Sheet Forming Process Research Articles

On the improvement of material formability in SPIF operation through tool stirring action

Single-point incremental forming (SPIF) is a quite new sheet-forming process which offers the possibility to deform complex parts without dedicated dies using a single-point tool and a standard three-axis CNC machine. The process mechanics enables higher strains with respect to traditional sheet-forming processes, but particular attention must be given to the maximum forming angle. In this paper, a new approach is proposed to enhance the material formability through a localized sheet heating as a consequence of the friction work caused by elevated tool rotational speeds. AA1050-O, AA1050-H24, and AA6082-T6 were utilized, and the reached temperatures were recorded by thermocouples, fixed to the sheet using a metal structure. A significant increase in the material formability was observed for both materials, and new formability curves have been built at the varying of the utilized rotational speed.

Sequential coupled process-performance simulation and multi-objective optimization of thin-walled tubes

A sequential coupled nonlinear finite element analysis framework is developed to investigate the effects of sheet-forming process and product design parameters on energy absorption characteristics of thin-walled, multicorner tubes made of a magnesium alloy. Coupling stems from the use of analysis results from one simulation to form the initial state in the subsequent simulation. High fidelity coupled process-performance simulations are used in design of computer experiments for developing analytical surrogate models of process and performance responses as functions of product geometry and process parameters. Rupture, thinning and springback are treated as the manufacturing responses whereas mean crush force, maximum crush force, and mass are used as structural performance measures. An integrated process-performance optimization problem is setup and solved using the multi-objective genetic algorithm approach. The results of coupled simulations show the importance of manufacturing effects on the crush response while the Pareto optimal set highlights the tradeoff among process and performance attributes.

Numerical Analysis of the Stress/Strain Evolution in Incremental Sheet Forming and Stretch-Bending Processes

In this paper, the evolution of plastic strain and hydrostatic pressure evolutions in incremental sheet forming (ISF) processes to postpone fracture of the metal sheet is analyzed. A finite element (FE) model of an incremental forming operation and a conventional stretch-bending operation is performed. Experimental results of stretching tests are used as reference. The local evolution of the straining and the distribution of hydrostatic pressure are studied in detail. Different step downs and friction coefficients are simulated and their results are discussed.

On the use of Back-drawing Incremental Forming (BIF) to improve geometrical accuracy in sheet metal parts

Industrial interest about Incremental Sheet Forming (ISF) process is growing in the last years. Up to a few years ago, two main investigation ways were proposed, the former aimed at analysing the process mechanics, the latter at reproducing some “case study” geometries. In industrial applications, if the long cycle-time can be neglected in small batches manufacturing, geometrical accuracy represents a relevant drawback, especially when the product has to be coupled to one another. For this reason, in the opinion of the authors, the low accuracy is the most relevant defect of ISF processes today. Among the techniques already set-up to reduce inaccuracy, the use of different material supports or the use of “arbitrarily modified” tool trajectories are probably the most known. In this paper a simple approach is proposed, based on the process self capability to correct inaccuracy when different steps of Incremental Sheet Forming are carried out on both the part surfaces. In particular, it is demonstrated that a relevant increasing in accuracy is obtainable at the second repeated step, while new ones do not reduce the inaccuracy sensitively. The above approach builds a new scenario since it allows to keep the basic equipment (without any support) and does not require any further knowledge concerning the material behaviour after the punch action. These aspects are deeply discussed in the next chapters.

Tool Directionality in Contour-Based Incremental Sheet Forming: an Experimental Study on Product Properties and Formability

The Incremental Sheet Forming (ISF) process offers a large variety in tool path strategies to obtain a particular final product shape. As fundamental understanding of the relevant deformation modes in ISF is growing, the selection of the tool path strategy may be shifted from trial-and-error towards more fundamentally based knowledge of the process characteristics. Truncated cones and pyramids have been fabricated by both unidirectional (UD) and bidirectional (BD) contour-based tool path strategies, considering different wall angles and materials (Mn-Fe alloyed aluminum sheet and low carbon steel sheet). It is shown that the induced through-thickness shear along the tool movement direction is clearly non-zero for UD, in which case the sense of tool movement is the same for all contours, while it is close to 0 for BD, due to the alternating tool sense during consecutive contours. Furthermore, the heterogeneity in product thickness, as observed for the UD strategy in [1,2], is avoided by using the BD strategy. It is verified that this difference in deformation may affect the mechanical properties in the walls of pyramids by means of tensile testing, but the results are material-dependent. For the aluminum alloy, the re-yield stress along the tool movement direction is smaller for BD in comparison to UD, and the fracture strain in large wall angle products is higher. For the steel, no statistically significant differences in mechanical properties between UD- and BD-processed parts are observed. Finally, for both materials a (slightly) higher limiting wall angle has been repeatedly measured using the BD tool strategy. In light of these results, the bidirectional tool path strategy is to be preferred over the unidirectional one, as thickness distribution and formability are more favorable, while both strategies require similar resources and processing time.

