Stretch Flanging Research Articles

Formability Analysis of Stretch and Shrink Flanging by Single Point Incremental Forming Based on Stresses

Flanges are commonly used in aircrafts to provide stiffness and support for the assembly Incremental Sheet Forming (ISF) processes have been approached to produce both stretch and shrink flanges as a low-cost alternative in the fabrication of a small number of parts and prototypes. This work analyzes stretch and shrink flanges of AA2024-T3 sheet with different geometries manufactured by Single Point Incremental Forming (SPIF). The numerical simulation using Finite Elements of the flanges allows evaluating the stress in successful and failed flanges. On the one hand, the formability of stretch flanges is usually evaluated in terms of principal strains within the Forming Limit Diagram (FLD). However, this approach does not seem to capture all the physics to explain the enhancement in formability observed in SPIF over the conventional forming. A formability analysis is performed in the field of stress triaxiality versus equivalent plastic strain, discussing the differences between successful and fractured specimens. On the other hand, for shrink flanging, the appearance of wrinkles is analyzed in terms of the compressive stresses along the flange during the incremental forming. This allows to determine a critical limit stress of winkling to predict the failure in practice for a given geometry and forming condition.

Accuracy of stretch flange formability prediction by considering strain gradient for actual car body parts

The effect of the strain gradient on the stretch flange deformation limit of steel sheets was investigated by hole expansion tests under various forming conditions and by FEM forming simulations. The hole expansion ratio was changed by changing the initial hole diameter and punch shapes. This study clarified the fact that deformation limit strain increases, as it does not depend on the initial hole diameter or punch shape, and thus the strain gradient increases. The deformation limit thickness strain of the stretch flange portion could be changed by changing the strain gradient. These results suggest that the formability of the stretch flange deformation area should be determined from not only the maximum principal strain but also the strain gradient. Then, the accuracy of stretch flange formability prediction by considering the strain gradient for actual car body parts in mass production was investigated. The maximum principal strain and the strain gradient of the parts were calculated by FEM or measured in the press part. These results suggest that the formability of the stretch flange deformation area for actual car body parts could not be predicted by not only the maximum principal strain or the thickness reduction, but also the conventional FLD. This investigation verified the accuracy of stretch flange formability prediction by considering the strain gradient for actual car body parts.

Numerical and experimental investigation of hole flangeability of AA6061 alloy

Hole flangeability is a measure of edge formability or local formability, and it is important for forming processes like stretch flanging. Hole flangeability of sheet metals is evaluated by determination of hole expansion ratio (HER) using a standard hole expansion test (HET). In the present work, numerical and experimental investigation of hole flangeability has been carried out on 1.6mm thick AA6061 alloy sheets. The material model plays an important role in the accuracy of predicted results in numerical simulations. The simulations have been carried out using two different work hardening laws (Swift and Voce). The failure has been predicted using the experimentally determined forming limit curve (FLC) of AA6061 alloy sheets. HER and thickness have been predicted along the edge of the expanded hole by simulation. The predicted HER and thinning from FE simulation have been validated experimentally by performing hole expansion tests according to ISO 16630-2009 using a conical punch. The predicted results have been found to be in good agreement with the experimental results.

Local and global forming potential of present dual-phase steel developments

Modern car bodies are constantly relying higher proportions of high and ultra-high strength steels. In the past, the increase in material strengths led to a conflict regarding the complexity of component geometries. Limits of material formability are reached and compromises had to be made in part design. By integrating application-oriented problems into the material development, these limits are expanded with respect to the given requirements. Within the present study, for several dual-phase steels with a tensile strength ≥800 MPa, practical laboratory tests are presented and their benefits in press shop application. In particular influencing factors on stretch flange formability are presented and discussed for example cutting parameters, anisotropy, time between cutting and forming or the orthogonal strain gradient.

