Page 50 - MetalForming-Dec-2018-issue

The Science of Forming By Daniel J. Schaeffler, Ph.D.
Sims Aren’t Everything
Simulation has made inroads into the metalforming community as an ideal way to predict what will occur when sheetmetal of known prop- erties is subjected to known deformation forces. It saves days of trial-and-error, helping to evaluate what-if scenarios associated with variables such as metal grade and thickness, binder and adden- dum designs, and blank shape.
It is, however, risky to blindly accept simulation output as infallible. Simu- lations are accurate, but only to the extent that the inputs represent reality as opposed to a simplified estimate of reality. Simplification begins with the imported CAD file. No doubt that the design in the file was accurate at one time, but most likely that dates back prior to tool construction. Once fabri- cated, tooling likely undergoes some period of adjustments to achieve a good part, such as, for example, using spotting blue to check a bearing. By grinding anywhere on the tool and especially on a draw bead, you’ve made a permanent change to the metal flow in that area. Best practice: Re-scan the
Danny Schaeffler, with 30 years of materials and applications experi- ence, is co-founder of 4M Partners, LLC and founder and president of Engineering Quality Solu- tions (EQS). EQS provides product-applications assistance to materials and manufacturing com-
panies; 4M teaches fundamentals and practical details of material properties, forming technolo- gies, processes and troubleshooting needed to form high-quality components. Schaeffler, who also spent 10 years at LTV Steel Co., received his Bach- elor of Science degree in Materials Science and Engineering from the Johns Hopkins University in Baltimore, MD, and Master of Science and Doctor of Philosophy degrees in Materials Engineering from Drexel University in Philadelphia, PA.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
tools once they produce good parts. This provides a record of the tool sur- face responsible for today’s parts.
Recent years have seen more atten- tion paid to the weight of forming tools, with the focus on making them lighter. This has led to structural analyses of the tooling to ensure mass only where it contributes to functionality. The expectation is that reduced mass does not impact tooling stiffness, and what remains is the minimum functional structure. This may be an acceptable strategy should the tooling surface be retained during the buyoff process. We know that issues with springback increase with stronger sheetmetals. In the hands-on process of minimizing springback during physical tryout, recuts become inevitable. Since each iteration removes tooling metal, tooling stiffness can be affected. In most sim- ulations, assumptions include rigidity of the tool, ram and bolster, with no deflection under load of any compo- nent. But rarely does this accurately represent reality.
The Modulus of Elasticity (E), an important input into forming simula- tions, is critical to obtaining an accu- rate assessment of springback. Text- book values for E include 210 GPa and 70 GPa for steel and aluminum, respec- tively. However, these values change based on specific grades or alloys. These textbook values typically are determined in tension. However, regarding springback, the modulus from unloading should be considered. According to some studies, the unload- ing modulus may be 25 percent less than the modulus determined during loading. Furthermore, the modulus changes based on the level of plastic strain seen in each region of a part.
Density represents another param- eter where, previously, textbook values may have been sufficient. Steel density
does not vary much between mild-steel grades, but higher-strength steels use non-iron elements in greater amounts, which changes the alloy density. Alu- minum alloys in the 5XXX or 6XXX series achieve their properties with different alloying approaches, with the resultant density a function of the type and con- centration of the chosen elements. The Aluminum Association Teal Sheets lists the density of different aluminum alloys.
Many simulations do not consider the effect of temperature when the assumption is made that room-tem- perature stampings remain at room temperature during the press cycle. When forming higher-strength alloys even under ideal conditions, the tem- perature at contact radii substantially exceeds room temperature. Work from Professor Altan’s R&D group at Ohio State University has shown that, in identical setups resulting in a maxi- mum temperature of 120 F when form- ing mild steel, temperatures of 175 F were reached when forming DP600 and AA5182-O, and DP980 reached 210 F with DP1180 heating to 300 F, again under identical thickness and forming conditions. At these elevated temper- atures, die sections will expand, which changes clearances and metal flow.
Although some lubricants are designed for higher temperatures, you may be choosing the wrong one if you think that you are performing only room-temperature stamping. Elevated temperatures seen at contact points can lead to lubricant burn-off, resulting in no lubricant at the most critical loca- tions. Among other things, this accel- erates wear, a circumstance not accounted for in the simulation.
Most simulation programs offer one input for a friction value, which we know varies across the entire part and is based on local conditions. Friction changes with contact pressure and
48 MetalForming/December 2018
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