VW—driven to lead

May 1, 2004
In 1992 Volkswagen AG (VW; Wolfsburg, Germany) began testing the laser as a possible heat source for the welding of automobile sheet metal, in the body-in-white condition.

With a committment to laser processing and dreams of a laser power house, volkswagen chooses disk over lamp-pumped and high-power fiber lasers for the near term

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In 1992 Volkswagen AG (VW; Wolfsburg, Germany) began testing the laser as a possible heat source for the welding of automobile sheet metal, in the body-in-white condition. The first welds were made on a Golf III body, using a 2.0kW CW Nd:YAG laser from Haas (now TRUMPF). Shortly thereafter VW commenced production line welding with five of these lasers in two plants on the Passat B5 model that had been specifically designed with laser welding in mind. This equipment is still in operation today. Although VW was not the first automaker to employ lasers in an on-line body-in-white operation (that honor goes to Volvo), VW is currently the leader in implementing laser welding technology for zinc-coated steel welding applications.

With rumors concerning VW's commitment to laser welding rampant in the industry, ILS approached the company requesting permission to interview Klaus Loeffler, currently director of joining for the VW brand. Loeffler is well known in the United States where, for seven years, he was responsible for TRUMPF's laser activities and its efforts in the auto industry.

For several years VW has kept its laser joining activity under wraps, preferring not to tip competitors off to the major technology advances the company was making. A year ago the company began to ease this restriction and Loeffler presented, for the first time, details of the technology status at the Laser 2003 meeting in Munich, Germany. However, non-attendees only learned of the talk by word of mouth, as proceedings did not include his talk. In January of this year at EALA, Loeffler presented an overview of the laser activity at the Wolfsburg facility, a talk he repeated at this year's ALAW meeting in Ann Arbor, MI.

Those unfamiliar with automotive manufacturing and especially laser in auto production may wonder why the VW revelations have caused so much industry gossip and curiosity. Simply put, VW is the world's leading user of high-power lamp-pumped, CW, Nd:YAG lasers for auto body welding. Many numbers have been bandied about but ILS has learned that currently VW operates more than 450 of these lasers in plants in four German cities and in nine other countries. Models that have been or are now being welded include the following: Lupo, Polo AO3 and 4, Jetta, Touran, Caddy, Passat B5 and 6, Eurovan, Phaeton, and Touareg. Audi, one of the Volkswagen brands, utilizes lasers on its products as well (A2, A3, A4, A6, A8). The same is the case for the other brands of the Volkswagen group such as Skoda, Seat, and Bentley. Body components welded on these vehicles include: doors, roofs, hoods, side frames, B-pillars, underbodies and trunk lids. These are made from zinc-coated steel and aluminum in case of the Audi A2 and A8.

If that isn't an impressive record, consider this: VW in Wolfsburg operates 150 4kW Nd:YAG lasers in one line to produce the body-in-white for the Golf V, the single largest high-power solid-state installation in the world. This operation was the subject of the ALAW presentation, following which ILS was granted an authorized, exclusive interview with Loeffler.

Robots position the Golf V body-in-white for laser welding/brazing.
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But first let us offer a description of the Wolfsburg Golf V joining operations, which began in 2003. Note we said joining, because the Golf V employs both laser welding and brazing applications. To start, Nd:YAG lasers weld and braze front and rear inner side panels using a headstock/tailstock weld fixture in the side panel lines. The braze joint is necessary because on one panel it is visible to observers. Then, in two framing stations, 14 4kW lasers, some with dual heads, weld and braze the side panels and roofs, producing 3400 mm of laser braze and 5340 mm of laser weld in 68 seconds in each station. At this point the body has been 100 percent laser processed. Further down the line laser robots make additional body welds in flexible cells that provide for quick changeover of product or for routine model changes.

Meanwhile, off-line, door inners or the front and back, left and right are welded using several laser/robots that employ a beam switch to maximize weld speed with a lesser number of lasers. Finally a laser robot system customizes the body by making penetrations for special orders.

All the above requires 150 4kW Nd:YAG lasers, one 1.0kW Nd:YAG, 250 laser welding heads, and three laser cutting heads, all running at 99.9 percent uptime. These are all networked for control in what Loeffler considers the most technically challenging of functions in this line. In a normal day 2400 bodies are welded with a total of 140 km of joint length. This takes about 42 MW of power for the lasers and 18 MW of power for the necessary cooling.

