Thursday, 23 June 2011 16:20

New Camera-Controlled Surface Laser Welding, and That’s Not All

Written by Jeremy Hayhurst

At the Laser World of Photonics 2011 trade fair held in Munich, Germany, May 23–26, Fraunhofer-Gesellschaft scientists presented some amazing new applications for lasers and related automotive technologies.

 

As any self-respecting tech geek knows, LASER stands for “light amplification by stimulated emission of radiation,” but only the geekiest know that self-lighting lasers are really optical oscillators, rather than amplifiers, and therefore the acronym should have been LOSER, for “light oscillation by stimulated emission of radiation.” It’s too late to change now.

Regardless, lasers have long since eclipsed the Buck Rogers image that brought them into the general consciousness in the late fifties. In a former life, I edited a well-known laser science textbook, so I have some familiarity with the concepts, but I was taken with the following discussion from Fraunhofer-Gesellschaft in Germany on how indispensible laser technology has become in automotive manufacturing. It’s about to get more so.

What’s not so clear is how this now proven but still pending technology, which has just caught the eye of the OEMs, will affect car designs and repairability for shops in the future.

Existing Laser Welding
Conventional laser-beam welding, which is being utilized to improve production efficiency using robotic welders, has evolved into an efficient production jointing method because it combines a high bonding quality with high speed processing. A high intensity laser beam is focused onto the working surface with a diameter of several tenths of a millimeter. The extreme high temperatures cause the auto body metal to instantly melt. A capillary filled with metal vapor develops, which may extend to the underside of the surface. If it does, a feature known as a full-penetration hole is formed. This hole is closed again by the weld puddle as the laser beam, or production line, travels forward. The full-penetration hole is an indicator of the strength of the weld by assuring that the whole cross-section of the sheets is used. However, seam irregularities like spatters, craters, or underfill repeatedly occur in this complex and highly dynamic process. They lower the quality of the welds or, in the worst case, render them unusable.

 

An important quality feature for the strength and quality of the weld seam is complete joint penetration. You have to use highly optimal procedures to avoid both insufficient and excessively strong joint penetration (with underfill).

Conventional production monitoring techniques register the full-penetration hole with cameras by acquiring images and evaluating them afterwards for quality assurance. However, for real-time process control you need to generate a feedback signal that can be processed significantly faster than has until now been possible. The reason is because of the very rapid movements of the full-penetration hole in the melt require frame rates of several kilohertz for an accurate analysis of the contour. So how could this be improved?

How would a perfect automotive weld be done?
As if controlled by an invisible hand, the welding head on the robot’s arm races along the sheet metal parts. Where the laser hits, sparks fly and the metal glows red hot. The process lasts just a few seconds. The outer door panel and the door frame are now welded together perfectly. A thin weld seam extends along the join, but it can only be seen on one side. From the other side of the welded car door the join is invisible. This is a perfect weld—the kind every car manufacturer dreams of, because it could be used anywhere on the car body. Expensive design work to hide the seam, such as folding the sheet metal or covering with trim, would no longer be necessary.

The news is that research scientists at the Fraunhofer Institute for Physical Measurement Techniques IPM in Freiburg, Germany, have already turned this car makers’ dream into reality.

Controlled partial penetration welding is how experts refer to the process in which the laser does not burn right through all the sheets of metal, in contrast to full penetration welding, where a hole briefly forms in the melt pool. Instead, the weld seam is controlled to penetrate the lower sheet without damaging the bottom surface. Up to now, however, it was not possible to precisely control this type of welding and produce a seam that reliably meets strength requirements. Surface welding, instead of penetration welding, allows a laser to produce a weld that is only visible on one side. But how do you control the laser power to prevent it burning a hole through the sheets of metal? The answer is a new camera system that analyzes thermal images in real time, controls the beam, and ensures a perfect weld.