Effects of Edge Heating and Strain Gradient on Stretch Flange Deformation Limit of Steel Sheet

To improve the stretch flange deformation limit of ultrahigh-strength steels, a partial edge heating method was investigated. The effect of the strain gradient on the stretch flange deformation limit when a partial edge heating method was applied was investigated by the hole expansion test and FEM analysis. The stretch flange formability was improved by application of a partial edge heating method because of the recovery of the shearing microstructure and the reduction of the hardness difference of microstructure. It was found that the improvement of the deformation limit strain for the strain gradient with edge heating depends on the strain gradient region. In the low strain gradient region, the improvement margin of the deformation limit strain with edge heating was small. On the other hand, in the high strain gradient region, the deformation limit strain was markedly improved compared with that without heating. It was confirmed that the strain gradient in the direction perpendicular to the maximum principal strain could be used for the prediction of the deformation limit and also the determination of the stretch flange fracture by the FEM analysis of actual parts even under edge heating conditions.

Effects of processes parameters on spring back, wrinkles and cracks in different flanging processes: A Review

Flanging is an important type of sheet metal bending process. In present review, an effort was made to explore the contributions of various researchers to drawn out the cause of defects in the flanging process. Several parameters such as geometrical, material and process parameters affect the formability of the flanging process. In straight flanging process, spring back (due to elastic recovery) is a major problem that affects dimensional and geometrical accuracy of a bent component. Several factors are responsible for springback such as – profile radius of punch and die, bend angle, thickness of sheet, clearance between punch and die, blank-holder force, Young’s modulus, n-value, yield stress, plastic anisotropy of materials, velocity of punch etc. In shrink flanging process, wrinkling is the prime concern that occurs due to the excessive compressive stress in the flange portion. It is mostly affected by mechanical properties of material (R-value, n–value etc.), geometry of the tools and blank, process parameters. In stretch flanging process, the effect of material parameters is found to be less as compared to geometrical parameters for crack initiation and thinning.

Stretch flange forming of steel alloys through FEM simulation

In sheet metal operation flanging is a common process in which one edge of the sheet is bent to the desired angle or the desired contoured shape and the other end keep in control either itself or by blank holder or pad force. Stretch flanging involves stretching the metal circumferentially while the bending line is concave. In the stretch flanging process, the initial flange length is a critical factor. Using commercial finite element software programmed Abaqus, a finite element simulation of the stretch flanging process was performed in this research. The impact of initial flange length on the radial strain, circumferential strain, and flange thickness distribution along the free edge of rectangular-shaped sheet metal blanks is investigated in this work. Aside from maximum radial strain, maximum circumferential strain and maximum thinning in the flange are also explored as effects of initial flange length. With a reduction in initial flange length along the free edge up to the punched centre, circumferential strain and flange thickness drop while radial strain increases. With increasing initial flange length, maximum radial strain, maximum circumferential strain, and maximum thinning are observed to rise. The formability of the stretch flanging process is shown to be considerably influenced by the initial flange length.

Characterization of the Stretch Flangeabitity of High-Strength Bainitic Steel: The Significance of Variant Pairs.

Variant pairs have an indispensable function on mechanical properties such as low impact toughness. Therefore, it was assumed that they would also affect the HER (Hole Expansion Ratio, an indicator to evaluate stretch flanging performance). To clarify this, a comprehensive analysis of the common influential factors in an 800 MPa grade low carbon micro-alloyed steel, i.e., the retained austenite, the M/A (Martensite/Austenite) island, the titanium precipitations, the grain diameter, the density of high angle grain boundaries and the textures, was first conducted. It was found that they did not match well with the HER, suggesting that they were not the governing factor for HER in this steel. However, the dominating crystallography groups and the variant pairing results indicated that they fitted well with the HER. In the samples with high HERs, the CP (Close Packed) groups dominated the transformation, wherein one individual CP group consisted of two or more Bain groups, whereas it evolved into the domination of joint CP groups and Bain groups for the low HER sample. Further analysis on the variant pairing features indicated that a correlation occurred between the HER and the high angle variant pairs. In the steels with high HERs, high-angle variant pairs of V1/V2, V1/V3 that transformed from the same CP group, particularly of V1/V2 pair, were mostly generated. They turned to V1/V9, V1/V10, V1/V12, V1/V15, V1/V17, and V1/V18 pairs from differential CP groups, especially the V1/V12 and V1/V15 pair for low-HER steel. This result showed that V1/V2, V1/V12, and V1/V15 might have accounted most for the HER in this steel. The underlying reason was that the V1/V2 pair was specialized in supplying a slip passage for dislocation transmission across a grain boundary with little resistance, whereas the dislocation transmission ability for V1/V12 and V1/V15 pair was particularly poor. Thus, to efficiently enhance the HER, one should regulate the variant pairs by augmenting the V1/V2 fraction and suppressing the formation of the V1/V12 and V1/V15 pair.

