Environmentally friendly surface pretreatment with power lasers for manufacturing fiber metal laminates (FML)

© Fraunhofer IWS Dresden
FML – Skin field

© Fraunhofer IWS Dresden
Fiber-Meta-Laminate, consisting of aluminum thin sheets and glass-fiber reinforced epoxy resin adhesive films

Large-area structuring with high power lasers can replace wet chemical surface pretreatment and at the same time ensures good adhesion for even bonding.

In the aviation industry, FML is regarded as a lightweight construction material with great future potential. It consists of several aluminum and fiber composite layers, each only a few tenths of a millimeter thick. Compared to pure metals, the new material offers not only weight savings but also improved burn-through and impact behavior and, due to the delayed crack propagation, improved fatigue behavior. However, the production of this semi-finished product is hardly automated due to the still very small quantities involved and is therefore both expensive and resource-intensive. As part of an aeronautical research project, Fraunhofer IWS therefore cooperated with other industrial and research partners on basic technologies for automated FML production. Research in Dresden focused on the development of solutions for bath-free surface pretreatment of metal foils using laser technology.

Solution

© Fraunhofer IWS Dresden
Aluminum thin sheet surface of AA2024 with laser structuring

© Fraunhofer IWS Dresden
Focused Ion Beam (FIB) - cross-section of the generated aluminum oxide layer

The pretreatment of adhesive joints with laser radiation is an established procedure. Until now, pulsed laser systems have always been used for material removal, as only they achieved the correspondingly high intensities for the evaporation of the metal. Classical cleaning and pre-treatment of adhesive joints with these laser systems, however, only process a few square centimeters.

In the project, however, the structuring of several square meters of surface was necessary and consequently a much more efficient laser system. For this reason, a powerful continuously emitting solid-state laser combined with remote technology is used for FML pretreatment.

A reproducible material removal can be achieved by very good focusing of the laser radiation in the kilowatt range with simultaneously fast spot movement over the substrate. In order to achieve good productivity, the laser spot moves linearly over the surface at up to 300 m/s. Thus, area rates of currently up to 1m² per minute can be achieved.

Results

© Fraunhofer IWS Dresden
Test sample for analysis of adhesion properties

© Fraunhofer IWS Dresden
Cyclic testing of 5-hole probe made of 3-2 laminate

Due to the high power densities and short interaction times, the native oxide layer, often mixed with inorganic contaminants, could be removed, the surface roughened and at the same time a much stronger and tighter artificial oxide layer could be built up.

At structural depths of approx. 10 µm, oxide layer thicknesses of up to approx. 2 µm could be verified for the aluminum alloy AA2024 (sheet thickness: 300 µm). The subsequently fabricated adhesive joint behaved in strength tests and ageing tests such as Filliform and salt spray tests comparable to test samples pre-treated with a classical phosphoric acid anodisation (PSA).

To validate the performance of laser-structured fiber-metal laminates, 3-2 laminates were also produced. Crack initiation and documentation of crack propagation and residual strength were performed on 5-hole samples under cyclic loading. It was possible to achieve up to 90% of the performance of the FMLs produced with PSA pretreatment.

Research funding

Part of this research was funded by the Federal Ministry of Economics and Energy on the basis of the decision of the German Bundestag within the project "AUTOGLARE - Advanced metal fuselage construction - glass fiber reinforced aluminum and automated manufacturing processes for high production rates in aircraft construction; sub-project: NFM-GLARE" (FKZ: 20W1517D)

Projects