FIGURE 3. These are the sides of the sample tested in HHLH configuration, in contact with insert 1 (left) and insert 2 (right).
Editor’s Note: A similar study involving uncoated sheet metal blanks was described in the article “Measuring friction for stamping of UHSS.” For more details on the researchers’ methodology, please refer to that article. Galvanized Iron Pipe
Forming of ultrahigh-strength steel (UHSS) involves high contact pressures and temperatures in the tool-sheet interface, leading to lubricant breakdown, galling, and tool wear. For this study, researchers analyzed how the coefficient of friction (COF) between physical vapor deposition (PVD)-coated tool steel inserts and zinc-coated UHSS DP1180 sheet lubricated with mill oil is influenced by lubrication thickness, sliding speed, contact pressure, and the die surface roughness.
The coating on the sheet is many times softer than the coating on the die, since the sheet coating’s function is to protect the stamped sheet metal components from corrosion. The die coating is intended to increase hardness of the die surface so it can form higher-strength sheet materials without being damaged.
For this study, researchers employed an average blank holder pressure of 1% to 3% of the ultimate tensile strength. The sheet-die surface interface was designed to prevent the edge of the blank from contacting the die surface. The 1-mm-thick DP1180 sheet metal was sheared into strips 600 mm long and 50.8 mm wide. The dimensions of the samples were selected to fit well into a horizontal hydraulic press (see “Draw bead restraining forces in sheet metal drawing operations”) acting as a draw bead simulator.
The die surface was represented with two square inserts with rounded edges, which had a 42- by 42-mm contact area with the sheet. The edges of the strip were not touching the die inserts. The inserts were fabricated from Caldie tool steel and coated with a PVD coating, which provides a very thin layer of very high-strength and high-hardness material to protect the base tool steel material from damage.
The strips were lubricated with 61AUS mill oil, applied consistently along the 200-mm by 50.8-mm testing area. A strip then was clamped between two 42- by 42-mm flat surfaces using the horizontal hydraulic press mounted on a heavy-duty steel table and clamped to the 100-kN Instron 5982 tensile testing machine (see Figure 1). After clamping, the strip was gripped by the upper hydraulic grip of the Instron tensile frame and pulled 150 mm upwards.
The COF in this test was calculated as a ratio of the pulling vertical force Fzapplied by the Instron machine to twice the clamping force Fx, taking into account that friction is applied on both surfaces of the strip: µ = Fz/2Fx.
Researchers studied the effects of testing speed, average contact pressure, and lubricant layer thickness. Each of these factors was varied at low and high levels. Average contact pressure was 11.3 MPa and 34 MPa, testing speed was 200 mm/min. and 1,000 mm/min., and lubrication was applied as 60 mg/ft.2and 110 mg/ft.2
Initial experiments involved testing two PVD-coated inserts with average roughness of 136 nm and 158 nm, respectively. The experimental COF measured during the test is illustrated in Figure 2 as a function of strip displacement between two clamped inserts. In this group of tests, the speed was high, the contact pressure was low, and the lubrication level was high. The test results had rather significant scatter, which likely was because the amount of coating peeled from the sheet side in contact with insert 2 was larger than from the side in contact with insert 1 (see Figure 3). This showed how much of an influence insert roughness has on friction and potential galling. Later in the testing program, researchers understood that lowering the roughness of the tool can significantly reduce friction. Therefore, the level of roughness in the initial testing was considered high.
The COF varied from test to test, so results were averaged to be more useful in existing commercial codes typically using one COF value. Average COF value for the test configuration was calculated by integrating the COF for each curve by displacement x, dividing the obtained integral by the total displacement of the grip X for the whole process, and then averaging the calculated COFs for the number of samples tested for a given configuration (see Equation).
Because the rougher surface of the coated insert led to significant peeling and COFs higher than reported for bare material, researchers stripped the initial PVD coating from the inserts, polished the substrate surface, and reapplied the coating. They measured the roughness of both inserts again, resulting in 82 nm for insert 1 and 104 nm for insert 2. This step made a significant difference in observed friction results (see Figure 4): With all other parameters at the same level as in Figure 2, the COF decreased to 0.048, the amount of zinc peeling decreased dramatically, and the repeatability of performed tests improved very significantly.
Both sides of one of the tested samples are illustrated in Figure 5. Clearly, the peeling effect is nearly eliminated, at least in the central portion of the sample. Changes in the sheet metal coating near the edges of the contact zone are very significant. Although it is nearly impossible to achieve uniform contact conditions along the whole tested surface, and contact conditions also vary significantly on the surface of production dies, the peeling of the zinc coating indicates high friction and potential galling, which could be avoided by reducing the roughness of the die surface.
The comparison of average COFs within studied parameter ranges indicated that the coated insert’s surface roughness and the contact pressure are the two most significant factors. Both cause more zinc to peel from the sheet surface because the very hard peaks of the coated die insert create a deep indentation into the relatively soft zinc coating. With deeper indentation, more strain occurs in the layer of zinc, and a deeper layer of coating potentially can be peeled off.
The effect of increasing roughness by a factor of 1.58 on COF can be estimated by comparing testing conditions (see Figure 6):
Analysis of the lubrication layer and contact pressure effects was performed only for low insert roughness. The effect of increasing contact pressure by a factor of 3 was quantified based on the following comparisons of results in testing conditions:
Researchers estimated the effect of increasing the lubricant layer from 60 mg/sft. to 110 mg/sft. using the following comparison of testing conditions:
The effect of increasing the testing speed from 200 mm/min. to 1,000 mm/min. was analyzed comparing the following testing conditions:
T. Altan,Sheet Metal Forming Processes and Applications (Materials Park, Ohio: ASM International, 2012).
T. Huth-Fehre, “Method and device for determining the thickness of transparent organic layers,” European Patent EP 1 287 310 81.
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