Laser: The Disk Advantage

Although TRUMPF Inc.’s (trumpf.com) laser division has a full array of industrial laser types—CO₂, disk, fiber, diode, rod—and while David Havrilla, manager of Products and Applications there believes that there are appropriate tools for appropriate tasks, when it comes to high-power applications typical of both OEM and supplier welding operations (think in the context of 4- to 6-kW requirements; typical cutting tasks are performed at lower power, say 1 kW), the disk laser oftentimes provides the sorts of advantages that cannot be readily overcome by the other types of lasers in their portfolio.

And yet before taking a look at the characteristics of the disk, it is worth noting that Havrilla is bullish about the potential of the direct-diode laser. He considers it to be “disruptive technology,” and that it will quell the debate that currently occurs in laser circles between partisans of the disk and fiber lasers when it comes out in its high-power, high beam quality form some three to four years from now. While direct diode lasers are available today, they don’t have the power and beam quality necessary for many automotive applications. He anticipates that those issues will be addressed within the next few years. The reason that he thinks the direct-diode laser will achieve great popularity is because of the fundamental efficiency of the system. He explains that whether it is a disk or a fiber laser, there is a “middleman”: diodes are used to pump a medium to generate the lasing. With the direct-diode laser they’ve taken out the middleman. Consequently, whereas the “wall-plug efficiency”—what you get out of a laser compared with the energy put into it—for a disk or fiber-type is on the order of 25% (and know that this is really a big gain compared to the efficiencies realized as few as five years ago), the direct-diode laser should have a wall-plug efficiency on the order of 40%. What’s more, it will cost less.

But there are a lot of cars and trucks and associated components to be made between now and then, so it is worth considering the capabilities of a disk laser.

Havrilla says that TRUMPF is on its third generation of its TruDisk disk systems (launched in January 2009). The development has followed a path of more performance out of a smaller package. That is, say it is a 4-kW system. The first gen required four cavities, with a cavity containing the disk, pump and four diode modules. The second gen halved that, so there were two cavities, each containing a disk and four diode modules. Now it is down to one. What’s more, the diode life has increased significantly, from about 20,000 to 25,000 hours to about 50,000 hours or more.

All told, from a size comparison, a current 4-kW system is 61% smaller than the first-generation laser. What’s more, the current laser costs about half of what the original one did. (Part of the reason why they’ve been able to reduce system costs is because the company now produces its own diodes in a facility in Princeton, NJ.)

A burgeoning application for disk lasers in the auto industry—particularly in North America, as applications are underway in Europe at companies including Volkswagen, Daimler, and PSA Peugeot Citroën—is remote laser welding. Essentially, this is an approach that has a standoff distance of from 0.5 to 1 m between the workpiece and the programmable focusing optics (PFO) package, which may be on the end of a robot arm. The PFO contains scanning optics for the X and Y axes. Through the use of mirrors, the beam is quickly located onto the workpiece surface as required: consider that if the focal distance is 1 m and the requirement calls for spots to be separated by 25.4 mm, it doesn’t take much in the way of adjustment to get there. One immediate consequence is that there is maximum beam-on time, which helps justify the cost of the equipment.

Another benefit Havrilla points to is the fact that the programmability means the shape of the weld is precisely the shape. That is, if a sinusoidal or a C shape is required, then the PFO facilitates generating that shape, especially as compared with a setup wherein a robot is manipulating a conventional laser welding head. To be sure, the robot could be programmed to generate a shape, but if the issue is throughput, then the amount of time necessary compared with the PFO setup would be far greater.

In practice what this means, for example, is not only can the weld shape be tailored based on the end-product strength requirements, but also the weld length. Consider a door opening. It is possible to have stitch welds every 50 mm around the door frame, yet switch to a continuous weld at the pillars, where strength is needed.

And this speed of the disk laser coupled with remote welding is key to another benefit, which is a greatly reduced factory footprint, especially as compared with the spot-welding equipment typical of body shops in North America. According to Havrilla, in European applications of remote laser welding, space savings is on the order of 30% to 50%.

Better welds faster with less floor space? And the downside of this is…?

Laser Welding Galvanized Material Made Simple

The problem with welding galvanized material is the zinc. When two coated sheets are brought together for welding, the zinc turns into gas, and the gas needs some place to go. Otherwise, the result is a bad weld.

So a solution that has been developed is to dimple the surface of the bottom sheet, so that when the top sheet is put on top the dimples serve as escape routes for the gases.

According to David Havrilla, mana-ger of Products and Applications, TRUMPF Laser Div., this dimpling has been demonstrated by lasers in the past, but in order to do it, two lasers have been proposed: a pulsed laser to generate the dimples and a CO₂ laser to perform the welding. Which is expensive, and somewhat unwieldy, particularly as compared to using a diode-pumped disk laser, where combined with on-the-fly remote welding, one laser can manage both tasks.