EXPERIMENTAL AND FINITE ELEMENT ANALYSIS OF NONAXISYMMETRIC STRETCH FLANGING PROCESS USING AA 5052

This paper deals with experimental and finite element analysis of the stretch flanging process using AA- 5052 sheets of 0.5 mm thick. A parametrical study has been done through finite element simulation to inspect the influence of procedural parametrical properties on maximum thinning (%) within the stretch flanging process. The influence of preliminary flange length of sheet metal blank, punch die clearance, and width was examined on the maximum thinning (%). An explicit dynamic finite element method was utilized using the finite element commercial package ABAQUS. Strain measurement was done after conducting stretch flanging tests. A Mesh convergence examination was carried out to ascertain the maximum percentage accuracy in FEM model. It is found through finite element simulation that the width of sheet metal blanks has a greater impact on the maximum percentage of thinning as compared to preliminary flange length, and clearance of the punch dies.

The Effects of Piercing Methods on Burring Formability under Practical Hole Diameter

Advanced High Strength Steels (AHSS) sheets are used for automobile bodies to comply with the demands for weight reduction and collision performance. However, as the tensile strength is increasing, the ductility of steel sheets is decreasing. Therefore the forming defects such as fractures may occur. In case of chassis components, it is needed to prevent stretch flange fractures from burring edge. There are following methods to improve burring formability. 1) Double Punching Method (DPM): punching blank coaxially with a pre-punched hole. 2) Humped bottom Punch Method (HPM): punching blank under tension using a punch with a humped part. However, there isn’t much previous evidence evaluating the burring formability of DPM and HPM under practical punching diameter for chassis components. In this study, the burring formability using DPM and HPM has been evaluated under 30mm punching diameters, through experiments and FEM analysis. The results showed that: 1) DPM: deforming mainly occurred in scrap side and equivalent plastic strain was reduced on the punched edge surface. 2) HPM: the burring formability requires both increasing stress triaxiality in blank and reducing equivalent plastic strain applied by the preceding bumped part. Under identical conditions, the same improvement effect of DPM was obtained with HPM.

Evaluation of stretch flangeability of dual-phase steels by hole expansion test

In this study, investigations on stretch flangeability were carried out on as-received DP 600 steel which had 22 volume % of martensite. Stretch flangeability is normally characterized by the hole expansion test and refers to the ability to avoid cracking during stretch flanging or hole expansion. Two intercritical annealing treatments (soaking temperatures of 760 °C and 800 °C followed by water quenching) were selected in order to obtain lower and higher volume fractions of martensite respectively. The influences of four different hole preparation techniques, electrical discharge machining, drilling, drilling + reaming, and punching, on the hole expansion ratios were investigated. Higher surface roughness at the surface of the hole showed inverse correlation with HER values. The highest hole expansion ratio of 156% was obtained for the as-received DP 600 steel with the hole prepared by electrical discharge machining. The HER values observed for various hole preparation conditions can be normalized with respect to the electrical discharge condition so that the HER values can be estimated when one of the HER values is known.

