Time efficient laser modification of steel surfaces for advanced bonding in hybrid materials

To minimize the energy consumption and CO₂ emissions of vehicles, mass reduction is an omnipresent objective in the automotive industry. A promising approach to reducing mass while simultaneously ensuring the required safety relevant strength properties is the use of hybrid materials like fibre-metal laminates (FML) or steel structures partially reinforced with carbon fibre reinforced polymers (CFRP) in modern car structures. This is made possible by combining the best features of metals and fibre-reinforced matrix materials, e.g. the impact resistance of metals with the high stiffness and lightweight properties of compound materials [1, 2].

Hybrid structures are manufactured by adhesive bonding, which offers several advantages over mechanical joining, such as even force transmission compared to riveted or screwed hybrid structures with stress concentrations at the joining points [3]. In addition, the joining of dissimilar materials can lead to contact corrosion. This can be prevented by using an adhesive, which acts as a galvanic isolator between the materials [4]. Furthermore, adhesive bonding has a higher potential for light weight structures [5].

In order to achieve a solid adhesive bond, the surface condition of the joints is of essential importance [6]. Contaminants such as oil deposits have to be removed by degreasing [7]. Another way to improve direct adhesion and promote mechanical interlocking between the joints is to increase the surface area by various techniques such as anodising, sand blasting or laser ablation [1, 4, 8]. Among these, laser ablation stands out for its high precision, efficiency, environmental friendliness and simple implementation as an automated method in industrial applications [9]. In contrast to chemical processes, laser ablation can also be used for local surface treatments, which is beneficial for strengthening or repairing of metallic structures with CFRP patches [10].

Pulsed laser sources are common in surface treatment and micromachining applications [9, 11]. The pulse energy \({\text{P}}_{\text{e}}\) and the pulse power \({\text{P}}_{\text{p}}\) determine the amount of machined volume [11], i.e. they can also influence the intensity of the surface treatment. Both depend on the pulse frequency f as shown in formulas 1 and 2 [11]. By increasing the pulse frequency f, the pulse energy \({\text{P}}_{\text{e}}\) and the pulse power \({\text{P}}_{\text{p}}\) decrease.

Feng et al. [6] showed that laser modification of steel changes the surface morphology on a macro scale, which results in a higher bond strength in adhesive joints compared to steel surfaces without laser modification. Huang et al. [8] attributed the enhanced adhesion strength to mechanical interlocking between the joints caused by the periodic microstructure surface structure at the interface.

The energy of a laser pulse is absorbed by the metallic material, which causes the material to melt until it reaches the vaporization temperature [9]. This leads to the formation of a plasma plume, which generates a high internal pressure, also known as recoil pressure [12]. Under this pressure, the molten metallic material is ejected from the plasma plume and falls back to the material surface where it forms a layer of re-solidified debris. The laser forms a crater which captures the remaining molten material [9]. The emerged craters and re-solidified debris build the laser structured surface morphology. Kurtovic et al. showed that laser treatment of metallic materials modifies the surface even on the nano scale [13]. A thin porous oxide layer is formed, which significantly improves wettability and thus additionally increases the surface properties for adhesive bonding [14].

When it comes to the manufacturing of hybrid components, e.g. semi-finished FML products, the surface preparation should be maintained as efficient as possible, which is controlled by travel speed of the laser beam over the metallic surface, or scan speed.

