Laser light for CFRP

© Photo | TRUMPF

Fiber-reinforced composites are increasingly popular, but tough to machine. Production engineers are increasingly turning to laser light as a wear-free, high-precision and highly productive choice of tool.

Carbon fiber-reinforced plastics (CFRPs) are all the rage, especially in the automotive, aviation and wind power sectors. They represent a key part of the solution to some of the most pressing issues of our age, including climate protection, e-mobility, resource efficiency and sustainability. These megatrends are major drivers behind lightweight construction and the use of fiber-reinforced composites.

Design engineers love these woven lightweight materials for their strength and rigidity. Production engineers are somewhat less enthusiastic thanks to the complexities involved in working with a composite of fibers and polymer. The problem is that these two materials behave completely differently in the machining process: a mechanical cutter can pass through polymer like a knife through butter, but it will still blunt its edge on the hard fibers.

It is this radical contrast between the properties of the two materials that makes it so difficult to choose the right method and the right tool, which is why industrial production engineers are currently seeking faster, more reliable and more productive methods of working with CFRP. 

Lasers are gaining increasing popularity as a suitable tool for dealing with fiber-reinforced plastics at many different points in the process chain, including cutting blanks and parts, ablating layers to prepare workpieces for adhesive bonding, and joining metal to plastic.

Laser cutting of CFRP

The biggest challenge of cutting fiber-reinforced composites is that the material is both stubborn and delicate at the same time. That poses a problem for all mechanical processing methods. Water jet cutting with abrasive substances is far from ideal since the fibrous cut edge can easily be damaged by abrasive particles and fibers may become detached from the matrix as the material takes in moisture. Cutting imposes significant forces on the workpiece, which often results in rough cut edges with protruding fibers.

Machine tools pose the same kind of risk, though the main disadvantage here is the high cost of processing the workpieces. The hard fibers quickly wear down the drilling and milling heads so that they must be replaced multiple times each shift. In addition, any change in the thickness or composition of the material being machined generally means switching tools. This retooling process takes time, and constantly purchasing new tools is an extremely expensive business. Guaranteeing consistently high standards of production quality under these conditions also requires constant monitoring, which is yet another cost factor.

In comparison, laser light offers clear benefits. As well as avoiding the issue of wear thanks to its non-contact nature, it also enables manufacturers to achieve reproducible results at consistent high quality. Laser engineers have recently succeeded in using preset parameters to enable automatic, in-line adjustments to cater to different material thicknesses and compositions without interrupting production. But there are plenty of other reasons why light is the best cutting tool for CFRP.

Vaporizing material from a distance

Laser beams do not exert any mechanical forces on the workpiece. That makes them a good choice for machining very thin or delicate CFRP parts with great precision. A beam of light can be flexibly tailored to changing contours and geometries because the machining optics make no contact with the workpiece – in fact they are more than 150 millimeters away from it. That makes it easier for lasers to get into tight corners.

Das Licht eines Festkörperlasers koppelt hervorragend in die Kohlefasern ein: Die perfekte Schnittkante bei diesem CF-Preform ohne überstehende Fasern erleichtert und beschleunigt das anschließende Handling. Foto | TRUMPF

The light from a solid-state laser couples easily into carbon fibers. That gives CF preforms a perfect cut edge without protruding fibers, making subsequent handling faster and simpler. Photo | TRUMPF

Laser cutting of carbon fibers (CFs) and carbon fiber-reinforced plastics involves a sublimation process, which means that the material is vaporized as soon as it is hit by the accurate, high energy beam. That means there is no molten material to be ejected and the resulting edge is smooth, with no fibers protruding from it. The heat-affected zone at the cut edge is minimal and – according to findings so far – has no impact on the mechanical properties of the part.

Natural edge for CFRP parts

The high-precision benefits of lasers also extend to cutting preform materials. Preforms are dry, semi-finished fabric products, essentially a kind of mat, which has not yet been hardened and is therefore still flexible. To produce the preforms, the mat blanks are placed in a tool that uses elevated temperatures to turn them into 3D preforms. The preforms are then infiltrated with synthetic resin – for example by means of resin transfer molding (RTM) – which then dries and hardens. This creates CFRP parts with complex, three-dimensional geometries.

Laser beams have no trouble with complex geometries. Photo | TRUMPF

Laser beams have no trouble with complex geometries. Photo | TRUMPF

Laser light cuts the preforms into near net shape at a speed and level of quality that other separation methods such as ultrasonic cutters could never achieve. Laser-cut preforms have a clean, clear, natural edge without any protruding fibers. This simplifies downstream handling and eliminates the need for finishing work such as grinding. That means the preforms can be placed in the RTM mold and infiltrated immediately after laser cutting.

The right laser for cutting CF, CFRP and GFRP

There are many different types of fiber matrix materials – but fortunately there are also many different types of laser beam source. Solid-state lasers are a great choice for cutting CF preforms and CF yarn and fabric because the laser energy couples easily into the carbon fibers. For CF less than half a millimeter thick, a single kilowatt of laser power is sufficient to reach a machining speed in excess of 20 meters a minute – two or three times faster than a water jet or milling tool.

