September 2011 / Volume 63 / Issue 9|
By Alan Richter, Editor
| ‘Lightweighting’ vehicles through the use of magnesium parts can improve vehicle mileage and impact resistance, among other benefits.
When reducing a vehicle’s mass to increase fuel efficiency and lower emissions, making suitable components out of magnesium alloys seems like a no-brainer. Being the lightest structural metal, magnesium is 75 percent lighter than steel and about 33 percent lighter than aluminum, according to Meridian Lightweight Technologies Inc., a Strathroy, Ontario-based die caster of magnesium auto parts.
Compared to aluminum, magnesium has a higher specific strength, ductility and impact resistance, stated the United States Automotive Materials Partnership, a consortium of the U.S. Council for Automotive Research, in its paper “Magnesium Vision 2020: A North American Automotive Strategic Vision for Magnesium.” The paper also reported that magnesium provides better damping and dent resistance vs. steel and offers higher strength, stiffness, thermal stability and thermal conductivity vs. plastic.
Courtesy of Lotus Engineering
In addition to boosting fuel economy and cutting emissions, material properties provide a host of benefits. According to the report, those include:
• Enhancing acceleration/deceleration and steering/cornering response when lightweighting the vehicle’s front by moving the center of gravity rearward,
• Minimizing squeaks and rattles because magnesium can be cast into one large part instead of the part being fabricated from numerous individual steel elements that are susceptible to rubbing each other and causing vibration, and
• Reducing manufacturing cost because a magnesium casting can be less expensive than the same component when made of steel, especially for annual volumes lower than 200,000. A casting has a lower tooling cost than a multiple-part steel stamping, which requires a die for each segment. For example, a 30-part steel instrument panel (IP) cross-beam requires 30 tooling items to be produced, whereas the cast-magnesium version needs only six.
Nonetheless, only about 12 lbs. of magnesium parts are in the average U.S. automaker’s vehicle, which represents about 0.3 percent of a vehicle’s material.
“The paper was written before the Great Recession, so I think that 12-lb. amount is down a little bit,” said Dr. Gerald Cole, the paper’s author and president of the consultancy LightweightStrategies LLC, Franklin, Mich. (During his career at Ford Motor Co., he worked both in research and a group called “Weight Engineering.”) Cole added that the other information in his paper hasn’t changed.
Essentially, magnesium is not more prevalent in vehicles because it costs more than competing materials. “Because of the competitive nature of the business, anything that is cheaper gets a bigger play,” Cole said.
Reduction in the use of magnesium components is also partly due to the North American market losing die casters through closures and mergers, noted Greg Patzer, executive vice president of the International Magnesium Association, Wauconda, Ill. “The financial situation that happened a couple of years ago and what happened to the auto industry had a huge impact,” he said.
In connection with an August 2011 report from Ducker Worldwide, Troy, Mich., about projected changes in the average light-vehicle material content in net pounds per vehicle (see table on page 41), Dick Schultz, project consultant for the consultancy, estimated that magnesium components will increase from about 8 lbs. in 2008 to 22 lbs. in 2025. The increase will be entirely for power train components, primarily intake manifolds, he noted. “Magnesium transfer and transmission cases may eventually replace aluminum ones,” Schultz said. “There will be no magnesium structural parts.”Current Components
In U.S. vehicles, magnesium components can be found in the:
• Chassis, such as the brake bracket and bracket assembly, brake pedal bracket, air bag housing and clutch pedal bracket and assembly;
• Interior, such as the seat base, IP support beam and reinforcement, steering wheel armature, and steering column bracket and hub;
• Exterior, such as the sunroof cover assembly, outside mirror armature and roof frame; and
• Power train, such as the alternator brackets, valve cover, cam cover and transfer case.
Cole noted that European vehicles tend to have more magnesium parts, with additional applications including a few road wheels and a chassis front-end carrier, a seat cushion and back, and a power train’s transmission case (manual and automatic) and even an engine block, which significantly reduces upfront engine weight. “The Europeans have more of an interest in drivability because of their mountainous roads,” he said.
Cole added that magnesium is more expensive in the U.S. than Europe because the International Trade Commission, which controls the import of materials in the U.S., has placed countervailing tariffs against magnesium from China, from which the vast majority of the metal comes. “Probably 85 percent of the world’s magnesium is produced in China,” Patzer said. “Duties are such that it effectively keeps that source out of the U.S. market. There’s only one North American producer, U.S. Magnesium.”
[Editor’s note: U.S. Magnesium LLC is based in Salt Lake City, which is an ideal location because magnesium occurs naturally in chloride. Therefore, places with highly concentrated salt water, such as the Great Salt Lake and the Dead Sea, are sources for magnesium.]
