Sheet Metal Stamping Research Articles

Study of the curling of tensile test specimens from the 6xxx aluminum alloy sheet

The 6xxx Al–Mg-Si heat-treatable alloy is lightweight, has good formability before baking-hardening, and enhanced strength after baking. The aluminum alloy is to replace steel in many automotive stamping sheet metal parts. However, under the normal hot rolling process, 6xxx aluminum alloy is prone to surface roping defects, owing to the uneven texture in the thickness direction. Aluminum with severe roping can also produce curling of the specimen in a uniaxial tensile test. The literature and related phenomena infer that the material thickness direction does not have a uniform r value. The r value measured by the test machine does not represent the actual whole r value of the material. Therefore, in this paper, the actual r values of the top and bottom surfaces of the material are derived from the geometry of the curl of the specimen. An alkali etching method is developed to layer the aluminum sheet’s thickness. The layer is used for tensile testing to verify the aluminum sheet’s actual r value and derive the difference in r value. Then, experiments and numerical simulations are used to show that the variation of thickness direction r is the leading cause of curl in the tensile specimen. Furthermore, the numerical simulation was used to evaluate the difference in r value of an aluminum sheet in each thickness layer and the effect of thickness layer on curl and then make a reasonable explanation and correction of the difference between the tensile test and simulation. Using this method can obtain a more accurate r value as the material parameter for each thickness direction of an aluminum sheet, which certainly improves the accuracy of numerical simulation and analysis for other forming processes.

Experimental characterization and finite element simulation of FDM 3D printed polymer composite tooling for sheet metal stamping

Experimental characterization and finite element simulation of FDM 3D printed polymer composite tooling for sheet metal stamping

Increasing the Forming Limits in Hole Flanging of Dual-Phase (DP) 1000 Steel Using Punch Rotation

Dual-phase (DP) steels are widely used in sheet metal stamping. However, they are typically characterized by low hole expansion ratios. Since hole flanging is very often applied to sheet metal parts, solutions for improving hole flangeability are needed. In this study, high-speed punch rotation is applied in hole flanging of DP 1000 to generate frictional heat and increase formability. The flanges were formed using a punch rotating at 8000 rev/min and varying axial feeds. A maximum hole expansion ratio (HER) of 3.6 is obtained in the tests compared to ~1.58 in conventional hole flanging. The high formability is explained by the high temperature recorded in the process. The effects of temperature and strain rate on the formability of DP 1000 are examined by tensile tests conducted at different conditions. The tensile tests show an increase in formability at high temperatures. Optical microscopy at the flange edge reveals a change in the microstructure of the steel from the characteristic dual phase to a martensitic structure with elongated grains.

Influence of the Parameters of Conical Cavity of the Discharge Chamber on the Efficiency of Electrohydraulic Stamping

Mathematical modeling of the electrohydraulic sheet metal stamping process using discharge chambers with a conical cavity was carried out. The influence of the parameters of the conical cavity of the discharge chamber on the plastic deformation energy of the plate and the shape of its deflection was investigated. It was determined that the height of the cone, a small radius of its upper base, and the location of the discharge channel near the upper base increase the plastic deformation energy of the plate to the greatest extent.

Advanced part design method for springback minimization of stamped sheet metal car body components

The requirements for high-strength and low-weight car body structures prove challenging for the stamping process and cause growing problems with the elastic springback. Compensation approaches such as the geometrical adjustment of the stamping dies by the amount of and in opposite direction to the springback can be an effective solution in many cases. However, this approach can only lead to satisfactory results if the scatter of the springback is considered, controlled and in the best case reduced. The reduction of the scatter can be provoked by adaption of the part design. Therefore, different methods such as the topology optimization method, the so-called line-of-force method and shape optimization can be applied. In the present work an advanced topology optimization method is suggested. Based on the decomposition of the springback deformation into membrane and bending components, the most efficient optimization of the part design is found relying on unconventional topology optimization. For the proposed adapted topology optimization approach the excellent results are also confirmed when applied to the industry-relevant case of a truck rear panel.

