Single-sided, non-contact laser welding combined with high-speed scanning optics reduces weld times and increases productivity
Peter Busuttil
Remote laser welding is well on the way to being an accepted alternative to conventional resistance spot welding in automotive body-in-white applications. According to industry sources, there are more than 60 installations grouped mainly in Europe and North America. The concept of remote laser welding is to take advantage of the technical and economic benefits of single-sided, non-contact laser welding and combine them with the benefits associated with high-speed scanning optics, which dramatically reduces weld times to increase overall productivity in the welding process.
These advantages are most notable when one looks at the duty cycles for conventional laser/robot welding where 20mm stitches can be made in 0.2-0.4 second with a repositioning time up to 3 seconds, and with equivalent weld time for remote welding, repositioning time can be only 0.2 second. The key advantage in remote welding is the reduction in positioning time that the high-speed scanning beam provides (see Figure 1).
FIGURE 1. The key advantage in remote welding is the reduction in positioning time that the high-speed scanning beam provides.
Comau Pico has been an early and innovative promoter of remote laser welding, with installations that, in conjunction with forward-thinking automakers, break new ground in body-in-white operations. Among the company’s first accomplishments are the development, along with Rofin Sinar, of a CO2 laser based remote welder that utilizes a scanning mirror that deflects the beam, at very high speeds, to multiple welding locations on a body component.
On a typical part for the Fiat Marea, remote CO2 laser welding is used to replace resistance spot welding, primarily to eliminate the cost of placing necessary adhesives in the rear tailgate door. In this operation the total weld time for laser stitches is 5 seconds. On another component of this vehicle welding of the door frame, 43 laser stitches produced in 30 seconds replace conventional resistance spot welding. In both applications, repositioning time is reduced up to 94 percent.
Having had experience with Rofin on these previous applications, Comau recognized that this system offered the following: high productivity (up to 120 welds/minute), high flexibility, reduced floor space (a major cost factor in most auto plants), and the ability to weld a wide variety of part geometries.
However, regardless of the remote welding system used, the parts must be clamped tightly at every weld location. This can make the design and build of the fixtures necessary and in some applications quite complex. On the other hand, what is better, more economical, and uses less space, 6 to 8 fixtures for spot welding or just one for remote?
Comau also realized that the traditional scan box on this remote welding system was limited in that it could not move the remote welding head. So Comau developed and patented a fast beam delivery system that provides four axes of motion when mounted on a conventional gantry style system (see Figure 2). In this way the new unit allows coverage of a greater volume, the capability to move the beam at high speeds relative to part instead of moving the part, and a reduction in the complexity and cost of associated tooling. This concept allows the users to weld anywhere in the gantry envelope window.
FIGURE 2. The traditional scan box is replaced by a patented fast beam delivery system.
Dubbed the Agilaser, a dozen or more of these remote laser welders are now being used at Fiat plants in Mirafiori, Italy, where they weld components for the Idea and Musa, in Melfi where they are used to weld parts on the Punto, and in Cassino where they weld parts of the new Stilo. Units are also installed at Renault in Novo Mesto, Slovenia, for welding components of the Clio and Twingo and at Saundoville, France, for the Megane. Sevel in Sevel, Italy, welds parts for the Ducato and Stola in Pomigilano, Italy, for the Alfa 159.
At the Mirafiori plant an Agilaser replaces a robotized resistance spot welding line with a two-fixture remote welding system for door assembly that produces a 15 percent reduction in capital investment and a 20 percent improvement in productivity, while decreasing the floor space required for the conventional four robot welding systems by 50 percent. The result of this is 30 percent reduction in cost per part using laser remote welding.
Renault uses an Agilaser to weld front door components of the C85 (see Figure 3), replacing a former unit that required 12 robot resistance welders that took 1050 m2 of floor space with a five robot station Agilaser that occupies 808 m2. Producing 93 right- and left-hand laser stitches, as opposed to 130 right- and left-hand resistance spots, two Agilasers produce components on a 66-second cycle. At Renault, an Agilaser makes 38 shaped laser stitches on the front doors of the C65 model using twin fixtures.
