Deep Drawing Of Sheet Metal

Lightweight composite materials processing

D. Dixit , ... M. Stabenau , in Lightweight Ballistic Composites (Second Edition), 2016

6.5.3.4 Others

Deep drawing of composites materials: Composite materials are widely used in different industrial fields, because of their good formability and their high strength to weight ratio. In the present work a triple-layered sandwich composite is investigated. Experimental tests at room temperature are carried out for the materials constituting the composite. A finite element model of a deep-drawing process of the composite is performed, where a finite strain constitutive model for the metal part, with material parameters calibrated to uniaxial tensile tests, has been implemented.
Sguazzo et al., 2014

Reference link

•: http://www.google.com/patents/US20120279402
•: http://www.faqs.org/patents/app/20110291366

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Metal Working: Stretching of Sheets

E. Doege , ... K.Y. Benyounis , in Reference Module in Materials Science and Materials Engineering, 2016

2 Stretch Forming and Deep Drawing

The deep drawing process is often a combination of stretch forming and deep drawing. At the beginning of the forming process, when the punch moves downwards and form out the bottom, a general stretch forming process results. When the bottom is formed, the deep drawing process follows, characterized by transfer of the drawing force from the punch through the cup wall into the flange. Within the flange results the main forming process with radial and tangential compression loads. Figure 1 shows the whole process of deep drawing, including the stretch forming and deep drawing processes.

During the deep drawing process, different stress conditions result in the material. Figure 2 shows a drawn cup with the different loading zones. The zones of interest are the flange, the side wall (cup wall), and its bottom. The stress condition in the flange area is a tensile load in radial direction and compression load in tangential direction. The stress conditions determine the forming process at deep drawing. The important stress condition in the cup wall is the point of transition of the punch radius to the cup wall area. This loading zone is characterized by a plane strain load, whereas the cup bottom has a biaxial stress load. The cup bottom and its biaxial stress load determine the stretch forming process.

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Modeling of metal forming: a review

Uday Shanker Dixit , in Mechanics of Materials in Modern Manufacturing Methods and Processing Techniques, 2020

1.2.5 Deep drawing

Deep drawing process manufactures cup- or boxlike products by pushing a flat sheet through a die with the help of a punch while holding the sheet in a blank holder. Fig. 1.12 shows a schematic diagram of the process. The blank holder force (BHF) and clearance between punch and die should be carefully decided. Excessive BHF causes tearing, and too less BHF causes wrinkling. Clearance is usually 7%–15% of sheet thickness. Too less clearance causes ironing; sometimes, it may be desirable. Anisotropy of the sheet has a large influence on the deep drawing ability of the sheet. Planar anisotropy, in which properties differ with direction in the plane of the sheet, causes earlike formation on the drawn product; this defect is called earing. Normal anisotropy refers to the situation in which properties in the thickness direction differ from those in the plane of the sheet. A large flow stress in the thickness direction compared to that in the plane of the sheet provides better performance in deep drawing as it avoids tearing. A measure of normal anisotropy is the plastic strain ratio, which is the ratio of true width strain to true thickness strain for a material strained in the longitudinal direction. Automobile manufacturers prefer a plastic strain ratio of 1.4 or more for steel sheets. A measure of formability is the limiting draw ratio (LDR), which is the ratio of the diameter D of the largest blank that can be successfully drawn to the diameter of the punch d. Theoretical limit for LDR is 2.7.

Besides wrinkling, tearing, and earing, other common defect is the orange peeling. Orange peeling is the generation of high surface roughness in the region of the sheet that has undergone large deformation. This defect is more prominent with large grain size materials. Ironing can eliminate this defect.

