Lightweight Vehicle Body Structures from MSC Smart Steel

By on February 4, 2019 in MATERIAL MATTERS

One of the driving principals of automotive engineering today is improving fuel efficiency, which thereby reduces carbon emissions. Many strategies have been implemented concurrently by the automotive OEMs, such as improved aerodynamics and adopting alternative powertrains, but the most widely implemented practice involves reducing vehicle mass.

More than ever, innovative designs and lightweight materials are playing a significant role in engineering teams designing competitive vehicles that do not compromise performance. While offering various degrees of mass savings compared with traditional materials, rarely do these innovations integrate into long-standing manufacturing and design practices.

There are often headwinds associated with implementing new technologies. Examples of headwinds include complex manufacturing and assembly processes, additional equipment, new fastening schemes or unproven computer-aided engineering (CAE) modeling techniques.

MSC Smart Steel® is a new multilayer steel laminate from Material Sciences Corporation (MSC) engineered as a direct substitute for vehicle body parts stamped from low-carbon steel. Typically, these applications are nonstructural close-out panels.

While offering up to a 35-percent mass savings compared with same thickness monolithic steel, MSC Smart Steel is produced as a coil, stamped in typical dies, spot welded with existing equipment and processed through standard electrocoat and paint systems—meaning minimal disruption to existing manufacturing systems. MSC Smart Steel has also successfully been chrome plated for class A components such as bumpers and trim. This is the first-ever spot weldable, low-density composite laminate to be used in an auto-body application.

This is the first-ever spot weldable, low-density composite laminate to be used in an auto-body application.

An Industry First
Following a five-year collaborative effort between Material Sciences Corporation and a strategic customer, MSC Smart Steel is now validated for vehicle implementation and is going into production on several 2019-2020 global automotive platforms. The concept involves a three-layer laminate with steel outer skins. The middle layer consists of a low-density conductive polymer core, which allows MSC Smart Steel to be stamped and spot welded—an industry first.

In the event that added strength or dent resistance is required, MSC Smart Steel can be made with an asymmetrical steel structure. For example, a typical medium-duty pickup truck bumper that is 1.6-mm steel can be produced with a 0.7-mm outer front metal skin for strength and dent resistance while the back-side metal skin is a thinner 0.3 mm. Material Sciences Corporation can also use different grades of metal, such as aluminum or steel, as the outer skins.

The total laminate results in an overall density reduction of approximately 35 percent compared with same thickness monolithic steel. The polymer core is about 15 to 20 times thicker than the core found in typical acoustic laminates, such as Quiet Steel, and is structural in nature as MSC Smart Steel exhibits high stiffness to mass.

Stiffness/Mass Savings
By adjusting the metal-to-core thickness ratio, the Smart Steel can be “tuned” to the stiffness/mass savings requirement of each application. MSC developed algorithms that predict stiffness for various core/steel ratios. For example, instead of engineering MSC Smart Steel to provide a mass reduction at equal stiffness, the laminate can be designed to match the mass of steel and offer improved stiffness.

One example is an automotive cowl plenum. A linear static analysis is performed using Optistruct Hypermesh on a cowl plenum stamped from 0.8- mm monolithic cold rolled lowcarbon steel. The same analysis was performed on cowl plenum stamped from 0.84-mm MSC Smart Steel. The MSC steel construction consists of 0.22-mm outer steel skins and a 0.42-mm low-density core. The result is a 30-percent mass reduction in the part while maintaining stiffness. (See Figures 2 and 3.)

In some cases, extra panel stiffness is beneficial for performance, but the added mass associated with increasing thickness to achieve this goal is unacceptable. By adjusting the metal-tocore ratio of the MSC Smart Steel, the panel stiffness can be increased while matching the mass of the original 0.8-mm monolithic steel panel. In the case of this application, implementing MSC Smart Steel containing outer metal skin thicknesses of 0.36 mm with a polymer core of 0.28 mm can improve stiffness by a factor of 1.9 while maintaining the mass of the original monolithic steel.

NVH Benefits
While MSC Smart Steel was designed to be a lightweighting innovation, the product does offer inherent damping and noise, vibration and harshness (NVH) benefits when compared with monolithic steel. The natural frequencies shift higher for a component stamped from MSC Smart Steel than monolithic steel.