Ultiple Property Cross Direction Control of Paper Machines

Cross direction (CD) control in sheet-forming process forms a challenging problem with high dimensions. Accounting the interactions between dierent properties and actuators, the dimensionality increases further and also computational issues arise. We present a multiple property controller feasible to be used especially with imaging measurements that provide high sampling frequency and therefore enable short control interval. The simulation results state the benets of multiple property CD control over single property control and single property control using full feedforward compensation. The controller presented may also be tuned in automated manner and the results demonstrate the eect of tuning on input saturation.

Investigation on the TRIP Steel Sheet Forming Processes with Respect to Punch Speed Loading-Paths

Loading path is one of importance factors that influence the formability of sheet forming process. In this study, the effect of punching speed loading paths (PSLP) on forming and spring-back processes of TRIP (transformation induced plasticity) steel has been investigated. Four kinds of loading paths with three punch speeds are introduced to verify the cup drawing and U-channel spring-back processes based on a constitutive model accompanying the strain-induced martensite transformation. The results show that higher punch speed results in the thickness uniformity of drawing cup and the spring-back angle of stamping U-channel increased with the same loading path. Furthermore, a given loading path (C4) not only increases the minimum thickness of cup but also decreases the spring-back angle of U-channel.

Optimised tool-path design to reduce thinning in ISF process

Although the competitiveness of the Incremental Sheet Forming process is deeply assessed, some specific aspects penalise its industrial application. In particular, the idea to take advantage of the bigger formability appears of a great interest but the non homogeneous thickness distribution reduces the industrial applicability. It is well known that the process allows to work in the critical “necking-to-tearing” space which results in a strong thinning before failure. However, a significant improvement in the thickness distribution can be obtained by using a modified tool trajectory. In fact, previous experimental investigations carried out by the authors showed that it is possible to influence the thinning phenomenon by applying a proper tool trajectory. Rather than the conventional tool path, which describes the tangent profile to the desired shape, the imposed wall angle can be modified in order to compensate the localised thinning. Unfortunately, this approach increases the process design complexity and suggests to consider the tool trajectory as a decision variable to be optimised in the process design. For this reason, an optimisation model is required in order to simplify the choice among all the possible alternatives. In this paper, a promising analytical model is highlighted and a user friendly procedure is set up to simplify the approach. Satisfactory experimental results which validate the proposed technique are also presented.

Influence of thickness on formability in 6082-T6

Incremental Sheet Forming is a promising process for the small batch manufacturing of sheet metal parts due to its flexibility and to the high formability achievable. In the Incremental Sheet Forming process a localised state of deformation is imposed by the CNC controlled motion of a simple tool. Although the process potentiality, its industrial application is still bordered by some drawbacks such as excessive material thinning, part inaccuracy or process time. In the last years, several works were carried out by the researcher all over the world mainly focused on material formability. However further efforts are required in order to overcome the above specified drawbacks. In particular, concerning the material thinning, the simple Sine Law allows to predict the actual sheet thickness in a rough way. In fact, the Sine Law takes into account the initial thickness and the wall slope angle, neglecting all the others process parameters. For this reason, a more robust tool which takes into account the whole process complexity is strongly necessary. In this paper a wide investigation on 6082-T6 aluminium alloy was carried out with the aim to determine the influence of process parameters on thinning behaviour. Finally, with the aim to obtain a reliable model to predict the thinning, the ANOVA analysis was executed on the experimental data designed by DOE. All the other details are reported in the paper.

Constitutive modeling for anisotropic/asymmetric hardening behavior of magnesium alloy sheets: Application to sheet springback

The constitutive model for the unusual asymmetric hardening behavior of magnesium alloy sheet presented in a companion paper (Lee, M.G., Wagoner, R.H., Lee, J.K., Chung, K., Kim, H.Y., 2008. Constitutive modeling for anisotropic/asymmetric hardening behavior of magnesium alloy sheet, Int. J. Plasticity 24(4), 545–582) was applied to the springback prediction in sheet metal forming. The implicit finite element program ABAQUS was utilized to implement the developed constitutive equations via user material subroutine. For the verification purpose, the springback of AZ31B magnesium alloy sheet was measured using the unconstrained cylindrical bending test of Numisheet (Numisheet ’2002 Benchmark Problem, 2002. In: Yang, D.Y., Oh, S.I., Huh, H., Kim, Y.H. (Eds.), Proceedings of 5th International Conference and Workshop on Numerical Simulation of 3D Sheet Forming Processes, Jeju, Korea) and 2D draw bend test. With the specially designed draw bend test the direct restraining force and long drawn distance were attainable, thus the measurement of the springback could be made with improved accuracy comparable with conventional U channel draw bend test. Besides the developed constitutive models, other models based on isotropic constitutive equations and the Chaboche type kinematic hardening model were also considered. Comparisons were made between simulated results by the finite element analysis and corresponding experiments and the newly proposed model showed enhanced prediction capability, which was also supported by the simple bending analysis adopting asymmetric stress–strain response.

