By: Annett Klotzbach
Fraunhofer IWS
Carbon fiber reinforced polymers (CFRP) are increasingly applied in the aircraft industry as well as the automobile industry. The main reason is the highly mechanical load on one hand and the low density on the other hand. Moreover, the corrosion resistance plus the damping behavior of the material can fully be utilized in highly stressed structures. However, the concept of manufacturing CFRP-parts near- net- shape does not substitute the need of cutting them. The different properties of fiber- and matrix-material constitute an ambitious challenge for the CFRP cutting process with a laser beam. Main influencing parameters are the different thermo-optical characteristics of fiber and matrix. The carbon fibers have a more than 20 times higher sublimation temperature than the decomposition temperature of the resin. Because of a good heat conduction of the fibers, the resin fitting at the fibers will quickly be decomposed, before the carbon fiber itself is cut (Fig.1). The theory to decrease the heat affected zone is to minimize the interaction time between laser radiation and material.
The benefit of the availability of high power and high brilliant laser sources enables ablation and/or cutting processes with high processing speeds. However, the relation between wavelength and absorption on non-isotropic materials on the one hand, the influence of the intensity and the processing speed on the other hand, require fundamental research.
Within the experiments a continuous-wave (cw)-single mode fiber laser was applied in comparison with a Q-switched pulsed laser system (50 W, 100 ns). Using the Remote technology (the laser beam will be deflected by two galvo – driven tilting mirrors) a minimized heat interaction time between material and laser beam can be reached. The setup of the cw-Remote processing is shown in Fig. 2.
The material to be processed was a 3 mm thick consolidated CFRP with epoxy resin. Its fiber volume percentage is about 50% with multidirectional orientation of the long fibers.
As a result two cross sections can be seen:
Fig.3
Cross section of CFRP using cw – laser & Remote processing
Pl = 3 kW, vaverage = 9 m/min, ET = 11 J/mm²
Fig.4
Cross section of CFRP using Q-switched – laser & Remote processing
Pl = 50 W, vaverage = 0,04 m/min, ET = 28 J/mm²
The Remote technology proved to be an excellent tool to cut consolidated CFRP with good cut qualities. Average processing speeds of more than 10 m/min can be reached using high brilliant and high power cw – lasers. The heat affected zone can be minimized by decreasing the interaction time. One possible way is to use Q-switched lasers with short pulse lengths. However, because of the limited overall laser energy, the average processing speed is quite low.
Fig. 1: REM image of CFRP, gas assisted laser cutting
Fig. 2: Experimental setup for Remote processing
