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Modeling savvy helps propel 3GAHSS into the passing lane

Aug 03, 2017 | 08:00 PM | Gregory DL Morris


Facing an impasse, some innovators retreat to the drawing board. Others, in pursuit of answers and insight, push back further and dive into the mysteries to be unraveled at the molecular level.

For mounting and successfully concluding a demonstration project to show how third-generation advanced high-strength steels (3GAHSS) can be used in a vehicle subassembly—specifically a side body structure—the United States Automotive Materials Partnership LLC (USAMP) has earned AMM’s 2017 Steel Excellence Award for Best Innovation-Process.

USAMP is a part of the U.S. Council of Automotive Research LLC, the collaborative automotive technology development company of FCA US LLC, Ford Motor Co. and General Motors Co. Since the research USAMP conducts is pre-competitive, commercial application may be years away. Even so, the collaborative work performed among automakers, suppliers, academia, and national laboratories has made major strides advancing 3GAHSS.

“Under the Department of Energy (DOE) funding, we only had to demonstrate applicability for a subassembly with four components,” Eric McCarty, technical project manager for USAMP, noted. “But we chose a side body structure with 46 parts.

“Vehicle side body structures have demanding performance requirements and are tested for side impact, pole intrusion, rear impact, and roof crush,” McCarty went on to explain. “They are also stiffness limited and tested for bending and torsional stiffness.

“The design and performance criteria make the side body structure ideal to demonstrate the value of 3GAHSS,” the USAMP project manager added. “The increased strength enables lighter weight through gauge reduction and the increased ductility enables component shapes not feasible in AHSS.”

The milestone project substituted two 3GAHSS materials for conventional AHSS in the side body structure and was able to achieve a performance-neutral design and 29-percent mass savings with material gauges below 1-mm.

USAMP applied an integrated computational materials engineering approach, which McCarty described as “a reverse way of doing design. Rather than finding a material to meet the performance, we were able to link materials models to tailor a material for the application,” he explained.

Two 3GAHSS alloys were developed with properties approaching the DOE targets for a high-strength, exceptional-ductility steel (1,200 MPa ultimate tensile strength, and 30-percent total elongation) and an exceptional-strength, high-ductility steel (1,500 MPa ultimate tensile strength, and 25-percent total elongation).

The production of the two 3GAHSS was expanded from small laboratory heats of approximately one pound used to generate coupons, to larger heats of about 450 pounds from which small-scale, T-shaped components were formed. These emulate part of an automotive B-pillar, a critical anti-intrusion component in the side body structure.

The researchers also developed an experimental method to measure the non-homogenous in-situ transformation of meta-stable retained austenite to martensite as a function of strain. They used stereo digital image correlation (DIC), an optical strain field measurement technique, coupled with high-energy, synchrotron x-ray diffraction at Argonne National Laboratory.

A state-of-the-art data model was developed for curating experimental data with associated metadata using the National Institute of Standards Technology’s DSpace data repository.

“The conventional way of measuring deformation is to take static samples at different strains and deform them, test them, deform them again, test them again, and so forth,” McCarty explained. “Our new method is dynamic, enabling real-time assessment of the deformation and transformation behavior of 3GAHSS as a function of strain. This is not the first time that x-ray diffraction was used,” he acknowledged. “But it is the first time that it was combined with DIC.”

The experimental technique enabled the project participants to model the very complex strain hardening and phase transformation behavior in 3GAHSS. “Additionally, we observed non-homogenous phase transformation with increasing strain, a phenomenon that would not have been captured with traditional test methodologies,” McCarty noted.

“The next steps would be to look into joining,” Larry Sak, head of materials, fasteners, and engineering standards at FCA US, and chair of the USAMP Steering Committee, said.

“Now that this project has been completed, we continue to support the research,” he added. “We are now rolling into a new project under USAMP to work on those other issues, such as joining.”

 “Steelmakers were involved in the research, which should help speed the development and implementation of 3GAHSS,” McCarty said, noting that 3GAHSS grades are now emerging in the marketplace.

 “The question now is very much one of scale.” Sak reiterated. “All the participants share in the knowledge. It will be up to our industry partners to go from lab equipment to the steel mill.

“There is no specific time frame on commercialization.” he noted. “Not only is that beyond the scope of this project, but as with any other material, there is a validation period that can take three to five years. The auto makers conduct tests for impact and durability, as well as fit and finish,” Sak said.



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