Ultrafast lasers facilitate leaner CFRP production
Dr Stefan Janssen, of Fraunhofer ILT, discusses the scanner-based laser processing technique developed within the Carbolase project
This article was co-authored by Dr Sebastian Oppitz, of the Institut für Textiltechnik at RWTH Aachen University, and Dr Matthias Morasch, of Scanlab
Fibre reinforced polymers (FRPs) are particularly popular materials for manufacturing products that are subject to strict weight and durability requirements, for example in the automotive and aerospace industries.
Carbon fibres are a common choice for these kinds of applications, where they are referred to as carbon fibre reinforced polymer (CFRP) elements, or simply ‘carbon fibre’. The bonded fibres are incredibly strong and stiff but at the same time, they are extraordinarily lightweight. However, they are both expensive and difficult to process. Joining CFRPs to other components creates unique challenges that manufacturers must overcome.
For the connection with other materials, holes are usually drilled in the CFRP modules as a first step. Metal fasteners called 'inserts', which may or may not be internally threaded, are then introduced into these holes (see figure 1). When used for this purpose, conventional drill bits quickly reach their limits when it comes to precision, machining speed and tool life. Fine structures for complex insert geometries are difficult to produce and not all areas are always easily accessible. The drill bits wear at a considerable rate, driving up the costs. And CFRPs are generally only economical in large batch sizes. In short: the challenges posed by the choice of materials and during production are very diverse.
Figure 1: A CFRP component with integrated fasteners. (Credit ITA)
Lasers for processing fibre composites
Fraunhofer ILT and the Institut für Textiltechnik (ITA) at RWTH Aachen University embarked upon a joint project entitled ‘CarboLase – Highly productive, automated and tailor-made just-in-time CFRP component manufacturing’, with the aim of simplifying this process chain in CFRP production. The project was a joint endeavor between Fraunhofer ILT, ITA, Amphos, Lunovu, and Kohlhage Fasteners.
The idea behind this project was to drill the holes into the fabric preforms using laser beams, allowing the inserts to be placed into the fabrics before the fibres are bonded. With the components already in place, the resin matrix then cures around them, thereby eliminating the need to adhere the inserts in the holes at a later stage.
The laser and scanner configuration used in the Carbolase project. (Credit: ITA)
Throughout the course of the CarboLase project, this step was then integrated into an automated process that incorporates the entire preform production process. This not only significantly reduces the duration of the process, but also enables the just-in-time manufacturing methodology to be used for the components while additionally lowering manufacturing costs.
An ultrafast laser was used for the key step, the laser-drilled holes. One of the primary focuses at this stage was to ensure that the laser could be precisely positioned. This was achieved by means of a galvanometer based scan system mounted on a moving robot arm. The benefit of this is that it offers incredible flexibility in terms of the lines and contours along which the laser can travel. The result is customised production of CFRPs, regardless of the batch size. And even if the dimensions of the drill-patterns are changed, a tool change is not necessary.
Laser and scanner configuration
To ensure that the ultrafast laser drilling process ran smoothly, it was essential that the laser and scan head were perfectly coordinated. The CarboLase partners therefore decided to use a galvanometer based scan system from Scanlab called ‘excelliSCAN’ with high-precision digital encoders for position measurement and an RTC6 control board. Thanks to the integrated SCANahead control system, the laser machining process could operate with optimal dynamics. The system automatically calculated all the parameters necessary to allow the scanner to work with maximum acceleration at all times. This actuation control system meant that the project partners did not have to choose between high precision and exceptional dynamics, resulting in significantly higher productivity and quality.
The scan head is able to achieve significantly improved contour accuracy, coupled with higher marking speeds and acceleration, compared with a conventional scan system, especially when traveling through sharp corners and circles. Because the control system fully exploits the galvanometers' dynamic potential, significantly reduced corner rounding is achieved when traversing 90° corners (see figure 2). Using conventional control systems with a tracking error can result in substantial rounding, especially when corners have to be traversed quickly.
Figure 2: At v = 1m/s with no delays implemented, a traditional scan control system with tracking error caused substantial corner rounding (left). SCANahead control was instead able to traverse the 90° corner while producing far less rounding (right). (Credit: Scanlab)
This last point makes a huge difference for complex hole contours with many corners. Unwanted pulse overlaps are avoided because the laser does not linger at the corners and quickly speeds away. This is a major advantage over conventional laser and scanner actuation systems, where the energy input may vary more significantly when traveling along contours with sharp angles. The part itself also benefits from this method because avoiding fusion zones maximises its service life and optimises its quality. In other words, the excelliSCAN system ensured that the project partners were able to achieve both high productivity and high component quality when drilling CFRPs. Using the system was simple because the automatically calculated scanner delays made tuning selections redundant.
Quality and productivity improvements
The CarboLase project clearly demonstrated the benefits of using laser drilling systems. A breakdown of the individual process steps illustrates how the production processes have been streamlined (see figure 3). Using ultrafast lasers to make holes can reduce the number of steps in the production process from six to four.
The preforms are prepared by unrolling the material, cutting it to size, stacking it and hot pressing it to bond the individual layers. It then took approximately 50 seconds to make each hole – for six inserts, drilling took 4.2 minutes. For comparison, it took 12.3 minutes to produce the functionalised preform geometry demonstrated in the CarboLase project (see figure 1) from start to finish.
This makes it easy to see that the laser process itself makes up a large proportion of the total processing time. The use of the highly dynamic scanner allowed this time to be reduced to a minimum.
Figure 3: Conventional processing of FRP parts (top) versus production process using an ultrafast laser drilling system (bottom) (Credit: ITA)
Combining the individual steps to produce the preforms in a single robot cell has opened the door to real just-in-time production of CFRP components, regardless of component geometry, required holes and batch size. To test the new method and demonstrate its technical feasibility, the project partners produced a demonstrator of a B-pillar component and subjected it to extensive mechanical testing, which it passed exceptionally well. In a series of both pullout and torsion tests, the joints produced using the CarboLase method performed better than those in CFRP components produced by conventional means.
Thanks to the interlocking connection between the inserts and the matrix material, the CFRP components produced using this new method can withstand a maximum pullout force up to 50 per cent higher than conventionally manufactured components with glued-in inserts. Depending on the component design, this improvement in mechanical performance offers the potential to reduce overall component thickness and weight.
In September 2019, CarboLase won the CAMX Award in the ‘Combined Strength’ category. The CAMX Awards celebrate innovations that promise to have a major influence on the future of composite materials. Decisive for the award was the integration of the drilling step at the beginning of the process chain and the resulting reduction in time- and cost-intensive subsequent process steps.
Simplifying the machine concept for further productivity enhancements
The impressive results have been a springboard for further projects. Investigations into the use of systems with multiple scan heads, to further reduce processing times, are currently in the pipeline. Furthermore, the machine concept could be pared down, for example by having a less flexible robot arm, in order to keep investment costs down without compromising on process quality.
The market prospects are promising, as numerous other applications besides the aerospace and automotive industries come into question, which are confronted with complicated structures and connections for fibre composite materials. There is obvious potential to be tapped in the toolmaking industry, the furniture industry and the sports equipment manufacturing industry. Other inquiries and avenues are also being investigated.
Dr Stefan Janssen is team manager for laser drilling and precision cutting at Fraunhofer ILT
Dr Sebastian Oppitz is the manager of additive and joining technologies at the Institut für Textiltechnik at RWTH Aachen University
Dr Matthias Morasch is a product manager at Scanlab