September 17, 2001
In the field of automobile exhaust systems, such components as manifolds, pipes, catalytic converters, and mufflers are joined either by the car manufacturer or by a subcontractor to form a subassembly ready for attachment.
In the field of automobile exhaust systems, such components as manifolds, pipes, catalytic converters, and mufflers are joined either by the car manufacturer or by a subcontractor to form a subassembly ready for attachment. Today, large numbers of these components are welded with a laser.
As an alternative technology to conventional joining methods, laser welding offers fast welding speeds, a high degree of automation, and a high rate of laser uptime.
In addition, a laser welding tool permits entirely new approaches in engineering and production. One such example is the 3-D laser welding of car bodies: In this sector, the laser is used to weld in areas inaccessible to resistance spot welding machines (welding tongs) because of the cavities behind the seam.
The most widespread application for laser welding in car exhaust systems is exhaust pipes. In continuous operation, the stainless steel—usually ferritic—exhaust pipes are produced from rolled coil material. Catalytic converters, in particular, require extremely high-quality welds. For instance, the exhaust pipes between motor and catalyzer must be free of spatter and oxide to prevent particles from breaking off and damaging the catalytic converter. The laser welded tubes satisfy all the stringent quality criteria required for today's exhaust pipes.
For adaptation to on-site installations, the laser beam is routed to the desired working position with the aid of one or more bending mirrors. The welding optics, integrated in the working position, focus the beam onto the welding seam.
Because the welding gap changes position continually, the focal position must be adjusted by this difference. This is accomplished with a seam tracking system. The deviation of the weld gap is detected and corrected with the aid of a tactile or optical system.
The potential that the laser has shown for welding automobile exhaust systems components now is being developed into innovative products. One example is a double-walled exhaust manifold from DaimlerChrysler AG. With its multiple shell design, the manifold attains its operating temperature much sooner. This not only reduces pollutant emission because the catalytic converter activates earlier, but also significantly increases the service life of the converter.
Through laser welding's selective, localized heat application, thermal stress on the component is reduced to a minimum. Thermal distortion is minimized, and the welded manifolds meet very tight mechanical tolerances.
At DaimlerChrysler's Hamburg, Germany, plant, four laser welding systems currently help to produce the latest generation of exhaust manifolds. Two additional systems for the same application have been installed at a subcontractor's plant.
Serial production has shown that uptime on the laser systems can be significantly higher compared to conventional welding techniques. Contamination through spatter and smoke can be up to 10 times lower, allowing clean and safe serial production.
Increasingly, the outer jackets of catalytic converters and mufflers are being produced from so-called short tubes. In a manufacturing line, precut sheets first are bent into a tube shape. Joining takes place in the downstream laser welding station, where a square butt joint forms the longitudinal weld.
On a four-station welding system at Weil Engineering GmbH in San Clemente, California, a laser is routed to four welding stations by means of a beam deflector. Up to 600 catalytic converters are produced per hour on this installation. One major advantage of short-tube processing is its flexibility, allowing tubes of different sizes to be worked. The system welds diameters between 75 and 200 millimeters, tube lengths from 50 to 600 millimeters, and material between 0.5 and 2 millimeters thick. It takes only 20 minutes for one person to retool the machine for a different diameter.
The laser is used to join deep-drawn and punched muffler parts at AP Automotive Systems Inc.
The laser also is at the center of new approaches being taken in the design and production of mufflers. For example, the basic idea envisioned by auto supplier AP Automotive Systems Inc., Granger, Indiana, for a new generation of mufflers was to produce them from simple, and therefore inexpensive, deep-drawn and punched parts. The laser would take over the task of joining the components (see Figure 1).
The material in this application is a ferritic stainless steel. As an alternative, aluminum-coated steel also can be used. The individual components are between 0.8 and 1.2 millimeters thick. In successive joining operations, up to six layers are welded together as a lap joint. The biggest challenge in welding the multilayer joints is to obtain a seam that is gap-free, nonporous, and spatter-free. A series of process optimization steps developed in conjunction with its laser supplier helped AP Automotive Systems begin clean and process-reliable production at a U.S. plant.
In addition to the challenge of developing a reliable welding process for production, another challenge for AP Automotive involved equipment design. The system had to be reasonably priced, flexible in programming, easy to operate, and robust, besides being based on technically proven components.
The implemented concept of the muffler welding unit comprises the core components laser, beam arm, and articulated joint robot, as well as the clamping fixtures, material supply, and outfeed. One of the chief benefits of this installation concept for the welding task in question is that a pressure roller actively clamps the sheet close to the weld point, thereby minimizing the gap.
Currently, installations of this type are being used primarily for 3-D welding of car bodies. Its application in the area of small-format components is obvious, and the system presented here could play a pioneering role for similar applications.
On this installation, a key component for beam delivery from laser to welding optics is the laser arm.
The machine concept, through the use of a standard industrial robot with an external beam guide, can keep cost from being prohibitive. The working range is designed so that several welding stations can be served by one robot and one laser. Accessibility in the area of the weld seam means that welding can take place close to the clamping fixture.
One fundamental characteristic of the design is the laser beam's position stability. The laser is mechanically interfaced with the laser arm and the welding optics. The equipment's lack of mobility is compensated internally in the system by the flexible laser arm.
Constructive collaboration among AP Automotive, the system integrator/robot manufacturer, and the laser manufacturer/welding specialist allowed the realization of this system and the successful start-up and production at a plant in the U.S. despite a short implementation time.
This example shows that the welding system in question is an attractive concept, both in technical and commercial terms, and can be of great interest for other applications.
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