Single point incremental forming: state-of-the-art and prospects

Incremental sheet metal forming in general and Single Point Incremental Forming (SPIF) specifically have gone through a period of intensive development with growing attention from research institutes worldwide. The result of these efforts is significant progress in the understanding of the underlying forming mechanisms and opportunities as well as limitations associated with this category of flexible forming processes. Furthermore, creative process design efforts have enhanced the process capabilities and process planning methods. Also, simulation capabilities have evolved substantially. This review paper aims to provide an overview of the body of knowledge with respect to Single Point Incremental Forming. Without claiming to be exhaustive, each section aims for an up-to-date state-of-the-art review with corresponding conclusions on scientific progress and outlook on expected further developments.     

Enhancing time efficiency on single point incremental forming processes

Single Point Incremental Forming (SPIF) is a promising manufacturing technology concerning the production of customized products, low batches or prototyping of ready-to-use parts, given its easy implementation and absence of dedicated tooling. The range of application is wide, covering many materials and virtually unlimited geometries. Indeed, nowadays’ process downsides are more related to high forming times and dimensional inaccuracy. There are many processing parameters that can be optimized to circumvent such problems. In this work, focus is given on the effect of increasing tool feed rate. To this end, a dedicated SPIF machine is employed. After forming steel and aluminum sheets, parameters like forming force, maximum wall angle and formability are assessed for a range of velocities from 1500 to 12,000 mm/min. Parameters like step down or tool diameters are kept constant for a clear comparison. It will be shown how the process can be fastened up without seriously compromising its feasibility.     

Strain evolution in the single point incremental forming process: digital image correlation measurement and finite element prediction

Incremental Sheet Forming (ISF) is a relatively new class of sheet forming processes that allow the manufacture of complex geometries based on computer-controlled forming tools in replacement (at least partially) of dedicated tooling. This paper studies the straining behaviour in the Single Point Incremental Forming (SPIF) variant (in which no dedicated tooling at all is required), both on experimental basis using Digital Image Correlation (DIC) and on numerical basis by the Finite Element (FE) method. The aim of the paper is to increase understanding of the deformation mechanisms inherent to SPIF, which is an important issue for the understanding of the high formability observed in this process and also for future strategies to improve the geometrical accuracy. Two distinct large-strain FE formulations, based on shell and first-order reduced integration brick elements, are used to model the sheet during the SPIF processing into the form of a truncated cone. The prediction of the surface strains on the outer surface of the cone is compared to experimentally obtained strains using the DIC technique. It is emphasised that the strain history as calculated from the DIC displacement field depends on the scale of the strain definition. On the modelling side, it is shown that the mesh density in the FE models plays a similar role on the surface strain predictions. A good qualitative agreement has been obtained for the surface strain components. One significant exception has however been found, which concerns the circumferential strain evolution directly under the forming tool. The qualitative discrepancy is explained through a mechanism of through-thickness shear in the experiment, which is not fully captured by the present FE modelling since it shows a bending-dominant accommodation mechanism. The effect of different material constitutive behaviours on strain prediction has also been investigated, the parameters of which were determined by inverse modelling using a specially designed sheet forming test. Isotropic and anisotropic yield criteria are considered, combined with either isotropic or kinematic hardening. The adopted constitutive law has only a limited influence on the surface strains. Finally, the experimental surface strain evolution is compared between two cones with different forming parameters. It is concluded that the way the plastic zone under the forming tool accommodates the moving tool (i.e. by through-thickness shear or rather by bending) depends on the process parameters. The identification of the most determining forming parameter that controls the relative importance of either mechanism is an interesting topic for future research.     

Finite element modelling and experimental investigation of single point incremental forming process of aluminum sheets: influence of process parameters on punch force monitoring and on mechanical and geometrical quality of parts