In his paper, Cole detailed that if all suitable parts in a vehicle with an inline six-cylinder engine were made of magnesium instead of aluminum, steel, iron, zinc or plastic, the amount of magnesium would total about 380 lbs. and 300 lbs. of weight would be shed. Although he noted that automakers can achieve 75 percent of improved fuel-efficiency standards for that type of vehicle through enhancements to power train, transmission and stop/start technologies, the remainder requires lightweighting. “Magnesium will play a role,” Cole said.
Courtesy of Ducker Worldwide
How much of a role is a matter of debate. In a study funded by the Energy Foundation (a partnership of major foundations interested in sustainable energy solutions) to identify potential mass reduction opportunities for a selected baseline vehicle to represent the crossover utility segment, which was the 2009 Toyota Venza, Lotus Engineering USA reported that it developed two vehicle architectures. The “Low Development” vehicle, using technologies feasible for a 2014 program start and 2017 model year production, achieved a 21 percent (611 lbs.) mass reduction (less power train) with 2 percent of the material being magnesium. The “High Development” vehicle, using technologies for a 2017 program start and 2020 production, reduced mass 38 percent (1,093 lbs.) using 16 percent magnesium.
Lotus Engineering, Ann Arbor, Mich., approached the study from a technical and commercial standpoint, with the former targeting vehicle aspects like structural integrity and crashworthiness, and the latter focusing on tooling, part and assembly costs, noted CEO Darren Somerset. “Lotus approaches an architectural study of this nature from the highest level, or the system level,” he said, adding that a material was selected for placement around the vehicle structure based on its ability to achieve the technical and commercial objectives. “The structure and architectural strategies were optimized to put the right material in the right place.”
Somerset noted that Lotus used magnesium for all the closures, which are also found on commercially available vehicles, such as the Lincoln MKT. Meridian Lightweight Technologies estimates that a magnesium casting similar to the one on the Lincoln MKT liftgate, when combined with an aluminum outer panel, is about 40 percent lighter than the same Venza components made from equivalent steel stampings.
“The beauty of magnesium is you can drastically reduce complexity of, for instance, an IP structure, which is typically made of many welded steel fabricated pieces,” Somerset said. “You can replace the whole subassembly with one casting.”
He added that purity in vehicle design is central to how Lotus engineers vehicles. “Lotus has always been about performance through lightweight, and the elegant, simple solution,” Somerset said. “It’s very much inherent in the Lotus philosophy and has been since Colin Chapman founded the company in 1952. And that’s one of the key architectural approaches we have taken on this particular study, where we have been very clever in the way we reduced complexity around the vehicle.”
Somerset cited the Venza’s body structure as an example where Lotus reduced the number of parts from more than 400 to 221. Part reduction also decreases the amount of associated tooling.
Table. Average light-vehicle material content in net pounds per vehicle
Courtesy of Ducker Worldwide
By optimizing the body structure system components, ancillary reductions can be realized in other systems, such as the suspension and interior, noted Gregg Peterson, senior technology specialist for Lotus. “We set out to do a high degree of integration,” he said. “We set out to stress virtually every component in the total vehicle.”
In addition to being able to integrate numerous pieces into one casting, Peterson pointed out that, compared to a stamped steel part, a magnesium casting enables a part designer to change part dimensions, such as thickness, on a linear-millimeter-by-linear-millimeter basis. The casting also minimizes scrap. “When you punch out a large portion of, say, a door frame or body side aperture, 20 to 30 percent of the material becomes scrap,” he said. “Whereas with a magnesium cast section, there’s minimal scrap.”
Of course, lightweighting a vehicle through use of such materials as magnesium also shrinks fuel consumption. Based on U.S. Department of Energy estimates of a 10 percent mass savings generating a 7 percent fuel savings, a total vehicle mass reduction of 33 percent, including the power train, for the 2020 High Development car results in a 23 percent reduction in fuel consumption, Lotus reported.Corrosive Consequences
In addition to having a higher raw material cost than competing lightweight materials, magnesium parts are generally coated to avoid galvanic corrosion, which adds cost. According to Cole of LightweightStrategies, galvanic corrosion is one of the key issues that limits magnesium use in vehicle. Magnesium is subject to galvanic corrosion when it is in contact with a dissimilar metal and there is an electrolyte present, such as salt water.
Another form of galvanic corrosion is from surface contamination with small particles of a dissimilar metal, causing severe pitting, Cole stated. This can occur from die lubricants that contain molybdenum disulfide and carbon, from heavy metals present in shot-blasting media and from iron particles transferred from dies during casting, forging, extrusion, rolling and stamping.
“With a multimaterial vehicle, you must make sure you’re not going to have corrosion issues, so those magnesium parts were coated,” Peterson said, referring to the ones in the Lotus study.
