bending under tension test Archives

Coating Friction

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 lubricants^S-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 2: Influence of die material on friction of galvanized DDS determined in the Bending Under Tension test. S-55

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 3: Surface morphology and coating cross section of 3 galvanneal coatings. Left: Zeta surface; Center: Delta-rod surface; Right: Delta-cubic surface S-56

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.

Friction and Friction Testing

Friction, Testing and Characterization

Friction is a restraining force that limits metal flow resulting from contact with another surface during sheet forming. Friction is influenced by the complex interaction between the sheet steel, lubricant, and tooling material, as well as many parameters of the forming system, some of which are shown in Figure 1.

Figure 1: Friction is influenced by the complex interaction between the sheet steel, lubricant, and tooling material, as well as many parameters of the forming system. (image modified from Citation K-13)

Friction changes throughout the stamping process as well as across the part, since there are changes in contact pressure, contact temperature, geometry, the sheet steel strength (work hardening), and even the sheet metal surface due to flattening of the peaks after flowing over beads and radii. There is no one friction value that applies to all geometries and forming scenarios that can be encountered.

Characterizing friction in a simulative test is similarly challenging, since there is no one simulative test that approximates all regions of an engineered stamping. For many years, tooling, design, and simulation engineers used one friction value for each galvanized coating / lubricant combination, and applied that value across every part stamped from that metal with that lubricant. The industry has evolved to using different tests to generate friction values appropriate for different forming conditions.

Simulative tests to characterize friction include test to reflect bending under tension, pulling through draw beads, and movement over flat surfaces. Depending on the test, it may be possible to evaluate the effect of tooling material, temperature, radii, and speed in order to better simulate production conditions. Figure 2 indicates which of these simulative tests apply to the regions of an in-process stamping. The value obtained in one test is a function of only those specific test conditions and should not be compared against values obtained in other tests.

Bending Under Tension test: In the Bend Under Tension (BUT) test, a metal strip is drawn over a fixed cylindrical pin with a pair of independently controlled hydraulic actuators offset by 90 degrees. Two load cells measure the pulling force and back tension force. (Figure 3)
Draw Bead test: Two sets of tests are done using a Draw Bead Simulator (DBS). In the first test, the force to pull sheet steel through a set of fixed draw beads is measured and reflects both the bending and unbending forces as well as the friction forces. Another sample is pulled through frictionless roller beads in a similar arrangement. Here, only the bending/unbending forces are active. Subtracting the results allows for determination of the forces due solely to friction. (Figure 4)
Strip Draw: In the Strip Draw test, friction is determined by pulling sheet steel through opposing flat platens. The restraining condition is controlled by the force applied on the flat platens. The influence of bending is not considered in this test. (Figure 5)

Other tests are also used to provide some assessment of formability. Dome Testing simulates stretch forming conditions and can provide a relative comparison between different materials and lubricants. The Twist Compression Test (TCT) is used to compare the performance of different lubricants under controlled, repeatable conditions. Lubricant breakdown, which results in adhesion between the tool and the sheet steel, can be evaluated in the TCT.

Figure 2: Pertinent Simulative Tests During Stamping (image modified from Citation S-42)

Figure 3: The Bending Under Tension Test (image modified from Citation S-42)

Figure 4: Draw Bead Simulator Testing determines the effect of friction from the differences in pulling force in two test conditions.
a) The strip pulled through fixed draw beads experiences restraining forces due to bending/unbending as well as forces due to friction.
b) The strip pulled through frictionless roller beads experiences restraining forces due only to bending/unbending.

Figure 5: The Strip Draw Test (image modified from U-6)

Coating Friction

Friction and Friction Testing

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