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Forming Modes
Drawing is the sheet metal forming process where the punch that creates the part shape forces the sheet metal to pull in from the flange area. In contrast with stretch-drawing or stretch forming, little metal thinning occurs in pure drawing. There is not a generally accepted definition for the term “deep drawing,” although some references describe it as when the depth of draw is greater than the diameter.
Drawability, or the ability for a sheet metal to be drawn into a cup, is assessed by the cup drawing test to measure the Limiting Draw Ratio, or LDR. Here, a cylindrical punch contacts and then pushes a circular blank into the die (Figure 1). The ratio of the largest blank diameter successfully drawn into a cup to the punch diameter used for drawing is the LDR.
Figure 1: In the cup drawing test, a punch deforms a circular blank into a cylindrical cup. The largest ratio of blank diameter to punch diameter successfully drawn into a cup is the Limiting Draw Ratio (LDR).
In the LDR test, metal in the circular blank flows over the die radius and into the cup wall. The metal movement from the flat blank to the vertical sidewalls is the only metal movement which happens, since there is no metal flow within the flat bottom region.
As shown in Figure 2, the flange of the circular blank undergoes radial tension and a circumferential compression as the flange moves in a radial direction towards the circular die radius in response to a pull generated by a flat bottom punch. Blank holder pressure is set to prevent buckles in the blank.
Figure 2: Tension and compression in a drawn cup. Dashed white arrows indicate the radial tension created during cup forming; orange arrows indicate flange compression as a greater amount of metal feeds into progressively smaller regions.
The steel property that improves cup drawing or radial drawing is the normal anisotropy or rm value. Values greater than 1 increase in the Limiting Draw Ratio. In contrast, the LDR is insensitive to the strength of the steel and the n-value. High-strength steels with UTS greater than 450 MPa and hot-rolled steels have rm values approximating one and LDR values between 2.0 – 2.2. Therefore, DP and HSLA steels have similar LDR values. However, TRIP steels have a slightly improved LDR deep drawability.T-2 Since the transformation of retained austenite to martensite is influenced by the deformation mode (Figure 3), the amount of transformed austenite to martensite generated by shrink flanging in the flange area is less than the plane strain deformation in the cup wall. This difference in transformation from retained austenite to martensite makes the wall area stronger than the flange area, thereby increasing the LDR. The benefit of the increased LDR is seen in Figure 4 which shows cups formed from different grades having the same tensile strength.
Figure 3: The cup wall in plane strain strengthens more than the shrink flange due to increased amounts of transformed martensite in TRIP steels.T-2
Figure 4: Cups formed from 590MPa tensile strength steels, highlighting greater draw depths possible with TRIP steels.T-2
In a study of different gradesC-9, laboratory cup drawing experiments show an approximate LDR of 2.0 – 2.2 for the DP steels tested (Figure 5). Note that a doubling of the yield strength has no effect on the LDR. An increased rm value of mild steel created a small increase in LDR over DP steel. The LDR of the MS (martensitic) steel evaluated may have been impacted by the reduced bendability going over the die radius. Figure 6 shows the cup draw depths possible for the grades reported in Figure 5.S-26
Figure 5: LDR tests for Mild, DP, and MS steels.C-9
Figure 6: Cups used in the testing reported in Figure 5.S-26
Even though r-value is the only steel property influencing the formability of drawn flat-bottom cups through its relationship with LDR, not all cups have flat bottoms. Some have hemispherical or other configurations for bottoms. Adding a dome-like shape to the cup results in a more complex forming operation which is now sensitive to material properties like n-value and microstructures.
Corners of box-shaped stampings and the ends of closed channels contain design features similar to drawn cups, providing insight on an analytical approach.
- The four corners of a box-shaped drawn panel should each be analyzed as one-quarter of a cup. Buckles forming in the binder area indicate compressive flow. The corners of the blank will form the same as a deep drawn cup.
- The side walls are formed by metal flowing from the binder across the die radius. The term for this metal flow is bend-and-straighten.
