DeVelopment of a Universal seLeCtive stripping solution for Aircraft coatiNgs

Project Information

VULCAN

Grant agreement ID: 821236

DOI

10.3030/821236

Project closed

EC signature date 3 October 2018

Start date 1 November 2018

End date 31 October 2022

Funded under

• SOCIETAL CHALLENGES - Smart, Green And Integrated Transport

Total cost

€ 554 921,25

EU contribution

€ 554 920,00

Coordinated by

CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE CNRS
France

Periodic Reporting for period 3 - VULCAN (DeVelopment of a Universal seLeCtive stripping solution for Aircraft coatiNgs)

Reporting period: 2021-11-01 to 2022-10-31

Summary of the context and overall objectives of the project

Aircraft parts always have a minimum of two (generally three) of deposited layers: a primer, a coat of
paint and a final coat. The maintenance of an aircraft exterior parts need the removal of each layer in a clean, controlled,
precise way, without affecting the substrate material. For surface preparation, the industrial challenge is the quality of the post-stripping surface. If stripping does not leave the surface clean and without damage, an additional finishing step is required. The development and control of stripping would allow gains: In the processing speed, Quality of the treated surface, Precision, Adaptability of the process ( i.e. the ability to remove layer by layer), Reduction of the production of hazardous waste, Cost. Current processes (Chemical, Abrasive, Advanced technologies (laser, FlashJet) does not fill completely actual requirements.

Vulcan project aims to develop an industrial laser shock stripping process able to selectively remove layer by layer the top coat of an aircraft paint stack.
It consists in the production of mechanical shock wave from laser plasma. In playing with laser beams parameters and multi-impacts configurations, we can introduce selectively tensile on interfaces to generate damages if stress level is enough. Without contact, process performed with ns laser pulse allows accurate sollicitation with an accuracy of 1 um in the depth. Stripping products could be recovered in spalls of layers. Process parameters are optimized using numerical simulation and material stack properties.
The VULCAN project objective is to go beyond traditional stripping using laser shock in order to fulfill all requirement of aerospace industry that are:
-Development of a dry stripping process Environmental friendliness (no use of chemicals and liquid solutions, reduction of production of hazardous wastes, no water consumption, limited worker exposure to toxic and hazardous substances, reduction of hazardous material disposal costs)
-Universal solution that can be applied on metallic and composite substrates
-Selective = controlled stripping of a thin layer without affecting the surface treatment or the prime, For metals = corrosion protection stays intact, for composites = no top coat or fibers damages No deformation or any kind of damage
-Automated for productivity benefits (cost effective, reproducibility, efficient speed)

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The vulcan project concerns selective stripping by shocks produced by plasma laser. The preliminary tests have showed the interest of the technique in terms of stripping rage but also its sensitivity to the parameters of the materials: substrate, thicknesses and aging (WP1). Approaches combining experiments and simulations have made it possible to define process windows and the versatility of the process to deal with all configurations(WP2). Demonstrations at different scales (WP3) also made it possible to evaluate the process impact of material and against repainting. LCA and TRL assesses futures applications (WP4). All results have been or will be published in scientific journal and presented in technical conferences and on social networks. Vulcan opens an important perspectives for the developpment of a new generation of paint adhesion test using laser Shock.

Laser stripping is a process which typically includes different forms of ablation phenomena. The presented work investigates a mechanical stripping process using high pressure
laser-induced shock waves in a confined regime. Power density is studied as a parameter for selective Laser Paint Stripping Process (LPSP) on specimens and for paint adhesion relations. A flashlamp-pumped Nd:YAG lasers with fixed spot size (4mm) is shot on single layer epoxy and several layers of polymeric paint applied on AA 2024-T3 (Aluminium), AL-Li 2060 and CFRP based substrates. After laser treatment, samples are investigatedwith optical microscopy, profilometer and chemical analysis (FTIR, DSC & TGA). The results show that selective laser stripping is possible between different layers of external aircraft coatings and without any visual damage on the substrate material. In addition, paint stripping is developed from the paint side of specimens with the usage of thermal layer for single layer of epoxy coating. In parallel to the experimental work, a numerical model has been developed to explain the background of the physical mechanisms and to evaluate the detailed stress analysis and interfacial failure simulation both for the laser stripping from the substrate side and the paint side applications. The work covers the background and specifications for the laser paint stripping. Afterwards, samples, tools and lasers are presented. In the third part of the manuscript, behavior of aluminum based materials, aluminum tape, epoxy primers, polyurethane top coats and CFRP under lasershock loading is conducted. In the fourth part, laser paint adhesion tests are demonstrated as function of different coatings’ layer thickness, specimen thickness, surface treatment, layer type, substrate type and thermal ageing. The common process window is defined by
sub-grouping the tested specimens for paint stripping applications in chapter 5 which is directly linked with the applied adhesion tests. In addition, stripping rates are evaluated for different laser technologies.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Paint stripping is an essential process in the aeronautical industry since aircrafts need restoration, maintenance and routine inspection during their life time. In the 1970s, the Air Force Logistics Command began inspecting alternative stripping methods to reduce the environmental drawbacks caused by the hazardous chemicals and waste disposed from the chemical stripping process. They found that plastic media blasting and laser stripping are the most applicable techniques due to the environmental concerns. Although plastic media blasting was found as a replacement to chemical stripping, there were still limitations for how it could be used. Due to high amounts of dust produced while operating plastic media blasting, sensitive components and machinery of the airplanes should be covered and protected requiring careful maintenance work both during blasting and after stripping operations. In addition, since plastic media blasting is not complete selective stripping process and in many cases damages the substrate surface, laser stripping appeared as the most promising technique. Environmental friendliness and the monitorable aspects of laser stripping process makes the application favorable for industrial applications.

Thanks to the sensitivity of the laser stripping procedure, it opened us up a new perspective of laser paint adhesion test with the same methodology. Specimens are investigated to see the effect of coatings’ thickness, specimen thickness, surface treatment, coating type, substrate type and thermal ageing. Hence, laser paint adhesion test can be an innovative tool for paint adhesion test domain (beneficial both for HAI and RESCOLL instead of using conventional paint adhesion tests). The obtained results focus on the capability of the process to control. Moreover, mechanical, chemical and interface properties are studied to understand the origin of the adhesive failure for different layer configurations. In addition, specific stripping phenomena is considered (damage ring and the effect of
thermal ageing) and compared with modeling via LS-DYNA software for a single epoxy layer samples.

Large Stripped Zones after laser impact

Overview of the results