High-power solid-state laser (YAG lasers) took a quantum leap during the last few years providing higher available laser powers, better beam quality, improved electrical efficiency and reductions in price leading to a new, highly productive welding technique called \"Remote Welding\".
For about two decades, conventional laser welding with solid-state lasers, i.e. YAG lasers, has made its way to become an established and reliable process for many production industries. One of the main drivers for laser applications has been the automotive industry. Examples are car body and frame manufacturing, engines and powertrain, seat production and many other parts which have been seeing an increased amount of laser technology for both laser cutting and - more importantly - laser welding.
Due to the flexibility regarding installation of solid-state lasers featuring beam sharing and flexible laser light cables for transporting the beam to the work piece, solid-state lasers can be ideally used in combination with robots. The development of such laser processes started with conventional laser applications where only the robot motion defined the processing geometry. In a next step, the robot motion was combined with the motion of highly-dynamic optical scanner system. This combination of technologies allows to utilise synergies of the flexibility of 3D robot processing and highest productivity from the dynamics of laser scanner optics. Robotic laser scanner welding advanced to become the benchmark for efficient and economic high volume production. As a result, productivity compared to conventional welding technologies is several times higher.
During the last years, solid-state laser technology has evolved from lamppumped rod systems to diode-pumped rod systems first, to diodepumped 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 lamppumping. 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 expected to increase to 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. Last, but not least, for users of Disk Laser this means that not only the performance of such devices improves, but prices for say a 4 kW Disk Laser 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 improved beam quality of Disk Lasers allows the design of new optical processing heads with longer focus distances - without sacrifices of processing speed or focus spot size. For example, the 3-times better beam quality of a 4kW Disk Laser (8 mm*mrad) over an 4kW lamped-pumped laser allows a 3-times longer focusing length - while maintaining a focus spot diameter of about 0.6 mm, which still is the typical size for deep penetration welding. Hence, new welding optics can use focus lengths of 0.5m and more, and therefore 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 allowed to increase the field size of scanner optics, which allow to 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. Figure 3 illustrates the principle for a three-dimensional scanner optics.
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.
TRUMPF's scanner controller systems can be coupled with a robot motion controller to be fully synchronised with the axes of a robot (Figure 4). This allows extremely fast material processing while the scanner optics is being moved in space by a robot to enlarge the processing space and access the part 3 dimensionally. The technology of coupling two systems enables so-called "processing-on-the-fly". Today, this is the most productive welding technology available. Due to the very fast "jumping" of the laser beam from weld to weld by means of the scanner optics no time for repositioning the beam is lost. The velocity of the robot path is typically faster than the effective processing speed of the weld process. When physically looking at the process, it is difficult for the human eye to follow the 'fireworks'. Technical data for a typical Programmable Focusing Optics PFO 3D as already used manifold within the automotive industry worldwide:
The availability of modern lasers and scanners by itself is only half way to become a widely accepted production tool. Still, such systems have to be handled under rough production conditions by operators. This requires the implementation of further features to account for this. Programmable Focusing Optics display high user friendliness and easy of use by means of several important features:
• Real "welding-on-the-fly": What you see is what you get. No manual calculation of superimposed motion paths
• Graphical, CAD-type programming system for offline programming
• Teaching on the shop-floor, online
• All connections between PFO and Disk Laser are 'plug & play'. There is no need for adjustments or software re-programming
• Automatic program synchronization of scanner programs. This is of high importance in case a scanner ever needs to be replaced due to accidental damage.
• Sophisticated health monitoring system of PFO scanner optics
• TRUMPF's Telepresence to remote-access all laser and PFO data for diagnosing, troubleshooting and maintenance. This highly powerful tool allows software uploads from a service center all the way to the PFO, without onsite service personal.
The welding performance of a robotic laser scanner system strongly depends on the actual laser power used (available from 1 to 10 kW) and the design of the scanner optics. Generally, the higher the laser power the higher the welding speed, provided all other factors remain constant. Most applications in the automotive area are concerned with welding sheet metal between 0.6mm and 1.5mm thickness. In case of welding two 1mm thick sheets together, Remote Welding with a 4kW TruDisk Laser achieves approximately 100mm/sec effective welding speed. Higher powers behave nearly linear. The real boost in productivity results from time savings to re-position the focus point from one weld to the next.
