield strength, and blank shape, with respect to the target bend angle to minimize the longitudinal bow and wrinkling of the product. The back-propagation neural network (BPNN) was used to identify two objective functions, while non-sorting multi-objective algorithm simulation was used to optimize the input parameters to minimize the objective functions. The results showed that the sheet thickness had the greatest effect on the minimization of the two objective functions, followed by the yield strength and blank shape, respectively.

Analysis of material behaviour and shape defect compensation in the flexible roll forming of advanced high strength steel

Roll forming is a fast and low-cost forming process with the ability to form a wide range of materials, especially advanced high-strength steels (AHSS) that have limited ductility. However, roll forming is only able to form constant cross-sections along the longitudinal direction of the component, while many automotive components are more complex in shape. Flexible Roll Forming (FRF) uses rolls that have the ability to translate or rotate during production, enabling the manufacture of a part that has a variable cross-section potentially useful for the automotive industry.Deakin University is equipped with a flexible forming facility that can prototype parts with variable cross-sections. The machine has been developed, patented and manufactured by dataM Sheet Metal Solutions GmbH. Deakin’s Flexible Forming Facility (DFFF) consists of a single forming stand with two opposing robotic arms connected to forming rolls. The flexibly controlled arms form the clamped sheet along a CNC defined bend line while the sheet remains clamped. This facility is capable of creating changes in the cross-section in both width and depth along the part. Moreover, the Deakin facility can form the part in both forwards and backwards directions along the longitudinal dimension of the part. In contrast, conventional FRF facilities use multiple stands of forming rolls, and the forming sequence occurs in a single direction along the part.The first part of this study compares material deformation in the DFFF in contrast to that of the conventional FRF setup. This study will determine the similarities and differences in the final formed part between DFFF and the forming conditions in conventional FRF. The second step of the study is to investigate the forming properties of the final part when the part has variable depth. An experimental part formed on DFFF was compared to a numerical analysis using COPRA RF FEA to investigate the material flow and the forming defects. The results show that the major shape error is wrinkling. A new forming concept using a blank holder tool was analysed, and it shows promising results in regard to wrinkling reduction.

Analysis of shape defects during flexible roll forming of steel/aluminum double-layered blanks

Flexible roll forming is an advanced sheet-metal-forming process that allows the production of parts with variable cross-sections. Double-layered blanks can have enhanced properties such as a high stiffness-to-weight ratio and high corrosion resistivity when combined with suitable materials. However, in the flexible-roll-forming process of double-layered blanks, additional shape defects and interface delamination can occur owing to their inhomogeneous mechanical properties compared with those of single-layered blanks. In this study, investigations on shape defects generated during the flexible-roll-forming process of steel/aluminum double-layered blanks were performed. Shape defects such as web-warping, wrinkling, and delamination at the interface were investigated on three different blanks with trapezoidal, convex, and concave shapes. The results show that the process-induced longitudinal strains significantly affect the occurrence of shape defects. Moreover, the longitudinal strains strongly depend on the blank shapes, their stacking order, and mechanical properties of the constituent layers.

Shape Defects in The Flexible Roll Forming of Automotive Parts

Flexible roll forming allows variable cross sections of profiles to be made by using adaptive roll stands. The most common shape defects that occur during flexible roll forming are edge wave and longitudinal bow. These two defects are necessary to consider together for relatively thin-walled blanks because they usually occur simultaneously during flexible roll forming. To understand the occurrence of these defects, finite element (FE) simulations based on the rigid shell concept were carried out using three blanks with different shapes: trapezoid, concave, and convex. The FE analysis on the flexible roll forming process was performed first, and then the gap of rigid shell has been decreased for reducing the longitudinal bow in roll formed blank. The reduction of longitudinal bow in the web part usually causes edge wave in the flange part. The occurrence of shape defects is investigated based on the relationships among edge curvature, longitudinal stress at the edge, and longitudinal bow height. When longitudinal bow is reduced, the longitudinal stress at the edge of roll formed blank is changed from tensile to compressive in convex blank but in concave blank, tensile residual stress increases. Edge waves occur in the flanges of trapezoid and convex blanks as a result of decreased longitudinal bow. Lab-scale flexible roll forming experiments show multi-stand forming with a leveling roll is desirable to reduce both edge wave and longitudinal bow.

Theoretical Analysis of Five‐Point Bending and Springback for Preforming Process of ERW Pipe FFX Forming

Flexible forming excellent (FFX) forming of electric resistance welding (ERW) pipe is an advanced roll forming method. In order to understand FFX forming mechanism and set reasonable roll parameters, five‐point bending process and springback process for preforming section of ERW pipe FFX forming are analyzed. Under various assumptions, the bending process of sheet metal is divided into full elastic bending stage and elastic‐plastic bending stage by analyzing the forming law and simplifying the contact state model of roll bending process. Based on the theory of plane bending theory, the five‐point bending springback model is established. Mathematical expressions of bending curvature and rotation angle of arbitrary particle on plate under arbitrary state are derived. The curvature calculation formulas of each particle on plate after springback are obtained, and then the calculation formulas of bending angle and rebound angle after springback are obtained. The theoretical analysis results are verified by the wide plate five‐point bending experiment. The theoretical value of springback angle agrees well with the experimental results. The maximum deviation of springback angle is 0.42 degrees. The results show that the five‐point bending model is reasonable. It provides theoretical support for the preforming model of flexible roll forming.

Effects of circular holes on the web warping of perforated blanks in flexible roll forming

To meet the demands of highly functional parts, high-precision manufacturing technology that can provide sound semifinal perforated components having variable cross sections is frequently required. When a relatively short blank is formed into a variable section by a flexible roll forming process, web warping such as longitudinal bows and crossbows can be generated in the formed products. In this study, the characteristic features of web warping occurring in the flexible roll forming of perforated blanks were investigated both numerically and experimentally. In order to investigate the cause of web warping, the stress and strain variations during flexible roll forming are examined in terms of the shapes of blank and hole orientations of perforated blanks. The results show that longitudinal bows and crossbows are primarily caused by the nonuniformity of longitudinal and transverse strains between the flange and the web. In addition, the degree of web warping is sensitive to the shapes of the blanks and the hole orientations. From the process analysis, a guideline for the perforation of holes in convex, concave and trapezoid blanks is proposed to reduce web warping.

The Effect of Flange Length on the Distribution of Longitudinal Strain during the Flexible Roll Forming Process

Flexible roll forming is an advanced sheet metal forming process which allows the production of variable cross-section profiles. In flexible roll forming process, nonuniform transversal distribution of the longitudinal strain can cause the longitudinal bow, which is deviation in height of the web over the length of the profile. To investigate the effect of flange length on the transversal distribution of the longitudinal strain, FEM simulations are conducted with different flange length for three blank shapes; trapezoid, convex and concave. The result shows that the longitudinal strain and longitudinal bow decrease with increasing flange length for a trapezoid and a concave blank. For a convex blank, the longitudinal strain and longitudinal bow increase with increasing flange length. To validate FEM simulation result, numerically obtained longitudinal strain has been compared with experimental results.