Sheet Metal Forming Research Articles

Enhancing formability in deep drawing: A 3D finite element analysis of plastic wrinkling in AA2090 Al–Li alloy

Deep drawing is a manufacturing process used to produce billions of metal containers, crucial for the aerospace industry in forming thin-walled components efficiently. However, wrinkling due to compressive instability remains a significant challenge in sheet metal forming. This study investigates plastic wrinkling in sheet metal forming to enhance the final drawn shape. A 3D finite element model using ABAQUS/Explicit was developed to examine the effects of mesh topology, blank holder force, and sheet thickness on wrinkling in AA2090 Al–Li alloy. The finding indicates that that an increase in blank holder force increases wrinkle number and amplitude. Additionally, it was found that a 2000 N blank holder force is necessary to prevent wrinkling.

Polymer Shaped Punches Produces with Fused Filament Fabrication to Improve Cup Accuracy in Sheet Metal Forming

Rapid tooling has become an effective solution for reducing time and costs in tool production. In sheet metal forming, polymer tools produced via additive manufacturing offer performance comparable to traditional tools. However, a key challenge in this area is compensating for the radial expansion of polymer tools during the forming process, which leads to reduced accuracy in the produced parts and limits the achievable forming depth. To address this issue, the authors of this study proposed a novel punch design aimed at containing radial expansion, thereby enabling greater drawing depth and improved part accuracy. Different punch geometries were designed with a re-entrant angle varying between 150° and 180°. Numerical simulations were conducted to evaluate the optimal geometry, identifying the 160° angle as the best option to compensate for radial expansion and reduce punch load. Experimental tests were then performed to verify the numerical results, demonstrating the potential of this new design producing cups with higher drawing depth and best radial accuracy.

An extended mesoscaled M-K model for predicting discrete ultimate strains and evaluating the effect of normal stress on forming limits

Due to the size effect, the formability of metal sheets diminishes and the ultimate strain points exhibit a more discrete distribution. This poses a huge challenge to the process design and manufacture of microparts. This paper presents a modified mesoscopic scale forming limit model based on the M-K model. The model extends the forming limit curve (FLC) into forming limit bands to predict the discrete nature of the ultimate strain. In addition, a method for applying normal stresses to increase the forming limit of thin metal sheets is proposed. The effects of normal stresses on surface roughening and flow stresses in thin metal sheets were evaluated and quantified by uniaxial tensile experiments under normal stresses. The results show that the normal stress suppresses the increase of surface roughness and improves the work hardening rate. The effect of normal stresses on forming limits was obtained using a constructed model. The results show that the predictions of the proposed model are able to include almost all the ultimate strain points. The forming limit bands are more inclusive and safer than the single forming limit curve. In addition, the forming limits of thin plates under normal stresses are significantly increased, which is attributed to the suppression of surface roughening and the increase of strain hardening index. The forming limit bands proposed in this paper provide a more reliable form of forming limit assessment for thin metal plates. The proposed method of applying normal stresses provides a robust and inexpensive process route to address the challenge of poor formability of thin metal sheets.

Identification of hardening and fracture behaviors of titanium alloy by a hybrid digital image correlation and finite element method

The reliable identification of deformation and fracture behaviors of titanium alloys is essential for accurate numerical simulation in sheet metal forming, cutting process and crash. In this study, an inverse method is proposed to identify both strain hardening and fracture behavior of titanium alloys by a hybrid experimental-finite element analysis. Uniaxial and notched tensile specimens are tested on the additively manufactured and cold rolled sheet Ti6Al4V alloys. The distributions of displacement and strain on the specimen surface during the loading are measured by digital image correlation method. An identification strategy, considering the stress–strain curve, surface strain evolution and variation of specimen width, is proposed to identify the strain hardening model parameters at large strain range. Finite element simulations are performed to update the model parameters by using the iterative inverse method. The parameters of fracture model are determined through experiments on tensile specimens in combination with numerical simulation. The experimental and simulation results show the feasibility and effectiveness of the proposed identification method, that it can be further used to characterize the plastic and fracture behaviors of other metallic materials in practice.

