Emission Reduction Possibilities With Structural Castings
Magna International and Ford Motor Co.’s multi material lightweight vehicle embodies the full possibilities of weight reduction using lighter materials and the metalcasting process.
Shannon Wetzel, Managing Editor
(Click here to see the story as it appears in the September issue of Modern Casting.)
Like many research and development projects, the Multi Material Lightweight Vehicle (MMLV) Program conducted by Magna International and Ford Motor Company, with funding from the U.S. Department of Energy (DOE), started with a “What if?” question: What if, in order reduce vehicle weight, we did a comprehensive redesign of the complete vehicle instead of focusing only on a single component or vehicle subsystem?
It turns out, a 23.5% weight reduction in a 2013 Ford Fusion can be achieved, leading to the ability to downsize the engine and improve fuel economy, leading to a 16% reduction in Global Warming Potential and 16% reduction in Total Primary Energy.
“Until you have the opportunity to design a vehicle from the ground up, you don’t really know what’s possible,” said Tim Skszek, senior manager, government partnerships and principle investigator, MMLV project, Magna International.
Magna International is an automotive supplier with 319 manufacturing facilities, including five aluminum vacuum diecasting plants. Magna has worked with its customers on achieving weight savings for specific parts and vehicle subsystems, but the supplier was curious how far weight could be reduced if given the chance to apply weight-saving principles across the entire vehicle. Based on this curiosity, it applied to lead a U.S. DOE-sponsored research program to lightweight a current model production vehicle and invited Ford to be a 50/50 partner.
For the program, which started in September 2011, Ford’s Fusion was chosen as the baseline vehicle. The Fusion is a top-selling mid-size car, and, in a change from other similar studies, Ford provided the weights and designs of its future 2013 release. Other weight-reduction studies have been conducted on cars that are one or two generations old because the OEM is uncomfortable releasing the current or advanced vehicle engineering details. In this case, Magna and Ford started with an advanced design associated with a 2013 model year vehicle, comprised of the most current materials and manufacturing processing available.
The partners established a goal of reducing the vehicle mass to that of the Ford Fiesta, while maintaining occupant safety and performance characteristics of the baseline Fusion vehicle. This would enable the use of the smaller three cylinder 1.0 liter EcoBoost engine used in the Fiesta. Reducing the engine size would further reduce weight and also equate to a reduction in fuel consumption. In total, the MMLV weighed 800 lbs. (364kg) less than the baseline 2013 Ford Fusion.
“The message was, if you go for a weight reduction, you have to go for sufficient quantity to enable use of a downsized engine,” Skszek said. “Forty percent of the fuel reduction and environmental benefits of the program are due to primary mass reduction, but 60% is associated with the ability to downsize the engine while maintaining the vehicle performance.”
To meet the weight reduction goal, Magna and Ford designed a bimetallic body-in-white (BIW) that was 65% aluminum and 35% steel. The MMLV BIW, Closure panels and Chassis subsystems are comprised of aluminum extrusions, die castings and stampings along with stamped steel fabrications. The baseline Fusion BIW was 100% steel.
In the MMLV, steel was placed in areas where requirements for energy absorption for side impact, roof crushing and frontal impact protection were high. Aluminum was placed around the steel—such as in the major panels, which were not structural. For areas that were structural, Magna utilized aluminum high pressure vacuum die castings
“Aluminum castings were integral to the design, and they were strategically placed for both stiffness and strength requirements,” said Jeff Conklin, engineering department manager, Cosma International, the metalforming division of Magna. “If we had used other processes, we wouldn’t have the stiffness and the weight reduction wouldn’t have been as significant.”
One of the objectives of the MMLV was that it would meet the same five-star occupant safety ratings as the consumer version of the Ford Fusion. The actual MMLV concept car was physically put through a limited number of front impact safety tests from the U.S. New Car Assessment Program (NCAP) and the Insurance Institute for Highway Safety (IIHS) and passed them without trouble .
The MMLV lightweight body features eight aluminum castings that were produced via high pressure vacuum diecasting. The use of high pressure vacuum diecasting is unique for high volume automotive production, but the process allowed engineers to design castings for structural areas of the vehicle.
“When the castings are used as a structural part, they must carry the load during a crash event,” Conklin said. “High pressure vacuum diecasting gives three times the ductility of conventional diecasting, which opens up a new market.”
Making the parts in a vacuum gives producers the ability to heat treat without causing blistering or trapped gas. These castings achieve an average of 15% elongation versus 3% elongation in conventional diecast aluminum parts.
