No abstract

Today, industrial usage of carbon fiber reinforced plastic (CFRP) is steadily increasing, with an amount of 67 000 ton/yr [“Carbon fibers and carbon fiber reinforced plastics (CFRP)—A global market overview,” Report, Research and Markets Ltd., Dublin, Ireland, 2013]. Products such as the Boeing 787 and Airbus A350 in the aerospace sector, as well as the BMW i3 from the automotive industry consist of more than 50% of CFRP in their structural weight. At the same time, these products also have comparatively high production volumes, e.g., >10 000 cars/yr in the case of the BMW i3 [“BMW undertakes global launch of i3 EV,” see http://www.ihs.com/products/global-insight/industry-economic-report.aspx?id=1065981621, May 20, 2014]. Therefore, a higher degree in automation and cost-efficiency is needed in production. Due to the highly abrasive carbon fibers, conventional machining processes result in short tool life and high costs. Therefore, laser cutting of CFRP as a wear-free alternative has lately become the focus of several research groups. Two different approaches are commonly chosen: cutting by short- and ultra-short pulsed laser systems to reach a process regime of cold ablation and cutting with continuous wave (cw) lasers at high cutting speeds. For the latter approach, it has already been shown that by increasing power and cutting speed, the heat affected zone (HAZ) can be reduced due to less time allowed for heat conduction [Bluemel et al., “Laser machining of CFRP using a high power laser—Investigation on the heat affected zone,” in Proceedings of 15th European Conference on Composite Materials, Venice, Italy (2012)]. Graf and Weber introduced a perpendicular heat flow model, calculating that the required intensity to cut 2 mm of CFRP with a HAZ of 10 μm using a cw laser is 109 W/cm2. The required cutting speed is 8.3 m/s [T. Graf and R. Weber, “Laser applications from production to machining of composite materials,” in Proceedings of EALA, Bad Nauheim, Germany (2012), pp. 289–299]. In this paper, experiments using an ultra-high power fiber laser system of 30 kW to cut CFRP laminates are presented. Although it is not possible to fully achieve the intensities proposed by Graf and Weber, the intensities of approx. 108 W/cm2 of the setup still allow for a practical validation of the CFRP cutting at very high laser power. Due to the high intensities, high cutting speeds per laser pass are necessary. A special experimental setup is chosen with a rotational movement of the specimen, reaching a feed rate of 85 m/s. The heat affected zone was considerably reduced to 78 μm with the 30 kW system, 10% lower than with a 5 kW system under comparable conditions. Although today no scanner systems are available that could handle these high intensities at such high cutting speeds, the experiments still show that processing of CFRP with cw laser systems at highest power has a potential in order to reduce the heat damage to the material.

The growing interest in carbon-fiber-reinforced-plastics (CFRP) leads to a rising demand for cost-efficient, alternative production methods, such as laser-remote-cutting. Since the cycle times, particularly in the automotive industry, represent a significant cost factor, this paper presents a method for reducing the heat-affected zone (HAZ) in the CFRP-laminate while maximizing the cutting speed. In pursuing this goal, in this paper, a comparison is made between a multi- and a single-mode laser system. In addition, simulation is used to investigate which mechanisms contribute to the formation of the HAZ and the effect of the delay times between the exposures on the HAZ. For the comparison of the beam sources, attempts are made with regard to the required cumulative energy per unit length to achieve a complete cut. The developed numerical simulation takes into account the thermal decomposition of the plastic matrix as well as the fibers, the heat conduction in the laminate, the flow of the gaseous decomposition products in the kerf and the heat transfer by forced convection on the cutting edge, the layer setup, the material properties as well as the laser parameters power, scanning speed, exposure number, and the delay times between the exposures. By means of Monte-Carlo-simulation, an optimization of the HAZ or the sum of all delay times is carried out. The experiments show that a complete cut with the single-mode laser requires a higher energy than the multi-mode system. It can be shown by the simulation that the primary target conflict in laser beam cutting of CFRP with several exposures exists between the average width of the heat-affected zone and the sum of the delay times between the exposures. The sum of the delay times is decisive in particular for the effective cutting speed. The simulation also shows that as a rule at least 50% of the width of the heat-affected zone is caused by the flow of the gaseous process emission within the kerf. The optimization could be used to calculate and validate that a skillful selection of the delay times results in a reduction of the HAZ or the sum of all delay times. By comparison of the beam sources, it can be seen that the aspect ratio between laser beam diameter and laminate thickness must be optimized. In addition to the optimization of the delay times, it is also necessary to adjust the laser power and the scanning speed from exposure to exposure.

No abstract

Mass production of carbon fiber-reinforced plastic parts has lately started in the automotive industry. Due to no abrasive wear in combination with a high degree of automation and ability for 3D processing, laser remote cutting is a suitable method for machining purposes in this context. In the automotive environment, solid-state lasers are favored because optical waveguides may then be used. In turn, the low absorption of the radiation of such lasers in the matrix material presents a drawback in terms of comparatively large heat affected zones (HAZ) and flaking at the cutting kerf. This paper deals with the question, if a laser absorbing additive can be used to enhance the absorption within the matrix material, while the optical properties in the visible spectrum are kept. For this purpose, an additive known from laser transmission welding has been added to the matrix material. Cutting experiments have been carried out while varying concentration of the additive. The investigations show that a significant reduction of the mean HAZ of 25% and the standard deviation (1 σ) of 56% can be achieved by adding 4% w/w of the additive to the resin. In addition to that, the flaking behavior can be avoided. Compared to adding soot particles, the optical properties of the laminate do not change in the visible spectrum, leaving the fiber textile visible.