“As we do not weld through the sheet, basically we cannot see what we are doing,” says Andreas Blug, project manager at Fraunhofer IPM. Their solution uses a ground-breaking camera that generates temperature images, which enables the system to recognize how deep the laser has penetrated into the sheets. Where it burns into the metal, causing it to melt, the images show a hot region. If the bottom of the melt pool reaches the gap between the upper and lower sheets, the conduction of heat is interrupted and a cooler point can be seen. This is the full-penetration hole. From the relative frequency of this full penetration hole the system calculates the penetration depth into the lower sheet. A software program then adapts the output of the laser to the specific requirements.

“The process is closed loop controlled in real time,” Blug explains. An extremely rapid camera system is needed for this.” The system is based on cellular neural networks (CNN). A tiny processor is integrated in each pixel. They all work simultaneously and speed up the analysis of the individual images enormously—whereas, in conventional image processing systems, a few processors process the data consecutively.

“In this way the system analyzes up to 14,000 images per second,” says Blug. This compares with the usual rate of only 1,000 to 2,000 images per second. Together with colleagues from Stuttgart University and Dresden University of Technology, the Fraunhofer IPM research scientists have now developed a prototype which perfectly controls the surface welding process, offering car makers a further great benefit in comparison with full penetration welding: zinc does not vaporize on the bottom side of the weld. The corrosion problems encountered on galvanized car bodies are therefore a thing of the past.

Laser Technology in New Applications
Conserving energy is a top priority for auto manufacturers today and laser technology is helping that goal. In addition to welding, lasers are  being used to process thin light-weight components made of fiber-composite materials, as well as to manufacture more efficient engines and more powerful batteries.

Lasers take the lead
Cars rolling off assembly lines today are cleaner, quieter and more efficient than ever before. However, ever-stricter environmental regulations and steadily rising fuel costs are increasing the demand for cars that further reduce their impact on the environment. In the U.S. this emerging target is the 62 mpg fleet average fuel standard that is causing concern among politicians and automakers alike.

Even present day customer demands are tough for manufacturers to meet: car bodies should be safe yet light-weight and engines durable yet efficient. Year after year, new models must be developed and built that can claim to be better, more efficient, and more intelligently designed than the last.

The race against time and competitors places high demands on manufacturers and their suppliers to innovate and laser technology is increasingly effective in meeting those goals.

Resistant to wear and universally applicable, laser light is an ideal tool in the manufacture of vehicles. Lasers can be used to join, drill, structure, cut or shape any kind of material. Surfaces can be engineered for motors and drive trains that create less friction and use less fuel. Lasers are not only a decisive tool in getting faster, more efficient and economical production, but also in making better energy-saving vehicles.

A weight-loss program in automotive manufacturing
Extra vehicle weight costs energy. Vehicles have to be accelerated and decelerated every time they’re driven, over the entire lifespan of the vehicle. To reduce weight, manufacturers are increasingly turning to the use of fiber-reinforced plastics, which are 30-50 percent lighter than the metals they are replacing. However, these new materials are difficult to process. Fiber-reinforced plastics are brittle, meaning conventional cutting and drilling tools are quickly worn out and the standard assembly techniques used for metal components are often not effective.

“Lasers represent an ideal alternative here,” explains Dr. Arnold Gillner of the Fraunhofer Institute for Laser Technology ILT in Aachen, Germany.

“Lasers can cut fiber-reinforced plastics without wear and can join them too. With the appropriate lasers, we can cut and ablate (remove material from the surface of an object) the components with minimal thermal side-effects. Lasers can also be used for welding light-weight components —a viable alternative to conventional bonding technology. We can even join fiber-reinforced plastics to metals with laser welding. The laser roughens the metal surface, while the plastic, briefly-heated, penetrates the pores of the metal and hardens. The results are very stable.”

Weight reduction can also be achieved with high-strength metallic materials. These, however, are also difficult to process.

