At BMW, the laser has a stronghold in the growing list of joining processes
Franz Joseph Gruber
It has been on the German roads for some time now, and there is hardly an insider who hasn’t heard or read about laser welding, cutting, and soldering applications in body-in-white construction. There is talk about the current BMW 5-Series and the standard production use of laser technology in its production. Using our “laser eye glasses” we had already put the predecessor model under the magnifying glass so there can be a comparison.
In the current 5-Series, laser joining involves the firewall, rear trunk lid, and hydroformed parts (which, using laser technology, were also integrated in the current 3-Series convertible). Our impression: laser use in body construction has not become dramatically greater, but instead far more demanding.
At this point, it is hardly necessary to mention that automotive manufacturing is one of the most important drivers for the use and distribution of industrial laser technology. Most applications, such as gear welding, have been established for years, so nobody talks about them any more. And things like laser cutting and/or laser softening of interior parts (even of textiles in the current 5-Series) or the laser welding of plastic vehicle key housings show the large variety of possible application areas involved with the automobile.
However, with laser use in automotive assembly, there has been something like a “royalty class” for a long time, namely standard production use in the body area.
The biggest incentive for thinking about new joining techniques in body construction is the continuous need for weight reduction. Achieving lighter weight increasingly requires a selectively mixed design that is based on different materials. However, different joining concepts using new steel or aluminum alloys requires a mixed assembly philosophy in matters of joining technologies. On the predecessor model of the 5-Series, the use of other supplemental methods increased by approximately 60 percent over the current model.
Laser welding of aluminum and laser brazing were added to the laser welding of steel. That means “more,” but still far from the dominance forecast by many laser proponents years ago. It’s not by chance that Hans Hornig, the manager of welding and joining technology at BMW AG, also has laser technology in his background: without a doubt, he is among the laser pioneers in the industry. In spite of that, or perhaps because of it, he is a critical second-guesser when it comes to sensible use of the laser. Hornig says, “For a long time, we have been pursuing product and productivity improvements by the selective use of laser technology. However, cost and quality goals absolutely must be achieved in the process.” In this way, he defines the conditions that limit the use of lasers in body construction.
European carmakers, such as BMW, are world leaders in utilizing laser welding extensively on models such as the 645 Ci shown.
Let’s take a look at the laser applications using the example of the current 5-Series (also the 5-Series touring car, the 6-Series, and the 6-Series convertible) that have made their way into standard production.
First would be the firewall, where it is a case of joining a sheet metal shell (firewall) with a crossbeam (hydroformed part). This is a laser application already used in the predecessor model-and yet almost nothing was transferable. The reason: the entire front end is a construction of 72 aluminum parts that required a complete rethinking of joining methods. There were two locations where the laser was indispensable because of accessibility from one side. One is the connection of the firewall (hydroformed part to sheet metal) and the other is welding several brackets and hoops on the hydroformed cross member, both already mentioned. Later, instrument panel parts to instrument holders are connected to the cross beam. For all the acknowledged laser experience of Hornig’s team, the idea of laser welding of aluminum in high-volume production required a lot of preliminary work.
The entire process chain was put under the magnifying glass to clearly define the process. The metallurgical phenomena of the aluminum alloys had to be studied, additional materials tested, and all process characteristics had to be studied, including strength and quality. After prototyping on a laser robot system at the Research and Innovation Center, the basis existed for secure implementation in standard production.
Laser welding of the BMW bulkhead.
Two manufacturing lines, each with a lamp-pumped 4kW Nd:YAG laser, have been providing for the firewall sheet metal-to-cross beam and brackets-to-cross beam connections more than 1000 times a day in two laser welding booths at the Dingolfing plant. With 42 seconds of pure production time, the systems do best with a cycle time window of 72 seconds. It is worth mentioning that welding is carried out with the twin spot technique and the use of filler metals. Hornig describes how established this process has become, “I would almost prefer welding aluminum to welding coated steel materials. If all the limiting conditions are managed, very high quality can be produced with excellent process safety.”
