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Unique spindle microstructures with an apex angle of ∼20° bring the ability of directional water collection to various biosystems (i.e., spider silk and cactus stem). This has great potential to solve the insufficient interfacial wetting for mechanical interlocking formation between polymers and substrates. In this study, the bioinspired spindle microstructures were easily fabricated through the deposition of molten materials by a nanosecond laser with an overlap ratio of 21% between laser spots and achieved superior interfacial wetting for commercial epoxy adhesive on aluminum substrates. Detailed analyses show that there are four mechanisms responsible for the superior interfacial wettability of bioinspired spindle microstructures: the Laplace pressure difference, newly formed aluminum oxide, the capillary effect, and no extra pressure from a trapped atmosphere. Consequently, the bioinspired spindle surface microstructures achieve a maximum improvement of ∼16 and ∼39% in interfacial bonding strength before and after water soak exposure compared to the as-received condition. Moreover, the stable interfacial wettability of bioinspired spindle microstructures ensures that the improved joint strength varied little with an increase in surface roughness from ∼1.7 to ∼12.8 μm. However, the interfacial wettability of common dimple microstructures deteriorated with an increase in surface roughness, which is indicated by the decreasing rule in the quadratic polynomial function of the interfacial bonding strength as the surface roughness increases from ∼2.1 to ∼18.2 μm.

The epoxy adhesive-galvanized steel adhesive structure has been widely used in various industrial fields, but achieving high bonding strength and corrosion resistance is a challenge. This study examined the impact of surface oxides on the interfacial bonding performance of two types of galvanized steel with Zn–Al or Zn–Al–Mg coatings. Scanning electron microscopy and X-ray photoelectron spectroscopy analysis showed that the Zn–Al coating was covered by ZnO and Al2O3, while MgO was additionally found on the Zn–Al–Mg coating. Both coatings exhibited excellent adhesion in dry environments, but after 21 days of water soaking, the Zn–Al–Mg joint demonstrated better corrosion resistance than the Zn–Al joint. Numerical simulations revealed that metallic oxides of ZnO, Al2O3, and MgO had different adsorption preferences for the main components of the adhesive. The adhesion stress at the coating–adhesive interface was mainly due to hydrogen bonds and ionic interactions, and the theoretical adhesion stress of MgO adhesive system was higher than that of ZnO and Al2O3. The corrosion resistance of the Zn–Al–Mg adhesive interface was mainly due to the stronger corrosion resistance of the coating itself, and the lower water-related hydrogen bond content at the MgO adhesive interface. Understanding these bonding mechanisms can lead to the development of improved adhesive-galvanized steel structures with enhanced corrosion resistance. [ABSTRACT FROM AUTHOR]

This work explores the mechanisms for the non-linear degradation of the lap-shear strength of adhesive-bonded 5052 aluminum alloy (AA5052) joints exposed to neutral salt spray (NSS) environment up to 1200 hours. The lap-shear testing results show that a mixed failure dominated by adhesive failure occurred in both unexposed and exposed adhesive-bonded AA5052 joints. The lap-shear strength of adhesive-bonded AA5052 joints declined sharply in the preliminary stage of NSS exposure (e.g. shorter than 240 hours), then it decreased moderately as the NSS exposure time increased. Differential scanning calorimetry (DSC), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and electrochemical noise measurement were applied to exploring the degradation mechanisms of the lap-shear strength of adhesive-bonded AA5052 joints. The adhesive properties remained nearly unchanged after exposure in the NSS environment. The decreased hydrogen bond at the AA5052/adhesive interface was responsible for the sharp degradation of the lap-shear strength of adhesive-bonded AA5052 joints in the preliminary stage of NSS exposure. In the later stage of NSS exposure (e.g. more than 240 hours), the surface oxidation of AA5052 substrates dominated the lap-shear strength degradation of adhesive-bonded AA5052 joints. [ABSTRACT FROM AUTHOR]