An automotive rear parcel shelf was stamped from monolithic steel and MSC Smart Steel at the same thickness (0.7 mm). The panels were evaluated for radiated noise and stiffness using a modal-impact test in the Material Sciences Corporation hemi-anechoic chamber. (See Figure 4.)

While MSC Smart Steel was designed to be a lightweighting innovation, the product does offer inherent damping and noise, vibration and harshness (NVH) benefits.

The boundary conditions for this test were similar to how the panel is mounted in the vehicle. The panel was impacted at 10 locations, and data was collected through two accelerometers, as well as a microphone one meter from the panel. The data suggests that MSC Smart Steel can offer NVH advantages in panels that may be energized by wide frequency bandwidth inputs such as a subwoofer mounted to the rear parcel shelf. (See Figure 5.)

The green response curve highlights the reduction in radiated noise from the panel stamped from MSC Smart Steel. This is a result of the constrained layer-damping mechanism created by placing the engineered polymer core between the two metal skins. Through added stiffness and damping, the sound quality, when the subwoofer is in operation, will be improved as a result of the damped mounting surface. This may also eliminate NVH countermeasures and stiffening brackets resulting in additional mass and cost savings.

Little Manufacturing Disruption
MSC intended the integration of MSC Smart Steel would involve minimal disruption in the manufacturing process. For example, it is well documented that switching from a steel auto body to aluminum will involve fairly significant modifications to the manufacturing process.

Updated body shops (new automation, replacing spot welding with mechanical fastening methods), additional pretreatments and coatings, material handling and aluminum shelf-life concerns are just a few of the issues that have to be addressed when switching to aluminum. This equates to additional cost, complexity and development time for each platform. MSC Smart Steel saves the mass, but all the equipment and processes in place with steel can stay.

Extensive spot-welding evaluations have been completed for MSC Smart Steel in various electrogalvanized (EG) zinc-coated and uncoated 2T and 3T weld joints. (See Figure 6.)

All these tests yielded acceptable and consistent weld nuggets and weld lap shear tensile results. During shear testing, the load is carried through both skins of the MSC Smart Steel structure, and the parent metal fails at completion of the test. Moreover, standard weld controllers and transformers (AC and DC) can be used to weld MSC Smart Steel within a minimum 2000-amp process window.

Smart Steel Welding
Adaptive welding technology has also proved very successful when welding MSC Smart Steel. This is possible as a result of the highly engineered conductive core between the two metal skins. Figure 7 and 8 show micrographs of die penetrant and a nugget cross section for a 2T spot weld joint. Recast structure between the two skins of MSC Smart Steel and the base metal is visible. During coach peel destructive testing, consistent buttons are pulled that meet automotive OEM and American Welding Society (AWS) diameter requirements.

As the outer metal skins of the MSC Smart Steel are cold rolled steel, the structure is designed to be formable in standard stamping dies. The polymercore applications that require thinner substrate may be less formable, but most target applications are within the formability range of MSC Smart Steel. Two examples of stamped automotive components are shown in Figure 9 and the image at the beginning of this article.

These highlight formability, as well as the ability to successfully trim through all three layers of the laminate with standard tooling. The material is produced and provided to customers as a master coil, similar to standard steel applications. MSC Smart Steel can be processed through many die setups, including transfer and progressive. Pick-and-place material handling robots, coil feeders and oilers are all compatible with MSC Smart Steel.

Engineers at Material Sciences Corporation’s Application Research Center in Canton, Mich., developed algorithms to determine and optimize the ideal metal-to-core ratio for each application. Once established, custom data property cards and techniques are shared with customers who can use their own finite element analysis (FEA) predictive tools to predict performance of MSC Smart Steel in their specific system-level durability, crash and NVH models.

As the outer metal skins of the MSC Smart Steel are cold rolled steel, the structure is designed to be formable in standard stamping dies.

For more information on MSC Smart Steel or other engineered lightweight and acoustic laminates, please contact Material Sciences Corporation at smartinfo@materialsciencescorp.com.

Authored by Matt Murphy

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