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Magnesium Alloys

Ian Polmear , ... Ma Qian , in Light Alloys (Fifth Edition), 2017

Sheet formability

Sheet forming process involves stretch forming and deep drawing in which magnesium sheet undergoes a variety of strain paths. Stretch forming and deep drawing are often used to obtain information about specific features of the sheet formability. Fig. 6.32 shows the stretch formability of different magnesium alloys measured by the Erichsen cupping test at room temperature, which is indicated by limiting dome height (LDH) values. The stretch formability of magnesium alloys containing RE elements, lithium or calcium, as well as the texture-modified AZ31 alloy, is better than or comparable to those of 5xxx and 6xxx aluminium sheet alloys at a similar strength level. In general, the stretch formability of those magnesium alloys that have low yield strengths (100–170 MPa) is comparable to that of some commercial aluminium alloys (LDH ≈ 7–11), but higher-strength magnesium alloys exhibit poor stretch formability.

Forming limit curves for annealed sheets (1.5 mm thick) of AZ31 and ZE10 alloys, tested at a speed of 1 mm s⁻¹ and at room temperature and 200°C, are shown in Fig. 6.33. For the purpose of comparison, the forming limit curve for the annealed sheet of the Al–Mg–Mn alloy 5052 is also shown in the figure (Section 2.1.3). Both magnesium alloys have equiaxed recrystallized grains of approximately 10 μm in diameter. AZ31 has a strong basal texture, while ZE10 has a weakened basal texture with basal poles spreading along the TD. Sheets of these two alloys have forming characteristics that are inferior to the aluminium alloy sheet at room temperature. The formability of AZ31 at room temperature is very poor. An increase of testing temperature to 200°C leads to a significant increase in formability under strain paths of both negative and positive minor strains. The ZE10 alloy has a better formability than AZ31 at both room and elevated temperatures under conditions of plane-strain and biaxial stretching. It also has a better deep drawability than AZ31. Testing results from other temperatures such as 150°C and 250°C indicate that the formability of ZE10 at room temperature is even higher than AZ31 at 150°C under the condition of plane-strain or biaxial stretching.

While ZE10 has better formability, it has a greater susceptibility to earing than AZ31. It is unclear whether this problem is related to its low Lankford value (r). In general, low R values tend to increase the tendency to fracture around the cup corner and are not beneficial for deep drawing. The problem may also be related to the rather complex stress states occurring during deep drawing, which may result in operation of various deformation and failure modes depending on the alloy microstructure. Apart from the failure near the cup corner, failure also occurs frequently near the top of the cup, especially in alloys with a strong basal texture and strong tension–compression asymmetry. This has been attributed to bending/unbending-induced twinning/detwinning near the die radius during deep drawing. The relationship between drawability and R value is not straightforward, but it is generally accepted that magnesium alloys with random texture and accompanying low-planar anisotropy have better drawability at room temperature. Lower R values also correlate with better stretch formability.

The formability of magnesium sheet can be improved by high-temperature hot rolling, reverse rolling (reversing the sheet direction after each rolling pass), or cross rolling. For example, reverse-rolled AZ31 sheet has more isotropic mechanical properties than unidirectionally rolled sheets due to its more homogeneous grain structure. Cross rolling of AZ31 sheet leads to a weakened basal texture and enhanced formability.

Only limited cold forming can be carried out with sheet alloys and typical minimum bend radii vary from 5 to 10T, where T = thickness of sheet, for annealed material, and from 10 to 20T for the hard-rolled condition. Thus, for even simple operations, hot forming within the temperature range 230–350°C is preferred. Under these conditions sheet can be formed by pressing, deep drawing, spinning, and other methods using relatively low-powered machinery. A number of deep drawn automotive sheet panels in alloys such as AZ31 have been produced for prototype testing. Indicative weight savings are 50% compared with the same panels made from steel, and 15% when compared with aluminium. This alloy has also been shown to exhibit superplasticity if processing is carefully controlled. If the overall cost of producing magnesium alloy sheets can be reduced, this feature may offer unique opportunities for forming complex automotive body panels.

