Characterization of Interfacial Corrosion Behavior of Hybrid Laminate EN AW-6082 u CFRP

The corrosion behavior of a hybrid laminate consisting of laser-structured aluminum EN AW-6082 u carbon fiber-reinforced polymer was investigated. Specimens were corroded in aqueous NaCl electrolyte (0.1 mol/L) over a period of up to 31 days and characterized continuously by means of scanning electron and light microscopy, supplemented by energy dispersive X-ray spectroscopy. Comparative linear sweep voltammetry was employed on the first and seventh day of the corrosion experiment. The influence of different laser morphologies and production process parameters on corrosion behavior was compared. The corrosion reaction mainly arises from the aluminum component and shows distinct differences in long-term corrosion morphology between pure EN AW-6082 and the hybrid laminate. Compared to short-term investigations, a strong influence of galvanic corrosion on the interface is assumed. No distinct influences of different laser structuring and process parameters on the corrosion behavior were detected. Weight measurements suggest a continuous loss of mass attributed to the detachment of corrosion products

Transferability of the Structure–Property Relationships from Laser-Pretreated Metal–Polymer Joints to Aluminum–CFRP Hybrid Joints

The transferability of structure–property relationships for laser-pretreated metal adhesive joints to laser-pretreated metal–carbon-fiber-reinforced plastic (CFRP) bonds was investigated. Single-lap shear tests were performed on hybrid AW 6082-T6–CFRP specimens pretreated with the same pulsed laser surface parameter sets on the metal surface as previously tested, AW 6082-T6–E320 metal adhesive joints. The fracture surfaces were characterized to determine the type of failure and elucidate differences and commonalities in the link between surface structures and single-lap shear strengths. Digital image analyses of the hybrid specimens’ fractured surfaces were used to quantify remaining CFRP fragments on the metallic joint side. The results indicate that high surface enlargements and the presence of undercut structures lead to single-lap shear strengths exceeding 40 MPa and 35 MPa for unaged and aged hybrid specimens, respectively. Whereas for the metal–polymer joints, the trend from high strength to weakly bonded specimens is largely continuous with the degree of surface structuring, hybrid metal–CFRP joints exhibit a drastic drop in joint performance after aging if the laser-generated surface structures are less pronounced with low surface enlargements and crater depths. Surface features and hydrothermal aging determine whether the specimens fail cohesively or adhesively

Einflussfaktoren und deren Auswirkung auf das Adhäsions- und Alterungsverhalten von Titan-Polymer-Klebungen

Übergeordnetes Ziel der Arbeit war es, die für die Anbindung zwischen Metall und Polymer relevanten Mechanismen und Einflussfaktoren zu identifizieren und systematisch zu unter-suchen, um eine Bewertung der einzelnen Faktoren vorzunehmen. Mithilfe von unterschiedlichen Vorbehandlungsverfahren und Titan/Polymer-Kombinationen wurde die Oberflächenstruktur und Chemie systematisch verändert und umfassend charakterisiert. Der Einfluss der Oberflächenstruktur, Chemie und Belastungsrichtung auf die Adhäsionsfestigkeit und Alterungsbeständigkeit der Klebungen wurden diskutiert und folgende Rückschlüsse gezogen: Die (nano)mechanische Verklammerung liefert den Hauptgrund für langzeitstabile Klebungen. Chemische Wechselwirkungen können dagegen aufgrund der Schwächung der Bindungen durch Wasser nur kurzzeitig zur Adhäsionsfestigkeit beitragen. Auf Grundlage dieser Erkenntnisse wurde ein erstes Modell der Anbindungs- und Alterungsmechanismen entwickelt und anhand einiger Fallbeispiele validiert

