By Rudolf Weber and Volkher Onuseit
The benefit of CFRP for lightweight construction in automotive and airplane industries is widely accepted. Impressive pictures of high-performance cars and airplanes with numerous high-tech looking carbon fiber parts are familiar to everybody.
However, industrial large-volume application of CFRP requires efficient and high-quality processing. And of course, the laser is a very promising tool. Its advantages and disadvantages have been discussed in numerous papers in the last few years.
There are two main issues which limit the use of lasers for cutting, drilling and surface structuring today:
On the one hand carbon has no fluid phase at ambient pressures below about 100 bars. Therefore, material can only be removed when it is evaporated. This means that the very large volume specific enthalpy of about 85 J/mm3 is necessary for laser processing of carbon. This value allows an easy calculation of the average laser power necessary to remove a certain volume in a given time (the time is usually given by the bookkeepers). As an example, cutting of 2 mm thick material with a kerf width of 0.2 mm with industry-typical 100 mm/sec requires several kilowatts of average power.
On the other hand, the high evaporation temperature of carbon of about 3600 K together with the low damage temperature of about 800 K of the matrix often results in a large thermal damage. During the laser interaction the heat flows along the fibers and destroys the surrounding matrix. Model calculations to determine the minimum possible thermal damage were done at the Institut für Strahlwerkzeuge (IFSW) of the University of Stuttgart. The blue and red line in Figure 1 show the calculated minimum possible extent of the matrix damage and matrix evaporation, respectively, as a function of the intensity.
The comparison with the experimental data points collected from various literature proves the validity of the model. It is also seen that it seems to be easy to create more damage than necessary. However, in addition, the calculations give an optimum intensity range for laser processing of CFRP, shown as green area. The lower boundary of this optimum range of about 108 W/cm2 implies using short-pulse lasers which are usually run at high repetition rates in order to achieve high average powers. Unfortunately, the repetitive pulsing can cause heat accumulation effects if the beam does not move fast enough over the workpiece. This will be even accentuated in the next few years when the average power of commercial short-pulse lasers will reach the required Kilowatt level at repetition rates of several Megahertz.
At the IFSW an example of large volume application was drilling of several hundred of 1 mm diameter blind holes in 4 mm thick CFRP which were used for structure integrity diagnostic in stringers of airplanes. A small amount of damage of the surrounding material could be accepted in this case. The main issue was to reach the correct depth of slightly less than 4 mm (into the lowest fiber layer). And, of course, a drilling time of less than 10 s had to be achieved with a low-invest laser system.
The IFSW approach was to use a 7 W, 515 nm wavelength marker system in combination with a new and highly efficient “enhanced helical drilling” method.
The Trumpf TruMark 3000 laser marker system that was used is shown in Figure 2. For processing of CFRP it is highly recommended to modify commercial systems. An adapted exhaust system (seen on the top, sealed with the yellow sticky tape) prevents carbon dust to enter into the laser case. Furthermore the marker system was mounted in a small closed-housing processing station with an efficient exhaust system with an activated coal filter: The health hazard risk of the pollution from the laser process is not yet clarified.
The “enhanced helical drilling” method takes benefit of the thermal damage, and of the fact that the volume specific enthalpy for evaporating the matrix material, is about a factor of forty lower than that of the carbon fibers. The principle is shown in Figure 3. First, an outer circle groove with a diameter of 1 mm is created in one single turn in order to thermally detach the surrounding from the material which has to be removed. Subsequently the inner circle groove with a diameter of 0.6 mm is created. The strong heat accumulation in the area inside the inner circle causes evaporation of the matrix material. The evaporation pressure removes the remaining fiber fragments. Finally, the material between the outer and the inner groove is removed in a helical beam path (solid line with arrow) with a beam displacement of 10 µm between the turns. One such compete ablation procedure is referred to as “1 pass.”
Figure 3 shows the drilling progress for an increasing number of passes. The focus position was adapted for each pass as sketched in Figure 4.
The required 3.9 mm of drilling depth was reached after 11.5 s. This is already very close to the requirement. Up to a factor of seven, higher efficiency was measured in the beginning of the process when compared to pure evaporation drilling. The quality of the surrounding of the holes and the hole walls was within the required tolerance. The cross-sections show that the quality of the walls of the holes is independent of the hole depth. The walls are almost parallel. However, increased quality – and shorter drilling times – are expected when further optimizing the process as will be discussed in the end of this article.
Apart from the significantly increased efficiency the use of heat conduction for processing has another advantage: The poor conductivity perpendicular to the fibers yields very good quality of the bottom of the hole and with it, a very good control of the hole depth. This was important for the current task as the drilling should reach the lowest fiber layer without creating through holes.
In conclusion, the method “enhanced helical drilling” used at the IFSW allows increasing the productivity, up to a factor of seven, together with reasonable hole quality. The next step is further optimization of the process. Focus will be on adapting the local feed rate, i.e., increasing it for the outer circle to reduce thermal damage in the surrounding material and decreasing it for both the inner circle and helical beam path, in order to enhance the heat accumulation effect in the material which has to be removed. This might also allow to completely omit the inner circle which would further reduce the drilling time. In addition, it is planned to transfer the enhanced helical drilling process to a multi-10 W ps-laser system and a kW-ps system which was recently demonstrated at the IFSW. Furthermore, this processing scheme will be applied to other processes such as surface structuring and cutting.
Dr. Rudolf Weber is head of the materials processing department of the IFSW of the University of Stuttgart and Dipl.-Ing. Volkher Onuseit is head of the precision manufacturing group in the material processing department of the IFSW of the University of Stuttgart.