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The Conventional Spinning of Sheet Metals for Fabrication of Metallic Parts and Structures

Mei Zhan , Pengfei Gao , in Reference Module in Materials Science and Materials Engineering, 2020

Stress and strain characteristics

The conventional spinning of cylindrical part is similar to deep drawing process in terms of the workpiece shape change. The initial blank is a plain disc and formed to a cylinder by the roller following one pre-set path. During forming process, the workpiece is usually divided into four different feature zones, i.e., non-deforming zone, deformed zone, deforming zone and flange zone, as shown in Fig. 4. Jurković et al. (2006) have systematically analyzed the stress and strain states in these feature regions. During analyses, it is assumed that there is no reduction of wall thickness in conventional spinning. The stress is decomposed into three components based on cylindrical coordinate: axial stress $(σ_{Z})$ in the direction parallel to rotation axis, tangential stress $(σ_{T})$ in the direction tangential to the mandrel and parallel to the blank, and radial stress $(σ_{R})$ in the radial direction. The detailed stress and strain states in different regions are illustrated in Fig. 4.

The non-deforming zone is compressed by the tailstock, so its stress and stain states are all performed as compression in axial direction and tension in the other two directions. Element in deformed zone is mainly drawn by the roller in axial direction. So, the stress state of deformed-zone is single-dimensional tension in axial direction. Its strain state is compression in radial direction and tension in axial direction. Element in deforming zone undergoes the press of roller and constraint of flange, so the stress and strain states are both tension in axial direction and compression in the other two directions. Based on the assumption that the flange is deformed without wrinkling and thickness reduction, the element in flange zone is considered as plain strain state without the axial strain. So the element undergoes positive radial stress and negative tangential stress, and whose strain state is compressive in tangential direction and tension in radial direction.

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Forming technology for composite/metal hybrids

J. Sinke , in Composites Forming Technologies, 2007

In-plane deformations

The applicable in-plane deformations are very small and deep drawing operations cannot be applied. Only some limited stretching is feasible, in particular when the laminates are not directly stretched in the fibre directions but at an off-axis angle (e.g., in bias-directions). In that case the principle of the forming limit curve (FLC) can be used to predict the failure of the laminate. This curve presents the forming limits, e.g. expressed in strains for in-plane strain combinations. For that case the limits in fibre directions are set by the failure strains of the fibres.

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Superplastic Forming (SPF) of Complex Sheet Metal Parts and Structures

Guofeng Wang , in Reference Module in Materials Science and Materials Engineering, 2020

SPF Process and Quality Control Method

SPF process

The SPF process has been applied in many aspects.

In sheet forming, it mainly includes superplastic inflatable forming, superplastic deep drawing, etc. Among them, superplastic inflatable forming uses high pressure gas as the forming medium while superplastic deep drawing uses steel dies. In bulk forming, there are mainly superplastic forging and superplastic extrusion.

The most widely used process method is superplastic inflatable forming. In the superplastic state, the plastic deformation resistance decreases sharply, the plastic deformation ability is greatly improved and almost no strain hardening occurs, which is approximately a viscous flow state.

Method for controlling the thickness of superplastic formed parts

During the SPF, the sheet has excellent filling performance, so it can form parts with high shape accuracy. Fine sharp corners, grooves and bosses can be formed as a whole structure at once time. However, due to the uneven distribution of the stress-strain field of the sheet in SPF, the thickness will be significantly different, which has become one of the key issues that limit the application of this process. The non-uniformity of thickness is mainly reflected in two aspects. One is the effect of uneven deformation in the freely bulging part, where the strain rate sensitivity index (m) is the main influencing factor; the other one is that the uneven deformation of formed part due to the die constraints and friction.

In order to improve the thickness distribution of superplastic formed parts, some effective methods to control the process are:

(1): Direct-reverse forming method. Direct-reverse forming is divided into two steps: reverse forming (pre-forming) and direct forming (final forming). The first step is reverse forming, that is, reverse pressure is applied to deform the sheet in the direction of the preforming die, the original thick place is pre-thinned, which alleviates the thinning concentrated at the corners of the lower die and uneven thickness. Reasonably designing the preforming die can play a role in dispersing deformation to a large extent. Then, direct forming is performed, the reverse air pressure is removed and the direct pressure is applied, so that the sheet is deformed toward the final forming die until the die is completely attached. It should be noted that the surface area of the sheet after reverse forming must be controlled within the final formed part. Fig. 10 is the schematic diagram of direct-reverse forming.

