11/15/2016 | 4 MINUTE READ

BMW 7 Series CFRP: corrections and missing details

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Images of the hybrid B pillars being pressed plus joint design, adhesives technology and BMW’s insights into the future needs for CFRP in automotive.

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In the November issue of CW, I wrote about my tour of BMW’s new 7 Series and carbon fiber reinforced plastic (CFRP) parts production at their plant in Dingolfing, Germany. BMW was so very gracious and I am grateful for the time and attention they gave to me.

It is a testament to their leadership in CFRP technology that many people have read the tour article and commented. Contributing writer Bob Griffiths noted that Manchester, UK was mistakenly listed as the seat of BMW subsidiary Rolls Royce, when, in fact, the headquarters and manufacturing are in Goodwood, near Chichester, Sussex, UK.

Also, communications personnel for Hexcel (Stamford, CT, US) pointed out that even though the article does credit Hexcel Austria as the source for the prepreg used in the 7 Series hybrid B pillars, it describes the epoxy resin as “from an undisclosed supplier”. In fact, the B-pillars are made with Hexcel’s own rapid-curing HexPly® M77 epoxy resin. This was announced in a 2016 Hexcel press release with approval from BMW. 
 

Missing Details
The process of writing an article is imperfect and frequently does not allow inclusion of all the information I would want. For this article, top on that list was an image or two of the hybrid B pillars being pressed. Though I was unable to provide these in the article, they were published in a March 2016 presentation given by Dr. Joachim Starke to the European Composites Industry Association (EuCIA, Brussels, Belgium). In the picture at right below, you can see the peel ply/foil on top of the prepreg that gets removed at the end of the process.

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The BMW 7 Series’ hybrid B pillar joins preheated and preshaped CFRP prepreg and steel substrate together and cures in a single press step. SOURCE: Slide 30, “Carbon Composites in Automotive Structural Applications” presented by Dr. Ing. Joachim Starke, BMW Group.

The presentation includes a number of interesting details, such as the concept for galvanic corrosion prevention in the Carbon Core BIW, its “swimming” CFRP joint design and braided CFRP roof bows, as well as differences between CFRP in the i3 and 7 Series models (click on photos below to enlarge).

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SOURCE: Slides 23, 24, 27 and 37, “Carbon Composites in Automotive Structural Applications” presented by Dr. Ing. Joachim Starke, BMW Group.

Adhesives in the BMW 7 Series Carbon Core
Another find is the article by Dobrivoje Jovanovic, Dr. Nicolas Morel and Denis Souvay of Sika Automotive AG (Romanshorn, Switzerland) in the 2016 preview issue of Lightweighting World magazine. Titled “Optimizing Bonding & Reinforcing Solutions” the article basically reviews the company’s adhesive solutions for automotive, including its SikaPower Mixed Bonding Excellence (MBX) technology developed to join steel, aluminum and CFRP while providing separation to prevent galvanic corrosion.

Galvanic corrosion occurs when two different metals are in contact in the presence of an electrolyte. Even though carbon fiber is not a metal, it is an electrical conductor with a significantly different electrochemical potential vs. steel or aluminum. Galvanic corrosion occurs at the joint between the two materials, and though it will not erode the carbon composite, it will attack steel and aluminum. Water is the most common electrolyte that enables the electrochemical corrosion process.

The images above show how BMW designed joints in the Carbon Core BIW to avoid galvanic corrosion, but this wasn’t the only issue. Sika explains its MBX systems also address the issue of delta-alpha — i.e., mismatched expansion and contraction properties. In composites, we are quite used to thinking about issues due to carbon fiber’s different coefficient of thermal expansion (CTE) vs. other materials. For the BMW 7 Series multi-material BIW, this can produce high residual stress in the adhesive between the aluminum, steel and CFRP as assemblies are heated and cooled during normal body shop processes. These residual stresses can reportedly lead to adhesive failure, substrate failure and/or substrate deformation. Sika was able to develop its SikaPower MBX products to perform in both “fixed” steel-aluminum joints and much thicker “swimming” steel-CFRP joints, without cracking during high temperatures from spot welding and paint oven cycles.

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Sika has developed a solution to the “delta-alpha” issue of differing CTE materials joined in mixed-material automotive body structures. SOURCE: Lightweighting World and Sika AG.

The article goes on to describe the ultra-high modulus Sikaflex-UHM technology it developed for the BMW i3 and i8 models, as well as technology that combines Sika adhesives with molded reinforcement carriers bonded in the vehicle body with structural foam and/or Sika High Strength Bonding (HSB) products. These solutions function to provide local strength “patches” to reduced-thickness lightweight shaped steel structures, similar to BMW’s use of CF/epoxy prepreg in the 7 Series hybrid B pillar.

Future of CFRP in Automotive
Going back to Dr. Starke’s presentation for EuCIA, the last few slides are prefaced “Demands for Future Lightweight Design with CFRP” with the following goals:

  • Reduced material cost
  • Reduced part production cost
  • Optimized composite properties per material & process (simulation and fiber sizing are stressed)
  • Further weight reduction but stable process (reliable)

It appears the trade group MAI Carbon (Augsburg, Germany) has presented a vision and research results which aim at reducing 2010 CFRP part process costs by 90% with a part cost below 18 €/kg (≈ $8/lb) by 2020. Of course, this assumes 1-minute cycle times and carbon fiber cost below $5/lb. However, a load path optimized roof bow developed using the skeleton design method and fabricated via injection molded thermoplastic reinforced with recycled carbon fiber reportedly achieved:

  • Properties exceeding actual BMW i3 structure
  • Production cost cut by 74%
  • Tooling cost cut by 80%
  • Cycle time of 75 seconds.

It looks like BMW will continue to drive CFRP technology forward across design, simulation, materials, process, recycling and more. Stay tuned.

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SOURCE: Slides 40-41, “Carbon Composites in Automotive Structural Applications”
presented by Dr. Ing. Joachim Starke, BMW Group.


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