Prediction of Crash Performance of Adhesively-Bonded Vehicle Front Rails

<div class="section abstract"><div class="htmlview paragraph">Adhesive bonding provides a versatile strategy for joining metallic as well as non-metallic substrates, and also offers the functionality for joining dissimilar materials. In the design of unibody vehicles for NVH (Noise, Vibration and Harshness) performance, adhesive bonding of sheet metal parts along flanges can provide enhanced stiffening of body-in-white (BIW) leading to superior vibration resistance at low frequencies and improved acoustics due to sealing of openings between flanges. However, due to the brittle nature of adhesives, they remain susceptible to failure under impact loading conditions. The viability of structural adhesives as a sole or predominant mode of joining stamped sheet metal panels into closed hollow sections such as hat-sections thus remains suspect and requires further investigation. As modern vehicle design is primarily driven by CAE (Computer-Aided Engineering), it is important to ensure that the experimental behaviors of adhesively-bonded components can be satisfactorily predicted. With the stated issues in mind i.e. gathering insight into the performance of adhesively-bonded steel hat-section components under impact loading and simulation of its behavior using an explicit FEA code such as LS-DYNA, a systematic experimental and numerical study is carried out comprising: (a) testing of single lap shear joints in a UTM and prediction of the average mechanical behavior of the joints till failure using a cohesive zone material modeling approach for the adhesive with independent Mode I and Mode II fracture criteria; (b) axial impact testing of double-hat section components with conventional spot welds, the same components with purely adhesively-bonded flanges in lieu of spot welds, and hybrid components with adhesive-bonding as well as sparse spot welds, and prediction of the detailed impact responses of the components mentioned; and (c) finally, implementation of the adhesive-based joining strategies in front rails of a validated finite element model of a commercially produced unibody passenger car and assessment of its performance vis-à-vis the baseline vehicle in full frontal NCAP test mode against a rigid barrier.</div></div>

A Novel Method for Friction Coefficient Calculation in Metal Sheet Forming of Axis-Symmetric Deep Drawing Parts

Friction is one of the important factors in sheet metal forming. It greatly affects dynamic behaviors of metal sheets and stress and strain distributions in the metal sheets. In this study, deformation characteristics, stress–strain distribution, and change law of symmetrical parts in the process of deep drawing are analyzed using a new theoretical model based on the plastic flow law and partitioning the forming area. In the model, the least-square method is used to linearize the friction coefficient in nonlinear problems and reverse the calculation of friction coefficients to interpret the friction coefficient. To evaluate the model, the friction coefficient in sheet metal drawing of axis-symmetric deep drawing parts under various friction conditions was measured using a self-developed measuring system. The comparison between the experimental results and the calculation using the model shows a good agreement. The results show that the drawing force increases with the increase in punch depth; the friction coefficient decreases with the rise in punch depth. The friction coefficient obtained by fitting is relatively stable, and the average error is less than 3%. Using the friction coefficient model in finite element simulation analysis, it shows that the thickness and blank shape errors are less than 5%. The novel method studied in this paper shows great significance in support for theoretical research, numerical simulation research, and sheet metal stamping performance evaluation.

Laser welding for stamping blank reinforcement: Signal monitoring for digital certification

Sheet metal stamping is widely used in the automotive sector, where the thickness of the blanks is critical to reduce the final weight of the manufactured part. Nevertheless, in specific locations, reinforcements are required for achieving the desired properties. In view of this necessity, laser welding is proposed to reinforce blanks prior to the stamping operation, which enables to increase the mechanical resistance of the stamped part in the required areas without penalizing the total weight. HCT780C (also called DP780) is the material employed in the present study and T-joints are analyzed. The welding process is monitored by means of a coaxial camera and a two-colour pyrometer. In parallel, the cross-sections of the welds are analyzed to evaluate any internal defect or lack of fusion and shear tests are performed to ensure the correct functioning of the weld. The combination of both results provides the opportunity for developing a digital certification of the welding process, which will enable the verification of future welds without the need for experimental studies.