Fig. 10. The schematic diagram of direct-reverse forming.
(2): Reverse forming by movable punch. As shown in Fig. 11, after the sheet is formed upward to a certain height, the punch consistent with the size of the part is moved into the bulging arc surface, then pressurized in the opposite direction to make the sheet attach to the punch. The bottom material flows to both sides in advance, when the reversed pressure is applied, the round corner of punch are attached to the sheet firstly, the subsequent thinning is small, so that the thickness after forming is relatively uniform. However, in the first step, the height of the dieless forming should be controlled moderately. If it is too large, the workpiece will be wrinkled, while if it is too small, the improving effect of thickness distribution is not obvious.

Fig. 11. The schematic diagram of reverse forming by movable punch.
(3): Pre-machining method. As shown in Fig. 12, the blank is pre-machined to different thicknesses, then the pre-machined blank is bulged into a shell. This method can achieve a fairly uniform thickness distribution, but the pre-machine of blank is difficult. At present, the blank shape is mostly determined by the addition and subtraction method. Finite element simulation is performed with blanks of the same thickness, then the difference between the thickness distribution of the formed part and the target value is measured, finally, the size of the blank is modified.

Fig. 12. The schematic diagram of pre-machining then forming method.
(4): Punch-assisted forming method. Fig. 13 has a punch device that can move up and down. The movable punch can deform the area which adjacent to flange of the die first, and the area which in contact with the punch does not deform due to friction (therefore suppresses thinning). After the sheet deforms to a certain extent, the punch is returned and the deformation is completed by applying air pressure.

Fig. 13. The schematic diagram of punch-assisted forming method.
(5): Pre-forming method. Fig. 14 is the schematic diagram of pre-forming method. The initial blank is pre-deformed in an appropriate way to obtain a pre-formed blank, the center of the pre-formed blank retains the original thickness, and the side walls are thinned in advance to ensure that the overall thickness is uniform after the final SPF. The pre-forming method can effectively control the thickness uniformity of single-layer structure titanium alloy. This method is relatively simple and convenient for production and promotion. It can effectively control the thickness of single-layer structure titanium alloy formed by SPF. Therefore, it is suitable to manufacture parts with relatively low forming accuracy requirements or intermediate process parts. For parts with high forming accuracy, the forming parameters need to be optimized to reduce defects at the corners of the die entrance.

Fig. 14. The schematic diagram of pre-forming method.
(6): Non-uniform heating method. This method is used to form a varying temperature field on the sheet to reduce the temperature in the deformation concentrated area, thereby slowing the amount of deformation. As a result, because of the high temperature, the not easily thinned area during direct bulging are thinned first, and the easily thinned area are not thinned or rarely thinned because of the low temperature.
(7): Cover forming method. This is a method composite punch and die, bulging with a blank that is much larger than the size of the part, and then cuts off the redundant material. Because the material in the annular area around the punch is also involved in the deformation, the rounded corners of punch are equivalent to the dangerous parts in the simple punch expansion method, this part first attach to the die during the bulging process and the amount of thinning is small. Therefore, the most severely thinned part is transferred, so that the thickness distribution of the part is improved compared to the simple punch forming method, but this method is at the cost of material waste.

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Influence of Control Parameters on Tool Wear for Sheet Metal Stamping Die with Various Die Radius Arc Profiles

X.Z. Wang , S.H. Masood , in Reference Module in Materials Science and Materials Engineering, 2016

2.2 Finite Element Modeling

To analyze the wear work distribution along the die radius profile, a finite element model of the deep-drawing was established using Abaqus 6.8. As described in detail in a previous study (Wang and Masood, 2011), a two-dimensional half symmetry model was created for the tools (die, punch, and blank holder) and the blank. The dimensions and standard values of parameters of the model are listed in Table 1.

