Evaluating dry film lubricants for automotive applications Part II

This column was prepared by Hyunok Kim, a staff member of the Engineering Research Center for Net Shape Manufacturing (ERC/NSM), The Ohio State University, Professor Taylan Altan, director.

Stamping lubricants tend to fail if contact pressure and temperature at the interface are high. For example, depending on the stroke rate during progressive-die forming, dies may reach temperatures of 200 degrees F to 250 degrees F.

The ironing test, developed at the ERC/NSM, reproduces production conditions of contact pressure up to 94 kilopounds per square inch (KSI) and temperatures up to 300 degrees F to evaluate lubricant performance quantitatively. During the ironing test, lubricant performance can be evaluated at various temperatures on different sheet materials. In this test, the temperature and pressure at the tool-workpiece interface can be emulated by heating the ironing die and selecting a desired ironing ratio:

Where: t₀, t_f= Initial and final thickness

On the left side of Figure 1, a tool setup is shown in a hydraulic press with a CNC hydraulic cushion. The punch is stationary, and the ironing ring moves downward with the upper ram. The ironing ring's derails are shown on the right side. The ring is heated with a band heater that is insulated from a drawing die.

Figure 1 CThis tooling was modified from existing deep-draw tooling to incorporate an ironing ring. To obtain the desired temperature in the ironing die, a band heater is used. A mica sheet is also provided to reduce the heat loss from the ironing die to the die holder. Source: H. Kim, S. Chandrasekharan, M. Shirgaokar, V. Tangri, and T. Altan, "Performance Evaluation of Dry Film Lubricants at Various Temperatures Using Ironing Test," ERC/NSM Report No. 05-R-06, 2005.

Testing lubricants in real-world production conditions is difficult and expensive. Lubricants should be tested in production only after they have been proven effective in laboratory tests that emulate production conditions.

Recently the ERC/NSM conducted ironing tests to evaluate dry film lubricants with AISI 1018 sheet materials (initial sheet thickness, t0 = 0.083 inch) at room temperature and tool temperature (68 degrees F and 168 degrees F, respectively). Detailed specifications of tested lubricants are given in Figure 2. A phosphate coating was used for comparison and benchmarking purposes.

Figure 2 Tested Lubricants Specifications

Evaluation Criteria

Qualitative and quantitative analyses to determine lubricant effectiveness were tested on the following evaluation criteria:

• Maximum ironing load (the lower the load, the better the lubricant)
• Ironed cup surface topography after testing (the lower the surface roughness, the better the lubricant)
• Cup wall thinning (the lower the wall thinning, the better the lubricant)
• Lubricant buildup on die (good lubricants have minimum coating loss through die contact)

Test Results, Conclusions

Maximum ironing loads were compared for different lubricants and testing temperatures (see Figure 3a and Figure 3b). Lubricant C performed best regardless of temperature. However, as the testing temperature increased, lubricant B reacted similarly to lubricant C. Lubricant A displayed high friction at both testing temperatures.

Figure 3 The load-stroke curves were obtained from experiments at both room temperature (RT) and elevated temperatures (ET). Three tests were repeated to obtain an average value for each lubricant. The ET of 168 degrees F was selected to emulate the tool temperatures measured in real production conditions.Source: H. Kim, S. Chandrasekharan, M. Shirgaokar, V. Tangri, and T. Altan, "Performance Evaluation of Dry Film Lubricants at Various Temperatures Using Ironing Test," ERC/NSM Report No. 05-R-06, 2005.

Die lubricant buildup was indirectly quantified by measuring the specimen's initial and final weights. In Figure 4 lubricant performance ranks are summarized in terms of evaluation criteria.

Figure 4 Lubricant Performance Ranks

It should be noted that specimens coated with lubricant C were not ranked in surface analyses, because phosphate coating is difficult to remove with chemical agents to measure surface roughness. In comparison to the phosphate coating (lubricant C), lubricant B displayed superior performance to lubricant A with the exception of buildup. This study illustrates that lubricant weight directly affects die buildup characteristics.

By comparing laboratory test results with industrial results, the ERC ironing test was validated as a reliable method for evaluating lubricant performance under productionlike conditions.

Taylan Altan is a professor and director of the Engineering Research Center for Net Shape Manufacturing, 339 Baker Systems, 1971 Neil Ave., Columbus, OH 43210, 614-292-9267, fax 614-292-7219, www.ercnsm.org. The ERC/NSM conducts research and development; educates students; and organizes workshops, tutorials, and conferences for the industry in stamping, tube hydroforming, forging, and machining.

Reference
H. Kim, S. Chandrasekharan, M. Shirgaokar, V. Tangri, and T. Altan, "Performance Evaluation of Dry Film Lubricants at Various Temperatures Using Ironing Test," ERC/NSM Report No. 05-R-06, 2005.