• Laser-induced physiochemical modifications improve joint strength of cast Al by 374%. • Four treatment strategies were examined for joint strength by tests and simulations. • Bonding edges normal to loading direction greatly determines joint strength. • Interfacial shear stress from simulation is in an asymmetric M-shape distribution. • Prediction model based Goland-Reissner achieves an overall error less than 10%. Surface physicochemical modifications induced by laser ablation treatment (i.e. rough morphology, removal of surface contaminants and newly-formed aluminum oxides) enhanced joint strength by 374% and improved the fracture mode from interface to cohesive. To acquire desirable joint strength with high processing efficiency, this study further investigates the influence of laser treatment strategies (i.e. distribution and area of laser-treated region) on shear bonding strength of structural adhesive joints with cast aluminum, which is also essential for industrial applications due to incomplete treatment of partial area in bonding region missing laser ablation inevitably occurring when processing complex part surfaces with variable curvatures. Therefore, four typical treatment strategies were designed by assigning laser-treated regions with various areas in different edges of adhesive bonding region, and analyzed through experimental and simulational studies. Moreover, comparative investigations show that the treatment strategy of assigning laser-treated regions on two bonding edges perpendicular to the loading direction more effectively improved the joint strength compared to the other three strategies, which is explained by an asymmetric M-shape distribution of interfacial shear stress from simulations. Furthermore, a theoretical model based on Goland-Reissner equation was established to predict joint strength in four treatment strategies with an overall error less than 10%. [ABSTRACT FROM AUTHOR]

• Laser ablation treatment is effective for adhesive bonding of metallic materials. • Laser-induced surface modifications reflect in morphology, chemistry and wettability. • Laser energy density is crucial to control the surface modifications in laser ablation. • Full wetting of adhesive upon laser-ablated surface is an important prerequisite. • Rough adhesive/metal bonding interface retards diffusion of corrosive solution. Surface treatments have been widely applied to a variety of metallic materials in order to modify their physicochemical properties and to improve both strength and durability of adhesive-bonded joints. Among available surface treatment methods, laser ablation has the advantages of efficiency, environmental friendliness and easy industrialization. In this work, the principle of laser ablation and its effect on the physicochemical properties of metallic materials are reviewed. The mechanisms accounting for the improved bonding strength and corrosion resistance of the adhesive-bonded joints fabricated from laser-ablated metallic materials are reviewed and discussed. Bonding strength of laser-ablated adhesively bonded joints increases with the increase of roughness and wettability of the metallic adherend surface, and the surface roughness contributes to the improved joint strength only when the adhesive is capable to sufficiently wet the laser-ablated surface to form mechanical interlocking at the bonding interfaces. Although the laser-ablated surfaces are more active with lower polarization resistances and higher corrosion current, which suggests that the laser-ablated surfaces are more susceptible to corrosion, the increased roughness by laser ablation plays a crucial role in the improved corrosion resistance of laser-ablated adhesively bonded joints, as rough surfaces effectively retard the diffusion of corrosive solution. [ABSTRACT FROM AUTHOR]

Adhesive bonding provides a solution for joining sheet molding compound (SMC) and metals. In this study, three typical automotive structural adhesives (i.e. polyurethane adhesive A236, epoxy adhesive 120HP and acrylic adhesive M10) were applied to joining galvanized steel and SMC to investigate the compatibility of adhesive type and adherend materials in terms of the lap-shear tensile strength of as-fabricated joints. In addition, adhesive-bonded joints after immersion in 63 °C deionized water for 0, 7, 14 and 21 days were mechanically tested. Results show that the joint strength gradually decreased with increasing exposure time. Among the adhesive-bonded joints fabricated from three adhesives, the lap-shear strength of A236 joints is the highest before and after water soak. It is also observed that the fracture mode of A236 joints switched from SMC substrate failure to the interfacial failure between galvanized steel and adhesive after exposure, while 120HP joints and M10 joints always fractured at the galvanized steel/adhesive interface during lap-shear tensile testing. The fracture surface observations indicate that the corrosion of galvanized steel extends inward the bonding area with an increase of exposure time, which is verified with an energy dispersive spectrometry (EDS) and can be explained by the moisture diffusion along the galvanized steel/adhesive bonding interface. [ABSTRACT FROM AUTHOR]

