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Smart Joining and Measuring are the Basis of BMW’s Lightweight Material Mix

| Editor: Alexander Stark

Recently, the BMW plant in Dingolfing allowed selected journalists very profound and interesting insights into its joining and measuring technology approach to lightweight body construction.

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BMW´s lghtweight material mix is in the focus.
( Bild: Pixabay / CC0 )

Expert presentations illustrated how the modern lightweight body-material mix in the premium segment can be implemented productively, with almost no manual labor and in a high quality. A subsequent tour of the respective production halls provided practical impressions to the theoretical information. BMW relies on modern measurement technology from the very beginning, e.g. scanners that convert a complete car body into a 3D image very quickly. Deviations from the target values can be identified at the earliest possible stage and production receives corrective measures in good time. Plant manager Dr. Andreas Wendt: "The overarching objective is to ensure the long-term stability of production and the highest possible degree of quality. We can achieve this by using intelligent measuring and joining technology, even for the CFRP material mix consisting of high-strength steel and aluminium."

Modern Material Mix Relies on Smart Joining Mix

“Mixed construction places new demands on joining technology,” says Thomas Richter, who is in charge of joining technology at BMW. Welding is not adequate for joining CFRP. The most widely used method is friction-locked gluing of the connections. According to Richter, BMW built up a great deal of know-how with the production of the BMW i3 and BMW i8 models. Adhesive technology creates a mechanical connection between car body parts made of different materials without direct contact and also ensures a high degree of corrosion protection. Complementary processes such as riveting and innovative screw connections increase the stiffness of the body parts and makes them more stable. Robots guarantee for the required process reliability.

At BMW in Dingolfing, the car body construction with CFRP, high-strength steel and aluminium is accomplished by using various bonding processes as well as rivets or thread-forming screws in combination with a robot. This picture shows the assembly of various components on a 7-body frame in the CFRP-steel-aluminium-mix.
At BMW in Dingolfing, the car body construction with CFRP, high-strength steel and aluminium is accomplished by using various bonding processes as well as rivets or thread-forming screws in combination with a robot. This picture shows the assembly of various components on a 7-body frame in the CFRP-steel-aluminium-mix.
( Source: BMW )

The Right Amount of Adhesives Avoids Problems

In addition to the material mix, improved components were the driving force behind the innovations of the company’s production technology. Less components, for example, reduce the weight of the vehicle. A single component instead of the previously umpteen individual parts creates additional precision solely due to the reduced tolerance chain. It also reduces the stocking of the individual components and all issues that many individual parts can cause. Nevertheless, integrated components require additional and geometrically more complex connecting points to adjacent components. For example, the application of adhesives depends on a particularly precise dosage, says Richter. This ensures both the strength of the connection and avoids soiling of car body construction systems and paint baths, for example with adhesive residues.

Joining of Roof Frames in Car Body Construction and Paint Shops

The experts from Dingolfing emphasize that the BMW 7 Series marked the debut of a hybrid construction approach consisting of CFRP, aluminium and ultra-high-strength steel in automotive series production. The strength and stiffness of the passenger compartment reached a new level — whilst reducing vehicle weight. In the field of joining technology, the BMW Group realizes the mechanical connection not only by gluing but also by means of blind rivets and semi-tubular rivets without a pre-punched hole for insertion. Lightweight constructions on the roof and the parts that are to support the roof also reduce the vehicle's center of gravity and at the same time improve driving dynamics, emphasizes BMW. In addition, CFRP reinforcements, which can also be found on the roof frame, are intended to ensure the safety of the vehicle in the event of a rollover.

Thermoplastic Support for CFRP Reinforcement

The CFRP component is enclosed in steel, which makes the roof frame much more stable without adding weight. The special feature of this innovative reinforcement is that the CFRP component is hollow. The blow-molding core required to manufacture the CFRP component consists of a thermoplastic plastic tube. According to BMW, the tube is initially used as a carrier, which is braided with CFRP fibers. After the resin matrix is cured in a RTM process, the plastic core can be removed. BMW points out that market leaders usually do not remove the cores again. In Dingolfing, however, the removal of the process aid plastic carrier also contributes to the weight reduction of the car body.

Temperature Control Overrides Expansion Differences

To connect the roof frame to the body, it is important to consider that the CFRP, steel and aluminium will behave differently later in the paint dryer. While heating to 180 °C has no effects on CFRP at all, steel and aluminium expand at a significantly different rate, explains BMW. This temporary increase in length is particularly significant at the ends of the roof frame, i. e. at the joints to the front and rear frame. In car body construction, therefore, only the B-pillar, approximately in the middle of the roof frame, may be mounted with rivets and a relatively small adhesive surface. The roof frame is only pre-fixed at the mounting points of the front and rear frame. Adhesive, which is injected into the cavities between the steel sheet and CFRP after the body has cooled down, would then provide the friction-locked connection at these points and at the B-pillar. In addition, one can override the different expansion effects that accompany the material mix by intelligent temperature control, reveal the experts at BMW. This takes a little longer than with the methods used by other car manufacturers, but it also eliminates many issues.

