What Can Lasers Do With Composites?

What Can Lasers Do With Composites? image
June 9, 2015 | Source: Industrial Laser Solutions

The widespread use of advanced polymer composites in the aerospace industry is leading to their increased use in other industries—most notably in the automotive industry, where weight reduction is of increasing importance. Composites are by nature entirely inhomogeneous; therefore, their physical properties change significantly over very small areas. In the case of carbon fiber-reinforced polymer (CFRP)—the most widely used employed composite material—the physical properties of the fiber and the matrix are hugely different, as carbon fibers absorb all light wavelengths very efficiently and conduct heat very rapidly, whereas the epoxy matrix absorbs and conducts far less well. Research activity in this area is reviewed here; although some progress appears to have been made using specialized cutting and drilling techniques, there does not appear as of yet to be a clear solution as to how a laser can effectively cut this "worst-case" material at realistic industrial speeds. Similarly, epoxies being thermoset polymers are not thermally weldable, so conventional laser-based polymer welding techniques are not feasible, either. However, pressure from both the aerospace and transport industries to improve cycle times is moving the composites industry towards thermoplastic composites and this review shows that the prospects here are brighter, with lasers already in use for material consolidation. Some recent work on laser welding thermoplastic composites is also presented.

Weight reduction via the use of polymer composites is now gaining rapid acceptance throughout the transport industries—25 percent by weight of the current A380 aircraft is made from composites, and this is set to increase to 50 percent for the upcoming A350 [1] (see cover photo). It is, however, the automotive industry in particular that is pushing composite component manufacturers hard to increase volumes, reduce cost, and decrease manufacturing cycle times.

Composites are made up of individual constituent materials; at least one matrix and one reinforcement material are required, and the matrix surrounds, supports, and maintains the relative position of the reinforcement. A number of combinations are possible and this results in a unique range of mechanical and physical properties, allowing a designer a wide choice of properties for a particular component or structure. This structure, however, is the basic problem with processing composites with lasers, as 1) each component has very different physical properties, and 2) their properties are highly anisotropic.

Because of the spatial and temporal control of laser beams, laser processes are most effective on regular homogenous surfaces, where the beam material interaction phenomena are stable and repeatable in all directions. Although this is not always the case in some engineered materials, small inconsistencies in metals or polymers such as porosity or grain boundaries are usually of a size that does not trouble a high-power laser beam. However, composites are, by their very nature, entirely inhomogeneous and their properties vary dramatically in all three dimensions, depending on the particular location and direction of the laser beam. This is particularly true in the case of continuous fiber-reinforced composites. What's more, if a laser engineer was to choose from a list of possible fibers for reinforcement of polymer composites, the last ones to be chosen would be those that are currently the most widely employed-carbon and glass. Not only do they have dramatically different melting points from the polymer matrices, they also have very different absorption properties and, worse still, carbon and glass have very different absorption properties from one another. This "worst-case scenario" of incompatibility is largely responsible for the real problems that are seen in applying the well-established flexibility of laser processing to the manufacture of lightweight thermoset polymer composites. The advantages in manufacturing flexibility that might accrue from the implementation of laser processes into the manufacturing of composite components are so significant, however, that there are currently many programs worldwide looking for solutions to the issues outlined above.

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