The most common source of galvanic corrosion occurs from fastening, Cole noted, especially when using steel fasteners. The most effective remedy is to select compatible joining materials and reduce the cathodic surface area. However, sometimes automakers have to invent unique designs. For example, the front-end assembly bracket for the Ford F-150 truck and the engine cradle for GM Corvette required significant field design work for the joints and the washer, spacer and bolt materials, he added, noting that it’s less of problem in Europe, where aluminum fasteners are allowed.
One magnesium alloy that offers “very good corrosion resistance” is AZ91D, according to Meridian Lightweight Technologies. AZ91D is the most common alloy for high-pressure die casting, and is typically used for power train and mechanical components where toughness is more important than deformation capability.Flammability Concerns
Prior to being placed in a vehicle, even a near net-shape magnesium casting must be machined. Although magnesium is not considered a difficult-to-machine metal, magnesium chips are flammable and the chips produced when finishing magnesium parts can be more prone to catch fire than chips produced when roughing because they are thin and have high surface-to-volume ratios. Also, magnesium in the presence of water-soluble cutting fluids without inhibitors can produce hydrogen, which is flammable, and excessive dwell times and low feed rates enhance the fire hazard.
Special fire suppression equipment may be required because the potential for sparks exists when cutting, especially if the workpiece is not electrically grounded. “One company had an optical device that picked up the glow if a magnesium chip caught fire and dumped in oxygen and burned up the little chip before it could fall into the pile of chips and spread the fire,” Cole said.
Because magnesium dissolves in water and oxidizes, adding to the flammability issue, magnesium should be cut dry or with a coolant that contains hydrogen suppression or antioxidation additives, Cole explained. “As well, filters must be carefully maintained to limit the buildup of fines in the coolant, inhibitor concentration has to be continuously monitored, and all machining cabinets require hydrogen monitors. With the right maintenance management, machining magnesium is easy,” he said.
Courtesy of Lotus Engineering
The magnesium industry intends to keep making inroads into the automotive sector even though it has a significantly smaller presence than aluminum and steel, said Patzer of the International Magnesium Association. That’s because of the high volume of vehicles produced and the considerable amount of material in a vehicle compared to other magnesium-containing goods. “It’s an industry focus in that it has the greatest potential for the largest increases in tonnage use of the metal just by the nature of the product,” he said. “It takes a lot of cell phones to equate to any significant tonnage.”
Ultimately, the market will decide the appropriate material mix that helps meet future vehicle emission and fuel-efficiency standards, such as the one recently proposed by the Obama administration to raise the corporate average fuel economy standards for U.S.-sold cars and light trucks to 54.5 mpg by 2025.
“The Lotus objective is to use the most appropriate material for the part design, and we don’t care if it’s steel, aluminum, magnesium or unobtanium as long as it works and is commercially feasible,” Peterson said. “The real trick to getting this right is to design a cost-effective, lightweight vehicle and make sure you integrate parts and do all systems simultaneously. If you piecemeal the operation, you’re not going to get to the point where you can, say, change the suspension because you’ve lightened the vehicle.” CTE
About the Author: Alan Richter is editor of CTE, having joined the publication in 2000. Contact him at (847) 714-0175 or email@example.com.
Courtesy of Fraunhofer IWU
Opening the door to resource-friendly car makingCourtesy Fraunhofer Institute
Courtesy of Lotus Engineering
Lightweighting with composites
play a role in lightweighting vehicles. In its paper about the study of potential mass reduction opportunities for the Toyota Venza crossover utility vehicle, Lotus Engineering USA reported that composites have been used on high-volume production vehicles, such as the Saturn and early GM minivans, but the applications are being expanded into structural areas on niche and specialty vehicles. Those materials include composite sheets, multiple-layer composites using sheet materials such as aluminum or glass fiber on either side of a foam core and carbon-fiber composites.
Other possible applications include long fiber-reinforced polypropylene for the trunk floor and long fiber-reinforced polyurethane for load floors to reduce part count and assembly time while maintaining structural stiffness.
In the study, Lotus primarily used glass-reinforced composites. “Carbon-fiber composites and titanium were ruled out because of the project’s commercial constraints in this vehicle segment,” said Lotus’ Darren Somerset.
The company kept the shape of composite parts fairly simple, but included some curved features, such as in the rear seat kick-up area where the seat forms were molded in, noted Lotus’ Gregg Peterson. “We used lightweight material that was structural and eliminated a number of very heavy parts that you typically see in rear seats like seat risers and seat supports,” he said.
Fraunhofer Institute for Machine Tools and Forming Technology IWU
International Magnesium Association
Lotus Engineering USA
Meridian Lightweight Technologies Inc.
CUTTING TOOL ENGINEERING Magazine is protected under U.S. and international copyright laws. Before reproducing anything from this Web site, call the Copyright Clearance Center Inc. |
at (978) 750-8400.