Comparative evaluation of formability in cup drawing and V-bending using additively manufactured tools

ABSTRACT Additive Manufacturing (AM) is rapidly transforming industrial processes by introducing advanced methods for designing, manufacturing, and prototyping key components across various industries. Its true potential is realized in integrated component repair via reverse engineering, significantly reducing rework, maintenance costs, and time. With growing demand, AM technology continues to evolve, incorporating intelligent printing systems and diverse applications in sectors like automotive, sheet metal forming, healthcare, aviation, and consumer goods. AM also aids in optimizing geometric and process parameters for tools such as dies, punches, and blank holders, enhancing economic batch production. This study focuses on the Finite Element Analysis (FEA) of two commercial operations: bending and deep drawing, using DC04 and AA6060 materials. The analysis identifies geometric precision, thinning, and formability. In steel, increasing the bottom radius from 3 mm to 20 mm results in an 11% difference in thinning, with a maximum of 0.67 mm. For aluminium, thinning varies minimally between 0.68 and 0.75 mm. Increased sheet thickness limits formability in both materials. FDM-printed tools were used for FEA simulation, followed by stress analysis of dies to assess design safety, small batch production, and dimensional conformity.

Application of the Gradient-Boosting with Regression Trees to Predict the Coefficient of Friction on Drawbead in Sheet Metal Forming.

Correct design of the sheet metal forming process requires knowledge of the friction phenomenon occurring in various areas of the drawpiece. Additionally, the friction at the drawbead is decisive to ensure that the sheet flows in the desired direction. This article presents the results of experimental tests enabling the determination of the coefficient of friction at the drawbead and using a specially designed tribometer. The test material was a DC04 carbon steel sheet. The tests were carried out for different orientations of the samples in relation to the sheet rolling direction, different drawbead heights, different lubrication conditions and different average roughnesses of the countersamples. According to the aim of this work, the Features Importance analysis, conducted using the Gradient-Boosted Regression Trees algorithm, was used to find the influence of several parameter features on the coefficient of friction. The advantage of gradient-boosted decision trees is their ability to analyze complex relationships in the data and protect against overfitting. Another advantage is that there is no need for prior data processing. According to the best of the authors' knowledge, the effectiveness of gradient-boosted decision trees in analyzing the friction occurring in the drawbead in sheet metal forming has not been previously studied. To improve the accuracy of the model, five MinLeafs were applied to the regression tree, together with 500 ensembles utilized for learning the previously learned nodes, noting that the MinLeaf indicates the minimum number of leaf node observations. The least-squares-boosting technique, often known as LSBoost, is used to train a group of regression trees. Features Importance analysis has shown that the friction conditions (dry friction of lubricated conditions) had the most significant influence on the coefficient of friction, at 56.98%, followed by the drawbead height, at 23.41%, and the sample width, at 11.95%. The average surface roughness of rollers and sample orientation have the smallest impact on the value of the coefficient of friction at 6.09% and 1.57%, respectively. The dispersion and deviation observed for the testing dataset from the experimental data indicate the model's ability to predict the values of the coefficient of friction at a coefficient of determination of R2 = 0.972 and a mean-squared error of MSE = 0.000048. It was qualitatively found that in order to ensure the optimal (the lowest) coefficient of friction, it is necessary to control the friction conditions (use of lubricant) and the drawbead height.

A study on anisotropic hardening of 7075 aluminum alloy based on non-associated flow rules

The accurate description of anisotropic plastic deformation is key to accurately predicting the stamping forming of metal sheets. The anisotropic yield criterion, when based on the assumption of isotropic hardening, often leads to significant inaccuracies. To address this issue, this paper observes the anisotropic hardening phenomenon through non-associated flow rules and evaluates the anisotropy of 7075-O aluminum alloy. Through a series of tensile tests, we determined the mechanical properties of 7075-O aluminum alloy in three distinct orientations. To describe the metal hardening behavior, we employed the Swift-Voce hardening criterion. From the hardening curves in three different directions, it was found that AA7075-O exhibits plastic anisotropy. Based on the VUMAT subroutine, finite element simulation of AA7075-O tensile tests was conducted through Abaqus. Compared with the Hill48 model, it was found that the simulated values of the S–Y2009 anisotropic hardening model have a higher degree of agreement with the experimental curves. The S–Y2009 anisotropic hardening model was adopted to predict the earing behavior of AA7075-O during circular cup deep drawing. The root mean square error between the predicted values of the S–Y2009 model and the experimental values was only 0.1795, which is far smaller than that of the Hill48 yield model. Therefore, the SY2009 model has important guiding significance for the stamping forming of metal sheets.

