A Different Look at Rules of Friction

October 1, 2008
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Last month’s column described five types of friction tests that simulated five common forming modes. Over 180 sets of tests were run to evaluate each steel/lubricant combination for six steel surfaces/coatings and six lubricants, with each of the five friction tests. At first glance, the tests simply yielded 180 different coefficients of friction (COF). Closer evaluation of the data, however, reveals two distinct groups of forming modes that responded in opposite direction to the lubricants.The first group of forming modes contains the Strip Pull test (flat binder sliding), Draw Bead Simulator (multiple bend/unbend) and Die Radius Friction test (single bend/unbend). These forming modes duplicate steady-state conditions. An element of sheetmetal with its lubricant enters the deformation zone, performs its function and then exits the deformation zone. A new element with new lubricant follows the “spent” element. In addition to als having a fresh supply of material and lubricant, the deformation rate is rapid.

The second group of forming modes contains the Punch Radius test (flat-bottom-punch radius) and Hemispherical Rotation test (punch stretching). In contrast to the first group, the initial material and lubricant in the deformation zone remain in contact

Draw bead simulator tests
Fig. 1—Three consecutive Draw Bead Simulator tests in Fig. 1A show nonconstant pulling force for sheetmetal with no lubricant. In Fig. 1B, a small amount of poor rust-preventive liquid eliminates the pull-load variability.

with the die and fresh material or lubricant is unavailable. In addition, the rate of deformation is much slower relative to the first group. The data also show that an excellent lubricant for group one is not the best for group two and vice-versa. Attempting to correct lubrication problems in one group of forming modes may result in transferring the problem to the other group of forming modes.

Other lubrication studies have provided useful press-shop data. One misconception is that not adding a lubricant at the press shop is running the part dry. If the part were truly dry, the steel would be colored red. The rust-preventive solution added as the final step at the metal processor is a lubricant—though most likely one with poor friction conditions. Application of better-quality lubricants is possible depending on supplier capabilities.

A completely dry (no solutions or lubricant added) electrogalvanized coated steel was tested using the Draw Bead Simulator. Is this a good reproducible condition for forming? Studies have shown that forming without any lubricant is highly variable. Fig. 1A contains pull loads recorded for three consecutive dry electrogalvanized steel strips. Note the wide fluctuations in pull load. In contrast, Fig. 1B shows how the pull loads become very consistent (more robust) when the same electrogalvanized steel is tested with a thin coating of very poor lubricant.

weld-then-break sequence
Fig. 2—The first Draw Bead Simulator test strip of electrogalvanized steel coated with a good lubricant produced galling. The above weld-then-break sequence of the coated sheetmetal asperities created a sound wave that sounded like the screech of chalk pulled across a blackboard.

Note the initial high starting load peak of the first test conducted in both Figs.1A and B. This is not a static to dynamic change in friction because it does not appear in the subsequent two samples. Before each test, the draw beads are cleaned and polished. The initial load peak occurs when zinc from the steel coating transfers from the test strip and coats the draw beads. This suggests that before evaluating new or repolished dies for forming coated sheetmetal, run a number of blanks to precoat or season the die for stabilization.Some coated steel can become a problem even when formed with a good lubricant. The draw bead simulator has a standard bead-to-bead gap of material thickness plus 0.003-in. Fig. 2 shows the Draw Bead Simulator test of a different coil of electrogalvanized steel and a good lubricant. The graph shows the sound wave of a screech (chalk pulled across a blackboard) as the strip pulls through the draw bead. This is the onset of galling, where the asperities of the electrogalvanized coating weld to the draw bead and then break off. If continued, a plow of zinc will build up on the bead and create scoring in the sheet.

Lubrication studies by Dr. Harmon Nine, formerly of GM Research, show that one of the two surfaces in the fiction couple (die or sheetmetal) has to be rough and the other smooth. If both are rough, the asperities of each surface interlock, causing a high friction force, welding and galling. If both surfaces are very smooth, no lubricant remains in the work zone under high pressure and heat. One smooth surface (usually the die) allows the asperities of the other surface (workpiece) to slide with minimum resistance. The rough surface brings lubricant into the deformation zone. This suggests that forming a highly polished sheet requires a roughened die surface or a dry barrier lubricant that maintains complete separation between the die and the sheet.

Mr. Norbert Izworski, formerly of Ford Motor Co., conducted a very interesting study with two lubricants—one with a high COF and one with a low COF. With the two lubricants placed one on top of each other, which lubricant controls the forming operation? The results showed the lubricant placed on the sheet first (in the valleys) controls the process. The die pushes the second or top lubricant out of the deformation zone. Under further pressure, the asperities of the sheet begin to deform and decrease in height. This forces the first lubricant in the valleys between the peaks to form a lubricant layer over the deformation zone and control friction during deformation.

The above studies provide a logical, but not surprising, look at how lubricants function. Yet, lubricants can make or break many a stamping. Even worse, successful achievement of the dimensional consistency requirements imposed today on stampings depends on the correct choice and use of lubricants. Unfortunately, lubricants all too often are a commodity chosen only by price. MF

Industry-Related Terms: Surface, Thickness, Die, Draw, Form, Forming, Layer, Run, Strips
View Glossary of Metalforming Terms

Technologies: Quality Control

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