Getting autobody welding down cold
Cold metal transfer helps Volkswagen meet customer demands
Volkswagen's automobile manufacturing facility in Saxony, Germany, was spending more time and effort on post-weld finishing operations than it wanted to, but because of the requirements the company was dedicated to meeting, not much automation was possible. Through its research, Volkswagen decided to invest in cold metal transfer welding, which helped the company save time and effort while keeping up its product quality.
Metalworkers perform finishing work on the weld seam of an A-pillar.
The bodywork for the Phaeton® and Bentley Continental GT® limousines is built at Volkswagen Sachsen's Mosel facility in Saxony, Germany. The purchasers of these models expect perfection in terms of travel comfort and safety, as well as classic handcrafted quality. In other words, the bodywork finish has to make a visual impression that complements the cars' sheer exclusivity.
Such expectations demonstrate why this manufacturing operation performs fairly small bodywork production runs, with a relatively low degree of mechanization. The vehicle-building craftsmen at Volkswagen at Mosel have to juggle several tasks relating to quality, deadlines, and costs.
Combining Vehicle Quality, High-performance Structural Components
The buyers' insistence on prestige is the dominant and recurring theme driving automobile manufacturing at the Mosel facility. Bentley and Phaeton bodies use high-strength galvanized sheet steel, and several mounted parts are made from plastic and aluminum. Only 25 percent of the manufacturing process can be automated. To compare, the Lupo® and Polo® production lines are 90 percent automated.
Seams and joints comprise a key work area for these vehicles. They tie up a high proportion of labor and logistical resources. Engineer Andreas Krüger, who was the head of autobody construction for the D-Class from 2004 to 2005, described the job as controlling a "magic triangle" of quality, cost, and time management.
The load-bearing structural components were of most interest when it came to finding a more efficient welding process at the Mosel facility. This group of components—the undercarriage, the rear-wall crossmember, the drain channel in the engine-bay bulkhead, and the A-pillar—is the crucial point where forces acting on the vehicle superstructure intersect (see Figure 1).
The C-pillar is the strut that makes the biggest difference in an accident—but its weld zone also is exposed, so it needs to be strong and attractive.
One of the most important of these points is the C-pillar, where joint quality is significant because it has to be just right in stability and appearance (see Figure 2). In an accident, this strut makes the most difference in terms of safety. In addition, the weld zone here is exposed to view, with a lustrous glow shining in the car's reflective paintwork. The welding system used on the C-pillar has to fulfill both of these requirements.
All three parts to be joined in the C-pillar vary in thickness. The deformation and sinkage parallel to the seam, which often resulted from the high thermal input of gas metal arc welding (GMAW), required postweld machining. Although the facility's skilled metalworkers were adept at machining and removing weld spatter—both of which were required—production engineers realized that such labor was taking up too much time per car body and employee. These operations also ate up time of facility experts, who were needed urgently in other departments.
This led Krüger's team to try to find a way to reduce labor time and effort.
Defining Goals, Plans, and Partners
Inside a Bentley's finished bodywork are 4.3 meters of laser-welded and 18 m of fusion-welded seams (the Phaeton has almost as many), so welding accounts for a high proportion of value added to these vehicles. This was an even bigger reason for technicians to choose a welding system partner critically.
The facility already had a good client-vendor relationship with Austria-based welding systems provider Fronius Intl. GmbH. At the beginning of the second half of 2004, Fronius approached Krüger regarding a possible solution to the labor-intensity problem on the C-pillar.
The approach Fronius offered involved cold metal transfer, a process designed to offer lower heat input than conventional GMAW and no spatter.
Krüger decided to learn more about this process when he visited Fronius at the EuroBlech fair in Hanover in October 2004. While at the tradeshow, Krüger presented a list of requirements for a new welding process. In the course of the first meetings, Krüger specified his exact objectives and gave Fronius key phase-by-phase time-goal data:
- Reduce the number of employees needed per day, corresponding to a two-hour reduction in working time per car body.
- Conduct testing over several days during weeks 52 and 53 of 2004, before the start of series production and during a holiday period. If Volkswagen had waited until its next window of opportunity, spring 2005, Krüger and his colleagues would have lost an opportunity to jump ahead of the competition.
Soon a plan was in place:
- Fronius would perform a test phase at its facility with its own demo systems.
- Testing would take place with demo systems in Volkswagen's central R&D department in Wolfsburg.
- Equipment would be installed in the Mosel facility.
- Technicians would adjust and optimize the cold metal transfer seam.
- The product series would be deployed for the first and second stages of the work flow sequence.
Despite the thorough conceptual design, Volkswagen knew it was taking a risk: If the equipment didn't meet its expectations, it may have to dismantle and remove it.
Data and Perspectives
For Andreas Krüger, the main advantage of cold metal transfer technology is its 20 percent to 30 percent lower heat input. Here he points to a weld seam on the C-pillar of a Bentley Continental GT.
In January 2005 the company started using cold metal transfer and began an experience curve and testing phase.
Test data provided the company with encouraging data: According to Krüger, the strength of the cold metal transfer-brazed seam was excellent—even higher than that of the base metal. One car body per month, he said, was subjected to destructive testing. In addition, the company conducted random samples by using micrographs, and other locations were tensile-tested, as well as visually and ultrasound-examined.
In the end, the company met more than its objectives, according to Krüger. Skilled employees were trained and ready for redeployment to other challenging tasks sooner than expected.
"We all heaved a sigh of relief when the report came in about the satisfactory test results. The bottom line is that it's been a great success," Krüger said.
"The main advantage is the 20 to 30 percent reduction in the heat input," he said (see Figure 3). "Given the relatively thin sheets we're dealing with here, the principal effect of this is to reduce sheet sinkage by half."
Processing sheet metal in this way is well worthwhile, Krüger said, because it reduces the labor time required for raising and smoothing operations on the C-pillar. Repeated interruptions in the arc, caused by controlled short-circuiting, help reduce heat input. Now metalworkers need only raise the sheets a maximum of 0.1 millimeter.
Cold metal transfer helped Volkswagen perform spatter-free welding.
Eliminating labor time for spatter removal has helped improve the manufacturing operation’s overall efficiency.
The company also has reduced its postweld spatter removal operations (see Figure 4 and Figure 5). Because the wire electrode is retracted by digital control at a frequency of up to 70 hertz, one droplet of melting electrode ends up in the seam being welded. This cuts the amount of rework for spatter removal to zero, which, Krüger said, is helpful when it comes to luxury-class car bodies.
Krüger also found cold metal transfer to be a step up from conventional GMAW in Volkswagen's operations.
"CMT does away with a lot of clamping and logistics work because there is so much less heat distortion," he said. With all of these benefits, the company plans to introduce a second, complete cold metal transfer seam in its products.
"We're the first automobile manufacturer to use the process as standard," he said. "That gives us a prototype of how the process works that we can apply to other models as well."
Volkswagen Sachsen GmbH, Glauchauer Strasse 40, 08058 Zwickau, Germany,49-375-552740, www.volkswagen.com
Fronius USA LLC, 10503 Citation Drive, Brighton, MI 48116, 810-220-4414, fax 810-220-4424, www.fronius.com
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