Hybrid joints consisting of metals and fiber-reinforced polymer composites exhibit highly desirable properties for many lightweight design applications. This study investigates the potential of additively manufactured surface structures for enhancing the bond strength of such joints in comparison to face milled and laser structured surfaces. Titanium samples with different surface structures (as-built surface, groove-, and pin-shaped structures) were manufactured via electron beam melting and joined to carbon fiber-reinforced polyether-ether-ketone (PEEK) via adhesive bonding and thermal direct joining, respectively. Bond strength was evaluated by tensile shear testing. Samples were exposed to salt spray testing for 1000 h for studying bond stability under harsh environmental conditions. The initial tensile shear strengths of the additively manufactured samples were competitive to or in some cases even exceeded the values achieved with laser surface structuring for both investigated joining methods. The most promising results were found for pin-shaped surface structures. However, the hybrid joints with additively manufactured structures tended to be more susceptible to degradation during salt spray exposure. It is concluded that additively manufactured structures can be a viable alternative to laser surface structuring for both adhesive bonding and thermal direct joining of metal-polymer hybrid joints, thus opening up new potentials in lightweight design.

Recent research efforts in the automotive industry have been focused on the integration of high-strength steels within lightweight vehicles by using improved joining techniques. The present work falls in this subject area and is focused on the analysis of adhesive bonded dual-phase steel/epoxy joints for the automotive industry. Two quasi-static loadcases were considered, i.e. single-lap and T-peel tests, and various surface preparation strategies were evaluated. In particular, the mating surfaces were pre-treated by using pulsed laser irradiation with a fiber laser (1064nm) and comparisons were made with degreasing and sand blasting. Moreover, the effects of bondline thickness and adhesive type were also assessed. To this aim, two epoxy adhesives with fairly different mechanical behavior (i.e. strain hardening versus elasto-plastic) were deployed for joints fabrication. Finally, T-peel tests were also carried out after sample cycling under controlled high humidity and temperature (i.e. accelerated aging). The obtained results highlighted the beneficial effect of laser irradiation on the joints' mechanical behavior under both static and hydrothermal loadings.

The aim of this work is to investigate the effectiveness of laser irradiation on steel substrates for adhesive bonding. The evolution of surface topography and wettability for various pulse fluencies was investigated. The most suitable processing conditions were then selected and adhesive bonded steel/epoxy single-lap and T-peel joints were prepared for static testing. Moreover, in order to assess joint strength against environmental attack, an additional batch of samples was subjected to hydrothermal cycling before testing. The obtained results were compared with baseline samples fabricated using degreasing and sand blasting surface pre-treatments. Post-failure SEM analyses of fracture surfaces were undertaken to survey the locus of failure within the joint bond-line. Results indicated that laser irradiation effectively enhances the overall mechanical behavior of the joint under shear and peel loading. In addition, it allows to retain joint strength after exposure to aggressive environmental conditions.

Aluminium is one of the most popular construction materials in machine and equipment manufacture as well as vehicle and aircraft construction. Particularly, in automotive and aircraft industries, the adhesive bonding of aluminium requires the pre-treatment of the adhesive surfaces. In this study laser pre-treatments were used to laterally control the surface roughness and clean the substrate surfaces by forming fresh aluminium oxide layers. In order to keep the adhesive properties stable over time, the laser pre-treated aluminium surfaces were subsequently coated with weak polyelectrolytes. The applied polyelectrolytes lower the driving forces for the adsorption of unwanted surface contaminations and provide reactive amino groups for the subsequent coupling of reactive adhesives. The surface topographies of the laser-treated aluminium surfaces were investigated in relation to the applied laser parameters (such as pulse frequency, and laser power) by means of scanning electron microscopy (SEM) and light-microscopic techniques (confocal microscopy). The adsorption of the polyelectrolytes was studied by X-ray photoelectron spectroscopy (XPS). Inverse water contact angle measurements using captive air bubbles were carried out to study the wettability (hydrophilicity/hydrophobicity) of the modified aluminium surfaces. Single lap joint tests carried out on joined AlMg3 sheets showed that the shear strengths can be significantly increased by pre-treatment with laser and coating of the alloy surfaces with weak polyelectrolytes. Furthermore, the application of polyelectrolytes improved the stability against corrosion. The article shows the increase of tensile shear strengths at adhesively bonded single lap shear samples after laser pre-treatment and also an increase in long-term stability due to of the combination of laser pre-treatment and coating with polyelectrolytes. Adhesive bonds of laser treated samples with and without polyelectrolyte coating have a higher stability against corrosion compared to untreated samples.

