Page 42 - MetalForming Magazine May 2023

Metal Matters By Daniel J. Schaeffler, Ph.D.
Alloy-Property Variability Belongs
in Your Workforce-Development Plans
When you consider the many variables that impact stamp- ing success, few play as large a role as the properties of the chosen sheet metal. Many metal formers start a job believing that if the numbers on the material-certification sheet meet the specifications, then any future stamping problems will never trace back to the sheet metal. Reality takes over during production, however, leav- ing them to wonder what to do when they get so-called “bad metal.” Mini- mizing these challenges requires a little forethought as well as having workers knowledgeable about what to look for when examining the material.
Preproduction Homework
Many companies include metal forming simulation as an integral part of their workflow before die build begins. Value-added jobs likely will be more complex and use advanced mate- rials—exactly the parts benefitting the most from simulation.
Key to the simulation matching real- ity are the inputs. Each simulation uses three mathematically defined repre- sentations of sheet metal flow: the yielding criteria, the hardening model
Dr. Danny Schaeffler, with 30 years of materi- als and applications experience, is president of Engineering Quality Solutions (EQS) and chief content officer of 4M Partners. 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 is the metallurgy and forming technical editor of the AHSS Application Guidelines available from Worl- dAutoSteel at AHSSinsights.org.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
and the failure conditions. Simple mod- els may be adequate for mild steels, but advanced models may help as shops move to tackling increasingly challenging jobs. See the article by Cut- ting Edge columnist Dr. Eren Billur published in the June 2021 edition of MetalForming for more information about the simulation inputs.
Simulations are accurate, but only to the extent that the inputs represent reality as opposed to an oversimplified estimate of reality. The value of running advanced simulations decreases when using the same inputs and methods for advanced materials as those used for mild steel. Ensure that your simulation technologist uses a proper materials characterization; this also holds true when outsourcing this service. The technologist, either at the metal former or at the contracted simulation firm, may use inputs that are easy to obtain rather than the correct ones. Producing reports isn’t an issue, but these might indicate a misleading level of safety. Remember that the colors displayed by the simulation report only indicate the various thresholds set in the software, and “green” does not necessarily indi- cate that everything is good. When run- ning a die into production, a metal for- mer might wonder why it cannot run good parts; simulations based on faulty inputs may be the culprit.
Homeline Tool Commissioning
Ideally, metal formers incorporate any modifications learned during sim- ulation into the CAD file used for tool construction. Encountering problems at this stage might arise from avoidable issues. Building tools to the modified CAD file helps, but the hard-tool process often endures additional changes to the blank shape as well as features such as draw beads and radii, which alter metal flow. Feeding back
these changes to the simulation tech- nologists will allow them to confirm that the forming process remains robust for the full range of potential material properties. Also, be sure to alert the die-process group of these modifica- tions, as company silos can prevent the process group from learning that their tools have common rework themes in program after program.
Production Stamping
Repeat this important phrase: “Robust for the full range of potential metal properties.” Simulations often consider only one set of material prop- erties, either obtained from the mate- rial card built into the chosen software program or transcribed from the cert sheet. It’s possible to receive these val- ues, but question whether the values are realistic for the material supplier providing production quantities.
While sheet metal suppliers might target specific material properties, they may struggle to attain those values repeatably. Instead, ensure that the properties have a distribution that falls within the material-grade definition. The specification likely constrains these requirements, but multiple suppliers might provide different property dis- tributions. Therefore, metal formers should strive to develop stamping processes that will accommodate the full range of material properties that could be received, not just what has been dialed in based on restricted inputs during a handful of trials (see the accompanying figure).
And, remember that properties other than those listed on the cert sheets impact stamping success. Despite the importance to formability of n-value and uniform elongation, cert sheets rarely contain this infor- mation—unless the shop orders the material to a specification that explic-
40 MetalForming/May 2023
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