Tape laying head from AFPT in combination with a diode laser and specially developed homogenizing optics from Laserline. (AFPT)
The subject of alternative materials for vehicle design and construction is as repetitive and as seemingly endless as waves on a beach. Like those waves, there are variations—although relatively limited—on the familiar theme, so the industry can ponder the applications of aluminum, magnesium, a broad spectrum of steel strengths, materials recycled from an array of sources, and, increasingly, carbon-fiber-reinforced plastics (CFRPs).
Each has its pluses and minuses, but some, notably CFRPs, can present particular processing challenges. Research is under way at several industrial and academic centers, however, to ease those challenges via the use of lasers.
The Institut für Strahlwerkzeuge (IFSW) at the University of Stuttgart is one of the world’s leading research establishments for the implementation of laser technology for industrial applications, including automotive. Its director, Prof. Dr. Thomas Graf, and his departments are working to ease CFRP processing difficulties and help increase the use of the material as a wider-spectrum metals alternative for automotive.
Unlike metals, which are physically homogeneous, composite materials are inhomogeneous, comprising polyglot properties that give them advantages (stiffness, light weight) but also bring processing difficulties. In theory, lasers offer a solution to the quick and efficient cutting, drilling, and welding of CFRP; but it is not quite that easy.
Said Graf: “On one hand, inhomogeneity influences the distribution of the laser beam; on the other, the heat conduction is very anisotropic and the parameters that are decisive for the ablation of materials are very different.”
Serious damage of the plastic matrix may be caused by incorrect process control due to the high heat conductivity of the carbon fibers and the different properties of carbon and plastic. The solution to that lies in the development of guidelines for optimized process control, said Graf, which is just what the IFSW is doing: “In particular, very high infeed speeds, high precision with high dynamics, and a precise focus position control are some of the challenges.”
Dr. Wolfram Rath, Product Manager for Laser Beam Sources at Rofin-Sinar Laser, Hamburg, states that the different properties in composite materials demand very different optical and thermal properties of the composite partners, which lead to “very irregular” processing results when processing via continuously emitting lasers. However, procedures with a shorter interaction time can help, “achieved either by a correspondingly short pulse duration of the laser beam or by correspondingly quicker processing speeds,” Rath said.
Various types of laser technology (solid state, fiber, CO2) and wavelengths are being tested for CFRP applications. Indications are that CO2 is the most promising, its wavelength being apposite for processing composite materials in operation with a continuous beam. However, Rath notes that ultrashort pulse lasers are of interest for such applications; the short pulses cause little heat input.
In cooperation with companies and other research institutes, the IFSW is working on the control of the physical basics in the interaction between laser beam and composite material as a prerequisite for productive, high-quality processes and suitable system concepts.
The Manager of Process Development at IFSW, Dr. Rudolf Weber, states that the institute is not only working on the cutting of CFRP materials but also other composites and their manufacture. Cooperative ventures are in place with Daimler, Audi, and Porsche.
This June will see about 200 exhibitors taking space at LASYS 2012, the trade fair for laser material, to be held in Stuttgart.