By Dr. Mohammed Naeem
Composites are structures in which two (or more) materials are combined to produce a new material whose properties would not be attainable by conventional means. An example is carbon fiber reinforced plastic (CFRP). Composite material is constructed from two materials, carbon fiber and polymer matrix, each having significantly different properties. The two constituents work together to give the composite unique properties, such as high strength and light weight, which are markedly superior over some metallic alloys.
Within the CFRP composite, carbon fibers are surrounded and held by a polymer matrix. Each constituent retains its own chemical, physical and mechanical properties. The strength and rigidity of the composite can be controlled by varying the amount of carbon fiber incorporated into the epoxy. This ability to tailor properties, combined with the inherent low density of the composite and its (relative) ease of fabrication, makes this material an extremely attractive alternative for many of different industrial sectors.
These composite materials are normally processed using traditional machine tools (i.e. mechanical cutting, drilling and milling machines). Recently, companies have been investing in water jet technology for cutting CFRP composites. Water jet can give a high quality cut but this has associated problems of causing delamination and requires a pilot hole to be drilled mechanically if the cutting process starts anywhere other than at the edge of the sheet. Other potential issues with water jet cutting are the disposal of waste products from the cutting process.
The nature of inhomogeneous material in property and structure makes the composites more prone to damage during machining process than conventional metals. Damage such as delamination, fiber pulling out, matrix chipping, heat damage and tool wear generally represent the main concerns when machining composites. These difficulties inspired the development of machining technology for achieving high machining speeds and product reliability. In view of the high tool wear and high cost of tooling associated with conventional machining, a non-contact material removal process using lasers offers a more attractive alternative. With lasers, one can also minimize dust, noise, extensive plastic deformation and consequently heat generation typically associated with conventional machining of fiber-reinforced (CFRP) composites.
Laser machining is based on the interaction of the material with an intense highly directional and coherent monochromatic beam of light. The material is removed predominantly by melting and evaporation. In the case of resin matrix material, it is also removed by chemical degradation.
The advantages from laser machining depends on the thermal nature of the machining process, which does not involve any mechanical force applied to the material thus minimizing the mechanical damage induced during the laser machine processes. Nevertheless, some difficulties can arise because of the difference in the thermal properties of fiber and matrix. The energy needed for the vaporization of the fibers is higher than that required for the polymer matrix; hence the laser power required for cutting CFRP composites will be strongly dependent upon the kind of fibers and their volume fraction. The volume of the fibers depends on the orientation of the reinforcing fibers, i.e. unidirectional, woven, or 0/90 degrees etc. Because the large differences, e.g. one or two orders of magnitude, in vaporization temperature and thermal conductivity between the two constituents, the challenge in laser processing composites is minimizing thermal damage to the epoxy matrix and retaining high processing speed simultaneously. In the past, process qualities, such as the heat affected zone (HAZ), charring, delamination and epoxy recession due to intense thermal damages, have been major obstacles in using laser machining for industrial applications to process composites.
Laser machining and surface texturing of CFRP composites
CFRP composites are increasingly using adhesive technologies to join different CFRP components. Prior to applying the adhesive to the surfaces of composite components, the surface is typically mechanically abraded using an abrasive disk or tool. This roughens the surface of the composite and increases the surface energy, thereby increasing the wettability of the surface. This improves the spreading of the adhesive and increases the strength of the joint.
Laser cutting and surface texturing were investigated using a fiber laser and the S94P system controller. Surface texturing is new novel solution using lasers. Figures 1 and 2 show SEM images of the laser cut surface and the thermal damage. The damage is limited to a single top layer of the composite sample. The carbon fibers directly below this layer show no sign of damage. Figure 2 The edge quality produced by the fiber laser is far superior to the edge produced by mechanical shearing. Figure 3 shows a comparison between the two different methods for cutting CFRP composite. The mechanically sheared sample shows the fibers protruding out of the bulk of the composite whereas the laser cut sample shows a very clean cut.
This investigation has shown the new generation of fiber lasers can be used in both the macro-application of laser cutting and a micro-application of laser surface texturing for adhesive bonding of CFRP structures. The cut edge quality from laser cutting is superior to mechanical cutting with less surface damage. Depending on the application requirements, this may remain an issue. Work is in progress to reduce this damage further and increase the cutting speed.
Surface texturing of composites is a viable replacement to mechanical abrading giving better control over the final structured surface. The contact angle of the composite material showed a marked reduction. The fiber laser systems have shown that they can machine CFRP composites with fine control over the depth of material removed and a high quality surface finish. This ability to machine composites on the micro scale gives fiber lasers a new role in the composites industry as a new tool for processing fine structures in CFRP composites.
If you are interested in the learning more about using a laser in your composite materials for cutting and drilling applications, please contact firstname.lastname@example.org.