Incidentally, the handling of the aluminum parts also requires a rethinking of logistics. The firewall metal sheets are supplied to the processing stations separately using a suspension conveyor to prevent possible deformations that would make the clamping process harder. At each station, two robots handle the placement and removal of the parts and thus serve the two laser cubicles in which a robot performs the handling of the laser beam welding head that uses the familiar roller clamping technique.
In addition, the hydroformed cross beams have a history of production laser process behind them where the two ends of the formed component are cut using a laser beam. Laser cutting with two 2kW CO2 lasers is an integral component of the hydroforming system concept, which is also done in Dingolfing.
Laser welding of the roof and side frame of the BMW 6-Series.
In the 5-Series, other laser weld seams are used in the area of vehicle safety. Active roll stabilizers provide for comfort and safety by stabilizing the chassis as it adapts to the respective driving situation. In this application the laser beam is used to join the stabilizer element using a standard 5kW CO2 laser system. Actually this was business as usual for the laser team because the laser was obviously the most cost-effective method.
In the 6-Series, which has the same laser applications in common with the 5-Series, the roof is also laser welded. An existing system (with two CO2 lasers and flexible mirror beam guidance), used to join the 7-Series roof-to-side wall connection, had the reserve capacity to manage the 6-Series quantities. According to Hornig: “We manage the laser welding of fully galvanized sheet metal parts, so the actual idea was to implement this application relatively quickly and carry it out on the current system.” Again an example of the cost aspects: if the system is already there, the cost-effectiveness calculation looks completely different. In this case, the laser was able to spot weld again.
There is additional laser welding in the 5-and 6-Series derivatives. The impact plate, a high-safety part in the rear of the vehicle, provides for the safe and proper introduction of force into the body if there is a crash, and also for trailer towing. These plates are slid into the long members and connected to the body. Previously, these impact plates were very complex drawn parts with up to 32 drawing steps, that is, very expensive parts. The new construction could be designated as modern tailored blanking: the impact plates now consist of outer and inner sheet metal and an inserted pipe-all connected to each other by laser welding on a separate system. In this case, the laser made possible a simpler construction with equally good function, which provided a 20 percent reduction in the parts costs. An outstanding example of what Hornig means when he talks about sensible use of lasers.
Laser brazing is used for the trunk lid of the BMW 5-Series.
Laser brazing also is used for the first time on the 5-Series for the two-part rear lid with an outer skin connection, where corrosion protection was the driving force toward laser use. It is actually a case of galvanized sheet metal and the zinc being accommodating to the brazing process. During laser welding in the old 5-Series the zinc coating caused problems because the melted base material lost its corrosion protection and an additional subsequent adhesive application was necessary. Not so with brazing, where visually appealing seams require no reworking. One brazing head with integrated external wire supply, guided by a robot and a 3kW Nd:YAG laser source are responsible for the laser brazing. At up to three meters per minute, brazing is carried out on a 3-D contour, which demands a lot of the robot and brazing head in terms of movement capability: a 3-D application par excellence.
Even if the laser seam length per vehicle has stopped growing dramatically, one thing becomes clear: the laser has earned its permanent place in body construction. You can’t get rid of it-at least not at BMW.
Laser welding of the side impact plate
Hornig says, “We got away from this idea of laser seam in meters a long time ago. In spite of that, I don’t share your opinion that there is a downturn. It is much more the case that the laser has claimed its permanent place in the series of joining processes used. Nothing more and nothing less. Over and over again, there are areas where the laser has its opportunity as the only or most cost-effective method. The possibility of accessibility to the joining location from one side is an advantage, for example, that hardly permits any alternative. Think about the firewall, or in general terms: plate to pipe-that is almost exclusively the laser.”
He concludes, “However, I can banish your fears: it hasn’t become less-but the laser applications have become more demanding. In 2006 I will be able to name all of the laser applications that we are using in the new 3-Series.”
This article was adapted, with the permission of the author, Franz Gruber, from the original that appeared in EuroLaser magazine.