In general fiber reinforced plastics feature a great variety of properties owing to the range of fiber types and matrix materials. The kind of intended application of the lightweight material demands an appropriate design and development of the material itself but also of the needed bonding joints within multi-material design. For certain areas of application textile reinforced plastics may be produced by several different manufacturing technologies and allow the realization of loaddependent as well as individually adjusted material systems. A thermoplastic matrix enables reduces for process times in manufacturing and allows subsequent forming of materials. Owing to the improvement in manufacturing processes, customized materials with certain compositions may be used for structural as well as crash and impact applications. Within the collaborative research center (hereinafter referred to SFB) 639 the development of complex structures from hybrid yarn textiles with a thermoplastic matrix is implemented within a specific interlocked process chain from the filament up to the component part. The pressing-related manufacturing process uses composite structures consisting of hybrid yarns out of glass and polypropylene fibers. This material combination is seen as a model material to develop and evaluate application-orientated solutions for the structural lightweight industry, focusing on the development and implementation of structural joints using this specific material type. The use of polypropylene and glass fiber is problematic in that it has poor adhesive characteristics due to the low surface energy that polypropylene offers. Therefore, it is a challenge to produce joints for structural applications . Besides this, it is interesting to investigate the influence of textile types which different amounts of glass fibers have on the adhesion quality of the joints, especially near the surface of the samples. The development and production of complex fundamental structures, like spacer fabric components, may necessitate the use of additional elements to implement complex joints . Furthermore, by incorporating metallic particles into the adhesive, the curing may be accelerated through induction heating.

Surface treatment and functionalization has become an extremely popular field, where many researchers from different disciplines combine their talents and efforts. In several cases, the ability to pattern surfaces at very small (sub-micrometer) lengthscales and to control the chemical and/or physical properties of surfaces at this level is crucial. These surfaces can be used in different application fields where improved optical, mechanical or biological properties can enhance the functions of products. A possibility to fabricate these surfaces is provided for example by laser processing techniques. However, generally these methods are sequential and are limited to structures with feature size between 10-30μm. In order to solve this problem, techniques involving largearea maskless surface structuring methods for rapid fabrication of two and three dimensional (2D and 3D) structures are required. An innovative solution for high speed surface patterning of periodic structures is Direct Laser Interference Patterning (DLIP). Here, periodic structures can be produced in different materials including metals, ceramics, polymers and coatings in a single step process. Until now laser treatment before adhesive bonding is done especially using commercial pulsed Nd:YAG or fiber laser with pulse duration between 100-300 ns . The heat input during the DLIP process is compared to other commercial laser structuring methods significantly lower and therefore also suitable for applications of thin metal or plastic foils. The DLIP method works with a high surface rate.

This article examines the development of a laser pretreatment method for glass fibre reinforced polypropylene surfaces for industrial applications. This work aims to create a reproducible surface for bonding low-energy polypropylene which adheres very poorly to most adhesives and forms to the matrix material for plastic composites. The combination of glass fibres with polypropylene in the form of hybrid yarns is intended to produce a low-cost and powerful engineered fibre composite with applications in high-technology industries. The key process is bonding the engineered fibre composite without modifying the material properties. This is done by adhesive bonding. For this purpose, various pretreatment processes were examined and compared, for example, surface degreasing, plasma pretreatment for final cleaning and activation, the use of peel ply and laser pretreatment. The laser pretreatment serves two purposes: the defined generation of different surface structures and the exposure of glass fibres to be able to exploit the adhesive properties of glass surfaces. Moreover, two processes of artificial aging were performed to simulate potential boundary conditions during future use thus ensuring well-founded assessment of the pretreatments. Possible maximum initial adhesive strength is not the only key factor in favour of a decision to use adhesive bonding. Rather is the resistance to aging in real ambient conditions relevant for the long-term usage and stable bonding behaviour. Finally, the surface pretreatment methods are compared with each other, assessed and critical issues of surface pretreatment and material are validated.