Laser Machining Composite Materials

Composites are engineered materials in which two or more materials are combined to produce a new material whose properties would not be attainable in more traditional materials, such as metallic alloys. The composite’s constituents work together to give the composite unique properties compared to those of a single material, such as high stiffness in one direction and flexibility in another direction.

Types of composite materials

The main types of composite materials that we are asked to laser process include fiber reinforced polymer (FRP) composites, metal matrix composites (MMC), and ceramic matrix composites (CMC).

Even within a class of composites, the characteristics can vary dramatically. Designers can vary the amount, shape, size, and orientation of the second materials. Figure 1 shows examples of various reinforcements used in creating composite structures.

Fiber	Filler	Flake	Particulate	Random
Figure 1: Examples of various forms that reinforcements can take in creating composite materials.

The basis of FRP composites is the matrix material, which can be either a thermoset or thermoplastic resin. Common thermoset resins include polyester vinyl ester and epoxy whereas common thermoplastic resins are polypropylene, PET, PEI, PEEK, and PPS. The reinforcing fiber is typically glass, carbon, aramid, or Kevlar®, irrespective of the type of matrix material.

MMCs are typically created from a lightweight metal (the matrix), such as aluminum or titanium, and a ‘reinforcement’ material consisting of either metal, carbon, or ceramic. Often the fibers are coated with a compound to help in bonding the reinforcement fiber to the matrix during fabrication of the composite structure.

Similarly, CMCs are fabricated by embedding a ceramic fiber from a material such as carbon, silicon carbide, alumina, or mullite in a ceramic matrix of the same material.

Benefits of using composite materials

Composite materials provide designers new possibilities. Typical benefits that come with composite materials include:

• Lightweight – Composites are typically lighter in weight compared to metals.
• High strength/stiffness – Composites can be engineered to be stiff in a specific direction or multiple directions, while flexible in other directions.
• High strength to weight – Composites offer higher strength-to-weight ratios than single materials.
• Corrosion resistance – Composites resist damage from the weather and harsh chemicals that degrade other materials.
• High impact strength – The structure of composites provides the ability to absorb high impact.
• Elevated temperature properties – MMCs and CMCs can provide higher strength and lower creep rate at high temperature than alloys.
• Fatigue resistance – The structure of MMCs and CMCs can be engineered for high fatigue resistance over a range of temperatures.
• Thermal expansion – MMCs can be designed to match the coefficient of thermal expansion of materials to which they are joined.

Machining of composite materials

By their design, the structure of most composites is less uniform than of plastics, metals and alloys, and ceramics. This inhomogeneous structure makes composites more prone to damage during machining than homogeneous materials. Delamination, pulling out of fibers, chipping of the matrix, heat damage, and tool wearing generally represent the main concerns in machining composites with conventional machining processes.

The difficulties of conventional machining processes has caused manufacturers to look for ways to cost effectively machine composites without damaging them in the process. In view of high tool wear and high cost of tooling with conventional machining, a non-contact material removal process like laser machining can be an attractive alternative for certain applications.

To date various laser sources including lamp pumped Nd:YAG, CO2, ultrafast short pulse, and fiber lasers have been used for cutting, drilling, and texturing composites.

At Prima Power Laserdyne, we have conducted detailed studies on various composites, with the greatest amount of development involving FRPs. The main goal of these studies has been to develop a better understanding of the fundamental issues of laser-composite material interaction. Laser processed features, such as cuts and holes, have been characterized in terms of properties of interest to laser system users: heat affected zone (HAZ), charring, delamination and epoxy recession during laser machining.

A series of experiments has been carried out to optimize laser and processing parameters to cut and drill both glass fiber (Figure 2) and carbon fiber (Figure 3) composites. This development work is ongoing but early results are promising in terms of low HAZ, charring and delamination of the fibers.

Meanwhile, preliminary trials involving cutting and drilling MMCs (Figure 4) and CMCs have also been performed.

Figure 2: Laser trepanned pattern of 0.75 mm diameter holes in two-ply (two layers of glass oriented at 90⁰ to each other) glass fiber composite. Note the slight charring of the epoxy matrix in the picture on the left. Holes are round at the entry (upper right) and exit (lower right) with no evidence of fibers protruding from the laser cut edge.

Figure 3: Laser trepanned pattern of 0.5 mm diameter holes (left) in a carbon fiber composite. There is no evidence of charring on either the entrance (upper right) or exit (lower right).

Figure 4: Micrographs of laser cut edge of 2 mm thick MMC. The composite uses an Al-Li alloy matrix reinforced with 20% by weight silicon carbide (SiC) particulate. The picture at the left is a low magnification view of the cut edge (light band in picture at the left) while the picture at the right is a close-up showing striations on the laser cut edge and absence of cracks.