A flexible yield criterion for strength modeling from biaxial compression to biaxial tension

Accurate strength modeling from equi-biaxial tension (EBT) to equi-biaxial compression (EBC) is critical for the plastic behavior prediction covering the wide-range of stress triaxiality encountered in sheet metal forming. To date, however, few yield criteria are available that can precisely model the initial yield and hardening behavior under six typical stress states between EBC and EBT, simultaneously. Furthermore, there is still a lack of a unified yield criterion for accurate strength modeling across various stress state ranges. To address the issues, a theoretical framework for constructing yield criteria dependent on stress states is provided and a new analytically described isotropic yield criterion is presented in this study. The flexibility in terms of the yield locus and application range is thoroughly explored to make the new yield criterion general. Subsequently, the isotropic yield criterion is extended into an analytically described anisotropic-asymmetric yield criterion. Furthermore, the extended yield criterion is applied to capture the initial yield behavior of DP980, AA5754-O, and AZ31 sheets, and the strain hardening behavior of QP1180 sheets at various stress states ranging from EBC to EBT along different loading directions. The predicted results from the extended criterion agree well with the corresponding experimental findings. The applications demonstrate that the proposed anisotropic-asymmetric yield criterion can effectively model the initial yield and hardening behavior of HCP, BCC, and FCC metal sheets under EBT, EBC, uniaxial tension (UT), plane strain tension (PST), shear (SH), and uniaxial compression (UC) in an analytical way.

Numerical modelling and experimental investigation of incremental sheet metal forming of Aluminum Alloy Al 3003-O

Abstract Incremental sheet forming (ISF) is an unconventional sheet metal forming technique that can be widely used in the automotive, medical, and aerospace industries. The final product is achieved by moving a rotating tool along a defined spiral or profile tool path controlled by a CNC milling machine. A truncated cone shape was modeled in single point incremental forming (SPIF) to investigate the influence of step size, and sheet initial thickness parameters on the forming force components Fx , Fy , Fz during the forming process.

Systematic analysis of geometric inaccuracy and its contributing factors in roboforming

Incremental sheet metal forming is a highly versatile die-less forming process for manufacturing complex sheet metal components. Robot-assisted incremental sheet forming, or roboforming, allows a wider range of tool motion, providing the capability to shape intricate components. This makes roboforming the most flexible variant of the incremental forming method. However, the serial arrangement of links and joints in a robotic manipulator results in low positional accuracy under forming loads due to insufficient structural stiffness. The stiffness of the machine frame and tool directly impacts the accuracy of the final formed profile. The impact of machine compliance on component shape in incremental sheet forming is substantial in roboforming. This work presents a methodology for systematic analysis of the factors contributing to the errors in the geometric shape of robot-based forming. Experimental and numerical methods are used to estimate the material springback, tool/tool holder deflections, and errors due to machine compliance. The CNC machine frame is relatively stiffer than the industrial robots, such that material springback is estimated based on the experimental trials on CNC for cone and variable wall angle cone profiles. Tool and tool holder deflections are estimated using finite element simulations. The analytical method using the Virtual Joint Model is used to model the joint stiffness, and consequently, the robot Cartesian stiffness is estimated to predict path deviation contributing to geometric shape errors. The proportional contribution of each factor in the overall deviation in the Roboforming is also quantified.

MODELING OF THE CUTTING FORCES IN END MILLING OF A METALLIC MULTILAYER MATERIAL

Multilayer metal materials are increasingly used by tool shops in the production of transfer tools for sheet-metal forming. In research, it is found that during the machining of these advanced materials, damage to the cutting edge often occurs due to jumps in the cutting forces. The size of the cutting forces when milling a multi-directionally layered metal material is influenced by both the direction of depositing the individual layer and the geometry of the cutting tool. The main goal of the research was to create a mechanistic model of cutting forces for the end-milling of unidirectional and multidirectional 20MnCr5/316L four-layered metal material. The results of the model were compared with experimental data, where a good agreement was found when milling unidirectional layered material and a slightly worse agreement when milling multidirectional material. The maximum observed deviation of the predicted cutting forces is 14.6 % for all the comparative tests.

Design and Analysis of a Controllable Magnetic Blank Holder System with Magnetorheological Elastomer for Sheet Metal Forming

Design and Analysis of a Controllable Magnetic Blank Holder System with Magnetorheological Elastomer for Sheet Metal Forming

Evaluating the performance of Bi58Sn42 mold produced by material extrusion additive manufacturing system for agile manufacturing

PurposeThe purpose of this study is to produce a low-cost sheet metal forming mold made from the low melting point Bi58Sn42 (bismuth) alloy by using an open-source desktop-type material extrusion additive manufacturing system and to evaluate the performance of the additively manufactured mold for low volume sheet metal forming. Thus, it was aimed to develop a fast and inexpensive die tooling methodology for low-volume batch production.Design/methodology/approachInitially, the three-dimensional printing experiments were performed to produce the sheet metal forming mold. The encountered problems during the performed three-dimensional printing experiments were analyzed. Accordingly, both tunings in process parameters (extrusion temperature, extrusion multiplier, printing speed, infill percentage, etc.) and customizations on the extruder head of the available material extrusion additive manufacturing system were made to print the Bi58Sn42 alloy properly. Subsequently, the performance of the additively manufactured mold was evaluated according to the dimensional change that occurred on it during the performed pressing operations.FindingsResults showed that the additively manufactured mold was rigid enough and proved to have sufficient strength in sheet metal forming operations for low-volume production.Originality/valueAlternative mold production was carried out using open-source material extrusion system for low volume sheet metal part production. Thus, cost effective solution was presented for agile manufacturing.