Improved SPIF performance through dynamic local heating

The limited accuracy of the single point incremental forming process [J. Jeswiet, F. Micari, G. Hirt, A. Bramley, J. Duflou, J. Allwood, Asymmetric single point incremental forming of sheet metal, Annals of CIRP 54(2) (2005) 623–650] has been identified as its major deficiency [J.M. Allwood, G.F. King, J. Duflou, A structured search for applications of the Incremental Sheet Forming process by product segmentation, Proceedings of the Institute of Mechanical Engineers, Part B, Journal of Engineering Manufacture 219(2) (2005) 239–244]. Improving the process by introducing either fixed [J.R. Duflou, B. Lauwers, J. Verbert, F. Gelaude, Y. Tunckol, Medical application of single point incremental forming: cranial plate manufacturing, in: Proceedings of the VRAP 2006 Conference, Leiria, September 2006, pp. 161–166] or programmable support structures [J.M. Allwood, N.E. Houghton, K.P. Jackson, The design of an incremental sheet forming machine, in: Proceedings of the Shemet 2005 Conference, Erlangen, April 2005, pp. 471–478; H. Meier, O. Dewald, J. Zhang, incremental forming of sheet metal by two industrial robots, in: AMST’05 Advanced Manufacturing Systems and Technology 2005, vol. 486, Springer, Wien, New York, pp. 437–444], tends to reduce the flexibility of the process and increases the complexity of the control system. In the presented research, the authors have opted for an alternative approach in which the material properties are differentiated by localised temperature variations. In this way, different zones can be created in the sheet metal part being processed. By means of a dynamic heat, input in the direct vicinity of the stylus, a ductile area with low-yield strength is generated. By synchronising the movement of the heat source over the sheet metal surface with the stylus feed rate and direction, and by using appropriate cooling of the surrounding area, a temperature gradient can be assured between this area and the workpiece zone where no deformation is envisaged. Effects of such strategy are reduced forces on the stylus and, in consequence, a better localised deformation and thus a higher precision. Furthermore, higher forming limit angles have been demonstrated for materials with low strainability at room temperature. An experimental setup has been built to test this dynamic, local heating principle and is described in the paper. The effects on the process performance are demonstrated by means of experimental results.

The Increased Forming Limits of Incremental Sheet Forming Processes

Incremental sheet forming is known to give higher forming limits than conventional sheet forming processes, but investigation of this effect has been impeded by the computational cost of process models which include detailed predictions of through thickness behaviour. Here, a simplified process is used to gain insight into the mechanics of a broad class of incremental forming processes. The simplified process is described and shown to give increases in forming limits compared to a conventional process with the same geometry. A model of the process is set up with a commercial finite element package, validated, and used to trace the history of a ‘pin’ inserted perpendicularly into the workpiece. The history of the deformation of the ‘pin’ demonstrates significant through thickness shear occurring in the direction parallel to tool motion. This insight is used to modify an existing analysis used to predict forming limit curves. The analysis shows that for a sheet with uniform proportional loading, the forming limit is increased when through thickness shear is present, and this is proposed as an explanation for the increased forming limits of incremental sheet forming processes.

Determination of Strain in Incremental Sheet Forming Process

A simplified method to determine the strain distribution during incremental forming of a cone is proposed in this paper. Because of the symmetry of the deformed part, the strain can be derived using the results obtained from a limited number of consecutive tool contours instead of going through the whole process. Comparisons made between the measured and simulated results show that the proposed method can be applied to determine the strain encountered in such kind of incremental forming process where axi-symmetric parts are formed.

