Laser transmission welding using carbon fiber reinforced polymers
The invention deals with laser transmission welding using carbon fiber reinforced thermoplastics (CFRTP). The technique is suitable for different thermoplastic composite material combinations. It is possible to weld reinforced composites onto glass fiber reinforced polymers and unreinforced polymers. Laser transmission welding offers significant advantages in terms of flexibility and processing speed compared with other welding techniques. The absorption of the laser beam occurs mainly on the carbon fibers, thus, adding additives for absorption is not necessary.
Carbon fiber reinforced composites are of increasing importance in the aerospace and automotive sector.
Different joining techniques for reinforced composites exist. Among them are the use of adhesives, rivets, induction welding, electric resistance welding or ultrasonic welding. All the available processing techniques have certain limitations. Adhesive bonds need extensive steps for surface preparation and cannot be tested for stability without destruction. Induction welding works only on conductive materials. Electric resistance welding uses a conductive mesh at the welding area which constrains the achievable geometries and remains in the material after welding. The necessity of adding absorbing additives to the joining partners is also a frequent problem with laser transmission welding.
Welding carbon fiber reinforced materials adds an additional challenge. Carbon fibers possess a different thermal conductivity than the surrounding polymer matrix and they are aligned within the material. The resulting anisotropic thermal conductivity strongly influences the shape of the seam.
Scheme of the laser transmission welding (Source: Peter Jäschke, LZH).
Scientists and engineers at the Laser Zentrum Hannover have developed a flexible alternative to the above mentioned techniques. Laser transmission welding using carbon fiber reinforced thermoplastics is an efficient and flexible processing technique. A, at least partially, transparent joining partner is welded onto an absorbing joining partner. The laser light passes through the transparent joining partner and is absorbed at the surface of the second material. Both joining partners are kept in close contact by clamping and the heat is transferred to the transparent partner via heat conductance. Scientists at the LZH have shown that laser transmission welding of carbon fiber reinforced materials is possible without such additives. Carbon fibers typically absorb the laser light better than the surrounding polymer matrix, welding is thus possible without the addition of further absorbing materials. However, the anisotropic heat conductance introduced by the carbon fibers has to be taken into account and, where necessary, compensated.
Laser transmission welding offers for example the following advantages:
The laser transmission welding of carbon fiber reinforced materials is suitable for a number of applications. The high geometrical flexibility and the possibility to weld without adding further absorbing materials is interesting in particular for:
Laser transmission welding with absorption on the carbon fibers was tested and developed at the Laser Zentrum Hannover. Laser transmission welding was evaluated with aircraft pins made from PA6.6. The laser transmission welding of these pins onto carbon fiber reinforced composites resulted in a significantly stronger bonding than the commonly used adhesive connection.
US patent: US8591693B2
Advanced laser welding of high-performance thermoplastic composites
Pyrometrische Temperaturüberwachung für flexible scannerbasierte Laserdurchstrahlschweißprozesse
Laser transmission welding of high-performance polymers and reinforced composites - a fundamental study