INTEGRATING NEURAL NETWORKS INTO SHEET METAL FORMING: A REVIEW OF RECENT ADVANCES AND APPLICATIONS

In order to predict defects, improve performance, and streamline operations, machine learning techniques are becoming ever more indispensable in manufacturing processes, mainly in sheet metal forming. Incorporating neural networks into the process of sheet metal forming is the subject of this article's exhaustive examination of recent developments and applications. Exploring datasets from a variety of sheet metal forming processes, numerous machine learning models, including ensemble and single learning techniques are investigated. The functionality of this method extends to various tasks, including the prediction of springback in cold-rolled anisotropic steel sheets. The review provides a conclusion section that presents the main implementation methodologies and how they address to some manufacturing issues.

Effect of Countersample Coatings on the Friction Behaviour of DC01 Steel Sheets in Bending-under-Tension Friction Tests.

The aim of this article is to provide an analysis of the influence of the type of hard anti-wear coatings on the friction behaviour of DC01 deep-drawing steel sheets. DC01 steel sheets exhibit high formability, and they are widely used in sheet metal forming operations. The tribological properties of the tool surface, especially the coating used, determine the friction conditions in sheet metal forming. In order to carry out the research, this study developed and manufactured a special bending-under-tension (BUT) friction tribometer that models the friction phenomenon on the rounded edges of tools in the deep-drawing process. The rationale for building the tribotester was that there are no commercial tribotesters available that can be used to model the phenomenon of friction on the rounded edges of tools in sheet forming processes. The influence of the type of coating and sheet deformation on the coefficient of friction (CoF) and the change in the topography of the sheet surface were analysed. Countersamples with surfaces prepared using titanium + nitrogen ion implantation, nitrogen ion implantation and electron beam remelting were tested. The tests were carried out in conditions of dry friction and lubrication with oils with different kinematic viscosities. Under dry friction conditions, a clear increase in the CoF value, with the elongation of the samples for all analysed types of countersamples, was observed. Under lubricated conditions, the uncoated countersample showed the most favourable friction conditions. Furthermore, oil with a lower viscosity provided more favourable conditions for reducing the coefficient of friction. Within the entire range of sample elongation, the most favourable conditions for reducing the CoF were provided by uncoated samples and lubrication with S100+ oil. During the friction process, the average roughness decreased as a result of flattening the phenomenon. Under dry friction conditions, the value of the Sa parameter during the BUT test decreased by 20.3-30.2%, depending on the type of countersample. As a result of the friction process, the kurtosis and skewness increased and decreased, respectively, compared to as-received sheet metal.

Wrinkling suppression in cryogenic forming of high-strength Al-alloy ultra-thin shells by controlling interface shear stress

Avoiding wrinkling defects is extremely difficult in the sheet metal forming of ultra-thin components made from high-strength aluminum alloys. A novel cryogenic forming is thus proposed for solving this very challenging problem, wherein a thicker cladding blank is stacked above the thinner target blank to establish interface shear stress for reducing the hoop compressive stress, so that the critical wrinkling stress is increased. The enlarged radial deformation is transferred and withstood by the increasing hardening and ductility at cryogenic temperatures. The wrinkling suppression mechanism is revealed through mechanical and numerical analyses. Systematic experiments were conducted for studying the feasibility of high-strength AA7075 in cryogenic forming with different stacking sequences of cladding and target blanks, cladding blanks including three material strength levels (AA1060-O, AA5052-O, and SUS-304), two thicknesses (3.0, and 2.0 mm), and blank-holder forces. The effects of the cladding blank and blank-holder forces were clarified as reflected by forming defects, thickness, and strain distributions. The mechanical and numerical analyses can indicate that contact pressure can be produced by the wrinkling tendency of thinner target blank and limitation of cladding blank to wrinkling, which is accompanied by a decrease in the hoop compressive stress and increase in the critical wrinkling stress. Therefore, applying a relatively thicker blank on the punch side can prevent the wrinkling of ultra-thin components. The wrinkling tendency decreases with increasing strength and thickness of the cladding blank, which results in an increase in deformation or even splitting, which can be solved by the cryogenic temperature. The thickness of the cladding blank can be reduced by increasing the blank-holder force, which further reduces the material cost. AA5052-O, which has a strength similar to that of the target blank, is more suitable as a cladding blank for AA7075-W because the balance preventing wrinkling, improving thickness uniformity, and controlling the forming force. A Φ200 mm hemispherical shell with an initial thickness of 0.3 mm was formed successfully, and the corresponding thickness-to-diameter ratio reached 0.8‰, which almost increased one time on the basis of direct cryogenic forming. This new approach can be used for fabricating ultra-thin components from high-strength aluminum alloys.