Ductile Forming Limit Stress in Sheet Forming Processes with Nonlinear Strain Path Involving Out-of-plane Forming

The ductile forming limit in nonlinear strain paths which include out-of-plane forming such as rolling was investigated for a 11 % Cr steel sheet. For strain paths with only a plane stress state, e.g., in-plane strain, the forming limit stress diagram (FLSD) has been proven to be effective, even if the paths are nonlinear. In this research, a method of extending the FLSD theory to strain paths which include out-of-plane strain is investigated experimentally and theoretically. An experimental procedure is used to measure directly the stresses at the onset of diffused and localized necking and at the onset of separation with rolling-stretching loading paths. Theoretically, an assumption is introduced to consider out-of-plane strain in the M-K theory. The experimental and theoretical results of the forming limit stress show good agreement. When large rolling strain is induced in first-stage loading paths, this research demonstrated that there are cases where the FLSD theory is not effective for predicting the ductile forming limit. Those cases are characterized by a stress state of the re-yielding point in the final loading stage. If the point is outside the FLSD curve, the forming limit stress is not on the FLSD curve but located near the subsequent yield surface. Thus, a modified FLSD curve can be defined by the lines connecting the outer line of the conventional FLSD curve and the subsequent yield surface of the loading path under consideration. With this extension, it is possible to demonstrate the applicability of the FLSD method to processes which include out-of-plane loading paths such as rolling.

Evaluation of Ductile Forming Limit Stress in Sheet Forming Processes and Measurement of Actual Stress at Fracture

A stress-based ductile forming limit theory applicable to nonlinear strain paths is investigated with a 11 % Cr steel which displays a highly anisotropic behavior in plastic deformation. The forming limit stress is evaluated by a theoretical method based on the M-K theory, and is also measured directly in some cases with nonlinear loading paths. The evaluated and measured stresses are compared with an experimentally-obtained forming limit stress diagram, where the stresses are converted from strains by Stoughton's method. The theoretically-evaluated limit stress corresponds not to the measured stress at the onset of fracture but to the stress at the onset of localized necking. The evaluated forming limit stress and experimentally-obtained localized necking stress demonstrate no dependency on strains paths, thus verifying the applicability of the theory to this particular material. The experimentally-obtained forming limit stress diagram was corrected using the above results, which has improved the discrepancies between the experimentally- and numerically-obtained forming limit stress.

ISF process model development

Incremental Sheet Forming (ISF) process modelling is studied. Main attention is paid to safe manufacturing. Experimental, numerical and theoretical study is performed in order to determine the material formability and the forming load components. ISF strategies for determining the Forming Limit Diagram (FLD) are pointed out. The obtained experimental results have been found to be in good agreement with theoretical considerations. The effect of plastic anisotropy of metal sheets is considered in theoretical models and test design. The obtained results allow to predict tool and material failure in ISF process. [Received 5 June 2007; Accepted 5 July 2007]

A structured search for applications of the incremental sheet-forming process by product segmentation

A methodology for identifying applications of a new production technology is proposed and tested. It is applied to the incremental sheet-forming process, showing a preference for low-volume high-value applications and identifying the importance of improving process accuracy. The methodology could in future help to identify novel applications.

Incorporation of Sheet-Forming Effects in Crash Simulations Using Ideal Forming Theory and Hybrid Membrane and Shell Method

In order to achieve reliable but cost-effective crash simulations of stamped parts, sheet-forming process effects were incorporated in simulations using the ideal forming theory mixed with the three-dimensional hybrid membrane and shell method, while the subsequent crash simulations were carried out using a dynamic explicit finite element code. Example solutions performed for forming and crash simulations of I- and S-shaped rails verified that the proposed approach is cost effective without sacrificing accuracy. The method required a significantly small amount of additional computation time, less than 3% for the specific examples, to incorporate sheet-forming effects into crash simulations. As for the constitutive equation, the combined isotropic-kinematic hardening law and the nonquadratic anisotropic yield stress potential as well as its conjugate strain-rate potential were used to describe the anisotropy of AA6111-T4 aluminum alloy sheets.

Reduced order cross-directional controller design for sheet forming processes

This paper presents a novel control strategy that efficiently addresses the issue of the large number of cross machine direction actuators with which sheet-forming processes are equipped. The approach relies on using the Karhunen-Loeve expansion (KLE) to model the disturbances affecting the properties of interest together with vector autoregressive processes to design a low-order representation of the disturbances. Subsequently, the control space is also reduced by appropriate transformations that rely on the use of KLE. Model predictive control is then implemented on the reduced order model and an optimal solution is obtained in that space, satisfying various types of actuator constraints defined on the full-order system and minimizing the cross-directional variability. The solution from the reduced order model is then projected back to the original space for implementation on the real plant. Simulation examples are included to illustrate the method.

Microstructure-Based Constitutive Modeling for the Analysis and Design of Aluminium Sheet Forming Processes

Microstructure-Based Constitutive Modeling for the Analysis and Design of Aluminium Sheet Forming Processes