Page 50 - Metallforming Magazine December 2020

The Science of Forming By Daniel J. Schaeffler, Ph.D.
Loose-Change Management Can Be Costly
TThe art-to-part development cycle for stamped automotive parts can stretch into several years. Along the way, there can be many changes to blank thickness, part shape and dimensions, and sometimes even the sheet metal grade. At every step of the process, it is critical to doc- ument a complete and detailed record of these changes, especially when the parts will be crash-tested.
Here's the story of one such impor- tant safety component—the windshield header of a convertible.
The Background
Like most convertibles, this vehicle had a relatively low forecasted sales tar- get. The OEM had not selected a pro- duction-intent source to supply steel during tool development, and instead chose to purchase steel from service centers. The windshield header usually is specified as a high-strength steel grade.
Offshore soft-tool and hard-tool development reportedly went well, encountering the usual bumps resolved with minor tooling modifications. For soft- and hard-tool tryout, the con- tracted Asian tool shops used steel shipped to them from U.S. service cen- ters. Some parts produced off of hard
Dr. Danny Schaeffler, with 30 years of materials and applications experi- ence, is president of Engineering Quality Solu- tions (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 next release of the AHSS Applications Guidelines available from WorldAutoSteel.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
tools were assembled into a vehicle submitted for crash testing.
The Problem
After a smooth hard-tool buyoff, the dies were shipped from Asia to the U.S. production-stamping facility, which was unable to form a single part without experiencing splits. A search through the paperwork revealed that, for at least one steel shipment, the service center sent mild steel to the die shop rather than the specified high-strength grade.
At this point the production team suspected that during hard-tool buyoff the die shop had evaluated a part stamped from mild steel, which is why the production stamper could not form good parts using the specified high- strength steel. Unfortunately, the fix was not as simple as changing the spec- ification from high-strength to mild steel. On a safety-critical component, such a change could involve additional component and full-vehicle crash tests.
The Hope
No one knew exactly when the inad- vertent mild-steel shipment was used during die tryout, so it was possible that the successful crash-test vehicle was manufactured using a mild-steel wind- shield header. If that happened, then the body structure obviously was robust enough with mild steel in that critical component. Therefore, only a simple paperwork change was needed to switch the production-intent grade to mild steel. However, if the crash vehicle con- tained a high-strength steel header, the stamping facility would need to figure out a plan to form these parts.
The Plan
All the while, the clock continued ticking. The vehicle manufacturer needed stampings immediately for ini- tial-vehicle assembly, and determining the steel grade in the header became
critically important. No available paperwork or documentation con- tained the necessary information. One thought: Cut a relatively flat area from the windshield header and submit it for tensile testing. However, no area was large enough to obtain an appro- priate coupon.
The next idea: Check the material composition, using a test that required only a 1-in.-square area of material. The manufacturer rejected this idea, requiring the vehicle to remain com- pletely intact; cutting out even a small area of the header was not an option.
Ultimately, the team opted to use a hand-held x-ray-fluorescence (XRF) analyzer to determine the steel grade used on the windshield header of the crash-tested vehicle, a test that left the vehicle structure whole.
The Result
From the XRF test the team found relatively high values of carbon and manganese in the header, indicating it had been stamped from high- strength steel. Although not the result anyone had hoped for, only two days had passed since discovering the initial problem. The rapid and definitive analysis allowed for a quick decision on how to move forward.
Considering where the communi- cation breakdown occurred, financial responsibility was determined and accepted with only minor grumbling. The paperwork trail broke down at the tool-build source, and it needed to absorb the additional cost of recutting the tools with a slight product conces- sion. On top of that, the shop’s repu- tation took a hit with its customer— the vehicle manufacturer, which suffered a delayed production launch while waiting for the rework.
A more robust change-control process with a complete paper trail could have helped avoid these issues. MF
48 MetalForming/December 2020
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