What does laser welding of the body-in-white offer Golf V drivers compared to the Golf IV? An 80 percent increase is static torsion stiffness, a 15 percent increase in dynamic torsion stiffness, and a 35 percent increase in dynamic bending stiffness. For VW laser welding/brazing has done the following: increased joining process speed, increased productivity, shortened cycle times, increased module strength, reduced weld heat distortion, narrowed weld flange widths reducing weight, offered joining flexibility, and reduced floor space. In the case of the latter, floor space for side panel assembly was reduced 50 percent and that for the underbody 33 percent.

This didn't come cheaply; as Loeffler pointed out, lasers can run to $500,000 each and the company is using 150 on the Golf V line. Also a workforce with the necessary training was not available locally leading to intensive education programs. Not to mention the huge power bill.

In the conclusion to his prepared remarks Loeffler stunned many among the 200 attendees when, addressing the future, he stated that VW has decided that the next generation of joining lines will likely be powered by the disk laser rather than the lamp-pumped Nd: YAG laser. Generally it has been assumed that the high-power ytterbium fiber laser would be the next laser technology for VW. In an exclusive interview with ILS Loeffler stated that the company had tested high-power versions of the fiber laser and determined that, at this time, that product was not ready to be integrated into VW body-in-white production lines. The two prime reasons given for this decision are the lack of fiber laser control systems and the missing required reliability, an important factor because the complexity of the joining lines is key to the success that has been achieved in Wolfsburg on the Golf V line.

Loeffler had created waves in the laser industry before. Last June at a Munich conference he laid out his concerns for the current resonators being used at VW, in terms of efficiency, floor space, ability to deliver the beam up to 100 meters through fibers, beam quality, and fast repair times. At that time many in the industry read this as disenchantment with lasers in general, which he contradicted in our interview saying that he was actually sending a positive message to the industry. Apparently one company heard him because, he says, the developer of the high-power disk laser, TRUMPF, was in his office to ask what it was that VW required to meet its future needs.

In his opinion, Loeffler believes that the TRUMPF disk laser, at 4 kW output power, is close to being ready for implementation for applications on the successor to the Golf V. In effect this disk laser (see sidebar) may be a drop-in replacement for the current lamp-pumped Nd: YAG lasers, except they will offer higher efficiency, better beam quality, and the ability to deliver the beam up to 200 meters. The latter is the first step in what Loeffler sees as fulfilling his dream of a laser power house, where centrally located, high beam quality lasers send power to welding heads, on demand.

Commenting on laser efficiency, which will be improved dramatically with the introduction of the more efficient disk laser, Loeffler suggests that the powerhouse concept can significantly reduce the number of lasers needed in future operations because the energy of a given unit can be directed effectively to where it is needed, only when it is needed. Now some lasers are not used to 100 percent of their time available potential.

Prospects for the fiber laser are still bright as VW, part of a consortium of German automakers, is already in discussions with potential vendors of products that will come closer to meeting its projected needs. VW is sharing its laser experiences with others in the consortium and Loeffler mentioned Audi, BMW, and Daimler Chrysler. One of them is planning heavy laser usage for an unnamed project.

Asked specifically if VW, and indirectly the consortium, were open to new, non-German suppliers, we were assured by Loeffler and others at VW that they are in discussions with other laser suppliers, one an Asian-based laser/laser equipment manufacturer. Loeffler says the company has laid out its wants with several European fiber laser developers and that he sees the progression of industrial lasers at VW to be disk, fiber, and direct diode. The latter he sees as the ultimate for on-line applications, eliminating the need for fiber beam delivery. He has confidence that the current beam quality issue with diode lasers will be solved in the foreseeable future, making them a viable high-power welding heat source.

Looking back at the company's original decision to use solid-state lasers rather than CO2 lasers for the original Passat B5 welding applications, Loeffler said that fiber delivery and the flexibility this offered was a major consideration. Yes, he said, the better beam quality and higher efficiency of the CO2 lasers were attractive, but the on-line requirements at Volkswagen made fiber delivery, rather than hard optic CO2 beam delivery, attractive. Further, about that time, 1992, the advantage of welding without shielding gas with Nd:YAG lasers became known. With the 1060nm wavelength of these lasers energy passes though vaporized metal (plasma) allowing direct impingement of the beam in to the workpiece. This is not true with the 10600nm wavelength of the CO2 laser.

