Revolutionising the revolutionary

Hopes of more affordable flights soared when the new Boeing 787 Dreamliner took to the skies last autumn. Featuring a carbon composite design, the passenger jet is lighter than its rivals, resulting in substantial fuel savings. But the composition of carbon fibre reinforced plastics (CFRP) – in particular, the high heat conductivity of the carbon fibres themselves – make them difficult to process using traditional techniques. Mechanical milling and drilling has been known to cause costly heat damage, chipping, delamination and tool wear. A research project between Liverpool John Moores University and global laser manufacturer JK Lasers set out to determine whether or not laser processing CFRP would help overcome these challenges. JK Lasers is a global company that provides innovative industrial lasers and laser processing solutions for the medical device, aerospace, automotive, electronics and semiconductor industries. With decades of industry knowledge and an extensive portfolio of highly successful lasers, the company helps manufacturers choose the most suitable solution for their cutting, welding and drilling requirements. “We believe there is enormous potential to process CFRP using lasers,” begins JK Lasers’ global key account manager, Dr Mohammed Naeem. “It is well-known that lasers are a non-contact process which substantially reduces the risk of tool wear associated with manufacturing CFRP. Above and beyond this, we wanted to demonstrate that lasers can effectively cut, mill and drill CFRP without compromising the material’s integrity. “We have a strong working relationship with the Photonics in Engineering Group at Liverpool John Moores University, so we set out together to conduct a series of trials, using lasers to process CFRP.” During the trials, a 200W fibre laser (JK200FL) and scanning head were used for trepanning, drilling and milling. The JK200FL was also used in cutting and drilling tests using a conventional cutting head. Laser has the edge Compared to mechanical cutting and milling, the JK200FL’s small spot size produced a much cleaner edge and caused minimal thermal damage. Key to controlling this damage was the laser’s high quality beam, which limited the interaction time with the material. JK Fibre Lasers can be on-off modulated over a wide frequency ranging - known as pulsing - which also helps prevent thermal damage during the machining process. Good thermal management also depends on the choice of assist gas. The team at Liverpool John Moores University identified the use of CO₂ gas as a potentially advantageous thermal management technique. By increasing either the cutting speed or the CO₂ assist gas flow rates, surface damage can be reduced when cutting CFRP prepreg MTM44-1. For the given feed rate of 0.5m/min, increasing the flow rate of carbon dioxide from 2lt/min to 12lt/min reduces damage by 60%. Increasing the cutting speed produces similar results. Increasing the cutting speed from 0.5m/min to 2m/min reduces the damage by nearly 70% at a flow rate 2lt/min. JK Fibre Lasers can currently cut CFRP thicknesses up to 2mm. In material thicknesses of 1mm or more, the research team devised a special spiral drilling strategy to allow the cut fibres and plasma to escape cleanly. This produced a high quality cut with very slight burn-back that was limited to the top layer of fibres only. In figure 1, each ring represents an individual scan path for the laser beam. The central dark area is the central slug of material that will fall out as soon as the hole has been cut. In these particular experiments the diameter of the central slug was 1mm with the final diameter of the outer ring measuring 2mm. The distance between the individual rings was either 200µm or 300µm. Figure 2 shows a partially drilled composite using this drilling mechanism. The final three outer cutting paths of the laser remain visible, as well as the individual witness marks of the pulses. The slug still sits in the centre of the hole. The laser parameters used to machine this hole were the JK200FL in modulation mode, 50µsec, pulse energy 10mJ, peak power 100%, 10% duty cycle, frequency 2kHz and 18W average power. Figure 3 is a magnified view of the lower portion of the hole in figure 2. It shows that the fibres are cut into short length bundles. Isolating the fibres in this way means heat builds up in the bundles removing the polymer matrix material through the mechanism of conduction along the short fibre lengths. In this particular example the fibre bundle lengths was 200µm. The short bundles of fibres were then ejected with the vaporised polymer material. “Fibre lasers can couple significant laser power into small focal-spot sizes that generate a very intense beam,” states Dr Paul French, senior lecturer with the Photonics in Engineering Group, Liverpool John Moores University. “Combined with lower costs and greater robustness, these laser sources offer a practical industrial tool for processing composites. The results have been very encouraging.” Growing industry awareness The trials have already stimulated considerable interest across several market sectors. Enquiries have primarily come from the aerospace and automotive sectors, with some medical companies also requesting further information. Applications discussed so far include drilling, laser trimming components, surface texturing prior to adhesive bonding, welding thermoplastics, joining CFRP structures, material cleaning and milling parts for repair. There is also the possibility that lasers can be used for edge finishing. Water can be prevented from entering CFRP components by melting a matrix layer onto the face of the cut edge. According to Dr Naeem, the interest so far is just “the tip of the iceberg.” He says: “CFRP is a highly versatile material that can be used in many industries. However, its widespread use has been limited by the difficulties associated with processing it. Our research changes that. “Using a JK200FL, we have successfully cut, milled and drilled CFRP without compromising the material’s integrity. This demonstrates that fibre lasers are an efficient, effective and potentially less costly way of processing CFRP. We hope it will revolutionise the way CFRP is processed in years to come.” www.jklasers.com