Simulation-based data reduction and data processing for sheet metal forming in the hybrid twin framework

In sheet metal forming, the interaction between virtual models and the real world remains challenging. Process simulations can exhibit significant errors, and reliable measurements are often scarce during early production stages. This study presents a hybrid twin framework that systematically unifies computer-aided design, simulation, and measurement data in an adaptive manner. Central to this framework is a reverse engineering algorithm that reconstructs and transforms the geometry of deep-drawn components from optical scan data into B-spline surfaces. The algorithm demonstrated high precision, indicating its suitability for process control and geometric analysis. The hybrid twin framework integrates virtual data from simulations and real-world data, as evidenced by a sensor concept for inline surface measurement. The framework ensures robust and redundant measurement concepts by estimating complete geometries from a few systematically preselected measuring points. This adaptive approach permits continuous updates and extensions to the database, accommodating both sparse inline signals and offline inspection data. This framework provides a conceptual model for integrating direct feedback interactions between virtual and physical environments, thereby enhancing the precision of analytical and predictive models in sheet metal forming processes.

Reforming toolpath effect on deformation mechanics in double-sided incremental forming

Incremental sheet metal forming is a die-less process known for its high flexibility, making it a suitable choice for fabricating low-batch, highly customized complex parts. In this paper, the deformation mechanism of double-sided incremental forming (DSIF) with single pass or reforming toolpaths and its effect on the deformation-induced martensite transformation kinetics are investigated by experiments and finite element (FE) simulations for a truncated pyramid part geometry for the first time. An FE model accounting for tool deflection is developed, considering the change in tools' position caused by the forces applied at the contact with the blank. This is done to enhance the simulation accuracy in comparison to experimental results. A comprehensive analysis is conducted with respect to the stress state change and strain accumulation during DSIF with the single pass and reforming toolpaths. The findings reveal that material points along the pyramid wall primarily deform by a combination of plane strain tension, plane strain compression, and shear, depending on the tools’ locations on the sheet surfaces in relation to the material point of interest. The reversal of the forming direction in the reforming toolpath causes material points to experience different stress states and more strain accumulation than the single pass case. Under the same forming temperature, this phenomenon results in a higher and closer martensite transformation on both forming and supporting surfaces of the final part when employing the reforming toolpath as opposed to the single pass case.

Numerical investigation of effect of pressure exponent for enhanced Coulomb friction model

The complexity of sheet metal parts has reached a level, that without supporting process planning applications almost impossible to design the process planning. Because technology is sharpened to the limit the slightest disturbance in the production results in a waste product. The lubrication condition used during the sheet metal forming is crucial to the success of production. Therefore, the need to model the resulting tribological system as accurately as possible has become increasingly important. This paper purpose with the physically based measurement of the so-called enhanced Coulomb model material parameter used in AutoForm software and the analysis of its effect.

Experimental correlation between bending crack evolution and load-drop criterion in tight-radius three-point bending test with GPa-grade steels

Determining the bending-fracture limit, also referred to as local formability, is crucial for sheet metal forming. However, this is limited by existing methods such as the VDA-238-100 standard due to limitations in its failure criterion based on the punch load. This study addresses this limitation by proposing a quantitative crack-based failure criterion and investigating its relationship to the load-drop point. Along with surface and cross-sectional optical analyses, an integrated line-scanning system for the VDA-238 test is developed, and advanced high-strength steel (AHSS) grades are analysed with yield strengths ranging from 980 to 1500 MPa in both the rolling and transverse directions. The results showed that, while the load-drop point marks a rapid shift in the crack-propagation direction, the true onset of crack initiation occurs earlier, with different patterns for each AHSS and direction. For all the AHSS and directions tested, a crack depth exceeding 1 % of the sheet thickness occurs at 90 % of the switch-off stroke in the VDA-238 test, suggesting an empirical tendency toward microcrack growth for a more accurate evaluation of the local formability based on early cracks.