FROM SPOT TO SEAM

For welding the body of the new Peugeot 3008, which will be launched in summer 2009, PSA Peugeot Citroën is using three TruDisk 6002 lasers as well as a TruDisk 4002 from Trumpf (Ditzingen, Germany). These disk lasers, with power ranges of 6 kW and 4 kW, are used to weld the doors and body reinforcements of the new Peugeot model at the assembly plant located in Sochaux, France.

Instead of individual spot welds, these disk lasers lay a full-length weld seam. ?This enables us to achieve high body stiffness in our new Peugeot 3008,? explains Jean-Charles Schmitt, Product and Process laser manager at PSA, who describes the benefits of laser welding.

During the last few years, solid-state laser technology has evolved from lamp-pumped rod systems to diode-pumped rod systems, then to diode-pumped disk, and then to fiber laser systems. This allowed a quantum leap regarding efficiency and beam quality.

The main reason for such improvements is the use of semiconductor diodes for pumping the laser crystal, emitting only one wavelength of light which is best absorbed by the crystal. Optical-optical efficiencies for such systems reach approximately 65 per cent for disk lasers today, enabling an overall "wallplug" efficiency of up to 30 per cent, improving by about ten times compared to lamp-pumping.

Another main advantage of disk lasers lies in the design of the laser-active crystal itself. For rod systems, the thermal impact of the pumping light causes thermal lensing effects which limit the achievable beam quality. New disk lasers are designed such that the temperature inside the crystal (a ?disk?, therefore the name disk laser) remains constant across its surface. Figure 1 illustrates the difference between the two types. Hence, the beam quality achievable with disk lasers can be much higher than that of a rod system, improving the beam parameter product (BPP) up to 6 times.

Due to improvements in the area of semiconductor pumping diodes the potential of disk lasers is not exhausted. While the first generation ?only? extracted 1kW of laser power out of one disk, today's generation already generates 2kW out of one disk crystal. Still, the potential for this technology is not limited and reached 4kW per disk towards the end of 2008.

Further, by combining several individual disk cavities, as illustrated in Figure 2, the total available laser power of a disk laser is virtually unlimited. The pumping beam from diode pumping stacks is reflected multi-fold via mirrors inside the cavity to pass up to 20 times through the disk. The disk ?converts? the optical pumping light into a laser beam for processing. Based on an existing 4-cavity design, a laser power of 16kW will soon be available. The beauty of this disk laser principle over the fiber laser principle is that there are no losses in beam quality when scaling up laser power. These improvements in beam quality and power also lead to significant advantages for the design of processing optics and allowed the development of high-power scanner optics.

It is hardly necessary to mention that indispensable features known from conventional lamp-pumped lasers have not changed: disk lasers offer closed-loop power control, are insensitive against back reflections returning from the workpiece, their availability (uptime) is greater than 99 per cent and due to their modular construction all components can be replaced and maintained in the field.

As the performance of these devices improves, the prices for 4 kW disk lasers are falling because less cavities are required to generate the same laser power. Hence, technology advancements will continue to enhance competitiveness over alternative welding technologies.

The use of the disk laser also allows PSA engineers to develop new design options. ?Compared to conventional spot welds, full-length weld seams in car body manufacturing requires less sheet folding and thickness due to a specific laser design, and thus saves about 5 kg of weight per vehicle,? states Schmitt. PSA also could increase the size of the rear-quarter windows, allowing better visibility for the driver and the passengers.

The four TruDisk lasers used in production supply ten processing stations, which the Trumpf LaserNetwork, in turn, supplies with the necessary laser power for welding. ?The LaserNetwork enables us to take optimal advantage of the available power from the four disk lasers and achieve capacities of almost 100 percent. With this network design, PSA does not need any backup laser source,? notes Schmitt, who adds, ?In case of a problem, we can easily switch the whole pool of stations on the three remaining sources, with little production loss.?

For future applications, the improved beam quality of disk lasers allows the design of new optical processing heads with longer focus distances ? all without sacrifice of processing speed or focus spot size. For example, the 3X better beam quality of a 4 kW disk laser (8 mm mrad) over an 4kW lamped-pumped laser allows a 3X longer focusing length ? while maintaining a focus spot diameter of about 0.6 mm, which is still the typical size for deep penetration welding.

This means new welding optics can use focus lengths of 0.5 m and more and therefore be classified as ?Remote Welding?. In turn, larger working distances reduce contamination of such optics significantly and prolong the lifetime of the protection glass, hence contributing to reducing running cost. Further, the emergence of high-beam quality lasers increases the field size of scanner optics that position the beam via movable mirrors driven by galvanometer motors. Programmability of such scanner optics enables processing of any weld shape within the processing area.

Due to the low mass of the mirrors, such optics are extremely dynamic and there is virtually no time loss to reposition the beam from one weld to the next. The construction of the Programmable Focusing Optics (PFO) 3D is such that all axes can position the beam in 3 dimensions at highest speed. All axes can reposition the beam in less than 30 milliseconds from one end to the other end. Coordinated motion between the axes allows the processing of any weld patterns, e.g. lines, circle, brackets, etc.

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Trumpf GmbH + Co. KG, P.O. Box 1450, 71252 Ditzingen, Germany, +49 (0)7156 303-31559, www.trumpf.com.