Aluminium Cast Iron Puts an End to the Tangled Mass of Parts

The bodywork of the 5 Series Sedan and the Touring, the BMW 6 Series Gran Turismo and the BMW 7 Series have an innovative design that sets new standards in terms of precision and crash safety and makes an important contribution to reducing vehicle weight, according to the BMW pioneers. In joining technology, the BMW Group is also breaking new ground in connecting this central component to other body parts by resorting to the mixed construction principle.

In the latest development stage, the rear longitudinal member of the BMW 6 Series GT consists of a single aluminium die-cast part. The predecessor 5 Series GT model still comprised 25 individual components per side. In this way the weight of the two members could be reduced by 15 kg altogether. This represents the most effective individual measure in the body-in-white of the new 6 Series GT. Furthermore, this design incorporates a particularly high dimensional accuracy. Manual straightening of the components with a "massage" device, which eliminates deviations from the nominal dimensions in a targeted and automatic manner, is no longer necessary.

This is because accurately dimensioned components are the basis for a precisely manufactured car body. Also, the fewer the number of components, the higher the manufacturing tolerances. Another advantage is that the aluminium die casting process offers a high degree of flexibility with regard to geometry.

Each Part Can Be Designed for Its Specific Load

The reinforcing joints designed for this component increase the stiffness of the vehicle. The construction principle in accordance with an intelligent mix of materials also ensures that each body part meets the respective design requirements, e.g. in terms of weight and stiffness. The rear longitudinal member is connected to body parts made of different materials at numerous connecting points. This places high demands on the joining technology. In a design with mixed construction materials, glued seams ensure that two body parts made of different materials do not come into direct contact with each other. Otherwise the parts would corrode. The glued seams also ensure high rigidity and durability.

The gluing process requires maximum precision: The joint flanges must be completely filled in order to create a tight connection. A lack of adhesive would impair the quality of the joint. Too much adhesive, would in turn lead to leakage at the joints and contaminate body construction systems and paint baths.

Highly Accurate Gluing

Extensive monitoring ensures that the exact amount of adhesive is applied. Cameras check the adhesive pattern on the flange; the final inspection is done by a visual control for adhesive leakage that might have occurred after the joining process, supplemented by a visual control system. In addition, the glued joints are reinforced by means of mechanical joining technology such as self-tapping screws. The latter automatically creates a thread when it is screwed into the aluminium die-cast part. Self-tapping screws also increase the load-bearing capacity and fix the joining flanges until the adhesives have cured, the experts from Dingolfing explain. Only then has the joint reached its final strength.

The self-tapping screws are also used on the rear longitudinal girder because it is a crash-relevant component. Automated and fully integrated in the assembly of the rear longitudinal girder, these screws connect the safety component to the adjacent vehicle structure at the side of the car, BMW says.

I Am Having a Ball: Ingenious Clip Based on the Lego Principle

In joining technology, the BMW Group is increasingly relying on its own development called "L3 go". This patented, high-precision fastening system consists of a ball and a clip. The quite plausible principle is similar to the Lego coupling: a ball and its counterpart, a clip, can be snapped together quickly and easily. The geometry of the ball also "pulls" the clip into the lowest possible snapping position. Depending on the application, the ball and the clip are made of plastic or metal. The manufacturing process for the ball and the joining technology for its attachment to the component are precisely adjusted to the specific application. There are basically no limits to the use of L3 go. Individual or multiple balls can be welded onto the car body on top of each other, pressed in or even applied using the 3D printing process. Then a component is positioned and fixed on this ball connection element in an uncomplicated way. The clip can also be released again.

Easier Installation for Greater Driving Comfort

In addition, the ball-based clip reduces the risk of injury during assembly when compared to protruding screws or pins. The L3 go made its debut in body construction in the current BMW 7 Series and is now also used in the BMW 5 Series and BMW 6 Series Gran Turismo. Depending on the required height, one or two balls are welded on top of each other. When the components are assembled, the L3 go serves as a replacement for a screw connection. By applying this method, BMW reduces production costs and simplifies the fastening processes immensely. Bores are not required, the company said. For this reason, the body panel does not have to be re-sealed with a stopper. This creates a more pleasant background sound during driving. The integration of the clip geometry into a component is already implemented in several other applications. In this way, the component can be fixed directly onto the ball without any clip. A large number of pending patents underscore the potential that the BMW Group sees in L3 go.

MINI-Design by Clip

L3 go is therefore very well suited if components have to be removed again. This opens up completely new possibilities for the individualization of vehicles. This year, the "MINI Yours Customised" offer will be launched: Customers of numerous MINI models will then for the first time be able to design and order selected retrofit products in an online shop. This customization program includes side scuttles in the outer area and decorative trim strips for the vehicle interior. These unique parts are produced by means of various 3D printing processes. When the decorative strips are printed, the ball connection element is printed directly on the underside of the panel. The MINI service partner or the customer can attach the new part to the vehicle in just a few simple steps.

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