In summary, higher LDR values are achievable in steels with greater values of the normal anisotropy ratio, rm. The absolute value of the LDR, however, also depends on the lubrication, blank holder load, die radius and other system inputs. Figure 7 compares a higher viscosity lubricant on the left with a lower viscosity lubricant on the right.S-26 Die radii need to be balanced: large radii promotes metal flow and may lead to wrinkles, while small radii restricts metal flow and may lead to splits.
Figure 7: The influence of lubricant viscosity on drawing. The cup formed with the higher viscosity lubricant is on the left.S-26
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Coatings
Friction during the stamping process is a key variable which impacts metal flow. It varies across the stamping based on local conditions like geometry, pressure, and lubrication, which change during the forming process. The tool surface influences metal flow, as seen when comparing the results of uncoated tools to those with chrome plating or PVD coatings.
The sheet steel surface is another contributor to friction and metal flow, which changes based on the type of galvanized coating. There are different types of friction tests which attempt to replicate different portions of the forming process, such as flow through draw beads of drawing under tension. Since these tests measure friction under different conditions, the numerical results for the coefficient of friction are not directly comparable. However, within a specific test, extracting useful information is possible.
A study S-54 evaluated the friction of seven deep-drawing steels (DDS), all between 0.77mm and 0.84 mm, with the coating being the most significant difference between the products. Table 1 shows the sample identification and lists the mechanical and coating properties of the tested products, which included two electrogalvanized (EG), one electrogalvanized Zn-Fe alloy (EGA), two hot dip galvanized (HDGI), and two hot dip galvanneal (HDGA) steels. The HDGA coatings differed in the percentage of zeta phase relative to delta phase in the coatings.
Table 1: Properties of DDS grades used in this friction study.S-54
Tests to evaluate friction included a Draw Bead Simulator (DBS), a Bending Under Tension (BUT) test, and a Stretch Forming Simulator (SFS) test. Dome height test and deep draw cup tests were performed to verify the friction behavior of the tested materials. Citation S-54 explains these tests in greater detail. Two different lubrication conditions were evaluated: “as” meaning as-received, and “lub” where the samples were initially cleaned with acetone and mill oil was reapplied.
Figure 1 summarizes the results from the three different friction tests. The relative performance of different coatings is consistent across the tests.S-54 For the tested materials, the HDGI coated steels showed the lowest average friction coefficient and a more stable friction behavior regardless of the lubrication conditions. Zn-Fe alloy coatings (EGA or HDGA) typically resulted in the highest friction. The BUT test generates the lowest strain level among three tests, while the DBS and SFS tests result in higher strain due to a more severe surface contact between tooling and specimen. Stretch forming test tends to result a lower friction coefficient mainly due to higher strain in the stretching process.
Figure 1: Friction test results for different coatings. The relative performance of different coatings is consistent across the tests. S-54
Coating and lubrication interact to influence friction. Draw bead simulator testing compared friction generated on 1mm cold rolled (bare), hot dip galvanized (HDG), and electrogalvanized (EG) deep drawing steel, lubricated with varying amounts of either mill oil, prelube, or a combination of the lubricantsS-68, as summarized in Figures 2, 3, and 4.
Conclusions from this study include:
- Prelube reduces friction on all tested surfaces, with the most dramatic effect seen on electrogalvanized surfaces.
- Above 1 g/m2, there is little friction benefit associated with adding additional lubrication.
- Adding heavier amounts of prelube on top of mill oil did incrementally reduce friction, but the effect essentially maximized at 1.5 g/m2 prelube on top of 1 g/m2 mill oil.
- Cold rolled (bare) steel showed a greater tolerance for dry spots than hot dip or electrogalvanized surfaces. Areas without any lubricant on HDG or EG surfaces led to sample fracture.
Figure 2: DBS Coefficient of Friction: Cold Rolled (Bare) Mild Steel.S-68
Figure 3: DBS Coefficient of Friction: Hot Dip Galvanized Mild Steel.S-68
Figure 4: DBS Coefficient of Friction: Electrogalvanized Mild Steel.S-68
The tool material influences metal flow and therefore friction, but its effect varies with the zinc coating on the sheet steel. The impact of tool steel, kirksite zinc, cast iron, cast steel and chrome plated cast iron on different coated deep drawing steels was evaluated using the Bending Under Tension test.S-55 The friction coefficient obtained using kirksite is lower than that obtained with the other die materials and is relatively independent of the type of zinc coating (Figure 5), reinforcing the caution usually applied stating that soft tool tryout will not be fully representative of what occurs later in the die development process. Supporting the conclusions of the prior study, this evaluation also showed that the HDGI coating tends to have the lowest friction coefficient, especially for cast iron with and without chrome plating (hard tool and production). Also observed was that an oil-based blankwash solution tends to have the highest friction coefficient among the tested lubricants, while a dry film has the lowest friction coefficient.