Laser surface treatment was used as a surface preparation method for adherend prior to adhesive bonding, which significantly improved the fracture mode from mixed to cohesive and consequently increased the lap-shear tensile strength by 27.5% and 28.2% for aluminum alloys AA7075 and AA6022 adhesive-bonded joints, respectively. Nevertheless, the laser-treated AA7075 joint exhibited a 13.9% higher strength compared to laser-treated AA6022 joint although both joints showed cohesive fracture modes, which was attributed to smaller adherend deflection of laser-treated AA7075 joint than that of laser-treated AA6022 joint. Digital image correlation technique was applied to monitoring the deflections of laser-treated AA7075 and AA6022 joints during lap-shear tensile testing, and the stress analysis of adhesive-bonded lap-shear joints was conducted by finite element simulation. It was found that the deflection angle of the laser-treated AA6022 joint reached to 6.1° at the peak tensile load and was ∼69% larger than that of laser-treated AA7075 joint (i.e. 3.6°), which resulted in a mixed stress state with higher peel stress (i.e. ∼8 MPa) on edges of adhesive bonding region and consequently led to a premature fracture due to the low peel resistance of adhesive-bonded joints. Furthermore, the effect of decreased adherend deflection on lap-shear tensile strength was investigated, and a linear increase of lap-shear tensile strength was found with the decrease of deflection angle for adhesive-bonded joints showing cohesive fracture modes. [ABSTRACT FROM AUTHOR]

Highlights • Pulsed Yb: fiber laser treatment enhances adhesive bonded strengths for Al alloys. • The improvement of laser treatment on joint strength increases with laser fluence. • Laser treatment at higher fluence induces a greater surface modification. • Laser-roughened surface mainly accounts for improved joint corrosion resistance. Abstract Surface treatment of the Al surface by pulsed Yb: fiber laser has proven to be effective in improving the adhesive bonded performance of sheet Al substrates. This study was conducted to expand the initial investigation to include sheet, extrusion, and cast Al alloys and measure the effect of laser ablation treatment on the bond strength both in ambient and after water soak. It is found that laser ablation treatment improved the joint strength for all three Al alloys through modification of the surface characteristics and exceeded the target bond strength. Moreover, the results after water soak support the conclusion that the laser ablation treatment exhibited a significant improvement in corrosion resistance of the adhesive-bonded Al joints compared to as-received condition and resulted in the required cohesive fracture mode even after water soak. The reason for this is attributed to the retarding effect of rough laser-ablated surfaces on water diffusion along the bonding interface of laser-treated joints. Besides, the improvement of laser ablation treatment on joint strength before and after exposure increases with the laser fluence. The optimized laser fluence for all three Al alloys is 26.3 J/cm2, which is attributed to a higher degree of laser-induced surface modifications. [ABSTRACT FROM AUTHOR]

Abstract: Recent trends toward economically fabricating lightweight vehicle structures while ensuring structural performance have led to the implementation of crash-toughened adhesive bonding in automotive industry. Adhesive bonding has been shown to offer better fatigue performance and greater flexibility in joining dissimilar materials when compared with resistance spot welding. While a great deal of effort has been focused on studying the performance of adhesive, there is an urgent need to understand the effect of curing conditions encountered in paint baking environment on the mechanical properties of adhesive-bonded workpieces. This study was carried out to determine the effects of curing temperature, curing time and enclosure conditions in the paint oven on the strength of adhesive-bonded structural steels. The enclosure conditions, simulating the exposure of the vehicle interior and exterior parts in paint baking environment, were classified into three types: fully-enclosed, semi-exposed and fully-exposed. Tests were designed and implemented to examine how the adhesive-bonded interior parts would cure in an electro deposition (ELPO) environment. The results showed that enclosure conditions influenced significantly the strength of the bonded steel because of variations in heat convection. The optimum curing temperature and time significantly depend upon the enclosure conditions of the joints. The increase of the curing time or curing temperature is beneficial for improving the strengths of the adhesive-bonded steel joints. [Copyright &y& Elsevier]