A method for evaluating the accuracy of curved surface flattening

Based on the analysis of the existing methods for estimating the surface flattening error, they can be roughly classified into two types of estimation methods: the relative area error analysis method and the relative length error analysis method. The advantages and disadvantages of these methods are summarized in the paper. Combining the general surface flattening methods and sheet metal stamping forming process, a more reasonable, scientific and comprehensive evaluation system for the accuracy of surface flattening is proposed. In line with the unfolding characteristics of the grid surface, the relative average area error of triangles is used instead of the traditional area error estimation method. The accumulated length error is replaced by the relative length error of each triangle in the estimation method, and the value of the change in the orientation (direction, position) relationship between the triangles is increased to assess the deformation error of the surface flattening method. The mass attributes of the patch are introduced. The minimum displacement energy method is used to supplement the evaluation of surface flattening accuracy. Finally, a comprehensive evaluation system is established and proposed, which takes into account the accuracy of local and global surface flattening.

FORMATION OF QUALITATIVE INDICATORS OF PRODUCTS BY ROLLING STAMPING PROCESSES

The influence of rolling stamping on the service characteristics of products is investigated in the work. Based on the analysis of deformation kinematics, stress-strain state, microstructure and evaluation of deformability of workpiece material, ways to increase geometric accuracy, vacuum tightness, electrolytic stability and mechanical characteristics of product material, as well as ways to improve the material of workpieces for their subsequent processing. The influence of active friction forces on the nature of the material flow during SHO was established, which contributed to the development of new processes that allow to bring the shape and dimensions of the workpiece as close as possible to the finished part. Thus accuracy of the sizes of details corresponds to 7-11th qualities of accuracy, and roughness of the processed surfaces makes Ra = 2,5… 0,63 microns. The process of reshaping the square billets into round ones by the method of SHO is effective, which increases the utilization factor of the metal and reduces the anisotropy of its mechanical properties. The characteristic of flat anisotropy λr, adopted in sheet metal stamping as a characteristic of the ability of the material to form scallops, decreases as a result of reshaping by 70-80%. The relative difference in yield strength in the plane of the sheet decreases from 0.10-0.15 to 0.03-0.05. The ultimate tensile strain increases by 8-10%, and the uniform uniform strain - by 5-8%. This improvement in the characteristics of the material reshaped by rolling blanks leads to the fact that when drawing cylindrical products, the value of scallops decreases by 2-2.5 times, and the value of the maximum degree of drawing increases by 10-15%. This reduces the relative difference in wall thickness along the perimeter of the elongated workpiece, and the change in wall thickness along its height becomes linear. Thus, the use of SHO processes significantly improves the quality characteristics of products.

МОДЕЛЮВАННЯ ПРОЦЕСІВ ВИРУБКИ-ПРОБИВКИ ПЕРЕДАВАЛЬНИМИ СЕРЕДОВИЩАМИ

During improvement of the quality of products and reducing its cost, sheet metal stamping production, being the basis for the aerospace industry, should more intensively introduce modern production technologies, especially design. There are a large number of factors influencing the stamping process (especially parts with complex geometry), more comprehensive consideration of which would allow to optimize such processes, thereby reducing the manufacturing cost and improving the quality. Currently the processes of forming and separation of complex parts by of an elastic pad are of interest from the point of view of optimization and the final determination of the nature of the behavior of the material. This includes the refinement of such parameters as the maximum permissible thinning, the strength of the die. Clarification of these and other parameters will significantly reduce energy required. Determination of these and other parameters of sheet metal stamping is possible due to application of the modern analysis methods. For numerical studies in the sheet stamping production, the variational method or FEM is the most suitable. Computer modeling makes it possible to investigate the behavior of the material, the kinematics of the workpiece movement during forming process, select the correct loading scheme for the workpiece, and also makes it possible to consider several options for the location of the workpiece in the die, which is very important for stamping thin sheet metal blanks. It provides a significant reduction of the time and costs for carrying out natural experiments, and decrease of technological preproduction preparation of sheet metal stamping. The development of a mathematical model based on the FEM makes it possible to determine not only the required parameters of the process, but also to consider the forming process during its certain stages, to determine the stress-strain state, indicating at the same time the problem zones of excessive thinning, loss of stability, the need to apply a die with a back pressure for cutting of thin sheet metal blanks. It allows to evaluate the quality of a ready product according to the calculated parameters, to use the results obtained for the design of elastic pad