Figure 5: Influence of die material on friction of galvanized DDS determined in the Bending Under Tension test.S-55
The surface phase in hot dipped galvannealed steel has a impact on friction. Whereas the surface of hot dip galvanized steel is essentially pure zinc, the GA surface may be zeta phase or delta phase. The iron content is the primary compositional difference: the zeta (ζ) phase contains approximately 5.2% to 6.1% by weight of iron, and the delta (δ) phase contains approximately 7.0% to 11.5% by weight of iron.G-21 Zeta phase is softer and less brittle than the delta phase, but has a high coefficient of friction.G-22 Even with a fully delta phase surface, additional optimization is possible to produce targeted surface morphologies.S-56 The two right-most images in Figure 6 are both of delta phase surfaces, with the cubic surface (right image) associated with better formability than the rod surface of the center image (Figures 7 and 8).
Figure 6: Surface morphology and coating cross section of 3 galvanneal coatings. Left: Zeta surface; Center: Delta-rod surface; Right: Delta-cubic surface.S-56
Figure 7: Formability of galvannealed surfaces evaluated through a square cup test.S-56
Figure 8: Formability of galvannealed surfaces evaluated through a Limiting Draw Height (LDH) test. Higher is better.S-56
Low annealing temperature or time can result in excessive zeta phase. However, longer and hotter annealing cycles increase the risk of powdering and flaking. Producing the correct balance of ZnFe phases requires control of time and temperature of the galvannealing process.
Formability
Strength is defined as load divided by cross-sectional area. In a tensile test, the choice of when the cross-sectional area is measured influences the results.
It is easiest to measure the width and thickness of the test sample before starting the pull. At any load, the engineering stress is the load divided by this initial cross-sectional area. Engineering stress reaches a maximum at the Tensile Strength, which occurs at an engineering strain equal to Uniform Elongation. After that point, engineering stress decreases with increasing strain, progressing until the sample fractures.
However, metals get stronger with deformation through a process known as strain hardening or work hardening. As a tensile test progresses, additional load must be applied to achieve further deformation, even after the “ultimate” tensile strength is reached. Understanding true stress and true strain helps to address the need for additional load after the peak strength is reached.
During the tensile test, the width and thickness shrink as the length of the test sample increases. Although these dimensional changes are not considered in determining the engineering stress, they are of primary importance when determining true stress. At any load, the true stress is the load divided by the cross-sectional area at that instant.
The true stress – true strain curve gives an accurate view of the stress-strain relationship, one where the stress is not dropping after exceeding the tensile strength stress level.
- True stress is determined by dividing the tensile load by the instantaneous area.
- True strain is the natural logarithm of the ratio of the instantaneous gauge length to the original gauge length.
True stress – true strain curves of low carbon steel can be approximated by the Holloman relationship:
σ = Kεn
where true stress = σ; true strain = ε, n is the n-value (work hardening exponent or strain hardening exponent), and the K-value is the true stress at a true strain value of 1.0 (called the Strength Coefficient).
True stress-strain curves obtained from tensile bars are valid only through uniform elongation due to the effects of necking and the associated strain state on the calculations. Inaccuracies are introduced if the true stress-true strain curve is extrapolated beyond uniform strain, and as such a different test is needed. Biaxial bulge testing has been used to determine stress-strain curves beyond uniform elongation. Optical measuring systems based on the principles of Digital Image Correlation (DIC) are used to measure strains. The method by which this test is performed is covered in ISO 16808.I-12
Stress-strain curves and associated parameters historically were based on engineering units, since starting dimensions are easily measured and incorporated into the calculations. True stress and true strain provide a much better representation of how the material behaves as it is being deformed, which explains its use in computer forming and crash simulations. Although sample dimensions are challenging to measure during a tensile test, there are equations that relate engineering units to true units. Conventional stress-strain curves generated in engineering units can be converted to true units for inclusion in simulation software packages.
