Automakers challenge current methods to produce lighter-weight vehicles

March 2012 - The latest vehicles debuting in car showrooms across America tout emissions-friendly mechanics and better mileage. As fluctuating oil prices affect consumers filling up at the pump and U.S. government initiatives call for less detrimental effects on the environment, car companies are revisiting traditional designs and introducing unique alternatives to current offerings.

Many car companies use steel for the majority of a vehicle’s structure because of its proven strength and dependability. However, as car makers seek to build more fuel-efficient cars, engineers and designers are working toward creating a lighter vehicle that uses less fuel.

“We use high-strength steel to make a very stiff structure and aluminum to balance the weight from the front to the rear of the car,” says Tom Plucinsky, product and technology communications, BMW North America LLC, Woodcliff Lake, N.J. “For the current 7 Series model, our big sedan, we use aluminum for the roof panels, front fenders, hood and doors. The rest of the roof is steel.” Plucinsky says by removing the weight from the front of the car, weight is evenly distributed throughout the front and back of the vehicle.

“Weight is important,” Plucinsky says. “Reducing weight is key and aluminum allows you to do that. Our customers want more features in their cars and while the weight of cars hasn’t gone down much, it’s the features that can weigh a car down—so we use aluminum for many of those.” Plucinsky says BMW uses aluminum for the front and rear suspension and engines are comprised entirely from aluminum.

Aluminum for BMW vehicles comes from a special metals foundry. “We can produce our own engine blocks out of aluminum heads and magnesium for structure,” he says. “We’ve been invested [using aluminum and magnesium] for quite some time now, since the mid-1990s.

“People don’t really realize how many materials go into the construction of a car,” he continues. “We not only have to make it lighter but also much stronger for crash protection. Using various steels and computing abilities, we can design a body with different materials in different areas and create a body structure that is dramatically stronger than 20 years ago but lighter using the more advanced high-strength steel.”

A different kind of steel
Takao Hayashi, senior manager, materials engineering at Nissan Technical Center North America, Farmington Hills, Mich., part of Nissan North America LLC, with headquarters located in Franklin, Tenn., says, “It is easier to apply aluminum to components such as the hood, trunk lid and doors.” However, some challenges carmakers can face include assembly concerns and welding. “Aluminum application can be more limited than steel,” Hayashi says, noting Nissan has made strides constructing a lightweight body. The Nissan Altima is one of the lightest vehicles available on the market. The company’s latest innovation is the gas-free, fully electric Nissan Leaf, with studies being conducted on how to recycle batteries years from now when they need to be replaced.

At first, Nissan worked with Japanese steel makers to develop the best capabilities using high-strength steel, Hayashi says. “Then we localized the high-strength steel grades available in the U.S. and global regions where Nissan plants are located, creating alliances,” he says, noting Nissan uses an in-house stamping shop and local stamping suppliers to work with the high grades of steel as well as a body shop with workers welding various steel grades.

In November 2011, Nissan published a paper on high-tensile steel applications the company plans on using in vehicles manufactured in 2013 and beyond. According to the report, Nissan has developed a 1.2 GPa ultra-high-tensile-strength steel with high formability as compared to the previous 980 MPa class high-tensile-strength steel.

Utilizing recycled sources
Mercedes-Benz, Stuttgart, Germany, recently began constructing vehicles that focus less on steel. “We introduced the all-new 2013 SL550 at the Detroit Auto Show in January, the body of which is made completely out of aluminum, except for the windshield frame, which is made out of steel for strength,” says Dan Barile, product and technology PR specialist at Mercedes-Benz U.S. headquarters in Montvale, N.J.

From an engineering perspective, the challenge in replacing steel with aluminum is all about balance. “The yield rate for aluminum is only one-third of steel’s,” says Dr. Stefan Kienzle, director for lightweight and advanced engineering technology at Mercedes-Benz. Kienzle used an example of a person riding a motorcycle. “Nobody would rely on protection from an aluminum helmet,” he says. “It’s not used for safety protection—so engineers must keep that in mind.” Kienzle adds a vehicle may have reduced density when using aluminum instead of steel, but aluminum is thicker. According to Kienzle, aluminum needs to be a minimum 10 percent thicker to have the same effect as steel. 

“If you want to make the vehicle lightweight, you need to reduce the density of the materials and thickness,” Kienzle says, pointing out reducing the thickness too much would result in unbearable vibrations while driving. “If you over-reduce the wall thickness of a vehicle made using high-strength steel, the lack of comfort for the driver from vibration would be unacceptable.”

As the company tests new designs and material combinations, Kienzle says Mercedes has “a lot of ideas to improve processes going forward such as improving tag time and our internal processes.”

Among such improvements, Mercedes emphasizes the importance of recycled aluminum in its current vehicles as well as cars it will produce in the future. “If you look at the various parts of a modern lightweight engine—most of which are aluminum, including the intensive parts of the combustion engine—the parts are made using 100 percent recycled aluminum. 

“For example, the crank case in an engine in the casting process is made of recycled aluminum,” says Kienzle, noting the major challenge Mercedes faces in North America is convincing suppliers that recycled aluminum saves the company resources as well as is beneficial to the environment.  “For the last Mercedes launch in Detroit, engineers used a lot of energy to convince suppliers to produce parts using recycled aluminum also in the body structure,” Kienzle says. MM