Stretch-flanging of AA2024-T3 sheet by single-stage SPIF

Flanging is a forming process that is commonly used in many high technology industrial applications such as the manufacturing of thin-walled aircraft structures as wing ribs that are formed using rubber-pad forming during the last forming stages with the aim of increasing the stiffness of the component edges. In addition, the performance of hole-flanged parts manufactured using incremental sheet forming (ISF) processes has been recently addressed, focusing especially in evaluating the formability enhancement, as well as the onset of failure utilising a variety of forming strategies. In this context, this work presents an experimental analysis of formability and failure within the forming limit diagram (FLD) of stretch flanges of AA2024-T3 performed by single point incremental forming (SPIF) for a wide range of process parameters. The numerical modelling of the stretch flanging process in combination with a theoretical framework based on Barlat's anisotropy plasticity criterion, allowed to assess the formability of the material within the triaxiality space, permitting to understand the stress/strain states attained in SPIF and the forming conditions upon which the onset of failure occurs. The results show that this triaxiality space, defined as the accumulative effective strain versus the average stress triaxiality, might be the most appropriated space for analysing the highly non proportional process of stretch flanging by SPIF.

Study on the influence of the yield surface shape in the hole expansion test

The hole expansion test has become popular since it allows to study the formability of metallic sheets, in particular the onset of necking in stretch flanging areas. The accurate prediction of strain localization and consequent fracture requires a proper description of the plastic behavior, particularly the anisotropic yield function. In the context of strain localization prediction, the yield criterion adopted plays an important role, particularly when using an associated flow rule, since the direction of the plastic strain tensor is modelled by the normal to the yield surface. In this work, the parameters of an advanced yield criterion are calibrated considering a wide set of experimental data, which includes results from uniaxial and biaxial tension tests. This enables establishing yield surfaces with similar shape in the plane defined by the stress components in the rolling and transverse directions and a null shear component in the same plane. However, their shape changes slightly when considering non-null values for that shear component. The numerical simulations of the hole expansion test demonstrate the impact of these slight differences on the thickness strain distribution and, consequently, in the instant and location of the necking.

Investigation on the Crack and Thinning Behavior of Aluminum Alloy 5052 Sheet in Stretch Flanging Process

Stretch flanging is the important sheet metal-forming process which is widely used in automobile and aerospace sectors. The formability of sheet metal depends on various parameters such as material properties, geometry of the tool setup and process parameters. In the present work, effects of different die radius and sheet width on deformation behavior of sheet are studied by FEM simulation and experiments. The predicted FEM results are presented in the form of edge crack location and its propagation, crack length, forming load and strain distribution in sheet along the die profile radius. This study indicates that crack length increases with increase in the sheet width, while the crack length decreases with increase in the die radius. It is found that the crack propagation in stretch flanging process is affected by the strain distribution in sheet and this distribution of strain depends on many parameters. The crack initiates during deformation of the sheet at the die corner edge and propagates toward the center of the sheet along the die profile radius. Simulation results are compared with the experimental one in terms of crack length and variation in sheet thickness. FE simulation results are found in very good agreement with experimental results. Fractography study is also presented in terms of size, shape of the dimples along with their distribution on the fractured surface.

Effect of punch profile on deformation behaviour of AA5052 sheet in stretch flanging process

Stretch flanging is a type of bending process widely used in automobile and aerospace industries. Forming of the stretch flange is mainly affected by three important parameters: materials of the sheet, the geometry of tools and different process parameters. This work focuses on the effect of punch profile on deformation behavior of AA5052 alloy sheet to form the stretch flange. Six punches of different geometry i.e. cylindrical, two stepped, three stepped, six stepped, conical and hemispherical are used. Results are presented in the form of edge crack in the sheet at edge corner and its propagation towards center, forming load comparison for different punch profile and distribution of radial and circumferential strain in the sheet. It is observed that the punch profile has a considerable effect on the deformation behavior of the sheet. Circumferential strain, radial strain and load requirement to form the flange are found to be minimum in hemispherical punch profile as compared to other punch profiles. Experiments are performed to validate the FE simulation results and results are found in very good agreement in terms of edge crack length. Fractography study shows uniform and large number of small size dimples at the fractured surface for hemispherical and conical punch profile.