Influence of geometric attributes on friction coefficient in sheet metal stamping

Abstract. This paper presents a test device to explore the influence of geometric attributes of the contact surface on a friction coefficient along with sliding speed and contact pressure. Friction tests were conducted on a third-generation high-strength steel QP980. The friction coefficients for different surface curvatures, contact pressures, and sliding speeds were calculated, and the influences of these factors were analyzed. The formula for calculating the friction coefficients between curved contact surfaces was derived. The relationship between bending-induced surface roughness increase and friction coefficient was established. An enhanced friction coefficient model with pressure, velocity, and curvature dependence was proposed. The enhanced friction model was applied to simulate the stamping of an automotive part, and a better correlation was achieved.

Design for FDM of flexible tooling for manufacturing aeronautical components by incremental sheet forming

Nowadays, industrial production is required to reduce industrialization times and development costs for new products while maintaining high quality standards. In this context, the development of new flexible manufacturing technologies has gained relevance in the last few years. The use of Fused Deposition Modelling (FDM) additive technique has been recently proposed in different industrial sectors for manufacturing rapid tooling (dies) to be used in conventional sheet metal stamping or stretching processes with a significant decrease in costs and time savings. On the other hand, Incremental Sheet Forming (ISF) technology is characterized by an enhanced formability of the parts thus manufactured as well as for the need of a small number of tooling, reducing costs compared to conventional processes such as hydroforming or stamping. In particular, its simplest variant, Single-Point Incremental Forming (SPIF) requires the use of backing plates, which do not require tight tolerances as their only function is to collaborate in the deformation process acting as a support point. Furthermore, the strength requirements are also not a limitation since the forces involved in SPIF are very small given the local nature of the deformation. In this context, the main objective of this work is the design of a modular tooling system, manufactured using the FDM additive technique, that allows the flexible manufacturing of different aeronautical components by SPIF.

Extraction of square box parts with lateral pushing and squeezing

Modern development of mechanical engineering sets the tasks consisting in increasing of quality of produced parts, development and introduction of waste-free and lowwaste technological processes. Non-waste and low-waste production reduces the cost of raw materials, which affects the production cost and leads to increased profits. New technological processes for obtaining square box parts can be used at various enterprises of the rocket and space complex, defense industry, aircraft construction, shipbuilding, instrument making, construction, energy, as well as at enterprises producing consumer goods. Also square box parts can be used as metal packing, i.e containers. This is a strong, reliable package that has a low weight and a number of advantages. Manufacture of this type of parts is used in mass and largescale production. In modern mechanical engineering, sheet metal stamping operations are widely used, in which up to 30% or more of metal is discarded when using round and profile blanks. It should be noted that 90% of the cost of finished parts is the cost of metal, so the development of manufacturing technologies in which raw materials are used rationally, is currently an urgent task. The article presents a method of manufacturing square box parts from a square sheet blank, which reduces or eliminates crowning in a square shell. The choice of a square blank is due to the fact that when it is produced, the utilization factor of the material during cutting is 100%. Reducing waste on a large scale leads to significant savings.The research was conducted within the framework of the grant NSh-2601.2020.8.