Although these CO2 laser systems are well established with documented up time and availability in excess of 98 percent, they are still not regarded as mainstream process capable due to their inherent CNC machine construction. Almost all automotive component and body welding is accomplished through articulated robots, and if lasers are to be used, the beam delivery of choice is through fiber optics.
As part of an effort to carry remote welding to the next level, Comau developed a 3D scanner as part of a European “Remo-weld” project and it was first shown two years ago at the Munich laser show. The prototype system is powered by a Rofin Sinar 4.0kW diode-pumped Nd:YAG laser with the beam delivered through a 400-micron fiber to the base of a generic robot. Comau Pico quickly realized the limitations of such a set up, especially with the advent of better beam quality (high brightness) lasers such as the disk lasers and fiber lasers.
In most remote laser scanners, an F-theta lens is used to keep the focus point of the beam at a fixed distance from the lens independent of beam orientation. This high-cost device has a large diameter, and the working volume is a little larger than this diameter because the F-theta keeps the beam relatively perpendicular to the lens with the sacrifice of reduced beam quality and a varying spot size. This also means that the scanner must be continuously repositioned and/or oriented by the six-axes robot with the dynamics of a standard articulated system. Comau Pico’s SmartLaser (see Figure 4) essentially integrates the optics into a hollow arm robot. This optical collimating/zoom module (a telescope) takes the place of the F-theta modules used on conventional laser scanner devices (see Figure 5).
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FIGURE 5. The optical collimating/zoom module takes the place of the F-theta modules used on conventional laser scanner devices.
The idea behind the resulting SmartLaser is to create a high-speed 3D system with acceleration 60 to 70 times higher than a robot with 8g dynamics within the welding zone. This system has a programmable focal distance greater than 750 mm, in fact 750 to 1200 mm, which greatly expands the working distance of the unit. This also means that all optical axes, X, Y & Z for the telescope and scanning head are fully integrated into the welding system with full off-line programming capability. The novel scanning head (see Figure 6) at the end of the forearm includes two optics, the first with 30 degrees of movement and the other with a full 240 degrees of movement. This assembly allows fast repositioning times with a two- to three-fold increase in depth of field and negligible spot size distortion.
FIGURE 6. The scanning head at the end of the forearm includes two optics, the first with 30 degrees of movement and the other with a full 240 degrees of movement.
Unlike other scanner remote welding units, all system axes, dynamic and optical, are governed a by a single standard axis controller through a teach pendant. This makes programming very easy with true full control of all weld velocities, beam orientation, and laser functions while repositioning.
The robot has a maximum tool center point speed of 1.5 m/s with an acceleration of 1.2 m/s2. The lens axis has a maximum speed of 4 m/s with an acceleration of 80 m/s2. The all-important positioning time in the Z direction for a 100mm distance is 70 ms for the SmartLaser versus 580 ms for a conventional robot with 2D scanner. This means that we can weld up to a remarkable 200 to 300 stitches per weld cycle.
The optical fiber and coupler is mounted before the arm instead of before the scanner and therefore experiences low mechanical stress and fatigue with less potential for fiber damage. This and other features reduce maintenance requirements.
Comau is now testing the first of the SmartLaser units in Europe, and before the first of the year will have one available for customer tests in the United States. The company expects to see the first orders from the auto industry early next year with installations before the end of 2007. It is conceivable that a time-shared multi/robot installation could be used to process more than one component serially, such that the laser is welding all the time as new parts are located in other work stations.
Conclusion
The concept of remote laser welding is still evolving, and systems are being streamlined to meet customer production demands.
Suppliers are working diligently to diminish the impact of high initial equipment cost and fixturing complexity. Developments in laser technology, such as shorter wavelengths and fiber and optic delivery, are under investigation, as is the ability to eliminate shielding gases.
Multi-cells using a common laser to maximize laser-on time for higher product throughput are a reality.
Peter Busuttil ([email protected]) is manager of process development - lasers at ComauPico, Southfield, MI.