Carbon fiber connection solution for BMW 7 Series

The complex processing of carbon fiber requires extremely high-level professional technology. BMW's intelligent lightweight structure concept "Carbon core high-strength carbon fiber core" was first applied to the new BMW 7 Series. This move is considered a milestone in the field of automotive engineering, changing the perception of lightweight body technology in this era, and also means better driving comfort and better dynamics.

The application of high-strength carbon fiber core can significantly and specifically reduce the weight of the vehicle body while improving the stability of the vehicle. In terms of body torsional rigidity, compared with the previous generation model, not only the body weight is reduced by 130 kg, but also the comfort and safety are improved. In addition, the intelligent hybrid structure and weight optimization technology ensure consistent weight reduction from the whole to every detail, achieving an almost perfect 50:50 axle load distribution and further lowering the center of gravity.

However, a major challenge faced in connecting metal materials to carbon fiber composites is electrochemical corrosion.

When two different metals are connected by an electrolyte, electrochemical corrosion will occur. Although carbon fiber is not a metal, it is also a good conductor and has a large potential difference with steel or aluminum alloy. Intermetallic corrosion occurs at the metal joints. Although it will not corrode carbon fiber composites, it will corrode steel and aluminum alloys. And water is the most common electrolyte.

Therefore, in the new BMW 7 Series, carbon fiber composite materials are only used in key structural parts and dry areas of the body. In addition, structural adhesives are used to bond different parts together, while also blocking the chemical corrosion path between carbon fiber and metal materials, thereby achieving anti-electrochemical corrosion.

For the connection process of metal and composite materials, in addition to the bonding of structural adhesives, it is also necessary to combine it with physical riveting. BMW has developed different connection processes for multi-material bodies, such as the "fixed" concept between steel and aluminum alloy, and the "swimming" concept between steel and CFRP .

Glue riveting process

For the bonding of composite materials and metal parts, since different materials have different thermal expansion coefficients, the structural adhesive must have good toughness.

In this regard, we increase the thickness of the structural adhesive in the swimming process to improve the bonding toughness.

By increasing the thickness of the structural adhesive, we can achieve better expansion compensation and higher elongation at break, but the bonding strength will be reduced, and the amount of structural adhesive used will increase, thereby increasing costs. Therefore, it is necessary to make adjustments based on actual conditions.