David Blass

Sauber und prozesssicher

Der Einsatz von Faserverbundkunststoffen (FVK) im Automobil ist spätestens seit der Bekanntmachung, dass der BMW i3 ein Life-Modul aus kohlenstofffaserverstärktem Kunststoff (CFK) besitzen wird [1], nicht mehr nur auf die Kleinstserie im Sektor hochpreisiger Sportwagen beschränkt. Es bestehen jedoch noch erhebliche Herausforderungen, diese Materialien zu fügen, da gebräuchliche Verfahren — wie etwa das Punktschweißen — nicht eingesetzt werden können und Techniken, bei denen mechanische Verbindungselemente verwendet werden, zu einer lokalen Zerstörung der Fasern und erheblichen Spannungsspitzen führen. Es empfiehlt sich der Einsatz der Klebtechnik, die allerdings eine Vorbehandlung voraussetzt, um Trennmittelrückstände und andere Kontaminationen von den Oberflächen zu entfernen [2]. Mit dem Einsatz von FVK in größeren Stückzahlen entsteht dabei auch die Notwendigkeit, Verfahren zu entwickeln, mit denen automatisiert und mit hoher Prozesssicherheit und -geschwindigkeit die Klebvorbehandlung erfolgen kann.

Clean and reliable

The use of fiber-reinforced plastics (FRP) in the automotive industry is no longer limited to the smallest-batch series of expensive sports cars, at least not since the announcement that the BMW i3 will have a Life-module made of carbon fiber-reinforced plastics (CFRP) [1]. But there are still considerable challenges as regards the bonding of these materials, as conventional procedures (for instance spot welding) cannot be applied and technologies using mechanical fasteners lead to a local destruction of the fibers and considerable stress peaks. The use of adhesive bonding is recommended which, however, requires a pretreatment to remove release agent residues and other contaminations from the surfaces [2]. With the use of FRP in larger quantities it also becomes necessary to develop procedures which allow automated bonding pretreatment with a high reliability and low process times.

A contribution to the qualification process of surface pretreatment methods: Sensitivity of mechanical tests to adhesion and delamination

ABSTRACT Focusing the adhesive bonding of thermoset carbon fiber reinforced plastics (CFRPs) due to release agent residues on a surface pretreatment prior to the bonding process is inevitable to guarantee sufficient adhesion. Based on different material and process-related phenomena, a huge variety of parameters (e.g. treatment time) influence the treatment effectiveness. This often leads to intense parameter studies in which pretreatment methods are qualified by different mechanical tests and which require significant effort in terms of time and resources. Those qualification procedures are sometimes standardized by different and mostly company-specific standards. To increase the efficiency of this qualification process by increasing the general validity, this paper presents a comparison of different mechanical test methods (namely single lap shear test, floating roller peel test, double cantilever beam (DCB) test, and cross-tension test) in terms of their sensitivity to the detection of adhesion or delamination defects, even if they normally test different aspects of a joint. However, it could be found that the most common test – the single lap shear test – can only reveal large defect levels, while different tests, e.g. DCB test, show a good sensitivity for all investigated failures and others show intermediate performance.

The impact of prepreg aging on its processability and the postcure mechanical properties of epoxy-based carbon-fiber reinforced plastics

Due to the intention to increase the product efficiency by weight reduction, the application of carbon fiber reinforced plastics and its processing becomes more and more important for the industrial production planning. The aviation industry mostly uses epoxy-based preimpregnated laminates (prepregs). These prepregs are stored in the refrigerated state and may be processed after defrosting only for a specific shelf-life or so-called out-time. This limitation in processing time reduces the efficiency of the entire production, for example due to the waste of material during production stops. To improve the production effectiveness the shelf-life should be as high as possible, to be able to avoid material waste during production stops. Therefore, the processability and the postcure mechanical properties of two representative prepregs were investigated during this study for different aging stages and correlated with the reactivity of the prepregs.

CFRP bonding pre-treatment with laser radiation of 3 μm wavelength: laser/material interaction

Laser radiation of 3 μm wavelength was generated by frequency conversion of an industrial IR laser and applied in the context of CFRP bonding pre-treatment. Reinforced and non-reinforced epoxy resins were treated with this radiation varying the relevant parameters such as laser power or treatment time. The interaction between laser radiation of 3012 nm and 1064 nm wavelength and matrix resin was analyzed mechanically (e.g. ablation depth), optically (such as fiber exposure) and chemically (e.g. contamination removal). The results gathered show that, even with the small achievable pulse fluences, a sufficient treatment of the specimens and a sensitive removing of the contaminated layers are possible.