So far, laser ablation to improve bonding properties has been implemented with scanning speeds far below the technical possibilities. Feng et al. worked with a scanning speed of 100 mm/s [6], Rotella et al. prepared the steel substrates for their lap shear tests at 500 mm/s [7], and Zinn et al. at 800 mm/s [4]. However, laser systems are able to operate at much higher scanning speeds. The maximum scanning speed of the laser system used for this work is 8000 mm/s [15], which implies a great potential regarding the efficiency of laser ablation. Rotella et al. showed that higher laser scanning speeds lead to a lower degree of surface modification due to the shorter interaction time between the laser beam and the surface [7]. In another study, Feng et al. found that a higher number of craters or dimples per square millimetre induced by the laser beam increases the shear strength of the adhesive bond [16]. At high scanning speeds, however, fewer craters are formed, which means that the scanning speed cannot be increased at will. Therefore, the purpose of this study is to achieve improved bonding properties of metallic surfaces that were laser treated with a high efficiency, as well as to find correlations between the process conditions of the laser treatment, the ratio between laser treated and untreated surface and the tensile shear strength of bonded joints.

There are almost no differences in chemical composition between the laser treated and untreated surface. The relatively small deviations are likely due to the accuracy of the measuring method. It should be noted that in an EDX, the chemical composition is not measured directly on the surface, but in an area just below it due to the penetration depth of the electron beam [19]. Thus, if a nanoporous zinc oxide layer was generated during laser ablation within the spots, it would probably be too thin to be verified by such a scan. To increase the depth of the oxide layer, a higher pulse energy is required [20]. However, it is more likely that the rough surface texture within a single laser spot, as shown in Fig. 2, is induced by the previously described resolidified debris than by the formation of a zinc oxide layer. This significantly increases the surface area in addition to the surface morphology caused by the laser spot formation.

When working with an increased scanning speed of the laser beam, the expansion of the surface is less pronounced due to a smaller number of laser spots. This means that the more laser spots there are in relation to the surface, the larger the surface itself and the better the bonding properties. This correlation is clearly shown in Fig. 9. It is evident that a higher density of laser spots on the steel surface leads to a higher shear strength of the bonded area. The relationship between spot density and shear strength is nearly linear, the coefficient of determination is approximately 0.92.

The evaluation of the spot density shows that, if a high scanning speed is required, the increasing distance between two laser spots shown in Fig. 4 can be compensated by raising the pulse frequency. With a pulse frequency of 45 kHz the surface area can be completely processed by the laser beam even at a high scanning speed of 4500 mm/s, which increases the shear strength by 15% compared to the untreated surface. However, as the pulse frequency f rises, the pulse power \({\text{P}}_{\text{p}}\) simultaneously decreases (see Eq. 2). Leone et al. [11] argued that once the pulse power falls below a certain limit, the laser ablation process becomes unstable. Thus, the possibility to work with a high scanning speed at high frequencies to improve the bonding properties is limited. In the range of lower pulse frequencies, in this case 15 kHz, the variation of the scanning speed has a high influence on the shear strength.

Semi-finished FML products are often further processed by deep drawing, for example car body parts like cross members or the B-pillar [21]. During deep drawing, different stress conditions and relative movements between the laminates occur [21]. Consequently, the maximum bond strength between the laminates is not ideal for every area of the FML. Hence, the scan speed variation can be used for the targeted generation of reproducible graded surface morphologies on the steel surface in order to optimize further manufacturing processes.

The SEM images (Fig. 8) reveal a primary cohesive failure behaviour on the laser spots, which is evidently the preferred fracture mode as it indicates that the joint strength has reached or exceeded the strength of the adhesive [9]. This is attributed to the increased surface area within the laser spots [6]. At a higher spot density, more adhesive residues remain on the surface, which further underlines the conclusion of Fig. 9. The glass fibres seen on Fig. 8d obviously come from the used adhesive, as described in 2.1. While residues of the adhesive are also visible on the untreated surface (Fig. 8e), their arrangement appears random in contrast to laser-treated surfaces. This would reduce the reproducibility and thus the possibility to grade the bonding properties of the steel surface. This is also confirmed by the high standard deviation of the shear strength of untreated steel surfaces in hybrid materials, see Fig. 7.

The influence of scanning speeds between 1500 mm/s and 4500 mm/s in laser surface treatments on the bonding properties of steel surfaces in hybrid materials was investigated.