New trends in sheet metal forming are rapidly developing and several new forming processes have been proposed to accomplish the goals of flexibility and cost reduction. Among them, Incremental CNC sheet forming operations (ISF) are a relatively new sheet metal forming processes for small batch production and prototyping. In single point incremental forming (SPIF), the final shape of the component is obtained by the CNC relative movements of a simple and small punch which deform a clamped blank into the desired shape and which appear quite promising. No other dies are required than the ones used in any conventional sheet metal forming processes. As it is well known, the design of a mechanical component requires some decisions about the mechanical resistance and geometrical quality of the parts and the product has to be manufactured with a careful definition of the process set up. The use of computers in manufacturing has enabled the development of several new sheet metal forming processes, which are based upon older technologies. Although standard sheet metal forming processes are strongly controlled, new processes like single point incremental sheet forming can be improved. The SPIF concept allows to increase flexibility and to reduce set up costs. Such a process has a negative effect on the shape accuracy by initiating undesired rigid movement and sheet thinning. In the paper, the applicability of the numerical technique and the experimental test program to incremental forming of sheet metal is examined. Concerning the numerical simulation, a static implicit finite element code ABAQUS/Standard is used. These two techniques emphasize the necessity to control some process parameters to improve the final product quality. The reported approaches were mainly focused on the influence of four process parameters on the punch force trends generated in this forming process, the thickness and the equivalent plastic deformation distribution within the whole volume of the workpiece: the initial sheet thickness, the wall angle, the workpiece geometry and the nature of tool path contours controlled through CNC programming. The tool forces required to deform plastically the sheet around the contact area are discussed. The effect of the blank thickness and the tool path on the punch load and the deformation behaviour is also examined with respect to several tool paths. Furthermore, the force acting on the traveling tool is also evaluated. Similar to the sheet thickness, the effect of wall angle and part geometry on the load evolution, the distribution of calculated equivalent plastic strain and the variation of sheet thickness strain are also discussed. Experimental and numerical results obtained allow having a better knowledge of mechanical and geometrical responses from different parts manufactured by SPIF with the aim to improve their accuracy. It is also concluded that the numerical simulation might be exploited for optimization of the incremental forming process of sheet metal.     

Experimental investigations on the single point incremental forming of a titanium alloy component combining static heating with high tool rotation speed

The present work focuses the attention on the Single Point Incremental Forming (SPIF) process of a scaled car door shell made by Titanium alloy (Ti6Al4V). The effect of a temperature increase contemporary due to electric static heating and tool rotation speed was investigated.Preliminary tensile tests allowed to define the temperature level to be assured on the sheet in order to determine a consistent flow stress reduction. SPIF tests were carried out adopting rotation speed in the range 800–1600 RPM, while simultaneously changing the pitch value in the range 0.5–1.0 mm. Temperature during the forming process was continuously measured in the central area of the blank using a pyrometer. In addition a digital image correlation system was used for measuring the strain distribution over the formed part.The combination of the two approaches (heating by both electric bands and high tool rotation speed) revealed to be a feasible solution for manufacturing hard to work materials like Ti alloys, since the investigated case study was successfully formed by SPIF. In addition, a positive effect of the tool rotation speed in stabilizing the necking (thus allowing to reach higher level of stretching) was recognised.     

Incremental forming of Cu-35Zn brass alloy

Incremental Sheet Forming (ISF) and particularly its Single Point (SPIF) variant has been studied intensively over the last years given the potential for low-cost prototyping and small batches production. Numerical and experimental works have been covering a widespan of materials and geometries. This paper fills an important gap regarding studies of the SPIF process applied to brass alloys, and particularly the commonly used Cu-35Zn brass alloy. Despite being a material widely used in industry for centuries, with excellent cold formability and innumerous applications, there is still no relevant information on the mechanical response and properties of this material under SPIF. This research is based in SPIF experiments with brass alloy sheets with different thicknesses (0.50, 0.70 and 1.00 mm), to obtain data such as forming forces or forming fracture lines to be compared against standard forming limit diagrams or against other materials under ISF. Other data like friction during the process was evaluated as well. Fifteen sets of experiments were conducted, using different values of step down (0.10, 0.50 and 1.00 mm) and two forming tools with diameters 10 and 15 mm.     

An assessment of various process strategies for improving precision in single point incremental forming

Single point incremental forming (SPIF) is a process with the capability to form complex geometries using a tool of very simple geometry, without the need for a matching die. However, large elastic springback resulting from the die-less nature of the process can cause problems if high levels of accuracy are required. The aim of this investigation is to use numerical modelling to investigate different strategies to improve the process precision. A finite-element (FE) model has been used to investigate the effects of adding a backing plate, a supporting kinematic tool and modifying the final stage of the tool path. The results show that the backing plate will minimise the sheet bending near to the initial tool contact location; the additional kinematic tool will reduce springback; and the extension of the tool path across the base of the sheet will eliminate the pillow effect. The cumulative effect of introducing these features to the process shows an improvement in the overall accuracy of the profile and in the thickness distributions of the final product. The results contribute to a better understanding of springback in SPIF.     