Manufacturable casting parts design with topology optimization of structural assemblies

This paper presents a design method for manufacturable casting parts based on topology optimization of structural assemblies, which considers the geometry requirement and the manufacturing constraint of die-set material cost. The problem formulation follows the previous work presented in multi-component topology optimization for stamped sheet metal assemblies (MTO-S). Based on the vector method combined with Heaviside function, the moldability constraints for casting parts is formulated. As the base structure of component is easily misidentified as an undercut structure by the moldability constraints in the structural assemblies, the component baseline is proposed to realize the automatic filtering of the “fake” undercut structures which can be extended to the parting line to obtain the form of two-mold design. Several numerical examples on compliance minimization of single-mold and two-mold casting parts are conducted to verify the validity of the proposed method. The optimized results show that there is no interior void for each component and the component manufacturability has been improved obviously. The setting of minimum-area bounding box (MABB) area constraint limits and the number of components will have a significant effect on the performance of the optimized structure. Users can achieve the desirable solution based on their actual demand by making trade-offs between the structural performance and manufacturing cost.

The research for the influence of properties of punch material on performance and productivity of cold sheet stamping

After analyzing the main trends, it was found that sheet metal stamping is widely used in many fields of mechanical engineering. And the destruction of the working parts of the stamp in serial or mass production significantly affects productivity. In the course of work punches are exposed to high power loading therefore they are made of wear-resistant steel with the increased durability and hardenability. But like any tool with a long load, the punch and die are almost always accompanied by the combined action of two destructive processes - wear and fatigue, and the service life is limited by the number of strokes. The influence of the properties of the punch material by the finite element method for punching a hole made of sheet material on the stamps of cold sheet stamping is investigated in the work. The study was performed fifteen times for five different materials. The punch was subjected to a load from changes in sheet thickness where the maximum stresses of possible displacements as well as deformations were monitored. Having the character of influence on stability of the tool. During long-term operation of the stamp there is a large concentration of stresses in the weakest zone of the punch, which causes the lower part to wrinkle, which creates forces acting on the axis of the punch. Such deformations usually lead to a reduction on one side of the gap between the punch and the die and encourage an increase in friction forces. The gaps that depend on the mechanical properties and thickness of the material, as well as the mode of operation of the press - the number of double strokes of the slider per minute. The dependence of the influence of the material of the punch and the gap between the punch and the matrix on the quality of parts and the performance of the stamp is considered. The maximum possible number of punches was determined, the calculation and analytical analysis of the tool stability from material change to sheet thickness was performed. As well as stability depending on the hardness of the tool material. The obtained research results were analyzed, substantiated and described in this article, as well as recommended to use the influence of the properties of the material of the punch in the design of dies.

Application of machine learning for acoustic emissions waveform to classify galling wear on sheet metal stamping tools

Galling wear in sheet metal stamping processes can degrade the product quality and adversely affect mass production. Studies have shown that acoustic emission (AE) sensors can be used to measure galling. In the literature, attempts have been made to correlate the AE features and galling wear in the sheet metal stamping process. However, there is very little attempt made to implement machine learning (ML) techniques to detect AE features that can classify non-galling and galling wear as well as provide additional wear-state information in the form of strong visualisations. In the first part of the paper, time domain and frequency domain analysis is used to determine the AE features that can be used for unsupervised classification. Due to galling wear progression on the stamping tools, the behaviour of AE waveform changes from stationary to a non-stationary state. The initial change in AE waveform behaviour due to galling wear initiation is very difficult to observe due to the ratio of change against the large data size of the waveform. Therefore, a time-frequency technique “Hilbert Huang transform” is applied to the AE waveform as that is sensitive to change of wear state and is used for the classification of “non-galling” and the “transition of galling.” Also, the unsupervised learning algorithm fuzzy clustering is used as comparison against the supervised learning techniques. Despite not knowing a priori the wear-state labels, fuzzy clustering is able to define three relatively accurate distinct classes: “unworn”, “transition to galling” and “severe galling”. In the second part of the paper, the AE features are used as an input to the supervised ML algorithms to classify AE features related to non-galling and galling wear. An accuracy of 96% was observed for the prediction of non-galling and galling wear using classification, regression tree (CART) and neural network techniques. In the last part, a reduced short time Fourier transform of top 10 absolute maximum component AE feature sets that correlates to wear measurement data “profile depth” is used to train and test supervised neural network and CART algorithms. The algorithms predicted the profile depth of 530 unseen parts (530 unseen cases), which did not have any associated labelled depth data. This shows the power of using ML techniques that can use a small data training set to provide additional predicted wear state on a much larger data set. Furthermore, the ML techniques presented in this paper can be used further to develop a real-time measurement system to detect the transition of galling wear from measured AE features.