Relationships Between Engineering and True Properties
Citations
Citation:
S-94. Society of Automotive Engineers Standards, SAE J3215, “Hydrogen Embrittlement Testing of Ultra High Strength Steels and Stampings by Acid Immersion“.
Formability
Most sheet metals have different “Global” and “Local” forming capabilities, so it is critical to understand their meaning to optimize grade selection, processing, and usage.
Historically, most fabricators needed to consider only global formability when designing and stamping parts. Drawing, plane-strain tension, and stretch forming are “global” forming modes where deformation occurs in the plane of the sheet over relatively large regions of material. Tensile failures or necking failures, where the steel progressively thins during forming, are a characteristic of global formability failures. The strains across a stamped part start low and evenly distributed but begin to concentrate as the punch approaches bottom-dead-center. Critical thinning (also called necking) occurs if the strains induced by the part shape and forming process exceed the forming limit of the chosen sheet metal. Forming simulation software packages have reliably shown the ability to accurately predict global formability concerns and hot spots with the use of inputs like tensile test data and the correct forming limit curve.
Local formability failure modes are an entirely different failure condition, where fractures occur out of the plane of the sheet in response to concentrated deformation created when forming localized features like stretch flanges, extruded holes, or bends around a radius too small for the selected steel grade. These failures typically occur without any observable thinning or necking (Figure 1). Forming simulation software that considers only the forming limit curve or maximum thinning as the failure criteria cannot predict local formability failures.
Figure 1: A fracture related to insufficient local formability. Note the lack of thinning near the fracture. H-5
Cutting conditions and edge stresses developed in blanking and slitting operations play a significant role in limiting local formability. Trim steel clearances, shear angles, trim steel materials, tool sharpness and design, steel rolling direction, and part design considerations are all important (Figure 2).
Figure 2: Cut edge uniformity influences edge quality.U-6
The absence of global thinning leading to the onset of localized necking highlights the importance of clearly defining the actual mode of failure before trying to identify possible solutions. Examination of the edge of the part where the failure is occurring is the best way to accomplish this. Figure 3 shows a photo of a typical local formability related edge fracture where no observable thinning/necking before fracture occurs.
Figure 3: A typical local formability edge fracture viewed from different angles, with no appreciable thinning prior to fracture.U-6
Some AHSS grades are more prone to these local formability failures. A reduced hardness difference between microstructural phases appears to improve local formability. At a given tensile strength level, there may be grades with improved global formability, improved local formability, or a balanced approach available. Choosing the optimum grade requires understanding of the manufacturing die process and the functionality requirements associated with the part and its application.
Figure 4 shows an edge fracture on a DP980 rocker panel. This panel experienced edge fractures during production, occurring in embossments along the edge of the part, with no evidence of necking at the fracture site. This is a classic local formability failure, reinforcing the need to inspect the conditions at the fracture zone of AHSS to determine whether the root failure is global or local formability. Process and die solutions differ based on the mode of failure.
Figure 4: Classic local formability related edge fracture on an embossment within a DP980 part.U-6
Grade dependency is highlighted in Figure 5, which compares edges of HSLA 50SK on the left and DP500Y/780T on the right after stretch-bend testing under conditions to produce fracture. The HSLA sample shows the characteristic thinning down associated with necking. The DP steel did not have a visible neck at the failure location.S-11
Figure 5: Left: HSLA 50SK, showing thinning at the fracture location, which is typical for global formability failures. Right: DP 500Y/780T, showing no thinning at the fracture location, which is typical for local formability failures.S-11
Mechanical tests now being employed to better quantify and characterize the unique local formability related failure modes associated with AHSS include:
Fracture-Limited Formability = Local Formability
In general, most sheet metal stamping failures result from exceeding the necking limit, and are termed global formability failures. However, there are important conditions which promote fracture before first developing a neck. These are of particular concern since such fractures cannot be predicted or anticipated based on conventional FLC and grid strain analysis techniques. Recent advances in metal forming software codes now allow input of appropriate data which substantially aids in these efforts. Still, there is no universally accepted shape of the Fracture Limit Curve.