Optimization of relative tool position in accumulative double sided incremental forming using finite element analysis and model bias correction

Double-Sided Incremental Forming (DSIF) uses two small, independently moving, hemispherical tools on either side of the sheet to form a desired shape by following a predefined tool path. This study was motivated by the observation that the relative tool position of the tools, specified in the tool path generation algorithm, affects the formed geometric accuracy. A methodology for defining the relative tool positioning in the tool path generation algorithm based on local part geometry is proposed using simplified Finite Element Analysis (FEA) and sample physical experiments combined with Gaussian Process modeling techniques. This approach can take into account the mechanics of deformation in DSIF explicitly and physical compliance of the DSIF machine implicitly. Physical experiments were performed to demonstrate the effectiveness of the proposed framework.     

Experimental investigations and optimization of forming force in incremental sheet forming

Incremental sheet forming process has been proved to be quiet suitable and economical for job and batch type production, which exempts expensive and complex tooling for sheet forming. Investigation of forming forces becomes important for selecting the appropriate hardware and optimal process parameters in order to assure perfection and precision of process. Moreover, lack of available knowledge regarding the process parameters makes the process limited for industrial applications. This research paper aims at finding out effects of different input factors on forming forces in single-point incremental forming (SPIF) process. For operation sustainability and hardware safety, it becomes critical to optimize forming forces for a given set of factors to form a particular shape. In this study, optimization of input factors has been performed to produce conical frustums with helical tool path using Taguchi analysis as design of experiment (DOE) and analysis of variance (ANOVA). The optimal experimental conditions for forming forces have been calculated as sheet thickness (0.8 mm), step size (0.2 mm), tool diameter (7.52 mm), tool shape (hemispherical), spindle speed (1000 rpm), feed rate (1000 mm/min) and wall angle (50^o). Effects of tool shape and viscosity of lubricants have also been investigated. An intensive understanding of the mechanism of forming forces has been presented, which shows that force trend after peak values depends upon instant input factors that can be categorized as a safe, severe and crucial set of parameters.     

High-Speed Incremental Forming Process: A Trade-Off Between Formability and Time Efficiency

Making use of “optimal experimental design,” the paper attempts to investigate individual and interactive effects of predictor parameters, namely tool size, pitch size, feed rate, spindle rotational speed, and blank thickness, on sheet formability in single point incremental forming (SPIF) process. For the sake of precision, a novel sensor system was developed and employed to detect crack as it initiates on SPIF test specimens. A novel benchmark for formability in SPIF was established by addressing normal strain along sheet thickness, maximum attainable forming angle, and the rate of variation in forming angle. The process was finally optimized in terms of maximum achievable formability and minimum processing time. Accordingly, high-speed forming (with forming speed of at least 5000 mm/min) was realized to be perfectly viable, whereas the sheet formability remains quite satisfactory (over 90% of the maximum value). The key role of high spindle speed (up to 3000 rpm) was also highlighted in this regard.     

An investigation into thickness distribution in single point incremental forming using sequential limit analysis

Single point incremental forming (SPIF), needing no dedicated tools, is the simplest variant of incremental sheet metal forming processes. In the present work, a simplified model of SPIF of a truncated cone, capable of predicting the thickness distribution, has been developed using sequential limit analysis (SLA). The obtained results were validated experimentally and compared with thickness predictions obtained from an explicit shell FE model implemented in Abaqus. It is shown that SLA is capable to solve the thickness prediction problem more accurately and efficiently than the equivalent FEA approach. As an application of the proposed model, the effect of the diameter of the hemispherical tool tip and the step down on the thickness distribution and the minimum thickness in a 50° cone is studied using SLA. By introducing bending and stretching zones in the wall of the cone, variations of the minimum thickness by changing the tool diameter and the step down are discussed.     