“Joining combinations of various materials allows us to make optimal use of the individual materials’ specific properties. But this proves to be difficult in many cases,” explains Dr. Anja Techel, Deputy Director of the Fraunhofer Institute for Material and Beam Technology IWS in Dresden. Her team believes in lasers: “With our newly-developed integrated laser tools, we can now even weld together combinations of materials, free of fissures or cracks.”

At Laser 2011, Fraunhofer scientists presented, for the first time, a new welding head capable not only of focusing with extreme precision but of moving back and forth across the seam with high frequency to mix the molten materials. When they harden, they create a stable bond.

Lasers Replace Chemistry
Lasers also save time and money in tool design. The molds used in the production of plastic fixtures and steering wheels, for example, have to be structured to give the finished component a visually and tactilely appealing surface. Most car manufacturers order a design from their suppliers, whose surface typically has the appearance of leather. Until now, the negative pattern used to create the design has been etched out of the steel tools used in injection molding—a tedious and time-consuming process. “With lasers, the steel surface can not only be patterned more quickly, but also with greater scope for variety,” explains Kristian Arntz of the Fraunhofer Institute. “We can transfer any possible design directly from the CAD model to the tool surface: What will later become a groove in the plastic is preserved as a ridge, while the surrounding material is vaporized. The process is efficient, fully automatic, and highly adjustable.”

Saving Energy, Low Friction Motors
Laser technology is also applied in engine optimization. Engineers strive to keep friction as low as possible in order to improve efficiency. “That is true not only for the electric engines currently being developed, but also for classic internal combustion engines and diesel motors, as well as transmissions and bearings,” says Arnold Gillner. Ceramic, high-performance coatings are especially desirable, because they are not only resistant to wear but also smooth, which generates less friction. Coated metal components have until now been prohibitively expensive, being produced in plasma chambers in which the ceramic was vaporized and applied to the surface of the components. Fraunhofer scientists have now developed a less expensive and faster method in which work pieces are coated with ceramic nano-particles, then treated with a laser. This finishing process has already been applied to gear wheels and bearings.

Lasers can also be used to make specific modifications to the properties of engine parts. “Friction between the cylinder wall and piston is responsible for a big part of a motor’s energy consumption. That is why we try to minimize it. This is especially important for engines featuring modern, automatic start-stop functions that are stressed by frequent ignition,” says Gillner. “To protect them, we ensure that the cylinder is always coated with a film of oil. Laser technology can help reduce friction with special structuring processes that improve oil adhesion.” Fraunhofer researchers aim to increase the engine’s life-span and reduce energy consumption in this way.

Improving Batteries for Electric Cars
Lasers can also increase the efficiency and life-span of electric batteries. That is good news for manufacturers and prospective owners of electric cars, since batteries continue to be the most expensive component of the vehicles. The engineers and scientists at Fraunhofer are currently working on various solutions to make batteries more durable and less expensive. One approach is to increase the surface area of the electrodes with appropriate coating in order to increase their efficiency. Another approach involves analyzing and optimizing production processes. Manufacturers produce batteries using one anode and one cathode cell, which they link. In theory that sounds simple, but in practice the fusing of copper anodes with aluminum cathodes creates brittle connections that break easily. That presents a problem in cars because of road vibration. With the help of lasers, researchers at the ILT have succeeded in forming durable connections between electrodes without creating the vulnerable brittle alloys. Researchers at the IWS in Dresden have developed an alternative solution in which a laser warms the surfaces and rollers press them together. “Using roll plating with lasers and inductive pre-heating, we were able to create very stable connections with high electrical conductivity, with only a minimal loss of power,” reports Anja Techel.

“The finished batteries are very efficient. And since only small amounts of electrical energy are transformed into heat, these batteries do not require as much cooling.”

The bulk of this article is sourced directly from Fraunhofer-Gesellschaft with additional edits. To learn more about Frauhofer, Europe’s largest application-oriented research organization, go to www.fraunhofer.de 

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