Local Formability: Bending
Researchers discovered that all the steel through the sheet thickness must exceed the forming limit for necking to begin. In tight bends where the inner surface is compressed, strains never reach the critical strains in the conventional forming limit curve, which is why necking is not seen on the outer bend surface above a critical r/t level, or the ratio between the radius and the thickness.
However, as strength increases, the fracture limit is closer to the necking limit determined by the FLC. Advanced High Strength Steels are even more sensitive to local formability failures due to the localization of strains which occur at the interface between hard and soft phases in the microstructure.
On thin, wide sheet, bending strains on the metal surface plot along the axis of plane strain. The Fracture Limit Curve in this location is higher than the necking Failure Limit Curve, but since the critical strains only need to be reached on the outermost surface, higher-strength steels have a greater risk of experiencing bending fractures. In these cases, the material fracture limit becomes the effective forming limit in deformation modes with severe through-thickness strain gradients, and this is not considered in the traditional FLD.
Local Formability: Sheared Edge Expandability (Hole Expansion)
In a hole expansion test, the strains at the edge of the expanding hole follow a uniaxial strain path until reaching the fracture limit. In lower-strength steels with clean, machined (undamaged) edges, expansion ratios over 300% might be possible. As mentioned above, higher-strength steels have a lower fracture limit. Still, undamaged machined edges provide the best conditions for high hole expansion.
Cutting operations like blanking, shearing, and punching all damage the edge and lowers the fracture limit, meaning that the fracture limit might be reached at even lower strains. Strain localization occurring at the interface between hard and soft microstructural phases highlight why some grades have still lower critical strains.
Figure 6: Higher-strength steels have a smaller gap between the necking Forming Limit Curve and the Fracture Limit Curve.
Summary of Global Formability
- The resistance to localized thinning (necking) is the key to global formability.
- Stamped parts get progressively thinner as the press stroke approaches bottom-dead-center. Rapid thinning in critical areas lead to the onset of localized necking. Global formability promotes the ability to reach deeper draw depths before initiation of necking.
- Measures of global formability include the work-hardening exponent (n-value), the uniform elongation value, and the total elongation value determined in a tensile test, along with the forming limit curve (FLC).
- Global formability failures occur when a through-thickness volume of the formed sheet steel exceeds this forming limit and begins to neck.
Necking failure typically should not occur if the global formability limit is exceeded on only the outer surface of steel bent over a radius or expanded at a cut edge and not through the full thickness of the metal.
- Uniform elongation measures the resistance to local necking. The global formability limit corresponds to Uniform Elongation in the strain state achieved in a tensile test sample. The Forming Limit Curve (FLC) encompasses all strain states.
- For mild and conventional high strength steels, there is a large difference between the necking limit and the ultimate fracture limit. This corresponds to the difference between Uniform Elongation and Total Elongation in a tensile test, where the value of total elongation is typically twice that of uniform elongation.
Summary of Local Formability
- The resistance to fracture is the key to local formability.
- Local formability promotes fracture resistance in response to creating local product features like stretch flanges, extruded holes, and tight-radius bends.
- Local formability tests which evaluate the stretchability of appropriately prepared edges by deforming them with a punch may have limited reproducibility between labs due to the influence of different sample preparation methods. Determining parameters like True Fracture Strain, Reduction in Area at Fracture, or Thickness Strain at Fracture with a well-defined tensile test typically results in repeatable and reproducible values.
- Local formability failure occurs when strains at the surface or edge exceed the ultimate fracture limit, a value that is higher than the necking limit determined by the Forming Limit Curve.
- Local formability failures occur more frequently on higher strength steels partly because of a smaller difference between the necking limit determined by the Forming Limit Curve and ultimate fracture limit.
- Work hardening and damage from edge preparation methods like shearing further reduce edge formability to values typically lower than indicated by the Forming Limit Curve, especially for AHSS. The strain localization at the interface between hard and soft phases in AHSS also contribute to an increased risk of local formability failures.
- The lack of a localized neck is a characteristic trait associated with of local formability failures.