Thermal modeling in electricity assisted incremental sheet forming

A thermal model was built to account for the effects of geometrical parameters of sheet specimen, process parameters and material parameters on the temperature increase of the sheet specimen in Electricity-Assisted Incremental Sheet Forming (EISF). In the EISF, the local area of sheet specimen contacting with a forming tool is heated by direct current, which flows through the forming tool to the sheet specimen. EISF experiments of two high strength steel sheets were carried out to validate the thermal model. The thermal model can be integrated into the control program of EISF system to achieve more accurate temperature control.     

Critical analysis of necking and fracture limit strains and forming forces in single-point incremental forming

Single-Point Incremental Forming (SPIF) is an emerging manufacturing process especially suitable to produce small batches of metal parts. Moreover, the enhanced formability of metal sheets deformed by SPIF makes this technology useful to those industrial applications requiring high deformation levels. In this sense, the precise setting of limit strains in SPIF in relation to the conventional formability limits of the material, as well as the influence of the process parameters on these strains, are essential variables to understand how and how much can be deformed the metal sheets in real production. On the other hand, the forming force in SPIF is an essential variable, especially for the design of dedicated equipment or for the safe use of adapted machinery. This paper revisits failure in SPIF by means of an experimental analysis of the influence of process parameters, such as the tool diameter, the spindle speed and the step down, on the formability in SPIF (spifability) of AISI 304 metal sheets, studied in the light of circle grid analysis. The work also involves the independent determination of conventional formability limits by necking and fracture under laboratory conditions by using stretching tests (Nakazima tests), in conjunction with stretch-bending tests performed in order to quantify the influence of the bending induced by the tool radius. Failure strains are experimentally obtained and compared in stretch-bending and SPIF tests, being the failure mode discussed in each case. Finally, the axial forming force evolution was recorded with the aim of analyzing the range of process parameters that would guarantee the safely utilization of the non-dedicated process equipment.     

Experimental Study on Springback of Sheet Metal Single Point Incremental Forming

Sheet metal single point incremental forming(SPIF)is a new technology for flexible process.The springback phenomenon in single point incremental forming has been discussed.Effects of forming angle and shape of the part are analysed using simple experimental method.Tool diameter,sheet thickness,step size,material parameters and the interaction of them are also analysed by using orthogonal test.The results show that the primary factor affecting springback is forming angle.In addition,springback is decreased when the specimen has a larger forming angle.The order of the four factors that influence springback is tool diameter,sheet thickness,step size and material parameters.The forming precision will increase if springabck is decreased by optimizing the forming parameters.     

Modelling the correlation between the geometrical features and the forming limit strains of perforated Al 8011 sheets using artificial neural network

In the recent years, sheet metals are produced with perforations in various shapes and patterns to improve the appearance of sheet and to save weight of components. As in conventional metal sheets, it is important to form the perforated sheet metals also within their safe strain regions to avoid the forming failures like necking, fracture and wrinkling. The Forming Limit Diagram (FLD) is an appropriate tool to determine the forming limit strains. The limiting strains of perforated sheet metals mainly depend on the geometry of the perforations and forming variables. This leads to large increase in number of test to be conducted with various geometry and forming variables for determining the forming limit strain for perforated sheets. Aiming to reduce the number of experiments needed, in this work, an Artificial Neural Network (ANN) model has been developed for forming limit diagram of perforated Al 8011 sheets based on experimental results and correlated with the geometrical features of the perforated sheets. This model is a feed forward back propagation neural network (BPNN) with a set of geometrical variables as its inputs and the safe true strains as its output. This ANN model can be applied for prediction of FLD of perforated sheet having any geometry.     

Improving geometrical accuracy for flanging by incremental sheet metal forming

Incremental Sheet Forming (ISF) is a manufacturing technology for individualized and small batch production. Among the opportunities this technology provides there is the possibility of a short ramp-up time and to cover the whole production chain of sheet metal parts by using a single reconfigurable machine set-up. Since recent developments proved that manufacturing of industrial parts is feasible, finishing operations such as flanging and trimming gain importance, which are an integral part of manufacturing process chains of many sheet metal parts. This paper analyses the technological capabilities of performing flanging operations by ISF. Due to the localized forming zone and the absence of surrounding clamping devices, ISF exhibits a different material flow than conventional flanging processes. In this paper, the influence of the tool path characteristics, the flange length as well as the flange radius is analysed in order to establish a process window and to compare it to the process limits of conventional flanging operations. Since geometrical deviations occur when flanging operations are performed by ISF, a new adaptive blank holder is developed, which acts in the vicinity of the forming tool and reduces unwanted deformation outside the primary forming zone. The experimental results show the benefits of the adaptive blank holder with respect to geometric accuracy. The established process window and the adaptive blank holder hence contribute to the applicability of incremental flanging operations, such that ISF can be used for all forming and flanging operations along the process chain.     