Adapted part design methods for springback minimization of stamped sheet metal car body components

In sheet metal forming, both the dimensional accuracy of the manufactured parts and the robustness of the forming process are significantly compromised by elastic springback effects. In contrast to traditional compensation methods mostly relying on modifications of the tool geometry, a part geometry-based approach reduces the amount of springback by appropriately manipulating the part stiffness through slight modifications of the part geometry. To date, part-geometric measures are mostly based on practical experience and trial-and-error processes resulting in a need for simple and effective simulation-based approaches. In the present work three novel simulation-based approaches for a suitable modification of structural component design are suggested relying on unconventional topology optimization, the so-called line-of-force method and shape optimization. First tests with a classical hat profile reveal that the optimized geometries, compared to the nominal one, lead to significant reductions of the springback after stamping and simultaneously to a notable decrease of the process variation when parameters of the forming process are varied slightly. For the adapted topology optimization approach the excellent results are also confirmed when applied to the industry-relevant case of a truck rear panel.

Numerical and experimental verification of an iterative coupling method for analyzing the Lorentz-force-driven sheet metal stamping process

The Lorentz-force-driven stamping process is a newly developed forming process, which combines the advantages of the quasi-static stamping and electromagnetic forming to a certain extent. It involves complex physical problems such as strong electro-magneto-mechanical interactions and considerable deformation. Therefore, it is still a challenge to accurately predict the electromagnetic parameters (coil current and Lorentz force, etc.) and workpiece deformation (forming height and material flow) during the stamping process for different forming conditions. To solve this problem, taking the cylindrical cup forming of 5052-O aluminum alloy sheet metal as an example, a numerical method based on the iterative calculation of a circuit-electromagnetic model and an electromagnetic-mechanical model is developed and systematically validated. Numerical tests show that, compared with the non-iterative model, the proposed iterative mode can significantly improve the simulation accuracy for different forming conditions (such as varied discharge voltages and initial gaps between the coil and the punch). It has been confirmed that the iterative procedure can well consider the effect of the increased distance between the coil and the punch on the calculations of electromagnetic and mechanical parameters. Furthermore, numerical and experimental investigations for a Lorentz-force-driven stamping system using a discharge circuit with an additional crowbar branch are carried out, showing the simulation data of discharge current and workpiece deformation agree well with that of experiment measurement. This confirms the applicability of the proposed method to different discharge circuits.

Multi-component topology optimization for thin-walled structures

This article presents a multi-component topology optimization method for thin-walled structures, which can be applied to two-dimensional plate components and three-dimensional tube components. Based on the theory of multi-component topology optimization for stamped sheet metal assemblies, an accurate calculation method for the Young’s modulus of joint elements is provided. To avoid the discontinuous derivative and improve the optimization efficiency, a non-zero gradient-based material interpolation scheme with penalization is constructed. The cost of die-set materials can be controlled effectively by the maximum size of components. Several topology designs based on minimized compliance confirm the effectiveness of the proposed method. The topology solutions illustrate that the proposed method can realize the structural design of thin-walled structures which can meet the die-set material cost requirement.