Table 1. Dimensions and parameters of finite element model (Wang and Masood, 2011)

Punch diameter, D _p	30 mm
Punch radius, r _p	5 mm
Die radius, R _D	Various
Die to punch clearance, c	2.1 mm
Blank holder force, F _B	Various
Draw depth, d	50 mm
Blank width, w	Various
Blank Length, l	150 mm
Blank Thickness, t	Various
Lubrication coefficient, f	0.15 (Mild oil)

Figure 1 illustrates the meshed finite element model in the simulations. The die, punch, blank holder, and blank are set as two-dimensional deformable parts. The whole model is meshed mainly with four node plane strain reduced integration elements CPE4R. A few linear triangular plane strain elements CPE3 are used for the tooling parts. To ensure adequate accuracy and save computational simulation time, the elements of the die radius and top blank region are meshed more finely than those of the rest of the model. The mesh sizes of the die radius and top blank are 0.03 mm×0.03 mm and 0.05 mm×0.1 mm, respectively.

The die and blank holder are fixed in all directions. A specified load is applied at the bottom of the blank holder as the blank holder force. The right edges of the blank and punch are fixed in the x direction because of the symmetrical requirement of the model. Contact pairs are established based on surface to surface contact algorithm between die and blank, blank and blank holder, punch and blank.

A series of steps with loading was applied on the punch to drive the deep-drawing process to stretch the part. These procedures are referred to as the pre-processing procedures. After specifying parameters of the solution, Abaqus would solve the specified problem. The tool wear work results are obtained from the post-processing of the simulations.

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Deformation-Induced Compressive Instability

Heng Li , Mingwang Fu , in Deformation-Based Processing of Materials, 2019

4.2.1 Compressive Instability Classification

As shown in Fig. 4.1, the wrinkles take place in different material deformation processes. For sheets in the deep-drawing process, wrinkling is the one of the most commonly seen defects and can take place in both the side-wall and flange area. In addition, wrinkling may occur in the flange area in sheet spinning, along the feeding direction of a flat strip under extrusion, and in the circumferential direction of an annular thin film under torque. As for circular tubes, the wrinkles also happen in axial compression, bending, and hydroforming processes. Therefore, it is important to classify the various wrinkling phenomena based on their common characteristics to have a better understanding of the compressive instability.

As shown in Fig. 4.2, classification can be done through the features of external force. Generally speaking, wrinkling is induced by uniaxial compression, shearing, unequal stretch tension, and pure bending. Under different deformation scenarios, the stress state of deformation can be accordingly classified as uniaxial compressive stress, shear stress, compressive stress in conjunction with tensile stress, and nonuniform compressive stress.

The classifications may cover the major stress states that induce the wrinkling instability in the material deformation process. Wrinkling of tubes under axial compression is a typical case of uniaxial compressive stress. In the process, in addition to the axisymmetric wrinkling mode shown in Fig. 4.1G, the diamond mode and the mixed mode also occur with varying length-to-thickness and diameter-to-thickness ratios, as shown in Fig. 4.3. Shear load-enforced wrinkling is a major defect to be considered in the design of stainless-steel plate girders [14,15] and the important membrane structural parts in space exploration [16–18]. The compressive stress is induced along the diagonal direction in the shear-loading area of a 45 degree angle to the shearing direction. As for the unequal tension condition, the most representative case is the Yoshida test [19], as illustrated in Fig. 4.4. In this test, tension deformation is imposed along the diagonal direction of a square sheet metal, causing an unequal tension field, and then induces the transverse compressive stress resulting in the wrinkles. The bending moment also can induce nonuniform compressive stress at the intrados of a bent tube, and then cause periodic ripples along the inner ridge [20]. With different diameter-to-thickness ratios (D/t), the encountered buckling modes are multiple, including the axial ripples, the diamond mode, and the kink [21], as shown in Fig. 4.5.

From the perspective of the boundary constraint that influences the wrinkling significantly, the material deformation processes can be divided into the processes under simple boundary conditions (SBCs) and complicated boundary conditions (CBCs). The detailed difference is shown in Table 4.1. The above examples all belong to the SBCs and their wrinkles initiate and evolve freely. For many deformation processes, however, the CBCs is encountered during the deformation processes such as deep drawing, tube shear bending, tube RDB, and tube hydroforming process, as shown in Fig. 4.6. In these cases, the loading condition, stress state, and boundary constraint are more intricate and confusing when buckling initiates and evolves, which makes the wrinkling prediction more challenging.

