Adhesives Helping OEMs Push the Performance Envelope with Lightweight Multi-material Structures

Posted on 4/4/2016 9:57:59 AM By Marc Benevento

Passenger car manufacturers constantly seek a perfect balance of performance and fuel economy without compromising safety.  Nowhere is this more challenging than the luxury performance segment, where consumers demand cars that not only deliver them from point A to point B, but that provide an exhilarating experience in doing so.  To achieve this level of performance, manufacturers and suppliers are using a broadening array of materials to construct lightweight and rigid vehicle bodies.  Adhesives are an important enabling technology, and are used not only to bond dissimilar materials that cannot be welded, but to stiffen metals that have been joined by spot welding.  New entries in this segment from Acura, BMW, and Cadillac provide examples of how adhesives are enabling innovation in this space.

Acura vehicles, which have increasingly used high strength steel in efforts to shed weight over the past 15-20 years,  have begun to incorporate light metals, such as aluminum and magnesium, to further reduce weight.  Aluminum has been employed to reduce the weight of closure panels, such as hood and door skins.  Structural adhesives have enabled Acura to eliminate the use of spot welds and bond an aluminum outer panel to a steel inner on the 2014 RLX, thus reducing weight 17% versus steel/steel construction.  The adhesive not only bonds the two panels, but creates a “lock seam” that prevents galvanic corrosion and deformation due to differences in thermal expansion of the two materials.  Additionally, the reduction of mass at the outermost surfaces of the car help move the center of gravity inward, improving handling.

Cadillac is taking an even more aggressive approach when it comes to the use of light metals and adhesives to reduce weight and improve performance of premium vehicles.  The steel intensive ATS and CTS employ 88m and 118m of structural adhesives in their white bodies, respectively, to strengthen and stiffen the structure for the purpose of improving performance and safety.  As outlined in Automotive Engineering, the company has developed several joining and manufacturing techniques for advanced and lightweight materials, and the new, top of the line, CT6 sedan puts them to use.  The CT6 sports a multi-material body that is 2/3 aluminum and 1/3 steel, the combination of which Cadillac feels offers a more optimal solution than using one material exclusively.  In addition to spot, arc, and laser welding, the CT6 has about 180 m of structural adhesives, and the resulting body weighs in at 3700 lbs, saving an estimated 200 lbs versus steel intensive construction.

BMW has taken perhaps the most aggressive approach of  any light vehicle manufacturer in terms of exploring advanced lightweight materials in volume production.  Following the well-publicized i3 and i8 electric and hybrid vehicles featuring carbon fiber composite intensive construction, BMW has elected for a multi-material approach on the all-new 7 series that will hit the road this year.  The “carbon core” construction, as BMW labels it, marries carbon fiber composite to a body that is primarily steel using adhesives and rivets in a highly automated process.  This uses the expensive carbon fiber only where it provides the most benefit, such as parts that contribute to strength and rigidity high in the body.  This not only reduces mass, but lowers the center of gravity, which improves handling and stability.  In addition, parts of the front structure, including as the mounting points for the shock absorbers, as well as many closure panels, are made from aluminum, which assists in achieving a 50/50 weight distribution on the front a rear axles, a hallmark of BMW handling.  Despite having a retail price above $100,000, BMW applies lightweight materials where they provide the most economical solution to challenges confronted by the design team.  Carbon fiber is used rather sparingly, and only where it provides the most value for the money.  Confidence in structural adhesives as a primary joining method is imperative to taking this approach, which BMW estimates to save nearly 300 pounds in each 7-series body.

The examples above are just a few of many that show a clear trend in the luxury vehicle segment toward multi material structures.  As global fuel economy regulations continue to become more stringent, expect these materials and construction techniques trickle down the product lineup to more mainstream vehicles.  Adhesive manufacturers will continue to benefit as manufactures and part supplier look for innovative ways to incorporate lightweight technology into higher volume production.

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