Compensation of Contour Distortion in Stretch-Flanging Metal Sheets

Stretch-flanging commonly appears at the concave edge of the panel part. Sheet thickness tends to decrease at the center of flange attributed to the outflow of metal flow, and hence causes a radial shrinking of the material. This shrinking pulls the ends of the flange and makes the adjacent surface overcrown. In this paper the effect of punch profiles on a laboratory scale profile, which assimilates the front fender part adjoining the head light, was investigated for the stretch-flanging process. Both the concave and convex punch profiles were considered. SUS 304 stainless steel sheet of 0.6 mm thick was used as the model metal sheet. DynaForm software was used in simulating the stretch flanging process and followed by experimental verification. The results show that a depression angle of 4.4° and an elevation angle 2.6° can produce lowest crown-contour for the concave and convex punches, respectively. The concave punch also causes less thinning at the flange center which makes it a favorable solution than that of the convex punch.

Effect of pre-strain on stretch flange deformation limit of steel sheets

The effect of pre-strain on the stretch flange deformation limit of steel sheets was investigated by hole expansion tests under various pre-strain conditions. Pre-strain was given by stretch forming with a cylindrical punch. The strain ratio and amount of pre-strain were controlled by changing the blank shape and forming height. The hole expansion tests were carried out with a conical punch and cylindrical punch in order to clarify the effect of the strain gradient. The effect of the strain ratio of pre-strain on the stretch flange deformation limit can be expressed by equivalent strain. In the case of higher strain gradients, the stretch flange deformation limit increased with increasing pre-strain. However, at lower strain gradients, the stretch flange deformation limit was almost constant. These results suggest that the formability of the stretch flange deformation area given by pre-strain should be determined from not only the total equivalent strain but also the strain gradient and the amount of pre-strain expressed by equivalent strain.

Binder force effect on stretch flange forming of aluminum alloy

ABSTRACTThe present research investigates the effect of binder force in flanging of aluminum alloy sheets. FEM simulation of stretch flange forming is realized for zero, constant, and fixed binder forces. The finite element simulation is carried out to evaluate strain (circumferential and radial), flange thickness, position of commencement, and extension of cracks due to variation of binder force. The investigations reveal that the binder force has a significant effect on punch load, flange thickness (percentage thinning), circumferential and radial strain distribution, and commencement and extension of edge cracks. The results of FEM simulation are validated with results obtained by experimentation, and an excellent agreement is obtained.

Mechanism of fracture in sheet metal cutting processes and its effect on sheared edge stretchability

In recent years, implementation of aluminum alloys, the advanced high strength steels and ultra high strength steels (UHSS) is quickly increasing in automotive industry. However, these materials are often sensitive to sheared edge cracking if stretching along the sheared edge occurs in such processes as drawing of panels with blanked windows, stretch flanging and stretch hemming of edges of the panel. This study is dedicated to development of experimental techniques necessary to account for sheared edge condition on material formability as well as reporting the experimental results and general trends. Analysis of the hole punching process indicated that uniformity of the cutting clearance is rather difficult to maintain, especially for UHSS material where cutting forces are substantially higher than for mild steels or aluminum alloys, and stiffness of the tool starts playing critical role. Therefore, the majority of experimental studies were performed as tensile tests of samples sheared along a straight line in a dedicated trim tool where special measures were taken to achieve consistency of the die clearance. Experimental results on sheared edge stretchability of aluminum alloys similar to 6111-T4 and UHSS steel DP980 are reported. The mechanism of fracture propagation in trimming and hole punching processes is discussed in conjunction with sheared edge stretchability. The rather unique mechanism of fracture observed for trimming of UHSS DP980 steel leads to burr breaking at the final stage of the shearing process.

Application of Non-Uniform Stretch Flanging Theory to Edge-Crack-Prevention Product Design

In order to quickly obtain an appropriate (semi-)product shape that prevents edge fracture in stretch flanging, the non-uniform stretch flanging theory proposed by Nagai was applied to the prediction of strain distribution observed in sheet metal forming of a product with a flat top and a concave wall. The strains calculated by the finite difference method along the flange edge are comparable to those obtained by finite element analysis. Furthermore, flange height h, corner radius R, and shape angle θ were selected as significant factors on circumferential strain εc along the flange edge. The investigation on the effect of those factors shows that εc increases as R decreases, h increases, and as θ increases. A further increase in θ, however, reduces the increasing rate of εc.