Taking Aluminum to New Heights

Despite Falling Sales, Auto Makers are the aluminum industry's fastest growing customer segment.

New applications designed to reduce weight, such as the roof on BMW AG's '09 7-Series, is a big reason for the massive switch to the lighter-weight metal in vehicles. According to the Aluminum Assn., the industry's trade group, BMW's '09 models use an average of 421 lbs. (191 kg) of aluminum per vehicle, leading the industry in use of the lightweight metal.

The Aluminum Assn. designates leading users in two categories: actual average pounds per vehicle, as is the case with BMW, and overall percentage of aluminum in an auto maker's product portfolio. Honda Motor Co. Ltd. tops the latter category. Its '09 models average 10.2% aluminum content.

The fifth-generation 7-Series contains more than 400 lbs. (181 kg) of the light metal, the trade association says. A BMW spokesman won't confirm that figure, but says 14% of the car's body skeleton is made of aluminum. “We do not talk about (overall) kilograms,” he says.

However, BMW does say it shaved 18.5 lbs. (8.4 kg) from the latest 7-Series by substituting aluminum for steel on the car's roof. The big sedan still bends the scales at a hefty 4,600 lbs. (2,087 kg), but to engineers, 18.5 lbs. is very significant, especially when it comes off the roof. That means it can lower the car's center of gravity and improve vehicle balance and handling.

Add that saving to those achieved by using aluminum for the doors, hood, shock towers, engine block, suspension arms and differential housing, and it's easy to see why the 7-Series hasn't ballooned to an unthinkable 5,000 lbs. (2,268 kg).

Aluminum is only two-thirds the weight of equivalent steel components. BMW engineers take advantage of that inherent lightness to maintain the auto maker's signature 50/50 front-to-rear weight ratio, as well as pack the new 7-Series with more engineering and comfort-oriented innovations.

Serious technical challenges in bonding an aluminum roof to a steel skeleton were solved by BMW engineers working with adhesive supplier Dow Automotive, a subsidiary of Dow Chemical Co.

Dow Automotive says this application demonstrates the effectiveness of adhesive as an assembly technology for both lightweight constructions and mounting processes, crucial factors for structural applications where dissimilar materials are used to provide significant weight-reduction advantages.

Because metals conduct heat differently and are susceptible to a specific type of corrosion when they are joined directly, structures that combine steel and aluminum cannot be fabricated using normal welding procedures. Different expansion and contraction rates of materials exposed to heat and cold is another crucial factor when creating structures that combine dissimilar metals.

“Here, Dow Automotive's new bonding system provides a sustainable solution in two respects,” says Orhan Imam, Dow Automotive market-development manager for 2K PU adhesives.

“Compared to currently available adhesive systems, it is flexible enough to offset the different expansion coefficients while simultaneously guaranteeing the necessary structural system stability at even higher temperatures,” Imam says.

Together with BMW, Dow Automotive developed the improved 2-component polyurethane adhesive technology based on its Betamate 2K PU adhesive. This is the first time such an adhesive bonding system has been directly used on the assembly line to build an aluminum roof module, Dow Automotive says.

In addition to its technological advantages with regard to flexibility and simultaneous ability to keep the bonded structure stable as the car moves down the assembly line, the solution developed for BMW also presents advantages during the assembly process and provides an economically attractive solution for auto makers, Dow says.

BMW says the adhesive is applied with a special process and later fully hardens and sets in the paint bake ovens.

Using an adhesive to bond dissimilar metals also provides optimum protection from galvanic corrosion, also known as dissimilar metal corrosion, engineers say. Spacer nubs are included in the roof panel to provide an optimum gap for the adhesive to work as designed.

Another material innovation in the 7-Series is the optional ceramic knobs offered for the transmission shifter, climate-control and radio switchgear.

The industrial material is similar to that used in scratch-resistant expensive watch cases. It's a pricey option at $650, but BMW says the scratch resistance keeps the controls looking new.

Among the new engineering features in the '09 7-Series is an integral active-steering system that makes BMW's big flagship handle like a much smaller car. New to this model is rear-wheel steering that can turn the back wheels up to three degrees. At low speed, the rear wheels turn in opposite direction from the front wheels, reducing the turning circle by up to 27.5 ins. (70 cm). At higher speeds, rear wheels steer in the same direction to increase stability.

BMW claims rear-wheel steering cuts the vehicle's yaw rate in high-speed maneuvers, adding to the comfort of rear-seat passengers.

The first full implementation of the FlexRay electronic networking system in a BMW helps tie all of the car's major driving systems together in an integrated chassis-management scheme. BMW belongs to the FlexRay consortium, which has seven core partners, including Robert Bosch GmbH, Daimler AG, Freescale Semiconductor, General Motors Corp., NXP Semiconductors and Volkswagen AG. The companies' agreements are scheduled to expire at the end of this year.

FlexRay helps the car adapt to changing road conditions and surfaces, providing quick inputs from dynamic stability and damping control systems, active roll stabilization and active steering. The FlexRay system allows communication to occur 20 times faster than was possible in the predecessor 7-Series model, networking up to 16 electronic control units.

The '09 7-Series also offers its first lane-departure warning system with active blind-spot detection. Warning drivers by vibrating the steering wheel and a blinking light-emitting diode in the relevant side-view mirror, the safety device even informs when another vehicle is not yet in the blind spot.

New to this model is a second-generation Autoliv Inc. night-vision system with a head-up display. It's capable of detecting pedestrians located close to the road. The system's infrared camera sees up to 1,600 ft. (488 m) in front of the car. It displays the images on the iDrive screen or HUD that is optional on U.S. models. Should a pedestrian walk in front of the vehicle, night vision displays a warning on the HUD. The night vision option is $2,600. The HUD is $1,300 extra.

Among the features stored on the car's 80-gigabyte hard drive is what BMW claims is the industry's first integrated owner's manual. Seeking to emulate the help function available on computers, the electronic owner's manual is accessed via the iDrive. Besides displaying information obtained in conventional manuals, the electronic version offers a host of multimedia animations, slide shows with sound and written explanations of the vehicle's systems.

Price of the standard-wheelbase 7-Series begins at $81,125. A fully equipped long-wheelbase 750Li can cost more than $100,000.

In that price bracket, BMW can afford to use more expensive lightweight materials and produce a vehicle that bristles with state-of-the-art engineering features.

But even less expensive vehicles, such as those produced by Honda, also will feature more aluminum content in the future as fuel economy takes on more importance.

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