Effect of annealing on formability of aluminium grade 19000

Forming limit diagrams are used by the stampers to solve sheet metal forming problems. In practice, sheet metals have been subjected to various combinations of strain. Necking during sheet metal forming, sets the limit to which the sheet metal can be formed. Forming limit diagram is an effective tool to evaluate the formability of sheet metal in various strain conditions. The information upon the formability of the sheet metal is important for both sheet metal manufacturers and users. In this work, a study has been made on the formability of aluminium 19000 grades annealed at three different temperatures namely 160 °C, 200 °C and 300 °C for sheet thickness of 2.00 mm. The tensile properties and formability parameters were experimentally evaluated and they are related to forming limit diagram. Strain distribution profiles obtained from the forming experiment have been analyzed. The fractured surface of the formed samples were viewed using scanning electron microscope (SEM) and the SEM images were correlated with fracture behaviour and formability of sheet metal. The sheet which is annealed at 300 °C has been found to possess good drawability and stretchability compared to other two annealed sheets.     

Efficient implicit simulation of incremental sheet forming

In single point incremental forming (SPIF), the sheet is incrementally deformed by a small spherical tool following a lengthy tool path. The simulation by the finite element method of SPIF requires extremely long computing times that limit the application to simple academic cases. The main challenge is to perform thousands of load increments modelling the lengthy tool path with elements that are small enough to model the small contact area. Because of the localised deformation in the process, a strong nonlinearity is observed in the vicinity of the tool. The rest of the sheet experiences an elastic deformation that introduces only a weak nonlinearity because of the change of shape. The standard use of the implicit time integration scheme is inefficient because it applies an iterative update (Newton–Raphson) strategy for the entire system of equations. The iterative update is recommended for the strong nonlinearity that is active in a small domain but is not required for the large part with only weak nonlinearities. It is proposed in this paper to split the finite element mesh into two domains. The first domain models the plastically deforming zone that experiences the strong nonlinearity. It applies a full nonlinear update for the internal force vector and the stiffness matrix every iteration. The second domain models the large elastically deforming zone of the sheet. It applies a pseudolinear update strategy based on a linearization at the beginning of each increment. Within the increment, it reuses the stiffness matrix and linearly updates the internal force vector. The partly linearized update strategy is cheaper than the full nonlinear update strategy, resulting in a reduction of the overall computing. Furthermore, in this paper, adaptive refinement is combined with the two domain method. It results in accelerating the standard SPIF implicit simulation of 3200 shell elements by a factor of 3.6. Copyright © 2011 John Wiley & Sons, Ltd.     

Development of tooling concepts to increase geometrical accuracy in high speed incremental hole flanging

Incremental Sheet Forming (ISF) has been developed as a flexible manufacturing technology for small batch production and prototyping. ISF can also be used to form additional features or stiffening elements such as hole flanges. Incremental Hole Flanging (IHF) operations seem to be a promising alternative to conventional hole flanging. If it was possible to exploit the extended formability of ISF while achieving accuracy and process times of conventional hole flanging, IHF could substitute conventional flanging operations in many cases. However, the long process times and limited geometrical accuracy hinder industrial take-up. In this work, two different tooling concepts which allow incremental hole flanging operations at high speeds are investigated. The first tool is designed as a single forming tool that offers high flexibility and a comparison to conventional Incremental Hole Flanging. The second tool consists of four forming tools to improve the geometrical accuracy of hole flanges. In order to achieve high speeds, the experimental setup is installed on a turning machine. Compared to hole flanging with a conventional CNC machine, the forming time to expand a hole from 50 mm to 100 mm could be reduced from 1680 s to 15.7 s. The geometrical accuracy of the parts formed with the second tool concept could be improved significantly (up to 3 times regarding to the mean surface deviation to at maximum speed). Furthermore, it is shown that forming at high speeds has no significant influence on the characteristics of sheet thickness, strain, forces or geometrical accuracy.