Because CO2 lasers have a long history of excellent uptime performance we asked about VW's experiences with the high-power Nd:YAG (which all happen to have been supplied by TRUMPF). To date among the more than 450 of these lasers in production, uptime is >99 percent. Loeffler says that coming to VW from a laser suppler gave him a different perspective on lasers in production applications. So one of his first actions was to determine exactly what was causing any recorded downtime on units in use. He found that many times the person making repair notes simply listed a laser problem, when, in reality, he found that the difficulties lay elsewhere: in robots, electrical, sensors, and in some cases with the materials being processed. As a result he now reports uptimes in excess of 99.5 percent and at Wolfsburg in excess of 99 .9 percent.

Such performance is outstanding in any installation and more so in an auto plant. This led to a question about the original concept for laser joining and the driver behind what turned out to be a massive investment by VW in laser technology. Credit for the first laser welding on the Passat in 1993 goes to Dr. Folker Weissgerber, vice-president for production, who had the foresight, and in retrospect the team that could achieve the spectacular results today at Wolfsburg and other VW locations.

VW chairman Ferdinand Piech gets credit for corporate support for laser welding when, in a now famous public comment, he mandated that all VW autos would have zero gap tolerance on the roof joint. No more sealers, no more visible joints.

Can you imagine the chairman of a Detroit Big Three automaker throwing down this gauntlet and then expecting to have it happen? Anyone familiar with Detroit can picture the laughs this would have produced in the stamping plants, where for years the refrain has been, "we can't hold the tolerances needed for laser welding".

We asked Loeffler if there was resistance in the design offices and the stamping plants to the chairman's dictates. The answer, "they wouldn't dare." However, Loeffler did indicate that future designers would require more education in the advantages of laser processing so that new car bodies can take advantage of the benefits the technology offers.

Loeffler sits in a unique position. As director of joining for the VW brand he has responsibilities beyond just the laser. So other joining techniques, such as resistance spot, epoxy bonding, diffusion bonding, and hybrid arc/laser, are all within his purview. His authority comes from his placement just three steps below the chairman. He is positioned so that his people can interact with concept engineering people and eventually with managers on the production floor. It's a daunting, challenging, but stimulating job.

There is no question that the VW commitment to laser processing has made a dramatic change in laser technology. Frankly, the accelerated development of high-power fiber lasers, the disk laser, and in the future the high-power direct-diode laser are directly attributable to VW actions. Several years from now, when Loeffler's dream of a laser powerhouse is reality, we will all look back to give credit to VW.

Disk laser—the next hot technology

In concept the disk laser is a simple device, just replace a single crystal Nd:YAG rod with a thin disk of the Yb:YAG. In practice it gets a little more complicated, especially in the internal optics used to increase the gain in the device.

What caused researchers at the University of Stuttgart to consider the thin disk as a beam source was the persistent thermal lensing problem associated with rod-type solid-state lasers, especially as the output power demand increased to the multi-kilowatt level. With a rod, circumferential radial cooling the length of the rod is necessary to prevent thermal problems, even in a well-designed unit. With a thin disk, heat is directed axially through the thickness to a heat sink so that a homogenous temperature profile is attained with virtually no thermal lensing.

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TRUMPF has developed a disk pumping arrangement featuring a parabolic mirror system, a prism coupling unit, homogenizer, a beam collimator, and diode pumping to produce a device that can be scaled to multi-kilowatt levels at high efficiency (a single 600W diode laser stack produces 300W output power) and vastly improved beam quality (8 mm/mrad). And the output, up to 4 kW and higher, can be injected into a 200-micron fiber.

What this means is that Klaus Loeffler gets his wish for an improved high-power solid-state laser with the flexibility to deliver the beam up to 200 meters from the laser. Step one in his powerhouse dream.

For users, this means thick sheet metal can now be cut with solid-state lasers, higher processing speeds, shorter cycle times with lower total heat input, and more efficient power utilization.—DAB

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