Laser surface pretreatment for structural bonding and coating

Effective surface pretreatment is the key to long-term stable and durable bonds between different materials, e.g., used in structural bonding and coatings. Its purpose, besides a general cleaning of a joint surface, is to structure and functionalise the interface. Particularly attractive and fast, laser surface pretreatment enables simple, reproducible and waste-free processing of surfaces of different materials (e.g. metals, fibre-reinforced plastics). The laser pretreatment, the requirements for obtaining suitable surfaces as well as the optimisation for structural bonding and coating will be highlighted using different material combinations and application examples. For structural applications, metal-polymer and -FRP joining is very common. Metals are pretreated with a pulsed Nd:YAG laser and bonded with thermoset or thermoplastic matrices. For the purpose of forming protective metallic coatings on FRP components, the laser can be employed similarly. Various microanalytical, surface and mechanical characterisation methods are used to study the joint interfaces. Laser pretreatments of titanium, aluminium and steel resulted in nano- and microstructures that consist of solidified melt structures covered by oxidic films. Depending on the alloy, the morphology and chemistry of the surface structures varied. The optimal laser pretreatment leading to a stable interface depends strongly on the alloy itself, i.e., the reflectivity of the surface, the presence of contaminations, and the particular laser parameters. Generally, the formation of a stable, homogeneous nanostructure is a prerequisite for long-term stable adhesive bonding. Laser pretreatment of FRP, in contrast, leads to structuring of the surface by ablation of the matrix, thus exposing the underlying fibres. The metallic coating on the fibres is substantially more reliable than a coating deposited purely on the polymer matrix. Therefore, the laser pretreatment of metals and FRP for bonding and coating follow different concepts. The pretreated surfaces allow mechanical interlocking and chemical interactions leading to long-term stable bonds between different materials

Bonding and aging mechanisms of polymers on titanium alloys

Adhesive bonding is a key technology for structural joining as well as for modern lightweight multimaterials (e.g. fiber-metal laminates) in aerospace or automobile applications. It allows joining different materials with more homogenous load pathways compared to mechanical joining. However, it is quite demanding since service conditions often include both mechanical and environmental/chemical loads, for example humidity, salt-water or deicing-agents. Achieving high-performance joints with sufficient degradation resistance depends strongly on the surface pretreatment, especially for thermoplastics like PEEK (poly-ether ether ketone). The goals of the current investigations are two-fold: (i) To study the bonding of selected thermoplastic and thermoset polymers to titanium alloys and the aging mechanisms, and (ii) to develop surface pretreatments suitable for specific aerospace material combinations. Specimens have been investigated with various microanalytical, surface and mechanical characterization methods. Nd:YAG laser treatments as well as anodization allow for long-term stable joints between PEEK or epoxies like RTM6 and CP-Ti, different Ti-Al-V alloys or Ti-15V-3Al-3Sn-3Cr. Results show that the presence of a nanostructure with suitable morphology and chemistry on the metal surfaces is crucial for slowing degradation. Special experiments have been designed to separately modify surface structures at different length scales or the surface chemistry in order to elucidate the dominating bonding mechanisms. Chemical interactions are most important in the initial stage of the bonding for wetting and infiltration of the surface structures. For polymers largely "inert" against water, they account for the observed aging effects until only (nano-mechanical interlocking remains. Suitable processing windows for laser pretreatment of different titanium alloys have been identified and the understanding of prevailing bonding and aging mechanisms been advanced

Simulating the effect of microstructural inhomogeneity in UD carbon-fibre-reinforced thermoplastics on the mechanical behaviour

Calculation and modelling of structures made from continuous fibre-reinforced plastics (FRP) is usually based upon Classical Laminate Theory (CLT), in which inhomogeneous composite plies are considered as homogeneous material. For representing the elastic behaviour of FRPs the CLT-method is well suited. However, plastic deformation and damage behaviour particularly found in FRPs with thermoplastic matrix is affected by inhomogeneities. In this work a detailed analysis of the microstructure of a thermoplastic FRP with unidirectional carbon fibre reinforcement was conducted for generating parametric geometry models representative for different observed microstructures. For calculating the geometry models an algorithm has been developed, which allowed for modifying the fibre diameter distribution and the fibre volume fraction. In finite element simulations the effect of fibre arrangement on the mechanical response in terms of elastic, plastic, and damage behaviour has been investigated. The variation of fibre arrangements resulted in different localized plastic deformations and crack paths and also significant differences in the stress-strain curves. The outlined method of analysing and modelling different relevant microstructures of an FRP provides the basis for more realistic calculations of the mechanical behaviour of FRPs in multiscale models