Fiber-coupled three-micron pulsed laser source for CFRP laser treatment

We present a laser source providing up to 18 W and 1.5 mJ at a wavelength of 3 μm. The output is generated by frequency conversion of randomly polarized multimode radiation at 1064 nm of an Nd:YAG laser in a two-stage conversion setup. The frequency converter comprises an optical parametric oscillator and a subsequent optical parametric amplifier using PPLN as nonlinear medium in both stages. To implement fiber-based beam delivery for materials processing, we coupled the output at 3 μm to a multimode ZrF4-fiber. This source was then used to remove epoxy resin from the surface of CFRP samples.

Laser transmission joining of thermoplastic fasteners: Application for thermoset CFRP

A laser transmission-based joining process is investigated, joining one thermoplastic adherend (PA6) which is transparent for the applied laser radiation with another adherend being a thermoset carbon fiber-reinforced plastic. The application is addressed to use thermoplastic fasteners to join different materials to thermoset carbon fiber-reinforced plastic parts. The influence of different laser intensities and different amounts of introduced energies to the joint are investigated. Therefore, laser parameters such as scan speed and laser power are varied using a fiber laser and a scanner optic. The same laser source is used to pretreat the thermoset surface exposing the C-fibers before joining. The performance and the quality of the joints are evaluated by mechanical shear-tests and microscopic cross sections. The results show the significant influence of laser intensity and energy on damage-mechanisms but also the possibility to provide good quality joints reaching up to 13.8 MPa.

Vorbehandlung von Kunststoffen

Kunststoffoberflachen konnen mithilfe unterschiedlicher Verfahren gezielt aufs anschliesende Kleben vorbereitet werden. Die jeweils richtige Vorbehandlung soll die Substratoberflache sowohl auf den einzusetzenden Klebstoff einstellen als auch durch die damit erfolgte Konditionierung zur Prozesssicherheit der Klebung beitragen. Alle vorgestellten Vorbehandlungstechniken haben ihre Vor- und Nachteile. Welche Oberflachenvorbehandlung gewahlt werden sollte, hangt von folgenden Faktoren ab: Art und Menge der auftretenden Kontaminationen bzw. Randschichten, Art des Kunststoffs (Polaritat des Materials, Bestandigkeit des Materials gegenuber Losungsmittel, Warme etc.), Grose der zu klebenden Kunststoffbauteile bzw. Halbzeuge, Geometrie der Klebflachen (flach, komplex 3D geformt bzw. flachige Klebung, Stuckzahl der zu klebenden Bauteile (manuell, automatisiert), Taktzeit und Grose der Klebflache (Flachenleistung), HSE-Aspekte (Health, Safety and Environment Protection) und Fertigungsumgebung, Investitions- und Betriebskosten. Somit muss die gewahlte Vorbehandlungstechnik ein recht umfangreiches und individuelles Anforderungsprofil erfullen. Fur besonders hochwertige Klebungen mit extremen Anforderungen an deren Langzeitstabilitat werden zum Teil auch hybride Vorbehandlungsmethoden eingesetzt.

Transportwesen und allgemeine Industrieanwendungen

Im Automobilbau werden komplette CFK-Strukturen im Rahmen einer Serienfertigung ohne Einsatz zusatzlicher mechanischer Fugelemente geklebt. Die Herausforderungen an die Klebstoffsysteme variieren hierbei aufgrund der verschiedenen Anforderungsprofile, vor allem in Hinblick auf die Klebstoffeigenschaften und ihren Verarbeitungsprozess.

Composite Bonding Pre-Treatment with Laser Radiation of 3 µm Wavelength: Comparison with Conventional Laser Sources

To use the full potential of composite parts, e.g., to reduce the structural weight of cars or airplanes, a greater focus is needed on the joining technology. Adhesive bonding is considered favorable, superior joining technology for these parts. Unfortunately, to provide a structural and durable bond, a surface pre-treatment is necessary. Due to its high integration potential in industrial process chains, laser radiation can be a very efficient tool for this purpose. Within the BMBF-funded (German Federal Ministry of Education and Research) project GEWOL, a laser source that emits radiation at 3 µm wavelength (which shows significant advantages in theory) was developed for a sensitive laser-based bonding pre-treatment. Within the presented study, the developed laser source was compared with conventional laser sources emitting radiation at 355 nm, 1064 nm, and 10,600 nm in terms of application for a composite bonding pre-treatment. With the different laser sources, composites were treated, analytically tested, subsequently bonded, and mechanically tested to determine the bonding ability of the treated specimens. The results show a sensitive treatment of the surface with the developed laser source, which resulted in a very effective cleaning, high bonding strengths (over 32 MPa), and a good effectiveness compared with the conventional laser sources.