Demonstrating the potential of laser micro drilling in CFRP processing for aviation

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Richard Stähr and his colleagues are using laser drilling to produce perforated acoustic liners for insulating aircraft noise

Throughout the pandemic, the airline industry faced major challenges due to lockdowns and travel restrictions around the world.

After a tough couple of years, air passenger traffic is recovering and will continue to increase as it has in previous decades, while air cargo volumes have already exceeded pre-pandemic levels.

As air traffic increases, so does noise pollution, which can have serious effects on human health and the environment as a whole. Not only is the impact on the surrounding environment a problem, but passenger comfort also suffers. Regulations on aircraft noise emissions have also become much stricter in recent decades, constantly forcing aircraft and engine manufacturers to take action.

The engines are the main source of an aircraft’s noise, and numerous efforts have been made to reduce noise levels through active and passive measures.

A typical passive method for noise reduction is the use of perforated acoustic liners for sound insulation, as shown in figure 1 at the Airbus A300 engine inlet.

Figure 1: Micro-drilled acoustic liner at the inlet of an Airbus A300 engine (Credit: Wikimedia Commons/Dutch.Seb)

Among different designs, acoustic liners can consist of sandwich panels, with one perforated, micro-drilled upper skin layer (face sheet), a honeycomb structure inside, and a closed (sound-reflecting) backside face sheet. The components are bound together by an adhesive film between the face sheets and the honeycomb structure. Wherever operating conditions allow and weight savings are advantageous, the outer layers are made of carbon fibre reinforced plastics (CFRPs).

Lasers in, drill bits out

Despite the advantages of CFRP, disadvantages such as difficult machinability and high machining costs prevent its wider use. The main method currently used for perforating CFRP materials is conventional drilling with a drill bit. Despite years of experience with the technology, the process remains very demanding due to the high quality requirements and the sensitivity of the material. For the inhomogeneously composed carbon fibre-reinforced materials, this technique suffers from high tool wear, which can lead to inadequate quality and high tooling costs. Another limitation is the difficult realisation of bore diameters smaller than 1mm in industrial use.

Laser technology can draw on its strengths to address each of these issues. Not only is there no tool wear, but the achievable bore diameters are also in the tenths of a millimetre range, which can be particularly advantageous for the grazing flow conditions inside an engine. Also beneficial is the flexibility in terms of bore spacing and bore diameters that can be accomplished without tool changes by using remote machining. In this way, the properties of the acoustic liner can be adapted to the location and conditions of use.

Project miBoS

Because laser drilling is completely new to this particular application, it comes with a few new challenges. First and foremost, the cycle times of conventional processes, which are in the order of tenths of a second per borehole, must be achieved or, better still, improved. Typically, short- and ultrashort-pulse NIR-laser systems are the systems of choice because of their lower heat impact on the CFRP compared to continuous-wave lasers. They are becoming increasingly powerful, but the power must also be brought to the material surface without damaging it, for example, by suitable beam shaping and adapted process strategies.

In addition, there are specific challenges associated with the material. One of them is that the machining result is sensitive to thickness deviations of the material. Usually, an amount of energy is set at which complete perforation is certain to occur. If the perforation is finished earlier and the process is not terminated, the remaining energy will pass through the bore and cause unwanted markings that can damage the bonding or even the backside face sheet itself.

In the publicly-funded miBoS project (‘Micro-drilling of Sandwich Materials’), which is currently being carried out at the Laser Zentrum Hannover (LZH), these challenges are being addressed together with two industrial partners: Invent and KMS Technology Center. The aim is to demonstrate the potential of laser drilling and bring this technology closer to industrial application.

As mentioned earlier, improving efficiency is crucial. Therefore, the process is optimised for short cycle times, but attention is paid to low heat exposure and minimal taper of the bore. The project uses a nanosecond-pulsed laser from Trumpf Laser with a high pulse energy (EP=100mJ). The pulse energy is thus sufficiently high to be split into multiple beams, while maintaining the fluence on the material in an optimal range for ablation. The importance of laser type, fluence and other processing factors has already been highlighted in previous studies, as shown in figure 2. By simultaneously machining a large number of holes, cycle times will be significantly reduced. In addition, it is investigated which drilling strategy (percussion or helical drilling) works most effectively for a specific bore diameter. 

Figure 2: Cross-section micrographs of laser-machined CFRP highlighting the evolution in quality and kerf taper (Credit: LZH)

Process monitoring and control is used in several places in the miBoS drilling setup. One of them is the monitoring of the machining zone by means of thermography. The thermography images are processed and evaluated over time to assess the drilling progress of each hole. In this way, damage to the bonding of the sandwich structure is to be prevented. In addition to the work at LZH, the industrial partners are developing, among other things, optomechanical systems designed to make beam guidance and beam shaping flexible. Clamping devices are adapted so that curved components can be positioned dynamically with a robot. On the material side, the acoustic and mechanical properties of laser micro-drilled specimens are being compared to those of conventionally drilled specimens and evaluated.

When will it be available?

Following the project, the process still needs to be certified so that it can be approved for use in the aviation industry. Nevertheless, the results from this project are a big step in the right direction, especially because the application of the process is of course not limited to aviation, but can also be used in many other industries, for example automotive.

Richard Stähr is an R&D engineer in the field of laser processing composites and plastics at Laser Zentum Hannover.

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