In addition to CFRP and GFRP, lasers can also machine many other fiber composites including natural fiber composites and combinations of GF and CF

When it comes to CFR materials and glass fiber reinforced plastic (GFRP), a CO2 laser is the best choice because the glass fibers and matrix material are opaque to CO2 laser light, and therefore machinable. For materials with a thickness of two millimeters or more, a 5 kW laser can cut at a rate of 10 meters a minute, a figure that is once again two or three times faster than conventional methods.

In addition to CFRP and GFRP, lasers can also machine many other fiber composites including natural fiber composites and combinations of GF and CF.

Surface ablation of aramid fiber-reinforced polymer (AFRP). Video | TRUMPF

Ablation as a preparation stage for adhesive bonding 

Lasers can also be used to process CFRP in other ways than just cutting it. One of their main uses is ablation, in which laser beams vaporize the upper layer of the material with tremendous precision.

This form of processing is particularly useful as a preparation stage for adhesive bonding since this requires the top layer of paint to be removed or roughened. In this case lasers do an excellent job of machining precisely the area that is required, so only the necessary amount of material is ablated. Once again, one of the major benefits of laser light is the flexibility it offers – It can even prepare curved parts for adhesive bonding by precisely tracking their contours.

When it comes to machining large surfaces at high speeds, a CO2 laser is the preferred choice of beam source for adhesive bonding preparation. And in recent years engineers have gained access to a new tool for machining small, precisely defined areas at high quality with the emergence of ultra-short pulse lasers designed for industrial use. These lasers generate light pulses with a duration of just a few picoseconds or even femtoseconds.

They enable engineers to remove just a few nanometers of material from tiny areas without causing melt burrs and without causing thermal damage to the material. This technology marks another big step forward in the precision of machining processes.

Joining metal and plastic

Riveting and gluing are currently the standard methods of joining CFRP and metal. Lasers can be used to create the holes for the riveting process and to ablate the top layer of paint as preparation for adhesive bonding. However, both riveting and gluing involve the addition of a further step to the production process and require the use of additional materials.

Laser light is a fast, wear-free and non-contact tool that gives design engineers the freedom to create complex geometries

In addition, both methods suffer from inconvenient geometric limitations, for example the need to attach a joint flange, or a part geometry that allows easy access for the riveting tool. Production engineers have therefore been seeking more elegant joining solutions. Once again, lasers have provided the solution, this time in the form of ultra-short pulsed laser light, which can be used to join metal and fiber-reinforced plastic tightly together.

Fusing thermoplastics with metal

To securely join metal to thermoplastic polymer, an ultra-short pulse laser prepares the metal mating part by creating an undercut structure.

Securely joining metal and fiber composites: an ultra-short pulse laser creates an undercut in the metal part, ensuring that the polymer and hot metal fuse together properly. Photo | TRUMPF

Securely joining metal and fiber composites: an ultra-short pulse laser creates an undercut in the metal part, ensuring that the polymer and hot metal fuse together properly. Photo | TRUMPF

The structured metal part is then heated to a temperature above the melting point of the thermoplastic. This step can be carried out by an inductor, an oven or a different laser. When the hot metal and the polymer are pressed together, the thermoplastic begins to melt. The thermoplastic flows into the laser-generated undercut, creating a secure connection to the metal part when it cools down – all without requiring any additional filler material.

The static and dynamic stability of the joint are both higher than that achieved by gluing. This method can also be used to create liquid-tight joints without requiring additional seals, which can be advantageous for applications such as pressure tanks and body shell components. The typical joining rates are approximately six square centimeters a second for aluminum, and slightly less for steel.

Structuring inserts

Design engineers from the automotive and aviation industries often design metal components in fiber-reinforced polymer. Known as inserts, these are threaded sleeves or metal parts that are used to create connections to other parts, for example a hinge for a CFRP trunk lid.

Surface ablation of aramid fiber-reinforced polymer (AFRP). Photo | TRUMPF

Surface ablation of aramid fiber-reinforced polymer (AFRP). Photo | TRUMPF

Once again, an ultra-short pulse laser can be used to create an undercut, enabling a neat connection to thermoset polymers. Before the parts are infiltrated, the engineers place the structured metal part in the preform fabric. It is then infiltrated with resin and allowed to harden, once again forming a tight joint.

First sheet metal, now CFRP

Having established itself as a standard tool in sheet metal working, the laser is now ready to follow the same path in the field of CFRP machining. The key benefits of laser light for CFRP are basically the same as in sheet metal production: laser light is a fast, wear-free and non-contact tool that gives design engineers the freedom to create more complex geometries than those achievable with mechanical methods. At the same time, lasers enable the amount of energy applied to the workpiece to be controlled so precisely that they can comfortably handle the delicate machining of extremely thin materials.

The industrial ultra-short pulse laser – a relatively recent addition to the production environment – even provides the option of  “cold” machining, in other words a form of machining that applies virtually zero heat to the workpiece. This has opened the door to a wealth of new ideas and applications. 

I can still see lots of potential for neater and more efficient machining of CFRP materials in just about every industry in which lightweight construction plays a significant role – and laser light will increasingly be the tool of choice. 

 

Photo | TRUMPF

Author:

Marc Kirchhoff
Industry Management Automotive at TRUMPF
Tel.: +49 (0)7156 303-30113
E-Mail: Marc.Kirchhoff@de.TRUMPF.com

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