Welding patterns are freely programmable by software. Some welding patterns used are shaped like the letter 'C'. Depending on the laser power, TruDisk lasers require less than 200 milliseconds to produce one weld, whereas Spot Resistance Welding typically requires two seconds for one weld. Many automotive users around the globe, among them large OEM's like Daimler, Audi, Volkswagen and various OEM suppliers, already apply this technology for high volume production of various components and car bodies. Examples are doors, side panels, rear shelves, seats frames, and other subassemblies of high volume. Figure 6 depicts an example of welding application already used in the industry.
To illustrate the performance gain of Laser Scanner Welding, let's look at a comparison between classical Spot Resistance Welding and Laser Scanner Welding based on the example shown in Figure 6. The key advantage is the reduction of cycle time by a factor of approximately three. This increase in productivity was achieved with a TruDisk 4002 with 4kW laser power. Such significant improvements have to be aligned and coordinated with material flow inside the plant, therefore more than 4kW of laser power may often not be of further advantage.
Experience from various real applications has shown gains in productivity over conventional Spot Resistance Welding! However, to determine the exact increase of any given part various factors come into play:
Size of the part Number of welds per part / total weld length Weld location / distribution across the part Laser power available Focus spot size and work space of scanner optics Synchronised motion "on-the-fly" or "stationary" welding operation Robot reach
Shape, length and distribution of the individual welds may be part specific. Experience has shown that many non-circular laser welds show higher strength than round, circular welds known from Spot Resistance Welding. This is due to better distribution of forces of laser welds, not concentrating in one little spot (Figure 7).
Another issue for today's welding applications is high-strength steel. Yield strengths continue to increase and have already exceeded 1000 MPa. In principal, the higher the strength the higher the sensitivity to heat input. Thanks to the lower heat input of laser applications compared to Spot Resistance Welding and even more compared to MIG-welding, laser beam welding remains a preferred methodology for welding high-strength steel. Yet another side effect of reduced heat input is the reduced distortion of the part.
An important consideration for welding of steel materials is zinc. State-of-the-art steels for automotive body production are typically zinc-coated on both sides. Zinc vaporizes at about 900°C when the underlying steel is not even melted. Hence, two layers of zinc enclosed between two pieces of sheet metal generate high vapor pressure when welding. If there is no gap between the sheets this pressure leads to blowouts of molten material, mostly through the top sheet. In effect, the weld may be weakened and leaky. Thus, a gap is required to allow the vapor pressure to laterally escape between the sheets. Although, in principle, there are many possibilities to generate this gap, laser technology may offer the most flexible and versatile solution. The same equipment used for welding can solve this problem by applying a process called "Laser Dimpling" before the sheets are brought together. This additional process step can be conducted using the same laser and scanner equipment as for welding later, modifying the typical process steps as follows (Figure 8):
1. Laser Dimpling of one sheet in areas of later welding
2. Loading of top sheet. Dimples maintain constant gap for zinc degassing
3. Laser Scanner Welding
Dimples can be produced very cost-efficiently using the same equipment, and with very high repetition rates. A dimple can be produced in about 10 milliseconds.
Laser technology today is a widely accepted technology and capitalises on high system flexibility, yet great throughput for high-volume production. Advancements of disk laser sources and scanner optics enable stable and highly-efficient remote laser welding processes. Given the significant advantages in processing time, less equipment is needed in comparison to other welding technologies and hence higher production throughputs can be achieved with less equipment and floor space.
This technology has already been successfully introduced in the European automotive industry in high volume car manufacturing and is expected to contribute to cost savings and higher flexibility for body shop applications and tier suppliers, today and in the future.
Thomas Schwörer has held various positions at Trumpf Laser since 1998 in the areas of laser welding and laser cutting. For many years he assumed a project manager position in USA, Germany and Mexico managing the largest industrial laser installations in the automotive industry. Since 2005 he has been managing the Remote Scanner Welding Technology for Disk Lasers.