Traditionally, high pressure vacuum diecasting has been limited to niche vehicles, but its capability to produce parts meeting crush-zone requirements has drawn interest from car manufacturers, particularly after the MMLV program resulted in a vehicle that reduced weight by nearly 25% while still passing critical frontal safety tests.
OEM interest in structural die castings for high volume vehicle segments has led to increased vacuum diecasting capacity in the marketplace. In August, Magna announced plans to build a new aluminum casting facility in Birmingham, Ala., at their KAMTEK facility. KAMTEK will produce lightweight structural parts based on customer demand from North American and foreign domestic OEMs.
“We are seeing more and more requests from customers to use this technology for high volume vehicle platforms,” Skszek said.
The die castings included in the MMLV concept car are left and right front shock towers, left and right hinge pillars, left and right kick down rails and left and right rear mid-rail castings.
According to Magna, the shock tower is the most common high pressure vacuum diecast body application. It combines a number of steel parts into a single aluminum component that weights 40% less. The cast aluminum shock tower reduced the weight from 7.5 lbs. for the baseline to 4.6 lbs.
The kick down rail, which is located below the hinge pillar, had the highest performance requirement of the castings. As a chassis reinforcement in the front of the vehicle, it had to show that in a crash, it would not exceed the intrusion specification into the passenger compartment foot well area. The high pressure vacuum diecasting process allows the casting to be heat treated for stiffness, leading to increased torque load capacity and torsional rigidity to the MMLV body structure. FEA analysis showed the cast kick down rail resulted in better intrusion characteristics in the vehicle compared to the baseline design.
The cast aluminum kick down rail combines five steel stampings lowered the weight from 13 lbs. to 10 lbs.
The hinge pillar casting integrates five steel stampings and is about 35% lighter than the baseline, reducing weight from 9.8 lbs. to 7.4 lbs. Finally, the mid-rail casting design integrates 12 rear shock tower and rail components into a single casting for a weight reduction from 12.5 lbs. to 9.2 lbs.
One of the biggest challenges in designing a multi-material structure is determining the joining methods between parts made in different metals. Contact between bare aluminum and steel can lead to galvanic corrosion, so mitigation strategies needed to be devised. Two aluminum-steel joining methods were used in the construction of the MMLV. In the traditional method used, an adhesive/sealant electrically isolates the materials (typically galvanized steel panels and hard-coat anodized aluminum casting surfaces) prior to being joined using self-piercing rivets. The protruding end of the rivet also is sealed. Afterward, the assembly is submerged in a phosphate bath and e-coated. In the alternative method, the steel components are e-coated and the aluminum castings are hard coat anodized before the adhesive is applied and the parts are connected with self-piercing rivets. In this case, a phosphate and e-coat treatment is not required after assembly.
“Going to an all-aluminum vehicle structure is expensive,” Conklin said. “But with a bimetallic structure, every joint needs an adhesive barrier, and every joint has self-pierced rivets because you can’t spot weld steel to aluminum.”
The joints are more complicated, but incorporating aluminum extrusions and castings with some thoughtfully located steel frames leads to a reduction of parts.
“Fewer parts to design means fewer tools to manufacture and less labor to join the parts,” Skszek said. “The number of parts has a cumulative effect. Less parts, means less bodies to make the parts and less assembly costs. It’s a gain in efficiency.”
The MMLV body-in-white design consists of 32 fewer components (12% reduction) than the baseline vehicle.
The MMLV is a concept car. It won’t go into volume production, and Ford has not adopted the design for future Ford Fusion models. But the program has successfully served as a showcase of the weight reducing possibilities achievable with existing materials and processes.
“All of the aluminum diecast applications demonstrated with the MMLV are being implemented with other customers,” said Randy Beals, global engineering specialist-casting, Cosma International.
The opportunity to design a car from the ground up for a customer will probably not present itself to suppliers like Magna, but the increase in application of an aluminum casting in a structurally significant area of the vehicle is promising.
“OEMs designing a new vehicle still set their weight targets,” Conklin said. “Based on their targets and requirements, we try to figure out the best location and application of castings where we feel we can get the best bang for our buck.”
After the successful creation and testing of the first MMLV concept car, Magna and Ford continued the program with a second version, this time with future concept materials and processes, such as magnesium castings and carbon-fiber parts. This version is only a “paper study” and was intended to identify the gaps that have to be addressed to enable use of the new materials as mainstream technology. It also poses another scenario for future research and development: “What if even lighter materials were used to make a vehicle body-in-white?”