Table 4.1. Simple Boundary Condition (SBC) and Complicated Boundary Condition (CBC) [23]

Classification	Simple Boundary Condition (SBC)	Complicated Boundary Condition (CBC)
Load	Simple loading paths For example: Axial compression of a tube Pure bending of a tube Uniform internal pressure of a tube	Complex loading paths and history For example: Deep drawing Tube rotary draw bending Tube shear bending Tube hydroforming
Friction	Take no account of friction	A variety of frictional contact
Contact	Take no account of contact	Multiple tooling constraints Perturbation of friction and clearance Complex contact nonlinearities
Description	Easily be analytically described For example: Fixed-support Free-support Simple-support	Cannot be analytically described For example: Changing in time and space Dynamic effects

During the deep-drawing process, wrinkles can occur in both the flange area and side-wall area. The stress state of the flange area is similar to the Yoshida buckling test, of which the tensile stress is generated along the tangent direction and the compressive stress is generated along the circumferential direction. However, influenced by the friction induced by the normal constraint, the compressive stress in deep drawing is often uneven along the circumferential direction. In the tube shear-bending process, the tube is enforced by a shear force in the vertical segment. In the shear deformation zone, both sides of the tube are subjected to a compressive stress and thus wrinkles are likely to generate. For the tube rotary draw-bending process, the tube is bent by the clamp die, and its compressive stress will concentrate at the inner ridge along tangent direction. Thus wrinkles usually occur at the inner side of the bending region. For the tube hydroforming process, the compressive stress is produced along its axial direction by the feeding of punch. Then combined with the internal and external pressure, wrinkles are formed.

Generally speaking, most material forming processes suffer from compressive instability. Wrinkling parts include the plates, tubes, and membranes, while the loading conditions cover compression, shear, and bending, and the wrinkling waveforms are also different from case to case. In addition, the CBCs makes it a more challenging issue to understand the inner mechanism of wrinkling defects and realize its accurate prediction and effective control.

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Metal Working: Cold Rolling

P. Montmitonnet , ... K.Y. Benyounis , in Reference Module in Materials Science and Materials Engineering, 2016

3.2 Mechanical Properties and Microstructures

The most stringent requirements are put on cold-rolled strip if it is to be deep drawn. Stamping or deep drawing apply stresses tensile in nature, which may promote necking and fracture. Three characteristics are therefore desirable:

(i): A high strain hardening exponent (and if possible a high strain rate sensitivity) so as to oppose strain localization and necking: hence the importance of annealing.
(ii): A high Lankford coefficient, i.e., the metal should elongate more easily in the in-plane directions as compared to the normal direction ND, so as to resist thinning in tension and thickening in compression. Typical values of 2 or above are achieved for carbon steel, whereas values around 1 are obtained for aluminum alloys.
(iii): Furthermore, in-plane anisotropy should be kept as low as possible to avoid the earing defects.

These characteristics must be achieved in the cold-rolled coil, along with sufficient ductility for further processing. This is why annealing (batch or, more and more, continuous) is often performed at the end, or shortly before the end, of the cold rolling process. Increasing rolling reduction for homogenized-rolled AA5052 aluminum alloys result in, the equiaxed grains are elongated along the rolling direction obviously. Accumulation of rolling reduction increases the work hardening effect, which results in the enhanced strength and degraded plasticity. When rolling reduction is 87%, the ultimate tensile strength reaches 325 MPa but elongation is only 2.5%. There are much more secondary phase precipitates after annealing treatment. With an increase of annealing temperature, the amount of precipitates increases and work hardening diminishes continuously. The elongation is improved to ~23% but the tensile strength is decreased to 212 MPa after annealing at 300 °C for 4 h, which are comparable to those of as-homogenized alloy (Bo et al., 2015).

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